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Article

Dual-Modification Engineering of CoNi Alloy Realizing Robust Performance for Electrocatalytic Hydrogen Production

by
Yutong Ye
†,
Guorong Zhou
†,
Kaixun Li
and
Yun Tong
*
School of Chemistry and Chemical Engineering, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou 310018, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2023, 13(7), 1064; https://doi.org/10.3390/catal13071064
Submission received: 5 June 2023 / Revised: 28 June 2023 / Accepted: 30 June 2023 / Published: 1 July 2023
(This article belongs to the Special Issue Advances in Non-precious Metal-Based Electrode Materials for Electrocatalysis)
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Abstract

:
Anion modification and trace metal doping have been widely demonstrated to have unique advantages in regulating both electrocatalytic activity and the electronic structure of non-precious metal materials. Developing a simple and practical preparation strategy is critical, but it still faces challenges. In this paper, a novel type of dual-modification approach is put forward to rationally design the S, Pt-CoNi material, which can be grown directly on the nickel foam (NF) in a one-step electrodeposition process. The multiple advantages of having plenty of active sites, high conductivity, and a faster charge transfer endow the optimized reaction kinetic for HER. The prepared S, Pt-CoNi/NF catalyst displays excellent catalytic performance, and a low overpotential of 116 mV at 50 mA cm−2 and a small Tafel slope of 75 mV dec−1 are achieved. The coupled S, Pt-CoNi/NF||FeOOH/NF electrolyzer delivers a high current density of 100 mA cm−2 at the potential of 1.61 V as well as superior stability under alkaline conditions. Our work experimentally confirms the feasibility of constructing a dual-regulation strategy via one-step electrodeposition, and it also provides ideas for the controllable design of other high-performance electrodes for electrocatalysis.

Graphical Abstract

1. Introduction

The continued use of fossil energy has led to serious environmental pollution and an energy crisis, and this has compelled researchers to try to develop alternative renewable energy alternatives. As a prospective energy source, hydrogen has multiple advantages, such as high energy density (142 MJ kg−1), high calorific value, sustainability, and environmental preservation [1,2,3,4]. Hydrogen production from coal or the traditional methane steam reforming faces problems, such as low efficiency and secondary pollution. Thus, it is particularly important to develop simple and efficient hydrogen production technology in order to achieve sustainable production of renewable hydrogen. In this regard, electrocatalytic hydrogen evolution (HER) has proven to be a promising strategy for realizing large-scale hydrogen production by renewable electricity-driven means [5,6,7,8,9]. The application of electrocatalysts can significantly improve the energy conversion efficiency of HER, where platinum (Pt) is the most effective catalyst due to its outstanding bonding ability with hydrogen intermediates [10,11,12,13]. However, the limited availability and the high price of noble metals prevent their widespread applications. It is therefore crucial to effectively design precious metal materials for high-mass activity.
Recently, a large number of research interests have been devoted to exploring effective strategies, such as the construction of alloy, the design of single atoms, and the coupling of heterojunction, for realizing excellent catalytic performance at a low level of precious metals [14,15,16,17,18,19,20]. For example, a highly active PtSA-NiCo LDHs/NF is designed by anchoring Pt atoms on oxygen vacancies in order to form Pt-O5 coordination, showing a low voltage of 1.37 V at 100 mA cm−2 in glycerol oxidation-assisted hydrogen production [21]. In addition, Mai’s group developed a sub-nanometer platinum cluster modified 3D crumpled Ti3C2Tx MXene, which exhibits superior HER activity with a low overpotential of 34 mV at 10 mA cm−2 as well as high mass activity (1847 mA mgPt−1) [22]. On the other hand, a dual-modification strategy has gradually developed in order to optimize the coordination structure of precious metal-doped materials for the further improvement of performance [23,24,25]. For example, the co-modification of N-anion and Pt atom is applied on MoS2 nanosheets in order to modulate its electronic and coordination structures, achieving excellent alkaline HER activity due to the optimized water adsorption and the dissociation process [26]. However, although a series of studies have been reported and although significant progress has been made, the development of novel carriers with high intrinsic conductivity for anchoring trace precious metals is still a research hotspot.
Metal alloy with well-defined structures present a unique advantage for electrocatalysis or for other energy-related systems due to its special surface exposure metal sites, its controllable composition, and its high intrinsic conductivity [27,28,29,30,31,32]. Recently, many efforts have been made to dope trace amounts of precious metals into alloy materials, and they have achieved excellent catalytic properties [28,33,34,35,36]. For example, a porous graphene shell architecture is modified by an ultralow (0.66 wt%) Pt doped FeNi alloy nanoparticles by a facile strategy, which shows a superior specific HER catalytic performance, including low overpotential, a small Tafel slope, and high stability in 0.5 M H2SO4 [28]. Meanwhile, Huang et al. have reported a series of NiCoRux/SP materials via a low-temperature nucleation process. The optimal NiCoRu0.2/SP sample shows an overpotential of 59 mV at 10 mA cm−2 for HER. A detailed experimental result and characterizations confirm that the interface-induced electron transfer from atomic Ru to its surrounding Ni/Co does lead to significantly improved reaction kinetics [35]. However, there is still lack of simple strategy to realize the synergistic regulation of noble metal or anion modification on alloy systems.
Inspired by this, we put forward a facile electrodeposition method in order to achieve the dual modification of S-anions and Pt species on the CoNi alloy material. The S, Pt-CoNi electrocatalyst can be deposited on the NF supporter (S, Pt-CoNi/NF), which exhibits enhanced HER catalytic performance under alkaline conditions. A low overpotential of 116 mV at 50 mA cm−2 and a small Tafel slope of 75 mV dec−1 are realized by using S, Pt-CoNi/NF as an electrocatalyst. In addition, the assembled S, Pt-CoNi/NF||FeOOH/NF water electrolyzer displays a small voltage of 1.61 V at 100 mA cm−2 and excellent stability. Systematic research will prove that the excellent catalytic activity of the S, Pt-CoNi/NF catalyst is mainly due to multiple advantages of the improved catalytic centers and reaction kinetic processes for HER, and this work offers a new avenue for developing robust and effective electrocatalysts for the other fields of electrocatalysis.

