*Article* **Improving Fast Charging-Discharging Performances of Ni-Rich LiNi0.8Co0.1Mn0.1O2 Cathode Material by Electronic Conductor LaNiO3 Crystallites**

**Tongxin Li, Donglin Li \*, Qingbo Zhang, Jianhang Gao, Long Zhang and Xiaojiu Liu**

New Energy Materials and Devices Laboratory, School of Materials Science and Engineering, Chang'an University, Xi'an 710064, China; 2018031001@chd.edu.cn (T.L.); 2019131013@chd.edu.cn (Q.Z.); 2019231009@chd.edu.cn (J.G.); 2020131022@chd.edu.cn (L.Z.); 2021231096@chd.edu.cn (X.L.) **\*** Correspondence: dlli@chd.edu.cn

**Abstract:** Fast charging-discharging is one of the important requirements for next-generation highenergy Li-ion batteries, nevertheless, electrons transport in the active oxide materials is limited. Thus, carbon coating of active materials is a common method to supply the routes for electron transport, but it is difficult to synthesize the oxide-carbon composite for LiNiO2-based materials which need to be calcined in an oxygen-rich atmosphere. In this work, LiNi0.8Co0.1Mn0.1O2 (NCM811) coated with electronic conductor LaNiO3 (LNO) crystallites is demonstrated for the first time as fast chargingdischarging and high energy cathodes for Li-ion batteries. The LaNiO3 succeeds in providing an exceptional fast charging-discharging behavior and initial coulombic efficiency in comparison with pristine NCM811. Consequently, the NCM811@3LNO electrode presents a higher capacity at 0.1 C (approximately 246 mAh g<sup>−</sup>1) and a significantly improved high rate performance (a discharge specific capacity of 130.62 mAh g−<sup>1</sup> at 10 C), twice that of pristine NCM811. Additionally, cycling stability is also improved for the composite material. This work provides a new possibility of active oxide cathodes for high energy/power Li-ion batteries by electronic conductor LaNiO3 coating.

**Keywords:** Li-ion batteries; fast charging; LiNi0.8Co0.1Mn0.1O2; LaNiO3; electron transport; rate capability

#### **1. Introduction**

Li-ion batteries (LIBs) have received increasing attention for electric vehicles (EVs) and portable electronics due to their high energy density and long lifespan [1–3]. As a result, the development of LIBs with high capacity, high power/energy density, as well as long cycle life is necessary. The power density and energy density are significant parameters for LIBs [4]. Except for improving energy density, novel technologies and materials are needed to resolve the requirement for high power densities by enabling rapid charging-discharging rates without sacrificing cycling stability and energy densities. In particular, power density is critical for most applications, such as power grid stabilization and fast-charging EVs. Recently, despite the charging technology of LIBs having been intensively investigated, the current charging capability is still far from offering consumers the same refueling experience as conventional vehicles [5]. This is a significant reason, causing "range anxiety" for EVs owners and potential customers. Consequently, a higher charging rate with a shorter charging time is essential to achieve fast charging in the future [6–8].

Based on previous reports, multiple properties of the applied cathode, anode, and electrolyte materials affect the fast-charging capability of LIBs. Fast charging technology mainly depends on the transport rate of electrons and Li<sup>+</sup> between the LIBs components. To improve the fast-charging capability of LIBs, numerous research have been devoted toward reducing the diffusion length of Li+ and electrons by nanotechnology (such as

**Citation:** Li, T.; Li, D.; Zhang, Q.; Gao, J.; Zhang, L.; Liu, X. Improving Fast Charging-Discharging Performances of Ni-Rich LiNi0.8Co0.1Mn0.1O2 Cathode Material by Electronic Conductor LaNiO3 Crystallites. *Materials* **2022**, *15*, 396. https://doi.org/ 10.3390/ma15010396

Academic Editor: Sophie Tingry

Received: 3 December 2021 Accepted: 3 January 2022 Published: 5 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

nanofibers, nanotubes, or nanoparticles), and increasing the electronic and ionic conductivity by hybrid-composite design, Li-ion diffusion control, surface modification, and dopant manipulation [9,10]. Previous reports have demonstrated that electronic or ionic conductors can improve electronic or ionic conductivity, providing a positive impact on the fast charging of LIBs [11].

Compared with conductive carbon anode materials, the study on the influence of fast charging on the cathodes is still in its infancy [12]. At present, developing cathodes with high capacity, high operating voltage, and long lifespan is of great importance for practical application in LIBs. Traditional cathodes, such as LiCoO2, LiFePO4, and LiMn2O4, have limited reversible capacity (<200 mAh g<sup>−</sup>1), which cannot satisfy the requirement for high power and energy density LIBs [13,14]. Among numerous cathodes, LiNi0.8Co0.1Mn0.1O2 (NCM811) is considered as one of the most promising cathodes due to its high specific capacity (>200 mAh g<sup>−</sup>1), low cost, and high average redox potential [15,16]. Nevertheless, the inferior rate performance of NCM811 could be ascribed to the Li+/Ni2+ cation mixing, caused by the similar ionic radius between Ni2+ (0.69 Å) and Li+ (0.76 Å), resulting in reduced electrode kinetics and specific capacity [17–19]. Additionally, the poor cycling performance is closely related to moisture sensitivity and detrimental side reactions [20,21]. More importantly, the intrinsic poor electronic conductivity (10−<sup>5</sup> S cm−1) of NCM811 restricts its transport kinetics (e.g., rate performance) and cycling stability [22]. Moreover, these problems will worsen at high current density. To resolve the above problems, surface modification is considered as an efficient strategy to enhance the electrochemical performances of NCM811 cathodes. Among various coating materials, the majority of the lithium ionic conductors (Li3PO4 [23], Li2ZrO3 [24], Li3VO4 [25]) are focused on increasing the ionic conductivity of Ni-rich cathodes.

Furthermore, previous reports have demonstrated that electronic conductivity plays a crucial role in initiating the electrochemical process, and electron transport is critical to improving the electrode kinetics that dominate the power density of LIBs [26]. As a consequence, it is necessary to enhance the electronic conductivity of NCM811 cathode. It is well known that the high electronic conductivity carbon is beneficial to supply the electron transport pathway in the manufacturing process of LIBs. Thus, it is essential to add conductive agent (carbon black) into the conventional electrode materials. Moreover, many studies have reported that effective improvement of the electronic conductivity for NCM811 cathodes could be achieved by coating the carbon or conductive polymer. More recently, Sim et al. [27] prepared carbon-modified LiNi0.8Co0.1Mn0.1O2 cathodes using carbon black (Super P) as the carbon source, resulting in superior electrochemical performances. Furthermore, Zha et al. reported an efficient method to decorate the surface of LiNi0.8Co0.1Mn0.1O2 (NCM811) by combining with polyimide and carbon nanotubes. Compared with the modified-NCM811 (199.6 mAh g<sup>−</sup>1), NCM811 exhibits a higher initial discharge capacity (201.1 mAh g−1). However, it can be found that surface modification with carbon and conductive polymers is usually realized under an inert atmosphere and high temperature, leading to a poor rate capability or a loss in capacity. To improve the rate performance and specific capacity, it is necessary to achieve the coating process under an oxygen or air atmosphere, and the oxides could satisfy this demand.

The perovskite oxide LaNiO3 (LNO) has been intensively investigated in various applications in ferroelectric devices due to its highly electronic properties [28]. The rhombohedral structure of LaNiO3 oxide is metallic at all temperatures, and its high electronic conductivity (over 100 S cm<sup>−</sup>1) enough to act as an electrode [29–33]. Furthermore, LaNiO3 has been reported as a novel anode for LIBs, which exhibits superior electrochemical performances. More importantly, LaNiO3 could be annealed under an oxygen atmosphere. In this paper, we report the electronic conductor LaNiO3 as a coating layer to decorate the NCM811 surface for fast charging-discharging LIBs. The conductive LaNiO3 coating provides an effective electron transport pathway and serves as a protective layer that restrains the interfacial side reactions between the electrolyte and the NCM811 surface. Additionally, the impact of the LaNiO3 coating on the NCM811 cathodes is studied in detail.

#### **2. Materials and Methods**

#### *2.1. Synthesis of NCM811 Cathode Materials*

NCM811 was prepared via a typical sol-gel method as follows. First of all, stoichiometric amounts of Co(COOCH3)2·4H2O, Ni(COOCH3)2·4H2O, Mn(COOCH3)2·4H2O, and LiNO3 (with 5% excess) were dissolved together in ethyl alcohol to obtain a uniform solution. An excessive amount of LiNO3 was employed to compensate for possible lithium loss at high temperature. Afterward, acetylacetone (the molar ratio of transition metal ions to acetylacetone was 1:1) was added into the metal solution. The mixed solution was then evaporated with stirring in an 80 ◦C water bath. The obtained gel was dried at 100 ◦C and annealed at 450 ◦C for 2 h. After that, the acquired powder was ground, and calcined at 800 ◦C for 12 h in oxygen (denoted as pristine NCM811).

#### *2.2. Synthesis of LaNiO3 Surface-Modified NCM811*

A simple wet chemical method was employed to prepare the LaNiO3-modified NCM811 cathodes. Firstly, deionized water was used to dissolve Ni(NO3)2·6H2O and La(NO3)3·6H2O with the stoichiometric ratio of 1:1. Citric acid was then added under vigorous stirring to obtain the LaNiO3 transparent solution. Subsequently, the as-synthesized NCM811 sample was added into a required amount of LNO solution (from 0, 1, to 3 wt%), and the obtained suspension was stirred continuously at 80 ◦C to evaporate the water. Finally, the powder was dried at 100 ◦C and subsequently calcined at 700 ◦C for 3 h under flowing O2 to obtain the LaNiO3 surface-modified NCM811 materials. Based on the ratio of LNO to NCM811, the LNO-modified NCM811 materials were labeled as pristine NCM811, NCM811@LNO, and NCM811@3LNO, respectively.

#### *2.3. Material Characterization*

Powder X-ray diffraction (XRD, Bruker D8 ADVANCE) using a Cu target under 40 mA and 40 kV was used to characterize the crystal structure of samples. The XRD patterns were collected over the 2θ range of 15–90◦ with a step size of 0.02◦, and the scanning rate was 2.4◦ min−1. Furthermore, Rietveld refinement program—General Structure Analysis System (GSAS) software was used to further analyze the XRD data. Scanning electron microscopy (SEM, Hitachi S-4800) was employed to characterize the morphology. Elemental distribution on the surface of samples was analyzed by energy dispersive X-ray spectroscopy (EDS).

#### *2.4. Electrochemical Measurements*

Furthermore, 20 wt% carbon black, 10 wt% polyvinylidene fluoride (PVDF), and 70 wt% active material were dissolved together in N-methyl pyrrolidone (NMP), forming a slurry to prepare the electrodes. The slurry was cast on Al foil and dried at 100 ◦C. The mass loading of the active material was approximately 1–2 mg cm−2. The as-prepared electrode as the working electrode, Li metal as the reference electrode, and microporous polypropylene membrane (Celgard 2500) as the separator, assembling the CR2025-type coin cells in an argon gas-filled glove box. The electrolyte was 1 M LiPF6 dissolved in dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 vol/vol). After aging, a multichannel battery testing system (Neware Technology Co., Ltd., Shenzhen, China) was used to measure the electrochemical performances between 2.8 and 4.3 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out on a Princeton Applied Research VersaSTAT 3 electrochemical workstation. The EIS tests were conducted between 100 kHz and 10 mHz, and the voltage amplitude was 5 mV. The cyclic voltammetry was tested in the potential range between 2.8 and 4.5 V, and the scan rate was 0.1 mV s<sup>−</sup>1. The differential capacity versus voltage curves (dQ dV−1) were obtained according to the charge/discharge testing data of individual cycles.