2. Results and Discussion

2.1. The Characterizations of Structural and Morphology for the Catalysts

In this work, the synthesis process of S, Pt-doped CoNi alloy is shown in Scheme 1 and Table S1. During a simple electrochemical deposition process, the metal ions (Pt4+, Co2+, Ni2+) and thiourea occur, and they produce co-reduction on the NF surface by applying a negative potential in order to form the S, Pt-modified CoNi alloy layer. The simple S-modified CoNi alloy, S-modified Ni, and S-modified Co samples are also all electrodeposited onto the NF by similar methods in addition to the utilization of different electrolytes with different metal sources. To study the phases of the as-prepared samples, X-ray diffraction (XRD) is first performed at room temperature. Considering the influence of signals of nickel foam substrate, the carbon paper is further used as a working electrode in order to deposit the samples. The prepared samples are then peeled off from carbon paper for the measurement of XRD. As shown in Figure S1, in the Supplementary Materials, the XRD pattern of S-CoNi/NF shows some typical diffraction peaks, where the broad peak can be ascribed to the standard Co (JCPDS 05-0727) and Ni (JCPDS 04-0850) phases. In addition, the other two peaks belong to the carbon paper substrate. After the introduction of Pt species, the XRD pattern of S, Pt-CoNi/NF sample exhibits similar diffraction peaks (Figure S2), which suggests that the Pt species can be introduced into the CoNi alloy without causing significant structural change. These results confirm the successful preparation of Pt doped CoNi alloy material. Next, in order to study the morphology of the as-obtained samples, the scanning electron microscope (SEM) is further performed. As shown in Figures S3–S5, many S-Co, S-Ni, and S-CoNi nanoparticles with a size of 200–500 nm are uniformly grown on the NF substrate, which indicates the feasibility of the electrodeposition method. After the modification of the Pt species, the SEM images of S, Pt-CoNi/NF are measured, and they are shown in Figure S6. The S, Pt-CoNi/NF sample exhibits similar morphologies with many interconnected particles to that of the S-CoNi/NF samples, which indicates that the incorporation of Pt did not cause a significant change in the morphology.
Subsequently, the transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM) are further carried out in order to study the micro-morphology and the phase of these prepared samples. As shown in Figure S7, the TEM image of S-CoNi/NF sample displays a nanoparticle morphology with thin edges, which is consistent with the abovementioned SEM result. In addition, after the modification of Pt species, the S, Pt-CoNi/NF sample exhibits a similar morphology in the TEM images (Figure 1a–c). The nanoparticles with thin edges are interconnected to form the multi-level structure, which is beneficial for the exposure of catalytic active sites as well as for the adsorption of reaction species from the electrolyte. In addition, the HRTEM image of the S, Pt-CoNi/NF sample is shown in Figure 1d. The HRTEM image shows a lattice fringe of 0.204 nm at the edge of nanoparticle, which corresponds to the (111) crystal plane of the standard Ni/Co phase. Therefore, the phase of NiCo alloy can be formed by our developed electrodeposition method. Moreover, the energy dispersive X-ray energy spectrum (EDS) element mapping analysis further confirms the uniform dispersion of the Co, Ni, Pt, and S elements on the whole S, Pt-CoNi sample (Figure 1e). The appearance of the S element is related to the used thiourea, which proves that S, Pt co-modified CoNi material can be successfully prepared by this electrodeposition method. These results all demonstrate the successful incorporation of S-anions and Pt species into the CoNi alloy, and this is expected to have a unique property for electrocatalysis.