#### **3. Results**

#### *3.1. Physical Characterizations*

The crystal structure of pristine NCM811, NCM811@LNO, and NCM811@3LNO samples were investigated by powder X-ray diffraction (Figure 1a) combined with Rietveld refinement (Figure 1b–d). The main diffraction peaks of NCM811@LNO and NCM811@3LNO samples are similar to those of pristine NCM811, which belong to hexagonal layered α-NaFeO2 structure (R−3m space group), suggesting that the LNO coating does not affect the crystal structure of NCM811 [34]. Additionally, all three samples exhibit clear splitting of (108)/(110) and (006)/(102), which implies a well-ordered layered structure [35]. Compared with pristine NCM811, NCM811@LNO and NCM811@3LNO samples exhibit a negligible change in lattice parameters (Table 1), suggesting that La3+ is not doped into the bulk structure. The (003)/(104) peak intensity ratio is closely related to the degree of Li+/Ni2+ cation mixing in the Li layer according to the literature [36,37]. Interestingly, NCM811@LNO and NCM811@3LNO samples exhibit higher I(003)/I(104) values compared to the pristine NCM811, suggesting that the LaNiO3-modified LiNi0.8Co0.1Mn0.1O2 samples have lower Li+/Ni2+ disordering. Previous research results demonstrated that Li/Ni disordering is harmful to the kinetic diffusion of Li ion during electrochemical cycling, thus deteriorating the rate capability and discharge capacity [38]. Obviously, in the NCM811@LNO sample, no diffraction peak corresponding to the LaNiO3 can be seen, which may be caused by the low content of LNO material. In addition, the relatively weaker diffraction peaks between 30 and 35◦ could be identified as the crystalline LaNiO3 (JCPDS #33−0711, labeled with \*) in the NCM811@3LNO sample, suggesting the successful introduction of LaNiO3 nanocrystals to the NCM811 [39]. Based on these results, we can speculate that the NCM811@LNO sample contains crystalline LNO owing to the identical preparation method.

**Table 1.** The lattice parameters of all three samples.


All three samples are composed of well crystalline particles, and the average particle size is approximately 300–700 nm. Clearly, the pristine NCM811 exhibits smooth and clear particle surfaces (Figure 2a,b), whereas some small nanoparticles can be seen on the rougher surface of NCM811@LNO (Figure 2c,d) and NCM811@3LNO (Figure 2e,f) samples. Clearly, with increasing LNO coating content, the amount of small nanoparticles increases gradually on the NCM811 particles surface. Additionally, Figure 3 presents the EDS elemental mapping of the NCM811@3LNO sample. Obviously, Mn, Ni, Co, O, and La elements are homogeneous distribution on the particle's surface. In addition, the atomic percentages (at.%) of La, Ni, Mn, and Co are 1.51, 38.68, 4.62, and 4.71 at.%, respectively. According to these results, it is concluded that the LaNiO3 can be evenly coated on the NCM811 surface by a simple wet chemical process, forming a conductive coating layer. As a consequence, the electronic conductor LNO crystallites coating can facilitate the electron transport on the surface, restrain the direct contact between the electrolyte and the cathode surface, and thus reduce the transition metal ions dissolution and the interfacial side reactions.

**Figure 1.** (**a**) XRD patterns of all three samples; the Rietveld refinement of (**b**) pristine NCM811, (**c**) NCM811@LNO, and (**d**) NCM811@3LNO samples.

**Figure 2.** SEM images of (**a**,**b**) pristine NCM811, (**c**,**d**) NCM811@LNO, and (**e**,**f**) NCM811@ 3LNO samples.

**Figure 3.** EDS elemental mapping of NCM811@3LNO sample.

#### *3.2. Electrochemical Performance*

The first charge/discharge voltage profiles of pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes at 0.1 C (1 C = 180 mA g−1) are presented in Figure 4a. Compared with the charge and discharge profiles of pristine NCM811, NCM811@LNO and NCM811@3LNO electrodes do not show any additional voltage plateau. Table 2 shows the first coulombic efficiencies and charge/discharge specific capacities of all three electrodes. It is conspicuous that the NCM811@3LNO electrode exhibits an excellent coulombic efficiency of 85.10% and an ultrahigh first discharge capacity of 246.39 mAh g−<sup>1</sup> at 0.1 C, far surpassing the pristine NCM811 (82.12% and 194.67 mAh g<sup>−</sup>1). The significantly enhanced coulombic efficiency and specific capacity of the NCM811@3LNO electrode can be attributed to the electronic conductor LaNiO3 coating layer that provides the electronic conduction pathway between particles, leading to fast electron transport.


**Table 2.** The electrochemical performances of all three electrodes in the initial cycle at 0.1 C.

To further investigate the phase transition behavior during the first charge and discharge process, Figure 4b–d presents the corresponding dQ dV−<sup>1</sup> curves of pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes, respectively. All the dQ dV−<sup>1</sup> curves exhibit the phase transitions from hexagonal to monoclinic (H1 to M) and then to other hexagonal (H2 and H3). Clearly, NCM811@LNO (0.0068 V) and NCM811@3LNO (0.0032 V) electrodes demonstrate the lower potential difference of anodic-cathodic peaks compared to that of the pristine NCM811 (0.0096 V). The decreased potential difference of NCM811@3LNO electrode implies improved electrode reversibility and reduced electrochemical polarization according to the literature [40,41]. The above results indicate that surface modification with electronic conductor LaNiO3 crystallites is beneficial to improve the electrode kinetics, leading to increasing the first coulombic efficiency and charge/discharge capacity and decreasing the electrochemical polarization degree of NCM811 cathodes.

**Figure 4.** (**a**) First charge and discharge voltage profiles, and corresponding dQ dV−<sup>1</sup> curves of (**b**) pristine NCM811, (**c**) NCM811@LNO, and (**d**) NCM811@3LNO electrodes at 0.1 C rate.

Figure 5a exhibits the rate capability of pristine NCM811, NCM811@LNO, and NCM81- 1@3LNO electrodes. Additionally, the charge/discharge voltage profiles of all three electrodes at different rates are displayed in Figure 5b–d. Compared with the pristine NCM811, LNO surface-modified NCM811 electrodes exhibit obviously improved high-rate performance. It is conspicuous that a higher discharge capacity of 130.62 mAh g−<sup>1</sup> is retained after 35 cycles at 10 C for the NCM811@3LNO electrode, corresponding to a capacity loss of 1.12% per cycle from an initial discharge capacity at 0.1 C. In contrast, pristine NCM811 electrode decreases dramatically to 69.50 mAh g−1, corresponding to a capacity loss of 1.81% at the same condition. More importantly, NCM811@3LNO exhibits a superior discharge capacity of 213.48 mAh g−<sup>1</sup> when the current rate recovers back to 0.1 C, far surpassing the pristine NCM811 (147.63 mAh g−1). The above results demonstrate that the conductive LaNiO3 surface-modified NCM811 cathodes exhibit significantly improved high-rate charge-discharge performance and electrochemical reversibility, suggesting easier electron transport in the LaNiO3-modified NCM811.

The cycling performances of pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes at 0.5 C are displayed in Figure 6a. At 0.5 C, the NCM811@3LNO electrode exhibits an ultrahigh first discharge capacity of 213.23 mAh g−1, significantly surpassing the pristine NCM811 (177.86 mAh g<sup>−</sup>1). Pristine NCM811 exhibits obvious capacity fading with increasing cycle number, maintaining only 62.92% after 50 cycles, and the synchronous decay in the dQ dV−<sup>1</sup> peaks (Figure 6c) can be observed. In contrast, the NCM811@3LNO electrode shows excellent capacity retention of 87.87% at the same condition, and the corresponding dQ dV−<sup>1</sup> profiles (Figure 6d) overlap well among various cycles, suggesting outstanding electrochemical reversibility. Besides, Figure 6b shows the cycle performances of all three electrodes at a higher rate of 2 C. Remarkably, the NCM811@LNO electrode maintains 84.95% of its original capacity after 100 cycles, far surpassing 58.31% of the pristine NCM811. These results suggest that the electronic conductor LaNiO3 crystallites can provide the electronic conduction pathway between particles, restrain the direct contact between the electrolyte and the cathode surface, thus decreasing the transition metal ions dissolution and the interfacial side reactions, leading to superior cycling stability of the NCM811 cathodes.

**Figure 5.** (**a**) Rate performances comparison; charge and discharge profiles of (**b**) pristine NCM811, (**c**) NCM811@LNO, and (**d**) NCM811@3LNO electrodes at different rates.

**Figure 6.** Cycling performances of pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes at (**a**) 0.5 C and (**b**) 2 C; the corresponding dQ dV−<sup>1</sup> curves of (**c**) pristine NCM811 and (**d**) NCM811@3LNO electrodes at various cycles at 0.5 C.

To further investigate the redox process and the electrochemical reversibility of NCM811 cathodes during charge-discharge cycling, the first three cyclic voltammograms (Figure 7) were recorded between 2.8 and 4.5 V, and the scan rate was 0.1 mV s−1. All electrodes exhibit three redox couples. Compared with pristine NCM811, NCM811@LNO and NCM811@3LNO electrodes exhibit similar CV features, suggesting that the redox process of the NCM811 is not affected by the presence of LaNiO3 coating. The potential difference (ΔV) of oxidation-reduction peaks is closely related to the polarization degree of the electrode materials and the reversibility of the electrochemical redox reaction according to the literature [42]. Significantly, the potential difference of the pristine NCM811 electrode is 0.052 V, which surpassed the NCM811@LNO (0.044 V) and NCM811@3LNO (0.023 V) electrodes. Furthermore, compared with pristine NCM811, the CV curves of NCM811@LNO and NCM811@3LNO electrodes overlap well in the 2nd and 3rd cycles, suggesting that the LaNiO3-modified LiNi0.8Co0.1Mn0.1O2 electrodes have quasi-reversible electrochemical kinetics during the Li+ insertion and extraction processes [43]. These results suggest that the electronic conductor LaNiO3 coating are conducive to improving the electrochemical reversibility and reducing the polarization degree of the NCM811 cathodes, which are consistent with the results from superior cycling performance and high-rate performance of the NCM811@LNO and NCM811@3LNO electrodes.