2.2. The Characterizations of Composition and Valence States for the Catalysts

The surface chemical composition and electronic structure of catalysts are studied using X-ray photoelectron spectroscopy (XPS). As shown in Figure S8, the full XPS spectrum of S-CoNi/NF shows the compositions of Co, Ni, and S elements. After the introduction of Pt species, the XPS survey of S, Pt-CoNi/NF catalyst exhibits the presence of Pt, Co, Ni, and S elements, which confirms the successful incorporation of Pt species into the S-CoNi/NF sample (Figure S9). These results are consistent with those observed for EDS mapping images. In addition, the analysis of each of the elements are further conducted from the high-resolution XPS spectra. The Co 2p spectra of the S-CoNi/NF and S, Pt-CoNi/NF are shown in Figure 2a, and show similar characteristic peaks. The two peaks at 781.2 and 796.7 eV can be attributed to Co2+ 2p3/2 and Co2+ 2p1/2 [37,38], and the peak at 785 eV and 802.7 eV can be attributed to satellite peaks [39]. The similar characteristic peaks and the close binding energy of Co 2p suggest the same electronic structure of Co atom in both the S-CoNi/NF sample and the S, Pt-CoNi/NF sample.
As shown in Figure 2b, the Ni 2p spectra of the S-CoNi/NF and S, Pt-CoNi/NF catalysts show two peaks located at 855.7 and 873.3 eV, which are ascribed to the Ni2+ 2p3/2 and Ni2+ 2p1/2, while the peaks of 861.5 eV and 879.6 eV belong to the satellite peaks [40]. The peak located at 852.4 eV corresponds to the Ni0 signal, indicating the electron transfer from Pt to adjacent S-CoNi, which leads to the decreased valence states of Ni. This phenomenon can be further confirmed by the Pt 4f spectrum. Figure 2c provides information regarding the XPS spectra of Pt 4f, which can be divided into two pairs of peaks. The peaks observed at 70.4 eV and 74 eV are both ascribed to Pt0 species, while the peaks at 71.8 and 76.6 eV belong to the Pt2+ 4f7/2 and Pt2+ 4f5/2, respectively [41]. The appearance of the Pt2+ species confirms that the possible electronic interaction between the Pt species and S-CoNi material. In addition, the S 2p spectra of both S-CoNi/NF and S, Pt-CoNi/NF samples are also shown in Figure 2d. Two peaks with the binding energies of 162.4 eV and 164.4 eV originate from the S 2p3/2 and the S 2p1/2 of the metal-S bonds, while the peak located at the binding energy of 168.2 eV corresponds to the typical S-O peak. This S-O peak should be related to the partial surface oxidation when it is exposed to air [42]. In addition, the signals of metal bonds confirm that some metal sulfides (CoNiSx) should be present on the surface of the S-CoNi/NF sample and the S, Pt-CoNi/NF sample. All of the abovementioned experimental results prove the successful synthesis of S, Pt co-modified CoNi materials, which are expected to have excellent catalytic activity for HER.

2.3. The HER Performance of the Catalysts

Considering the unique structure, tcomposition, and morphological advantages of the prepared materials, the alkaline HER electrocatalytic performance of these samples is first assessed in a three-electrode system. The 1 M KOH solution is used as an electrolyte, while the prepared self-supporting electrodes are directly used for electrochemical measurements after proper cropping. Linear sweep voltammetry (LSV) is the first way of studying the apparent performance of the obtained catalysts. Figure 3a shows the LSV curves of the commercial 20% Pt/C, S-CoNi/NF, S, Pt-CoNi/NF, S-Co/NF, and S-Ni/NF at a sweep rate of 5 mV s−1. The S-Co/NF sample shows the worst catalytic performance. When using S-Ni/NF as electrode, the catalytic performance becomes much better than that of S-Co/NF. After the formation of binary metal alloy, the S-CoNi/NF sample shows a significantly improved catalytic performance, which indicates the unique advantage of dual metal sites for improving HER performance. In addition, the introduction of Pt species can further promote the catalytic activity of the S-CoNi/NF sample, which approaches the level of activity of the commercial 20% Pt/C catalyst, indicating the promising potential application of S, Pt-CoNi/NF for HER.
Specifically, the S, Pt-CoNi/NF catalyst exhibits a much better catalytic performance than the other control samples, and it only needs an overpotential of 150 mV to reach the current density of 138 mA cm−2. In contrast, at the same overpotential, the S-CoNi/NF catalyst is limited to 62 mA cm−2. At the same current density of 50 mA cm−2, S, Pt-CoNi/NF catalyst needs the lowest overpotential of 116 mV. The overpotentials are increased by 27 mV, 165 mV, and 62 mV relative to S-CoNi/NF (143 mV), S-Co/NF (281 mV), and S-Ni/NF (178 mV), respectively. Notably, the commercial Pt/C shows a smaller overpotential of 72 mV than S, Pt-CoNi/NF (Table 1), which should be the case because of the high loading mass of Pt nanoparticles on commercial Pt/C sample. In addition, the overpotential of S, Pt-CoNi/NF is 184 mV at a high current density of 300 mA cm−2. Figure 3b shows the Tafel slope of S, Pt-CoNi/NF (75 mV dec−1), which is higher than commercial Pt/C (49 mV dec−1) but smaller than the S-CoNi/NF (89 mV dec−1), S-Co/NF (203 mV dec−1), and S-Ni/NF (114 mV dec−1) catalysts. The smaller Tafel slope of S, Pt-CoNi/NF suggests its favorable electrocatalytic kinetics for HER after the dual modification of S-anions and Pt species.
The blank NF, Pt-NF, S, Pt-NF and S, Pt-CoNi/NF samples are also prepared and measured for HER in 1 M KOH solution. As shown in Figure S10, the blank NF shows poor catalytic performance for HER. After the deposition of Pt species, the Pt-NF exhibits significant improved catalytic activity. However, its performance is still worse than that of the S, Pt-NF sample, indicating the positive effect of S-anions in improving HER performance. After introducing the S and Pt elements into CoNi/NF sample, HER activity is further promoted. The highly efficient performance of the S, Pt-CoNi/NF sample can be confirmed by the corresponding smallest Tafel slope of 75 mV dec−1 among all prepared samples. In addition, as shown in Figure S11, the S, Pt-CoNi/NF catalyst exhibits the better HER catalytic performance, regardless of the overpotential at a same current density or Tafel slopes, than that of Pt-CoNi/NF sample, which indicates the key role that S-anions play in promoting catalytic activity. Thus, the modification of S-anions not only regulate the electronic structure of Pt-CoNi but can also act as an additional H* acceptor in order to accelerate the formation of H2. Those results suggest that the high activity of S, Pt-CoNi/NF sample is originating from the multiple synergistic advantages. Finally, the mass activity of S, Pt-CoNi/NF and commercial Pt/C (20 wt%) samples are also calculated by XPS result, as shown in Figure S12. The mass activity of S, Pt-CoNi/NF is approaching the commercial Pt/C, suggesting the feasibility of our electrodeposition method when attempting to introduce trace noble metal into active materials, although there is still much room for improvement in the HER performance.