**Figure 7.** CV curves of (**a**) pristine NCM811, (**b**) NCM811@LNO, and (**c**) NCM811@3LNO electrodes.

To study the electrochemical kinetic behaviors for the high-rate performance, EIS of the pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes were measured after 100 cycles at 2 C. Two semicircles and an inclined line can be seen in all Nyquist plots (Figure 8a). Generally, the solution resistance (Rs) is represented by the high-frequency intercept at the real axis [44,45]. The surface-film resistance (Rsf) is represented by the first semicircle at high-frequency, while the charge-transfer resistance (Rct) between the electrolyte and cathode materials is represented by the second semicircle at mediumfrequency [46]. The inclined line at low-frequency stands for the Warburg impedance (Zw), which is related to the lithium ions diffusion in the bulk of cathode materials [47]. Furthermore, the following equation was used to calculate the Li+ diffusion coefficient (DLi+) from the Nyquist plots in the low-frequency region:

$$\mathbf{D}\_{\rm Li^{+}} = \frac{\mathbf{R}^{2}\mathbf{T}^{2}}{2\mathbf{A}^{2}n^{4}\mathbf{F}^{4}\mathbf{C}^{2}\sigma^{2}}\tag{1}$$

where C represents the concentration of Li+ in the NCM811 cathode, F represents the Faraday constant, A represents the surface area of the electrode, T represents the absolute temperature, *n* represents the amount of the electrons per molecule participating in the electronic transfer reaction, R represents the gas constant, and σ represents the Warburg coefficient, which can be obtained by the following equation from the slope of the linear

fitting of resistance (Z ) vs. the reciprocal square roots of the frequency (ω−1/2) in the low-frequency region [48]:

$$\mathbf{Z}' = \mathbf{R}\_{\\$} + \mathbf{R}\_{\text{cf}} + \sigma \boldsymbol{\omega}^{-1/2} \tag{2}$$

**Figure 8.** (**a**) Nyquist plots with an equivalent circuit (inset) of pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes; (**b**) the relationship plots between Z and ω−1/2 in the lowfrequency region.

Figure 8b presents the relationship between Z and ω−1/2. Moreover, Table 3 exhibits the fitting resistance and calculated DLi<sup>+</sup> values. Compared with pristine NCM811, NCM811@LNO and NCM811@3LNO electrodes exhibit lower Rsf and Rct values. Additionally, the calculated DLi<sup>+</sup> value for the pristine NCM811 electrode is 1.17 × <sup>10</sup>−<sup>14</sup> cm<sup>2</sup> <sup>s</sup>−<sup>1</sup> after 100 cycles, whereas the DLi<sup>+</sup> value for NCM811@LNO and NCM811@3LNO is 4.61 × <sup>10</sup>−<sup>14</sup> cm<sup>2</sup> <sup>s</sup>−<sup>1</sup> and 3.21 × <sup>10</sup>−<sup>14</sup> cm<sup>2</sup> <sup>s</sup>−1, respectively. These results are consistent with the results from the cycling performances at 2 C (Figure 6b), indicating that the high conductive LNO coating layer is conducive to facilitating the electron transport, suppressing the side reactions between the electrolyte and NCM811 surface, subsequently improving the electrode kinetics and reducing the interfacial resistance.


**Table 3.** Impedance parameters of pristine NCM811, NCM811@LNO, and NCM811@3LNO electrodes after fitting.

#### **4. Conclusions**

In this paper, electronic conductor LaNiO3 crystallite surface-modified LiNi0.8Co0.1 Mn0.1O2 cathodes were prepared, and the effects of conductive LaNiO3 coating on the LiNi0.8Co0.1Mn0.1O2 cathodes were studied. The present work indicates that LaNiO3 nanoparticles are uniformly distributed on the NCM811 particle surface, which is beneficial towards improving the electrode kinetics caused by the fast electron transport and restraining the direct contact between the electrolyte and cathode materials, leading to excellent cycling performance and high-rate capability of LaNiO3-modified LiNi0.8Co0.1Mn0.1O2 cathodes. As a result, surface modification with high electronic conductivity oxide is an effective method to improve the electrochemical performances of Ni-rich LiNi0.8Co0.1Mn0.1O2 cathodes, which can also be extended to other electrodes for fast charging-discharging LIBs.

**Author Contributions:** Conceptualization, D.L.; methodology, T.L. and Q.Z.; software, J.G. and L.Z.; formal analysis, Q.Z. and J.G.; investigation, T.L. and X.L.; data curation, L.Z. and X.L.; writing original draft preparation, T.L.; writing—review and editing, D.L.; supervision, D.L.; project administration, T.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No. 22179011 and 21473014).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available from the corresponding authors upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Perspective* **Flash Sintering Research Perspective: A Bibliometric Analysis**

**Eva Gil-González 1,2,\*, Luis A. Pérez-Maqueda 1,\*, Pedro E. Sánchez-Jiménez 1,3,\* and Antonio Perejón 1,3**


**Abstract:** Flash Sintering (FS), a relatively new Field-Assisted Sintering Technique (FAST) for ceramic processing, was proposed for the first time in 2010 by Prof. Rishi Raj's group from the University of Colorado at Boulder. It quickly grabbed the attention of the scientific community and since then, the field has rapidly evolved, constituting a true milestone in materials processing with the number of publications growing year by year. Moreover, nowadays, there is already a scientific community devoted to FS. In this work, a general picture of the scientific landscape of FS is drawn by bibliometric analysis. The target sources, the most relevant documents, hot and trending topics as well as the social networking of FS are unveiled. A separate bibliometric analysis is also provided for Reaction or Reactive Flash Sintering (RFS), where not only the sintering, but also the synthesis is merged into a single step. To the best of our knowledge, this is the first study of this nature carried out in this field of research and it can constitute a useful tool for researchers to be quickly updated with FS as well as to strategize future research and publishing approaches.

**Keywords:** flash sintering; bibliometric analysis; field assisted sintering; knowledge structure; ceramic materials

#### **1. Introduction**

Flash Sintering (FS), an electric Field-Assisted Sintering Technique (FAST) [1] for the densification of ceramic materials at a greatly reduced temperature and time, has gained widespread attention since it was established in 2010 by Prof. Rishi Raj's group from the University of Colorado at Boulder. It basically consists of simultaneously applying heat and a modest electric field to a green body [2] placed on a furnace, allowing the current to totally flow through the sample. The main material requirement is that it should possess a negative temperature coefficient of electrical resistance so that the electrical conductivity increases while heating. At a given applied electric field, there is a critical temperature at which there is a sudden non-lineal rise of the conductivity of the material, which is normally accompanied by instantaneous densification as well as photoluminescence [3]. This signals the flash event and it is now accepted that it is initiated by a thermal runaway induced by Joule Heating [4,5]. Much effort is also being devoted to understanding the underlying mechanisms of FS. A few driven mechanisms have been proposed, but none of them can solely explain the flash phenomenon, which still remains elusive [6–11].

Nevertheless, FS has many practical advantages. For instance, it has been proven to be an ecofriendly and versatile methodology, as a wide range of materials from insulators to conductors can be sintered within seconds at furnace temperature much lower than those employed in conventional processing [12–17], thereby minimizing the energy footprint. Additionally, in comparison to other FAST techniques such as Spark Plasma Sintering (SPS) [1], it does not require any sophisticated experimental setup; basically just a furnace and a power supply are the two essential components to carry out a FS experiment [18].

**Citation:** Gil-González, E.; Pérez-Maqueda, L.A.; Sánchez-Jiménez, P.E.; Perejón, A. Flash Sintering Research Perspective: A Bibliometric Analysis. *Materials* **2022**, *15*, 416. https://doi.org/ 10.3390/ma15020416

Academic Editors: Mattia Biesuz and Peter Tatarko

Received: 9 November 2021 Accepted: 2 January 2022 Published: 6 January 2022

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An inner working atmosphere is not a prerequisite either, but indeed it is possible to tune it to study its effect over the final properties of the material [14,19,20]. Moreover, some flashsintered materials have been granted special properties [21,22] and it has been shown that it is possible to sinter unstable oxides with volatile components and complex composition while preserving their stoichiometry and properties [23–26]. Very recently, in 2018, it was reported that sintering and synthesis can be merged into a single step, giving rise to what has been named Reaction or Reactive Flash Sintering (RFS) and constitutes [27], by itself, another important branch of research.

Given the advantages of FS, the widespread attention that it has received not only from the scientific community but also from the industry is not surprising [28]. Since it was proposed for the first time a decade ago, this research field has deeply evolved, constituting a true milestone in ceramic processing. The number of peer-reviewed articles has continued dramatically growing year by year with an annual growth rate of about 46% (see Figure 1). To the best of our knowledge, bibliometric analysis [29], which is a powerful tool to quantitatively identify essential variables in a particular research topic (such as top authors, institutions, trends of publication, collaboration, networking, etc.) has never been carried out on FS or RFS. Hence, the aim of this work is to gain insight into the knowledge structure, research perspective, and trends of this fascinating research field. It is noteworthy that the readings of these review articles are highly encouraged [10,18,28,30,31], as a comprehensive literature review is out of the scope of this work. The Biblioshiny web interface of the Bibliometrix R package [32] has been employed to conduct the bibliometric analysis. Peer-reviewed articles and reviews retrieved from the Web of Science (WoS) database have been analyzed based on several bibliometric indicators [33], i.e., quantity (productivity), quality (impact), and structural (networking) indicators. The results derived from this bibliometric analysis allow one to identify the target sources and trend topics in order to strategize future research and publishing approaches. Those publications that most influence this research field have been pointed out. Authors, institutions, and countries in terms of impact and the number of publications have also been identified. Additionally, the social structure of FS is shown by the clustering and collaboration networking from countries to institutions to authors. Moreover, a brief bibliometric analysis is also presented independently for RFS, given its impact and the scientific growth that it has experienced in a very short lifetime (just 3 years).

**Figure 1.** FS annual and cumulative publications from 2010 to December 2021.

This article is structured in several sections. The section herein provides a basic and brief introduction about flash sintering, referring to the most relevant literature. The second section explains in detail the methodology used to carry out the bibliometric analysis. The third section provides the most relevant results of this work (target sources, authors, most influential publications, and hot and trending topics) along with data visualization and social networking. The last part of this section is dedicated to RFS. Finally, the conclusions and final remarks are presented.

#### **2. Methods**

This section presents the design of this bibliometric study, which is schematically depicted in Scheme 1. As explained as follows, it implicitly contains the standard stages of bibliometric analysis, that is to say, the study design, data collection, data analysis, visualization, and interpretation [34,35].

**Scheme 1.** Study design and workflow diagram.

#### *2.1. Document Search*

Clarivate Analytics Web of Science (WoS), one of the largest databases of peer-reviewed articles in different disciplines, was used for the data collection. The documents included in WoS related to the topic FS were retrieved according to the search strategy shown in Scheme 1, which also includes the specific search query words and Boolean operators. As a search topic, the words Flash Sintering were used, which implies that WoS documents that contain these words in their titles, abstracts, or keywords were scanned. The search was restricted to Flash Sintering, excluding other derived hybrid methodologies such as Flash Spark Plasma Sintering (F-SPS) [36,37], Flash Microwave Sintering [38,39], or Ultrafast High-Temperature Sintering [40], as has been properly indicated in Scheme 1. The reaction or reactive flash sintering documents were also excluded, as a separate bibliometric analysis is provided about this topic (see Table S5). The document types were limited to Articles or Reviews and the time span from 2010 to 15 July 2021, whereas the language was set to English. Finally, manual abstract assessment and screening were performed for each document, resulting in a list of 318 documents, of which the meta-data fields were compiled to conduct the bibliometric analysis.