In addition, the electrochemical impedance spectrum (EIS) is one of the important characteristic factors in evaluating catalyst reaction kinetics. Therefore, the EIS data is further collected by measuring the prepared samples at a potential of −0.08 V in order to evaluate the interfacial benefits. As shown in Figure 3c, the four samples all follow a similar kinetically regulated process, and they all exhibit various impedance characteristics. In the high-frequency region, where four different radius semicircles collectively form, the S, Pt-CoNi/NF electrode has lower charge transfer resistance compared to the S-CoNi/NF, S-Co/NF, and S-Ni/NF samples, but a higher one than the commercial Pt/C sample. This finding further supports the conclusion that the dual modification of S-anions and Pt species contributes to optimizing the charge transfer process, resulting in a faster reaction kinetic for HER (Table 1). It is known that stability is one of the criteria for evaluating the performance of catalysts and whether they have application prospects. In order to study the stability of the prepared S, Pt-CoNi/NF sample, a CV cycling test and a chronopotentiometry test are conducted in 1 M KOH solution. The CV cycling before and after 3000 curves confirm the same polarization curves, which suggests the high long-term stability of S, Pt-CoNi/NF for HER, as shown in Figure S13. In addition, the S, Pt-CoNi/NF sample can operate steadily over 15 h at the continuous testing of the current density of 60 mA cm−2, which suggests its exceptional durability for a HER test under an alkaline medium (Figure 3d).
The active surface area of the catalyst is an important indicator of the exposure number of the catalytic sites, which is directly related to the electrocatalytic activity. Therefore, the double-layer capacitances (Cdl) are further used in order to estimate the surface active areas of the prepared samples. The CV curves are recorded at a non-faraday range at scanning rates of 20, 40, 60, 80, and 100 mV s−1 (Figure S14). We then derived the current density values of CV curves that measured at different scan rates at the potential of 1.0 V, and we plot these values with the scan rates [43,44,45,46]. The corresponding Cdl values can be obtained by linear fitting, as shown in Figure 4a. The S, Pt-CoNi/NF sample shows the largest surface area among all of the prepared samples. Compared to S-CoNi/NF (6 mF cm−2), S-Co/NF (5 mF cm−2), S-Ni/NF (3 mF cm−2), and S-NF (0.75 mF cm−2), the S, Pt-CoNi/NF has the highest Cdl (6.2 mF cm−2) and ECSA (8.3) values (Figure 4b). These observations clearly show that the surface area of S, Pt-CoNi/NF is increased by a factor of more than eight times over that of the blank NF substrate, significantly raising the electrocatalytic HER activity. Figure 4c demonstrates that the S, Pt-CoNi/NF electrode has the best catalytic performance across all developed catalysts, even after the normalization of ECSA. The current density of S, Pt-CoNi/NF can approach 15 mA cm−2 under the overpotential of 150 mV (Figure 4d), which is substantially greater than that of S-CoNi/NF (7.5 mA cm−2), S-Co/NF (1 mA cm−2), and S-Ni/NF (7 mA cm−2). This indicates that the higher intrinsic activity of S, Pt-CoNi/NF originates from multiple factors.