#### *2.2. Bibliometric Analysis*

The data analysis was performed with the web interface Biblioshiny of the Bibliometrix R package (Version 1.4). The meta-data files extracted from the 318 documents were converted into R-readable files. Biblioshiny allows an analysis of three metric levels: Sources, authors, and documents. Hence, the most relevant sources, top authors, and documents in terms of productivity and impact can be identified. Additionally, the knowledge structure can be conceptual, intellectual, and socially analyzed, so that thematic mapping and evolution, co-citation, and collaboration networks can be built, among other things.

The metrics analyzed and the derived results from this analysis are schematically depicted in the workflow diagram in Scheme 1. The source analysis points out the target sources, which are the journals where most of the documents are published. The author analysis as well as their countries and affiliations allow one to identify not only the most prolific authors but also the social structure of the FS community, showing co-authorship, institutional, and international networking. The document-level study identifies the most important publications in the field, while the keyword analysis provides a general overview of the most-studied topics and evolution over the years, which makes it possible to identify mainstream and trend topics.

#### **3. Results and Discussion**

*3.1. Flash Sintering (FS)*

#### 3.1.1. General Descriptive Information

Table 1 summarizes the essential information extracted from the analysis of the set of 318 documents retrieved from WoS from 2010 to 2021, using the word search query indicated in Scheme 1. Detailed information about each metric level is commented as follows.

**Table 1.** Main information.


#### 3.1.2. Scientific Production

FS has constituted a breakthrough in material processing, and since its inception in 2010, it has become a hot topic and a new paradigm in ceramic processing. As it is shown in Figure 1, the number of publications has kept on growing, with an average of approximately 32 documents per year and an annual growth rate of 46.56%. Note that hybrid methodologies such as F-SPS and Reaction or Reactive FS (RFS) have been excluded and only peer-reviewed articles and reviews have been considered. Indeed, the cumulative publications since 2010 follow an exponential trend, as Figure 1 shows. If the topic development continues along the same trend, by a simple fitting to an exponential growth equation, the predicted number of publications in 10 years will be 4400, one order of magnitude higher than nowadays.

There are probably several reasons behind such dramatic growth in such a short period of its lifetime. The main reason is, of course, the scientific relevance of FS for the scientific community, mainly for those interested in ceramic research. Moreover, the simplicity of the FS experimental setup is significant, and as mentioned, just a furnace and a power supply are strictly needed, unlike other FAST methodologies that require complex and expensive equipment [1]. Despite its simplicity, FS is an extremely powerful sintering method that can be successfully used for most ceramic materials, from dielectrics (BaTiO3 [41–47] or (Bi0.2Na0.2K0.2Ba0.2Ca0.2)TiO3 [48]) to ionic (Zirconia, YSZ [2,49–52], CeO2 or doped-CeO2 [53–58]) or electronic (TiO2 [19,22,59–62], BiFeO3 or substituted-BiFeO3 [24,27]) conductors. Interestingly, it can be also applied for processing ceramic composites of complex stoichiometry, metastable phases, or materials constituted by volatile species at the temperatures required for their sintering such as YSZ-Al2O3 composites [63–65], different types of solid state electrolytes [25,66,67], BiFeO3 [68,69], or K0.5Na0.5NbO3 [26,70–73]. Moreover, ceramics prepared by FS present very interesting properties rarely reported for materials obtained by convectional procedures. For example, it has been observed that FS specimens deform plastically before fracture when compressed at high strain, due to their extraordinarily high density of defects, such as stacking faults, dislocations, and twins [22,74] or that chemically inert ceramics are converted into active catalytic compounds by enhancing the concentration and reactivity of the ionic species [21,75]. Furthermore, the understanding of the flash phenomena still remains elusive and requires contributions from different scientific fields. This challenge has raised the interest of many researchers with different expertise, including theoreticians and experimentalists [76–79]. Another significant feature of FS is its ecofriendly nature and potential to possibly scale-up, as it requires less energy than conventional processing processes [80] and, therefore, it contributes to reducing CO2 emissions. The possibility of working under continuous FS with rolling electrodes has been proposed in the literature [81]. This idea is already being explored at a larger scale by the company Lucideon [82] in a pilot plant. Very recently, the possibility of homogenously sintering 3D-complex shaped ceramics by the application of a three-phase power supply has been reported, giving rise to what has been named Multiphase Flash Sintering (MPFS) [83]. MPFS is presented as a feasible option to overcome the shape restrictions of conventional FS specimens. Moreover, the capabilities of FS combined with other FAST techniques are evolving in other interesting "flash-based methodologies" that have experienced great development as well. That is the case of Flash Spark Plasma Sintering (F-SPS), where pressure and pulsed currents are simultaneously applied, enabling the homogeneous and energetically efficient sintering (by different electrodes architectures) of both electric conductive and insulating materials [36,37,84–88]. Contactless Flash Sintering (Contactless-FS) is another flash-based methodology. Plasma electrodes are used instead of the traditional metallic wires. The plasma not only heats the material but also carries the current to trigger the flash, minimizing some of the thermal management issues encountered in conventional flash sintering due to the sample–electrodes contact [76]. Similarly, Flame-assisted Flash Sintering (FAFS) uses a flame as an electrode and heating source [89]. It has proved to be an effective technique for the sintering of ceramic coatings on metallic substrates. The combination of FS with Cold Sintering has resulted in Cold Flash Sintering (CFS), where the presence of relatively small amounts of liquids, such as water of acetic acid, on the pellets are used as electrolytes and enables the flash event even at room temperature [90,91]. Last but not least, the efforts made by the pioneers in the field, Prof. Raj and others, have been relevant by spreading the topic through the scientific community by inviting visitors to their labs, collaborating with other groups, giving lectures, and organizing successful International Conferences on the topic "Electromagnetic/Electric Fields in materials processing" such as those held on 2016 and 2019 in Tomar (Portugal) or the symposiums arranged by the Materials Research Society (MRS).

#### 3.1.3. Source-Level Analysis

As mentioned, just peer-reviewed articles have been considered in this literature set. The analysis reveals that the documents have been published in 63 different journals (Table 1). Nevertheless, a close examination of Figure 2, which includes the sources where most of the articles have been published, shows that FS documents are concentrated in a few journals, i.e., *Journal of European Ceramic Society* and *Journal of the American Ceramic Society*. Indeed, those two are the core journals where more than one-third of the entire collection has been published. Additionally, these two journals together with *Scripta Materialia (Letters journal of Acta Materialia)*, *Ceramics International*, and *Acta Materialia* have published more than 65% of the analyzed set of documents. These journals are mainly targeted towards ceramic materials as well as their relationship between processing, microstructure, and properties, which makes sense as the vast majority of flash-sintered materials are ceramics provided that a negative temperature coefficient of electrical resistance is possessed [28]. It is noteworthy that these five journals are top-ranked journals of the first quartile according to the Journal Citation Report (2020) in the category of Material Science–Ceramics (*Journal of European Ceramic Society*, *Ceramics International*, and *Journal of the American Ceramic Society*) or Metallurgy and Metallurgical Engineering-Science (*Scripta* and *Acta Materialia*). Reciprocally, these journals are also the most locally cited sources in the set of documents analyzed (see Table S1). A locally cited source is a journal included in at least one of the reference lists of the analyzed document collection. Additionally, there are also articles devoted to FS in general high-impact journals such as *Nature Communications* [74] or *Science Advances* [22].

**Figure 2.** Top 15 sources with 3 or more published papers.

This simple source-level analysis shows that FS research is mainly published in highranked, peer-reviewed journals and provides an idea about the high quality of the research carried out in this field and the interest that it generates and attracts within the scientific community.

#### 3.1.4. Author-Level Analysis and Networking

As shown in Table 1, the author-level analysis reveals that this set of documents involved 670 authors from 29 different countries. Seven of those authors published singleauthored documents, whereas the rest participated in co-authored documents. Generally speaking, each document is written by 2.11 authors on average and the Collaboration Index, defined as the total authors of multi-authored documents divided by the total number of multi-authored articles, is 2.2. Additionally, Figure S1 (Supplementary Materials) depicts

the country's scientific production map, where it is qualitatively shown that the USA, China, and Italy are the most involved countries.

Table 2 includes a list of authors that have published, so far, more than 10 articles directly related to FS along with their local *h*-index. The local *h*-index has been calculated using the conventional procedure (number *h* of publications that have been cited *h* times or more [92]) but only considering the set of 318 documents analyzed here. More details about authors with the highest numbers of local citations (citations included in this set of 318 documents) and global citations can be found in Table S2. It must be noted that those lists have been exclusively elaborated with the information retrieved from articles directly related to FS, as explained in the methods section. Those dealing with other similar techniques, such as F-SPS and others, have not been contemplated in the lists. In any case, Table 2 and Table S2 should not be considered as any type of author's ranking, as most authors in those lists have a significantly larger number of publications and citations in different related scientific topics that have not been included here. Moreover, some authors have been involved in FS almost since the early days, while others have become involved in recent times. As a way of example, the production of the authors included in Table 2 over the 11-year period of lifetime of FS is depicted in Figure S2, where the bubble size is proportional to the number of documents and the color darkness to the citations. Regardless of this, Prof. Raj not only introduced the FS topic for the first time in 2010, but he is the most prolific author in terms of the quantity of papers, citations, and the local *h*-index.


**Table 2.** Authors with the highest number of publications in the analyzed documents set along with their local *h*-index.

The high level of interaction among authors working in FS is very relevant, as shown in the collaboration maps of Figure 3. This type of figure displays the knowledge structure of the research field by providing a general overview about its social structure or, in other words, how the FS scientific community interacts at different levels (authors, institutions, and countries) [35]. Qualitatively, the bubble size in these collaboration maps represents the number of documents, while the strength of the relationship is represented by the links; the thicker, the stronger. Moreover, the position represents the influence, placing the most influential items, i.e., author, institution, or country, at the center of the maps. The different colors or clusters denote common collaboration networks or sub-networks. Figure 3a includes the collaboration map just among authors included in Table 2. Interestingly, all authors in Table 2, apart from Prof. Chaim, have very strong collaborations with others

from the same list. Actually, in some cases, they even belonged to the same research group. That is the case of Biesuz who was a former PhD student in Sglavo's lab (Trento University, Trento, Italy), Charalambous in Tsakalakos' lab (Rutgers University, New Brunswick, NJ, USA), H Wang and Phuah in Haiyan Wang's lab (Purdue University, West Lafayette, IN, USA), while Lebrun was a postdoc in Raj's lab (University of Colorado at Boulder, Boulder, CO, USA). Moreover, there is a significant movement of authors from one institution into another that is helping to spread the topic. For instance, Jha who is currently an Assistant Professor at the Indian Institute of Technology Kanpur was a former PhD student of Raj and, later on, a postdoc in Tsakalakos's lab. Dianguang Liuwas a former PhD student at Northwestern Polytechnical University (working with Wang, Yiguang) and then moved to Southwest Jiaotong University where he works with Jinling Liu. Grasso is currently a professor at the Southwest Jiaotong University after having worked at the Queen Mary University of London. Moreover, as seen in Figure 3a by the solid connectors, there are many fruitful collaborations among different groups, resulting in co-authored publications. As mentioned, the behavior observed in Figure 3a is limited to authors from Table 2. Nevertheless, it is quite general and can be extrapolated to the whole FS community, as explained as follows by Figure 3b,c, which represents the countries and institutional collaboration maps, respectively, and aims to provide a broader overview of the scientific landscape of FS. As depicted in Figure 3b, there are authors from 26 different countries with at least one mutual publication. Note that the whole FS community involved authors from 29 countries, as is shown in Figure S1. This highlights, once again, the strong interconnection of this scientific community. Similar to Figure 3a, the USA is the country with the highest number of publications and the strongest collaboration network. Authors from China and Italy also have important scientific production and collaboration networks. Interestingly, even authors from countries such as Germany, Spain, India, and France, among others, have become involved in FS more recently and, therefore, while their number of publications is not that high, they have very strong collaboration networks. For example, in the whole document set, only five papers have been issued by corresponding authors from Spain, all of them being internationally collaborative publications. Figure 3c shows the collaboration network at an institutional level. The FS community involves authors from 217 institutions. Thus, for the sake of clarity and visualization, the institutional collaboration map has been limited to those 18 institutions with the highest number of published papers. Analogously to Figure 3a, the bubble size of the University of Colorado at Boulder (USA) and Trento University (Italy) is not surprising; as mentioned, the mostproductive authors in terms of publications belong to these institutions, i.e., Raj, Sglavo, and Biesuz. Moreover, both institutions have strong collaboration networks worldwide. For instance, Trento University, Southwest Jiaotong University, one of the most prolific Chinese institutions, and Queen Mary University of London (UK) have established a strong collaboration sub-network. As commented for Figure 3a, Southwest Jiaotong University and the Queen Mary University of London are the current and previous affiliations of Prof. Grasso, respectively. At the same time, other researchers from Southwest Jiaotong University (Liu Dg and Liu Jl among others) work with researchers from other academic centers, such as the Northwestern Polytechnical University (China) and the University of Central Florida (USA). Additionally, Figure 3c also unveils a strong collaboration subnetwork within American institutions composed of Rutgers University, Purdue University, Argonne National Laboratory, the University of California San Diego, and the University of California Davis. In albeit extreme simplification, this subnetwork is partially shown in Figure 3a, proving again its similarities with the whole FS scientific community. Most of the works of this American co-authorship sub-network involve some kind of in situ measurements during FS or to flash-sintered samples, such as energy-dispersive X-ray diffraction [15,53,69,93].

**Figure 3.** (**a**) Selected authors from Table 2, (**b**) countries, and (**c**) institution's collaboration networks. \* UC San Diego: University of California San Diego, IPEN: Instituto de Pesquisas Energéticas e Nucleares, QMUL: Queen Mary University of London, SWJU: Southwest Jiaotong University, NPU: Northwestern Polytechnical University, NIMS: National Institute for Materials Science, Technion: Technion–Israel Institute of Technology.

All in all, Figure 3 highlights the significant level of interaction among research scientists and groups in FS. It is evidenced that authors involved in this research field are quite eager to collaborate with others from different institutions and even countries. Thus, internationalization is at the core of FS. This practice may be related to the complexity of the FS process that involves physical and chemical phenomena. Thus, its understanding and implementation demand an interdisciplinary approach from Physics to Chemistry and Engineering, and it requires of the participation of both experimentalists and theoreticians. Moreover, collaborations with experts from large scientific facilities (National Laboratories) in the monitoring of the FS process under in situ conditions are also quite noticeable. This strong collaborative networking as well as the International Conferences in Tomar (Portugal) and MRS symposiums, mentioned above, are helping to create a sense of community among researchers worldwide, and it is probably contributing to the fast development of this relatively new research field.

#### 3.1.5. Document-Level Analysis. Influential Documents and Trending Topics

This section studies the most influential documents of the collection with respect to their number of citations as well as the most commonly tackled topics in FS by keyword analysis.

Table S3 includes the top 20 most-cited documents, where the number of local and global citations as well as their ratio are shown. A local citation refers to the number of citations that one document has received from the analyzed document set (in this case, 318 documents and all of them related to FS), whereas global citations are the total number of citations from WoS that may include documents that are not necessarily associated with the analyzed research topic. Therefore, in principle, the higher local citations, the more important and influential the document is in a particular research field. This list is ranked by the paper of Cologna, Rashkova, and Raj reporting the first demonstration of FS, which triggered the development of this research field [2]. This paper constitutes a true milestone in ceramic processing and to date has received more than 474 citations, 270 of which correspond to local citations. This is followed by a single-authored paper of Raj, addressing the role of Joule Heating during FS [8]. It entails one of the first works devoted to explaining the underlying mechanisms of FS, concluding that the fast sintering rates achieved in FS cannot be solely explained by Joule Heating. This work closely links with another top-ranked paper (fourth place) authored by Todd et al. [4]. The authors modelled the electrical and thermal response of 3YSZ under FS conditions and established that the flash event is triggered by a thermal runaway caused by Joule Heating. Indeed, it is now well-accepted within the scientific community that the thermal runaway induced by Joule Heating is the actual phenomenon that initiates the flash. The third most-cited paper both globally and locally corresponds to the work of Cologna et al. published in 2011 [13], which deals with the demonstration of FS in alumina, a highly insulating material. Finally, special attention is deserved for the paper ranked fifth, as this is the second literature review dedicated integrally to FS [28] (to the best of our knowledge, the first review was published by Dancer in 2016 [30]). It was published in *Advances in Applied Ceramics* in 2017 and, to date, has received more than 207 citations, where half of them correspond to local citations. It is also worth mentioning that 75% of the top 20 most-cited documents in FS are published in either the *Journal of the European Ceramic Society* or the *Journal of the American Ceramic Society.* As mentioned in the source-level analysis section, both are high-ranked journals dedicated to the study of ceramic materials, which once again highlights the quality of the research carried out by the scientific community of this field.

In order to provide some perspective about influential works recently published, we carried out a similar analysis while refining the time span to 2020 and 2021. Thus, Table S4 presents the most global and locally cited documents in 2020 and 2021. Most of these publications deal with the understanding of the FS mechanisms in ceria and titania [62,94] or the correlation between the effect of the experimental parameters and the induced defects on the final properties and microstructure of the flash-sintered materials [51,95–98]. Many of these documents are co-authored by early-stage researchers, such as Lavagnini [51] and Storion [96], both PhD students at the University of São Paulo (Brazil), or Phuah [95,98] and Mishra [94,97] who recently obtained their PhD degrees from Purdue University (USA) and Forschungszentrum Jülich (Germany), respectively. This is an indication that many earlystage researchers are developing their careers in FS and are making important contributions to the field.

Keyword analysis is commonly used in bibliometric analysis to systematically identify the document content, trend topics, and research hotspots of a particular research field. Table 3 contains the top 10 Keywords Plus and Authors' Keywords. Authors' keywords are provided by the authors themselves, whereas Keywords Plus are generated by an algorithm, extracting words that frequently appear in the title's references and not necessarily in the title of the articles or as Author Keywords [99]. Very recently, a study carried out by Zhang J. et al. reveals that both types of keywords identify very similar research trends and knowledge structures [100]. Indeed, the list of words included in Table 3 for both categories as well as the prevalence order is quite similar. Note that trivial keywords, such as Flash Sintering or Flash Sintered, have been cleaned. An examination of these keywords reveals that zirconia is the most-studied material. As mentioned, the first demonstration of FS was carried out with this material [2]. Since then, zirconia powders of different compositions have been widely used as a model material to study the underlying mechanisms of FS as well as the driving sources triggering the flash event, which is also linked to other top keywords such as Joule Heating, thermal runaway or defects. As a way of example, a few works are cited herein [4,50,101,102]. Another important part of the research carried out in FS deals with the study of the properties of the flash-sintered materials, such as their microstructure, abnormal grain growth, defect structures, etc. [22,103,104], some of them granted with special properties. This also explains some of the top keywords obtained, e.g., "microstructure", "grain growth", "defect structures".


**Table 3.** Top 10 Keywords Plus and Authors' Keywords.

Figure 4a shows the Keywords Plus dynamic, representing the frequency of each keyword as a function of time (from 2010 to 2020), which allows for identifying trend topics and research hotspots. For instance, it can be observed that zirconia has always been a hot topic in FS. As mentioned above, it is now well accepted that the flash event is triggered by a thermal runaway. Since it was proposed in 2015 by Todd et al. in the already-mentioned paper entitled "Electrical characteristics of flash sintering: thermal runaway of Joule heating" [4], the keyword thermal runaway has continued growing. Therefore, the contribution of Todd R.I. et al. constitutes an important milestone in FS. From Figure 4a, is also depicted that ZnO is currently a trending topic, and in the last few years its annual occurrence has been increasing. In a similar way to zirconia, ZnO has been widely employed as a model material in FS [103,105,106]. This Keyword Plus dynamic analysis agrees well with Figure 4b, which includes the top 10 most-studied materials during 2020 and 2021. As expected, Zirconia leads the ranking, with more than 38 publications, followed by ZnO. Figure 4b also reveals that another important

body of work in FS is dedicated to the sintering of materials with technological interest, which development is being hampered by the high temperatures required or other kinds of difficulties in their processing such as the volatilization of some of their components. That is the case of different sodium and lithium ion conductive ceramics for solid-state batteries [67,107], the lead-free piezoelectric ceramic potassium sodium niobate [70,73], or ZnO-Bi2O3-based varistor ceramics [108–110]. We expect that this trend will probably be maintained during the next few years. That is to say that Zirconia and ZnO will continue being hot topics. As mentioned, the FS mechanisms are still clouded, and further studies about their understanding will certainly be carried out. Therefore, it is quite likely that these two materials will continue to be used as models in future works dealing with the underlying nature of FS. On the other hand, besides the reduced temperatures and times offered by the FS technique, it has also proved to be an engineering tool to grant special and unexpected properties to materials [22,75]. Thus, researchers will continue exploring these FS capabilities and, therefore, the study of the properties of the materials prepared by FS as well as the preparation of new ceramic materials that are hard or impossible to prepare by conventional procedures will be another mainstream area of research.

Materiales

**Figure 4.** (**a**) Keywords Plus dynamic and (**b**) top 10 studied materials during the two-year period 2020–2011.

All in all, we would like to emphasize that this keyword analysis provides a general and brief overview of the document content as well as of the major tackled topics and future research prospects in FS. It is probably quite trivial for researchers who have been working in the field for a while but may be useful to strategize future research as well as a starting point for those researchers who are not acquainted but keen on FS.

#### *3.2. Reactive Flash-Sintering (RFS)*

As mentioned in the introduction, another important branch of research directly related to FS is Reaction or Reactive Flash sintering (RFS). The foundations and the working mode are quite similar but instead synthesis and sintering are merged in a single step. RFS was reported for the first time in 2018, showing that a highly dense and pure metastable oxide can be prepared in a matter of seconds from a mixture of its basic constituents [27]. RFS soon garnered the attention of the scientific community and, since then, many documents have been published exploiting the capabilities of RFS. Indeed, Figure 5a shows the scientific production for RFS with a remarkable number of 32 publications in just three years, which implies a dramatic annual scientific production growth rate of 100%. Thus, due to the relevance of RFS, we decided to carry out a separate bibliometric analysis to sketch the general scientific map for RFS. The specific word search query as well as the general descriptive information from the analyzed document set for RFS can be found in Tables S5 and S6, respectively.