2.4. The Overall Water Splitting Performance of the Catalysts

Based on the excellent HER performance of S, Pt-CoNi/NF, we investigates its potential in hydrogen production by constructing a two-electrode configuration. The active FeOOH species can be deposited onto the NF by a simple impregnation-drying method. The S, Pt-CoNi/NF and FeOOH/NF are then used as both cathode/anode for the measurements of overall water splitting, while a 1 M KOH solution is also used as an electrolyte (Figure 5a). The assembled two-electrode system is denoted as S, Pt-CoNi/NF||FeOOH/NF electrolyzer in the following discussions. The LSV curve of the coupled S, Pt-CoNi/NF||FeOOH/NF electrolyzer is shown in Figure 5b, and exhibits a superior catalytic performance. The S, Pt-CoNi/NF||FeOOH/NF electrolyzer only needs a low potential of 1.45 V and 1.61 V at the typical current density of 10 mA cm−2 and 100 mA cm−2, respectively. In addition, the current density is about 300 mA cm−2 at the applied potential of 1.7 V. Thus, the high current density and low voltage of this S, Pt-CoNi/NF||FeOOH/NF electrolyzer proves the promising application potential for overall water splitting. In addition, the stability tests are performed by studying the chronopotentiometry test. The degradation of current density is insignificant over a long time test of 20 h for chronopotentiometry test (Figure 5c). These results prove the S, Pt-CoNi/NF sample has superior stability and a promising potential for hydrogen production by using electrocatalytic water splitting.
The yields of H2 production and reusability of the alloy for our prepared catalyst are further studied by analyzing the Faradaic efficiency of H2 and the post-characterizations. Firstly, the Faradaic efficiency of H2 is calculated by comparing the actually generated H2 with the theoretical value. As shown in Figure 5d, the generated volume of H2 increases with the test time, and the Faradaic efficiency of H2 is as high as 99%, which indicates the highly efficient performance of the prepared S, Pt-CoNi/NF electrode for HER. In addition, the post-characterizations, such as the XRD pattern, SEM image, and XPS results, are shown in Figures S15–S17. The morphology and phase of S, Pt-CoNi/NF maintain stability before and after the HER test. The compositions of Pt, S, Co, and Ni elements all exist in the spent sample, and this suggests the high stability of S, Pt-CoNi/NF for HER under an alkaline condition.
Based on the above systematic experimental results and physicochemical characterization of the prepared catalysts, the excellent catalytic performance of S, Pt-CoNi/NF can be attributed to three main aspects. Firstly, the introduction of highly-active Pt species can act as additional catalytic centers for the adsorption of the water molecule and hydrogen reaction intermediates, speeding up the catalytic reaction kinetics for HER. Secondly, the modification of S-anions help to modulate the electronic structure and surface property, which may be the additional acceptor of hydrogen reaction intermediates during the HER process. For some specific reactions, S (most often meaning the S2− anion) may be the strongest catalytic poison for noble metals, but, nonetheless, the introduction of S2− anion is widely proven to have a positive effect on promoting HER catalytic performance [47,48]. Thirdly, the intrinsic high conductivity of CoNi alloy can endow the faster electron transfer at the interface, contributing to the faster reaction kinetics that are observed. Finally, the special topography and three-dimensional (3D) electrode configuration can provide more active sites for catalytic reaction species, and can facilitate both the diffusion of electrolytes and the escape of gas products [49,50].

3. Experimental Section

3.1. Materials

Acquisitions of potassium hydroxide (KOH, >85%), nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, AR), and hydrochloric acid (HCl, AR) were from Country Shuanglin Chemical, Hanoi Chemical, and Aladdin, respectively. High crystal chemical company offered the absolute ethanol (C2H5OH, 99.7%). The cobalt nitrate (II) hexahydrate (Co(NO3)2·6H2O, AR), (H2PtCl6·6H2O, 99.9%), ammonium fluoride (NH4F, 98%), and thiourea (CH4N2S, 99%) were all bought from Aladdin. Ni foams with a 1.5 mm thickness were purchased from the SaiBo material network. Those chemicals were used without additional processing during our work.
Synthesis of S, Pt-CoNi/NF, S-CoNi/NF, S-Co/NF and S-Ni/NF samples:
In a typical synthesis process, several pieces of NF (2 × 1 cm) are pre-treated before use. The NF substrates are first put into acetone, water, and 3.0 M HCl for ultrasonication 10 min, respectively. Next, the NF substrates are taken out and dried in a vacuum oven. Then, the electrodeposition process is conducted in a three configuration, including a saturated Ag/AgCl electrode, platinum foil, and the NF substrate. The electrolyte for electrodeposition can be prepared by adding different chemicals. For the preparation of S, Pt-CoNi/NF, the 0.12 g Ni(NO3)2, 0.12 g Co(NO3)2, 3 g CH4N2S, and 1.5 g NH4F are added to 50 mL of deionized water (DI) in order to form a uniform solution. A total of 600 μL H2PtCl6 (10 mg/mL) is then added to the above solution before electrodeposition. For other S-CoNi/NF, S-Co/NF, and S-Ni/NF samples, the dual metal salts or single metal salt are added when the other chemicals are being kept the same. Next, the electrodeposition is performed by using NF as working electrode. The electrodeposition process is carried out at a constant voltage of −2 V for 400 s, and the corresponding products are labeled as S, Pt-CoNi/NF, S-CoNi/NF, S-Co/NF, and S-Ni/NF.

3.2. Synthesis of FeOOH/NF

The active FeOOH species can be deposited onto the NF by a simple impregnation-drying method. Firstly, 5 mM FeCl3 solution is prepared by dissolving FeCl3 into deionized water, and a clean NF substrate is then added to 50 mL of the above solution. The NF substrate is immersed in FeCl3 solution for a certain time. The NF electrode is then taken out and dried in an oven at 80 °C for about 1 h. Finally, the FeOOH/NF is successfully prepared for further electrochemical measurement.