**Figure 5.** (**a**) Annual and cumulative publications, (**b**) number of documents and total citations per source, and (**c**) country, author, and institution collaboration networks in RFS. The author and institution collaboration networks are limited to those with more than two publications and at least one co-authored document.

As can be observed from Figure 5b, RFS follows the same trend as FS in terms of targeted sources. *The Journal of the American Ceramic Society* and *the Journal of the European* *Ceramic Society* are where almost 50% of the entire collection have been published. Those two journals together with *Ceramics International* and *Scripta Materialia* published 80% of the entire collection. All of them are highly ranked journals according to Journal Citation Reports (2020) and accumulate most of the citations, which highlights, once again, the quality and relevance of the topic.

Nevertheless, the most-relevant publication of this set of documents is probably from *the Journal of Materials Chemistry A*. This journal published just one paper about RFS, but, by itself, it accumulates 20% of the global citations of the entire set. This document was authored by Gil-Gonzalez et al. in 2018 [27] and constitutes, to the best of our knowledge, the first demonstration of RFS. It has received 46 global citations with an average of 11.50 citations per year. Additionally, it is also the most-cited document of the set. Seventeen documents out of twenty-five have cited it, which makes this paper the most relevant one for the research topic of RFS. Details about the historical direct citation network in RFS can be found in Figure S3. Further details about authors, document impact, in terms of quality and quantity, as well as the most studied materials in RFS can be found in SI (Tables S7–S10). It is worth mentioning that according to Table S10, RFS has been mostly dedicated to the preparation of materials with technological interest. This is probably due to the advantages of reduced temperatures and times offered by the technique. A significant number of works have been dedicated to the preparation of high-entropy oxides [111–115], followed by ceramics such as the multiferroic BiFeO3 and related materials [27,80,116], solid electrolytes [117–119], or the lead-free piezoelectric potassium sodium niobate [120,121]. Analogously to Zirconia or ZnO in FS, the underlying mechanisms in RFS have been studied by high-resolution in situ measurements in the reaction of MgO and Al2O3 to form the spinel MgAl2O4 [122,123].

Figure 5c represents the collaboration networks in RFS from countries to institutions and authors. The RFS community is formed by 82 authors and just one of them published single-authored documents (Prof. Chaim from Technion Israel Institute of Technology). On average, each document is co-authored by 5.04 authors with a collaboration index of 3.38 (See Table S5). As shown in the inset of Figure 5c, the documents come from six different countries: The USA, China, Brazil, Spain, India, and Israel. All authors internationally collaborate, besides authors from Israel with two single-country publications [124,125]. More details about author clusters and institutions are shown in Figure 5c. Note that for the sake of visualization, the collaboration network is limited to authors with more than two publications and at least one co-authored document, and unfortunately, Figure 5c does not show the whole RFS networking. From a simple visual inspection, it can be identified that the most prolific authors in number of publications are Prof. Raj and Yoon from the University of Colorado at Boulder, with seven and six publications, respectively (see also Tables S7 and S8). Note that the bubble size is related to the number of publications. Additionally, Figure 5c also unveils the collaboration sub-networks within the RFS community. Analogously to FS, internationalization and collaboration are at the core of this scientific community. For instance, authors from the University of Colorado at Boulder strongly collaborate with Ghose from Brookhaven National Laboratory [117,118,122,123] and authors from Seville University (Spain) [27] and the Federal University of Sergipe (Brazil) [117]. Indeed, the most relevant works of RFS have been a result of these collaboration sub-networks such as the already-mentioned document of *the Journal of Materials Chemistry A* by Gil-Gonzalez et al. [27] or that by Yoon et al. [123], where RFS of MgO and α-Al2O3 were studied by in situ synchrotron measurements in Brookhaven National Laboratory. This document received 19 global citations, 12 of which are local citations, being one of the most-relevant documents for RFS. Another strong collaboration network is formed by authors from Chinese institutions such as Southwest Jiaotong University, Beijing Institute of Technology, and Northwestern Polytechnical University, who work with An from the University of Central Florida. Their research is primarily dedicated to the preparation of high-entropy oxides by RFS [112,113]. Researchers from Chang'an University working on the preparation of piezoelectric materials also form another cluster [121,126]. It is worth mentioning that China is the second most-prolific country in the number of RFS publications, just behind the USA, at 9 vs. 11 documents. Finally, another important cluster of authors is formed by researchers affiliated to the University of Illinois at Urbana-Champaign, whose works have been devoted to study the transformation of manganese oxides during RFS [127,128].

#### **4. Conclusions**

FS has constituted a truly breakthrough in materials processing, and since it was proposed for the first time by Prof Raj in 2010, the number of publications has grown exponentially. In this work, we report the scientific landscape of FS by bibliometric analysis, identifying key aspects and peculiarities of the FS community. The target journals where most of the FS papers are published have been pointed out. All of them are dedicated almost exclusively to ceramic materials and are highly ranked journals of the first quartile according to JCR. This highlights the quality of the research carried out in the field. Socially speaking, the knowledge structure has been depicted at different levels, from countries to institutions and authors. A detailed analysis of the interaction of the authors with the highest number of publications is provided. This unveils important collaboration subnetworks that describe the general social structure of the FS scientific community. One of the most striking features is the large number of fruitful national and international collaborations among authors involved in FS with co-authored publications. It seems to be part of the core of this research field. It may be related to the complexity of the flash event that requires the contribution of experts from different disciplines for its understanding and development. The most influential documents in terms of local and global citations have been also identified. A brief description about the topics addressed in those documents is presented as well. Finally, the most recurrent keywords that best describe the document content have been analyzed along with the most-studied materials in 2020 and 2021, identifying the most-tackled and mainstream topics in FS. They reveal that zirconia has always been a hot topic in FS and the documents frequently deal with the understanding of the underlying mechanisms of FS or the non-typical properties granted to flash-sintered materials, such as the abnormal grain growth or defects. As a future research prospect, it is predicted that these topics will be maintained in the next few years. Finally, due to the dramatic scientific growth experienced in RFS in a very short period of time, a bibliometric analysis is provided separately for RFS. A detailed collaboration network is laid out, interestingly showing that the most influential works in the field of RFS are the results of international or national collaboration between authors from different institutions.

All in all, the aim of this work is to draw a general picture of the scientific landscape of FS and RFS by a bibliometric analysis, where the target sources, the most relevant documents, hot and trending topics, and social networking have been identified. To the best of our knowledge, this is the first study of this nature carried out in the field of FS. We believe that this work can be of interest not only for researchers working in the field but also for those who are keen on but not acquainted with FS and RFS. It can be a useful tool to strategize future research and publishing approaches as well as to be quickly updated with this research field, in spite of the high number of scientific publications and the dramatic growth that the field is experiencing.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma15020416/s1, Figure S1: Country scientific production map; Figure S2: Production over time of authors included in Table 2; Figure S3: Historical direct citation network in RFS; Table S1: Most local cited sources; Table S2: Most local cited authors along with their global citations; Table S3: Top-20 most cited documents; Table S4: Most cited documents published in the last two years (2020–2021); Table S5: Word search query for RFS; Table S6: Main information about RFS document sets; Table S7: Authors with three or more publications and local h-index in RFS; Table S8: Most local and global cited authors in RFS; Table S9: Top-5 most cited documents in RFS; Table S10: Top Materials in RFS.

**Author Contributions:** Conceptualization, E.G.-G. and L.A.P.-M.; methodology, E.G.-G.; software, E.G.-G.; validation, L.A.P.-M., P.E.S.-J. and A.P.; formal analysis, E.G.-G. and L.A.P.-M.; investigation, E.G.-G.; resources, L.A.P.-M.; data curation, E.G.-G.; writing—original draft preparation, E.G.-G. and L.A.P.-M.; writing—review and editing, P.E.S.-J.; visualization, E.G.-G.; supervision, L.A.P.-M., P.E.S.-J.; project administration, L.A.P.-M.; funding acquisition, L.A.P.-M.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Spanish Government Agency Ministerio de Ciencia, Innovación y Universidades and FEDER grant numbers CTQ2017–83602-C2–1-R and CTQ2017– 83602-C2–2-R), Junta de Andalucía-Consejería de Economía, Conocimiento, Empresas y Universidad grant number P18-FR-1087, FEDER grant number US-1262507 and INTRAMURAL-CSIC grant numbers 201960E092 and 202060I004.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Article* **Study on Oxygen Evolution Reaction Performance of Jarosite/C Composites**

**Junxue Chen †, Sijia Li †, Zizheng Qu, Zhonglin Li, Ding Wang, Jialong Shen \* and Yibing Li \***

Department of Materials Science and Engineering, Guilin University of Technology, Guilin 541000, China; cchenjunxue@gmail.com (J.C.); lisj701023@gmail.com (S.L.); dadpro01@gmail.com (Z.Q.);

dahe121133@gmail.com (Z.L.); dingnvhuang@gmail.com (D.W.)

**\*** Correspondence: Jialong.Shen@glut.edu.cn (J.S.); lybgems@glut.edu.cn (Y.L.)

† These authors contributed equally to this work.

**Abstract:** In the electrolysis of water process, hydrogen is produced and the anodic oxygen evolution reaction (OER) dominates the reaction rate of the entire process. Currently, OER catalysts mostly consist of noble metal (NM) catalysts, which cannot be applied in industries due to the high price. It is of great importance to developing low-cost catalysts materials as NM materials substitution. In this work, jarosite (AFe3(SO4)2(OH)6,A=K+, Na+, NH4+, H3O+) was synthesized by a onestep method, and its OER catalytic performance was studied using catalytic slurry (the weight ratios of jarosite and conductive carbon black are 2:1, 1:1 and 1:2). Microstructures and functional groups of synthesized material were analyzed using XRD, SEM, FI-IR, etc. The OER catalytic performance of (NH4)Fe3(SO4)2(OH)6/conductive carbon black were examined by LSV, Tafel, EIS, ECSA, etc. The study found that the OER has the best catalytic performance when the weight ratio of (NH4)Fe3(SO4)2(OH)6 to conductive carbon black is 2:1. It requires only 376 mV overpotential to generate current densities of 10 mA cm−<sup>2</sup> with a small Tafel slope (82.42 mV dec−1) and large Cdl value (26.17 mF cm<sup>−</sup>2).

**Keywords:** jarosite; ammoniojarosite; electrocatalyst; oxygen evolution reaction; stability

#### **1. Introduction**

With the increasingly negative impact of fossil fuels on the environment, it is with a huge demand that modern science and technology need to pursue clean and sustainable energy [1,2]. Hydrogen production by water splitting, as a technology for producing clean energy, has caused extensive researches [3,4]. The electrochemical water splitting process includes the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. The HER reaction is a two-electron transfer process, while OER is a four-electron transfer process, whose higher energy barrier dominates the rate of the cathodic hydrogen production [5]. Therefore, the development and research of costeffective OER catalysts with high activity and long-periodic cycle stability is a primary task.

Noble-metal-based materials, including IrO2 and RuO2, are state-of-the-art OER electrocatalysts because of their high electrocatalytic OER activity both in alkaline and acidic solutions [6–8]. However, the large-scale application of IrO2 and RuO2 in OER is severely limited not only by the high cost but also by the scarcity of Ir and Ru [9]. Thus far, considerable research efforts have been devoted to the exploration of low-cost and highly active noble-metal-free catalysts to replace expensive and scarce precious catalysts [10–12]. Especially for transition metal Fe-based materials, including oxides/hydroxides [13–15], chalcogenides [16], phosphides [16,17], and nitrides [18], which have been investigated extensively as promising candidates for the OER.

Compared with these compounds, Fe-based polyanionic compounds [1–4], such as jarosite, are an earth-abundant natural mineral that belongs to the alunite supergroup with the formula AFe3(SO4)2(OH)6, where A represents different monovalent cations, such as

**Citation:** Chen, J.; Li, S.; Qu, Z.; Li, Z.; Wang, D.; Shen, J.; Li, Y. Study on Oxygen Evolution Reaction Performance of Jarosite/C Composites. *Materials* **2022**, *15*, 668. https://doi.org/10.3390/ ma15020668

Academic Editor: Marc Cretin

Received: 30 November 2021 Accepted: 11 January 2022 Published: 17 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

K+, Na+, NH4 +, and H3O+. At present, jarosite has been extensively studied by the acid leach mining industry due to the precipitation of jarosite in acidic media—a crucial step that allows for the physical separation of Fe3+ and other cations from the leach solution [19–23]. As a result, these refining plants produce large amounts of environmentally hazardous jarosite wastes that currently provide no commercial value. Fortunately, several researchers have explored the use of jarosite as a cathode in the lithium and sodium-ion battery [24–26]. According to our best efforts, there is no research on jarosite in OER so far. Therefore, exploring and tapping the potential of these environmentally harmful jarosite wastes in the field of OER is an attractive strategy to provide economic advantages for jarosite.

In this work, the design of applying jarosite to OER catalytic material is proposed. A simple one-step method was used to synthesize four various types of jarosite materials ((NaFe3(SO4)2(OH)6), KFe3(SO4)2(OH)6), (NH4)Fe3(SO4)2(OH)6, and ((H3O)Fe3(SO4)2(OH)6)). We explored the OER performance of these four catalysts under acidic, neutral and alkaline conditions. The electrochemical test results suggest that (NH4)Fe3(SO4)2(OH)6 shows the best OER activity among the four catalysts. When the weight ratio of (NH4)Fe3(SO4)2(OH)6 to conductive carbon black is 2:1, the overpotential of (NH4)Fe3(SO4)2(OH)6 is 376 mV at a current density of 10 mA cm−<sup>2</sup> in alkaline conditions. Although this performance is not comparable to that of precious metals (IrO2), there is a lot of room to improve the performance, which is our future research task. In short, exploring the application of jarosite waste in OER can not only protect the environment but also improve its economic value.

#### **2. Experimental Section**

#### *2.1. Materials*

Fe(SO4)·7H2O (99.0~101.0%), K2CO3 (99%), Na2CO3 (99.8%), (NH4)2SO4 (98.0%), NH3·7H2O (25.0~28.0%) and C2H5OH (99.7%) were purchased from Guangxi Dragon Technology Company (Guangxi, China). Conductive carbon black (Ketjenblack) and Nafion solution (5 wt%) were purchased from Suzhou Yilongsheng Energy Technology Co., Ltd. (Suzhou, China) All chemicals used were of analytical grade and there was no need to further purification.

#### *2.2. Preparation of (A)Fe3(SO4)2(OH)6, (A = K+, Na+ and NH4 +)*

FeSO4 and (NH4)2SO4 were dissolved in 250 mL and 100 mL of deionized water to form solution A (0.02 M FeSO4) and solution B (0.5 M (NH4)2SO4), respectively. Dilute H2O2 solution was added dropwise to solution A to oxidize Fe2+ to Fe3+ and then solution B was injected into the above solution with a water bath at a constant temperature of 95 ◦C and stirred magnetically for 3 h, meanwhile, the pH value of the mixed solution was maintained throughout at 1.5–2.0 with 1 M ammonia solution. Finally, the yellow precipitate (NH4)Fe3(SO4)2(OH)6 catalyst was collected, washed with deionized water and then dried in a vacuum at 80 ◦C. The main synthesis procedure for Na/KFe3(SO4)2(OH)6 is similar to that for (NH4)Fe3(SO4)2(OH)6, corresponding to the use of 0.5 M Na2CO3 and K2CO3 instead of 0.5 M (NH4)2SO4 solution and the replacement of the pH adjuster with 1 M Na2CO3 and K2CO3 solution, respectively.

#### *2.3. Preparation of (H2O)Fe3(SO4)2(OH)6*

A certain amount of FeSO4 was dispersed in 70 mL of deionized water, with stirring at 95 ◦C. H2O2 was used to oxidize Fe2+ to Fe3+. After it was completely oxidized, the reactants were transferred to a 100 mL reactor at 120 ◦C and kept for 12 h. The final product was collected after filtration and washed with deionized water several times.

#### *2.4. Preparation of Working Electrode*

Add 10 mg NH4-Fe3@KB-1((NH4)Fe3(SO4)2(OH)6 and conductive carbon black with a mass ratio of 2:1). These were dispersed into a mixed solvent of Nafion (30 μL), anhydrous ethanol (400 μL) and deionized water (600 μL). Form a uniform dispersion after ultrasonic for 30 min, then use a pipette gun to take the dispersion (4 μL), add to the glassy carbon electrode and dry. The working electrodes of other catalytic materials are prepared by using the same method.

#### *2.5. Characterization*

Scanning electron microscopy (SEM, Hitachi Works, Ltd., Tokyo, Japan) was conducted on S-4800 at 5 kV to perform microstructure analysis. The phase structure of the sample was analyzed by a X-ray powder diffractometer (XRD, X'Pert PRO, PANalytical B.V, Cu Kα, 40 kV, 40 mA, λ = 1.54056 Å, PANalytical B.V., Almelo, The Netherlands) at a scanning rate of 5◦·min<sup>−</sup>1. The functional groups of the samples were analyzed by Fourier transform infrared (FTIR, Thermo Nexus 407 spectrometer, White Bear Lake, MN, USA) and the laser Raman confocal microscope Raman spectrometer (Raman, Thermo Fisher Scientific DXR, thermoelectric company, 532 nm, White Bear Lake, MN, USA). Transmission electron microscopy (TEM, JEOL, Beijing, China) images were collected on Titan G260-300 at an acceleration voltage of 200 kV. Before BET and BJH measured via Surface area and pore porosimetry analyzer NoVA 1200e (Quantachrome Instruments, Shanghai, China), all samples were degassed for 5 h at 100 ◦C.

#### *2.6. Electrochemical Measurements*

All electrochemical measurements were conducted on a computer-controlled CHI 760E electrochemical workstation with a conventional three-electrode system. The glassy carbon electrode with a diameter of 3 mm was used as a working electrode, Ag/AgCl electrode and Hg/HgO electrode were respectively used as the reference electrode for the acidic (neutral) system and alkaline system, and graphite rods were used as a counter electrode. The measurements were performed in three different electrolytes, 0.05 M H2SO4, 1 M KOH, and 1 M PBS. All the potentials were converted to reversible hydrogen electrode (RHE) based on the formula ERHE = EHg/HgO + 0.0591 × pH + 0.098 and ERHE = EAg/AgCl + 0.0591 × pH + 0.1976. The polarization curves were measured at 5 mV s−<sup>1</sup> and iR-corrected. Tafel plots were calculated using the Tafel formula *η* = *b* log *j* + *a*, where *j* is the current density, *b* is the Tafel slope, and *a* is the intercept relative to the exchange current density. EIS measurements were conducted under a particular applied potential in the frequency range 0.1 Hz to 100 kHz. The electrochemically active surface area (ECSA) was estimated by the double-layer capacitance (Cdl). The time-current curve was measured at a fixed voltage corresponding to 10 mA cm−<sup>2</sup> of current density. All tests were performed at room temperature.

#### **3. Results and Discussion**

#### *3.1. Characterization of Samples*

Four various types of jarosite were synthesized, named NaFe3(SO4)2(OH), KFe3(SO4)2(OH)6, (NH4)Fe3(SO4)2(OH)6 and (H3O)Fe3(SO4)2(OH)6, respectively. The crystal structure of various types of jarosite was firstly investigated by X-ray diffraction (XRD). As shown in Figure 1a, the diffraction pattern of the as-prepared jarosite can be indexed to the hexagonal system with a space group of R3m, suggesting the successful preparation of the jarosite samples.

Figure 1b shows the infrared spectrum test chart of the jarosite. The infrared absorption peaks of different jarosite appear at similar positions. The peaks appearing at 469 cm−<sup>1</sup> and 502 cm−<sup>1</sup> are Fe-O peaks. The corresponding peaks at 624 cm<sup>−</sup>1, 1082 cm−1, and 1204 cm−<sup>1</sup> are SO4 <sup>2</sup>−. The broad and strong absorption peaks at 1004 cm−<sup>1</sup> and 3416~3700 cm−<sup>1</sup> are the stretching vibrations of –OH and the weaker absorption peak at 1638 cm−<sup>1</sup> is caused by the bending vibration of H2O [27,28]. A sharp peak appears at 1425 cm−<sup>1</sup> in the infrared spectrum of (NH4)Fe3(SO4)2(OH)6, which is regarded as the absorption of the –NH4 peak. According to the findings in the report [15], transition metal hydroxides have good OER catalytic performance and the presence of hydroxyl groups in jarosite makes it possible to have OER catalytic performance. This view is confirmed in the following electrochemical performance test.

**Figure 1.** (**a**) XRD patterns and (**b**) FT-IR spectra of various types of jarosite. SEM images of (**c**) KFe3(SO4)2(OH)6, (**d**) NaFe3(SO4)2(OH)6, (**e**) (H3O)Fe3(SO4)2(OH)6 and (**f**) (NH4)Fe3(SO4)2(OH)6.

It can be seen from the figure that KFe3(SO4)2(OH)6 (Figure 1c) and NaFe3(SO4)2(OH)6 (Figure 1d) have similar morphologies. Both of them are densely packed. The precipitation rate of KFe3(SO4)2(OH)6 is fast and the sample morphology has not yet been completely formed before it settles together. In the morphology of NaFe3(SO4)2(OH)6, it can be observed that they are stacked together in a rhombic structure, which has not yet been completely formed. It can be seen from Figure 1e that the particle diameter is larger and the shape is irregular. (NH4)Fe3(SO4)2(OH)6 (Figure 1f) is uniformly distributed in lumps of different sizes while particles do not appear to pile up, showing a larger specific surface area.

The nitrogen adsorption-desorption isotherm curves of the as-synthesized catalyst under various pressures were characterized with a Surface Area and Pore Porosimetry Analyzer NoVA 1200e., and the specific surface area and pore size distribution were calculated via Brumaire-Emmett-Teller(BET) and Barret-Joyner-Hallender (BJH) methods. As shown in Figure 2, (NH4)Fe3(SO4)2(OH)6 displays the highest BET surface areas of 6.5845 m<sup>2</sup> g<sup>−</sup>1, which is higher than that of KFe3(SO4)2(OH)6 (4.6879 m2 g−1), NaFe3(SO4)2(OH)6 (4.1587 m<sup>2</sup> g−1), and (H3O)Fe3(SO4)2(OH)6 (2.5179 m<sup>2</sup> g−1). The pore size distribution curves of the four materials in Figure 2b suggest the existence of a mesoporous structure (~8 nm). The large specific surface area of the catalyst is very beneficial for the exposure of catalytic active sites for OER.

**Figure 2.** (**a**) N2 adsorption-desorption isotherm curves, (**b**) the corresponding pore size distribution curves.

Transmission electron microscopy (TEM) was carried out to further identify the details of samples. Figure 3a,b shows the lamellar structure of the sample. It shows some branchlike structures, which can provide more active sites. The selected-area electron-diffraction (SAED) pattern (inset of Figure 3b) of (NH4)Fe3(SO4)2(OH)6 was also recorded. It displays the weak diffraction rings, which further explained how the prepared (NH4)Fe3(SO4)2(OH)6 possesses poor crystallization form. Figure 3c shows the recorded high-resolution transmission electron microscopy (HRTEM) image of the (NH4)Fe3(SO4)2(OH)6. The interplanar spacing of 0.287 nm was indexed matching the (006) crystal plane of (NH4)Fe3(SO4)2(OH), which is in good agreement with the XRD spectra. Furthermore, the high-angle annular dark-field scanning-TEM (HAADF–STEM) and its corresponding mapping were employed to analyze the distribution of the elements in the (NH4)Fe3(SO4)2(OH)6 catalyst. It shows that Fe, N, O, and S are evenly distributed across the entire nanoparticles without any noticeable segregation.

**Figure 3.** (**a**,**b**) TEM images (the inset shows SAED), (**c**) HRTEM image, and (**d**–**h**) HAADF-TEM diagrams of (NH4)Fe3(SO4)2(OH)6 and the corresponding EDS elemental mapping images.

#### *3.2. Electrochemical Analysis*

To increase the electronic conductivity of the jarosite, the catalyst slurry with a weight ratio of 1:1 (jarosite to conductive carbon black) was prepared and an OER polarization curve performance test was conducted. As shown in Figure 4a, when the current density is 10 mA cm<sup>−</sup>2, the overpotentials of KFe3(SO4)2(OH)6, NaFe3(SO4)2(OH)6, (H3O)Fe3(SO4)2(OH)6, and(NH4)Fe3(SO4)2(OH)6 are 412 mV, 400 mV, 424 mV, and 394 mV, respectively. Meanwhile, the Tafel slope of the (NH4)Fe3(SO4)2(OH)6 is 127.31 mV dec−1, which is smaller than those of the NaFe3(SO4)2(OH)6 (135.86 mV dec<sup>−</sup>1), KFe3(SO4)2(OH)6 (144.81 mV dec−1) and (H3O)Fe3(SO4)2(OH)6 (148.85 mV dec−1), which indicates that (NH4)Fe3(SO4)2(OH)6 shows an excellent OER activity among the four jarosite catalysts.

In addition, OER tests were carried out on four catalyst materials in the acidic (pH = 1 H2SO4) and neutral (pH = 7 PBS) solution. As shown in Figure 5c,d, the catalytic performance of (NH4)Fe3(SO4)2(OH)6 in the acidic and neutral solution is better than the other three materials. However, the OER performance of (NH4)Fe3(SO4)2(OH)6 in the acidic and neutral condition is far inferior to that in the alkaline condition. Therefore, we will take alkaline conditions as an example to focus on the (NH4)Fe3(SO4)2(OH)6 catalyst.

The electrochemical double-layer capacitance (Cdl) approach was applied to estimate the electrocatalytic active surface area (ECSA) from cyclic voltammetry curves at various scan rates over a small potential range. The (NH4)Fe3(SO4)2(OH)6 electrode possesses the largest Cdl of 15.49 mF cm−<sup>2</sup> compared to those of KFe3(SO4)2(OH)6 (6.69 mF cm−2), NaFe3(SO4)2(OH)6 (13.56 mF cm−2), and (H3O)Fe3(SO4)2(OH)6 (4.28 mF cm−2)

(Figures 5 and 6), showing indeed that a larger ECSA of (NH4)Fe3(SO4)2(OH)6 allows for more exposed active sites to promote OER performance.

**Figure 4.** Polarization curves for jarosite to conductive carbon black ratio of 1:1, (**a**) 1 M KOH (pH = 14) polarization curves, (**b**) Tafel plots derived from the Ph = 14 polarization curves, (**c**) 0.05 M H2SO4 (pH = 1) polarization curves, (**d**) 1 M PBS (pH = 7) polarization curves.

**Figure 5.** CV curves of (**a**–**c**) (H3O)Fe3(SO4)2(OH)6, KFe3(SO4)2(OH)6, and NaFe3(SO4)2(OH)6 at different scan rates, (**d**) Cdl diagram of (H3O)Fe3(SO4)2(OH)6, KFe3(SO4)2(OH)6, and NaFe3(SO4)2(OH)6.

**Figure 6.** (**a**) NH4-Fe3@KB-1, NH4-Fe3@KB-2, NH4-Fe3@KB-3 and IrO2 polarization curves; (**b**) NH4- Fe3@KB-1, NH4-Fe3@KB-2, NH4-Fe3@KB-3 at overpotentials reaching current densities of 10, 50, and 100 mA cm<sup>−</sup>2; (**c**,**d**) the Tafel slope and EIS diagram of NH4-Fe3@KB-1, NH4-Fe3@KB-2, NH4-Fe3@KB-3.

Different ratios of catalyst powder and conductive carbon black may affect the results. The different weight ratios of (NH4)Fe3(SO4)2(OH)6 and conductive carbon black (2:1, 1:1, 1:2) are prepared and the total mass of 10 mg is guaranteed. The samples are referred to as NH4-Fe3@KB-1, NH4-Fe@KB-2, NH4-Fe@KB-3 and IrO2. The OER polarization curve test was performed on them in 1 M KOH electrolyte saturated with oxygen, and the test results are shown in Figure 6a. Additionally, NH4-Fe3@KB-1 has better OER catalytic performance. When the current density is 10 mA cm−2, the overpotential of NH4-Fe3@KB-1 is 379 mV, and it is 15 mV and 34 mV lower than NH4-Fe3@KB-2 and NH4-Fe3@KB-3, respectively. Furthermore, (NH4)Fe3(SO4)2(OH)6 and IrO2 have the same overpotential when the current density is 100 mA cm−2. When the current density is 30 mA cm−<sup>2</sup> and 50 mA cm−2, the overpotential of NH4-Fe3@KB-1 is still the lowest (Figure 6b). NH4-Fe3@KB-1 has a higher current density with the same measurement conditions.

To get insight into the OER kinetics, the Tafel slope values were calculated from the steadystate OER polarization curves. As shown in Figure 6c, NH4-Fe3@KB-1 (82.42 mV dec−1) has the smallest Tafel slope. Figure 6d is the AC impedance (EIS) test results of NH4-Fe3@KB-1, NH4-Fe3@KB-2 and NH4-Fe3@KB-3. The charge transfer resistance of NH4-Fe3@KB-1 is significantly smaller than that of NH4-Fe3@KB-2 and NH4-Fe3@KB-3, which suggests the catalytic interface and the electrolyte have a faster charge transfer rate.

The slope was calculated to get the Cdl value and the test result is shown in Figure 7. NH4-Fe3@KB-1 has the largest Cdl value of 26.17 mF cm−2, indicating that the ECSA of NH4- Fe3@KB-1 is large. This is allowing more active sites to be exposed and promotes the catalytic process of OER. This also explains the good OER catalytic performance of NH4-Fe3@KB-1.

Additionally, durability was another significant parameter of the catalyst for OER. Through the i-*t* test, the stability of NH4-Fe3@KB-1 was evaluated, and the test was carried out for 48 h at a constant voltage of 0.68 V (vs. Hg/HgO) with a current density equal to 10 mA cm−2. The test results are shown in Figure 8a. With the increase in test time, the current density of NH4-Fe3@KB-1 increases slightly around 10 mA cm−2, which may be caused by the burst of oxygen bubbles generated during the test. Overall, NH4-Fe3@KB-1 still shows good stability. The structure and composition of (NH4)Fe3(SO4)2(OH)6 after

the stability test was also studied in detail, with the SEM image of (NH4)Fe3(SO4)2(OH)6 (Figure 8b) after stability test displaying a newly formed rice-like structure, indicating that the (NH4)Fe3(SO4)2(OH)6 catalyst may have undergone surface reconstruction during electrolysis. In addition, the XRD pattern (Figure 8c) of the (NH4)Fe3(SO4)2(OH)6 catalyst showed an amorphous feature after OER. As with many reported works [29,30], the catalyst undergoes a surface reconstruction accompanied by the appearance of an amorphous structure (e.g., oxyhydroxide species) during the OER process, and the observed amorphous feature was further analyzed by Raman spectroscopy. As shown in Figure 8d, the four Raman bands at 215, 275, 390 and 599 cm–1 represent the phase of FeOOH, which is wellmatched with the literature reports [31–33]. Therefore, it might be reasonable to conclude that (NH4)Fe3(SO4)2(OH)6 was transformed into amorphous FeOOH during the OER process.

**Figure 7.** CV curves of (**a**) NH4-Fe3@KB-1, (**b**) NH4-Fe3@KB-2, and (**c**) NH4-Fe3@KB-3 at different scan rates; (**d**) Cdl diagram of NH4-Fe3@KB-1, NH4-Fe3@KB-2, and NH4-Fe3@KB-3.

**Figure 8.** (**a**) The chronoamperometric curve for NH4-Fe3@KB-1, (**b**) SEM after stability test, (**c**) XRD after stability test, (**d**) Raman after stability test.

#### **4. Conclusions**

In this work, a simple hydrothermal method was used to successfully prepare the jarosite. Furthermore, (NH4)Fe3(SO4)2(OH)6 shows the best catalytic performance. The OER catalytic performance of (NH4)Fe3(SO4)2(OH)6 and conductive carbon black with different weight ratios were further explored. The OER catalytic performance is best when the weight ratio of (NH4)Fe3(SO4)2(OH)6 to conductive carbon black is 2:1. Additionally, NH4-Fe3@KB-1 has a lower starting potential of 1.42 V (vs. RHE) and Tafel slope (82.42 mV dec−1). It also showed a small charge transfer resistance and a large Cdl (26.17 mF cm<sup>−</sup>2). The raw materials for preparing the synthetic are easily obtained and are low in price. Experimental results show that jarosite has a broad development space and further research is needed to improve its OER performance.

**Author Contributions:** Conceptualization, J.C., S.L. and Y.L.; methodology, J.C. and S.L.; software, J.C. and S.L.; validation, Z.Q., Z.L. and D.W.; formal analysis, J.C. and S.L.; investigation, J.C. and S.L.; resources, Y.L.; data curation, S.L.; writing—original draft preparation, J.C. and S.L.; writing—review and editing, J.C.; visualization, J.S. and Y.L.; supervision, J.S. and Y.L.; project administration, Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data used to support the findings of this study are available from the corresponding author upon request.

**Acknowledgments:** The authors are grateful for the Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