3.3. Synthesis of Pt/C/NF

The commercial 20% Pt/C catalyst are directly deposited onto the NF substrate for the electrochemical test for HER. First, anhydrous ethanol (0.5 mL) and deionized water (0.5 mL) are mixed, before 20 mg 20% Pt/C powder are added. The mixture solution is then dispersed to form a uniform ink by ultrasonic for 30 min. Next, the prepared ink is sprayed onto a clean nickel foam by drying and spraying it several times. Finally, the prepared Pt/C/NF electrode is further dried in a vacuum oven for 12 h.

3.4. Material Characterizations

To evaluate the phase and structure of freshly obtained samples, X-ray diffraction (XRD) is done on a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd., Dandong, China). To investigate the surface morphology and element mapping of 3D electrodes, the scanning electron microscope (FESEM, S4800, Hitachi, Tokyo, Japan) is used. The transmission electron microscopy (TEM) is conducted on a JEM-2100F field-emission electron microscope (TEM, Tecnai G2 F20S-TWIN, Tokyo, Japan) operated at an acceleration voltage of 200 kV. The high-resolution TEM (HRTEM) images are performed on a JEOL JEM-ARF200F TEM/STEM (Tecnai G2 F20S-TWIN, Tokyo, Japan) with a spherical aberration corrector. To show the make-up and the chemical states of each element, X-ray photoelectron spectroscopy (XPS) measurements are performed on a PHI5300 apparatus using monochromatic Mg K as the radiation source.

3.5. Electrochemical Measurements

Electrochemical measurements are performed on a CHI660B electrochemical workstation (Chenhua Potentiostat Instrument, Shanghai, China) using a three-electrode system. The working electrodes are prepared samples, the counter electrode is a graphite rod, and the reference electrode is a saturated calomel electrode. The electrochemical measurements are conducted by cyclic voltammetry (CV) and by linear sweep voltammetry (LSV) in 1.0 M KOH electrolytes. The Tafel slopes of the prepared samples are calculated by transforming and then fitting the corresponding LSV curves by a calculation equation. The electrochemical impedance spectroscopy (EIS) experiments are carried out in the frequency range of (10−2–105 Hz) under an AC voltage model. The solution resistance (Rs) and charge transfer resistance (Rct) can be measured by analyzing the intercept and the radius on the horizontal axis. CV curve groups are further tested at scanning rates of 20, 40, 60, 80, and 100 mV s−1 in order to study the electrochemical double-layer capacitance (Cdl). The electrochemical active surface area (ECSA) is calculated by comparing the Cdl values of prepared samples with that of blank NF (ECSA = Cdl/Cs). The stability is evaluated by CV cycling at different curves and by an chronopotentiometry test at a fixed potential. The potentials are converted, and all electrochemical data are shown with an IR correction: E(vs.RHE) = E(vs.Hg/HgCl2) + Eө(Hg/HgCl2) (0.241 V) + 0.059 × pH.

4. Conclusions

In conclusion, we experimentally confirmed the feasibility of a one-step electrodeposition strategy in order to rationally prepare the dual S, Pt modified CoNi alloy catalyst on the NF substrate. The systematic characterizations prove that S-anions and Pt species can be introduced into the CoNi alloy, realizing the regulation of surface active sites and electronic structure. In addition, the detailed electrochemical measurements demonstrate the excellent catalytic performance of the prepared S, Pt-CoNi/NF catalyst under alkaline conditions. Remarkably, the low overpotential of 116 mV at 50 mA cm−2 and a small Tafel slope of 75 mV dec−1 are obtained by using S, Pt-CoNi/NF for HER. Moreover, the mass activity of S, Pt-CoNi/NF even approaches the commercial Pt/C material. After further coupling with FeOOH/NF as an anode, the assembled S, Pt-CoNi/NF||FeOOH/NF electrolyzer exhibits a low voltage of 1.61 V at 100 mA cm−2 and high durability for overall water splitting, thereby showing a promising application potential for hydrogen production. This work provides more insights into the development of high-performance electrocatalysts for energy-related systems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13071064/s1. Figure S1. The XRD pattern of S-CoNi/CP sample; Figure S2. The XRD pattern of S, Pt-CoNi/CP sample; Figure S3. (a–d) The SEM images of S-Co/NF sample; Figure S4. (a–d) The SEM images of S-Ni/NF sample; Figure S5. (a,b) The SEM images of S-CoNi/NF sample; Figure S6. (a,b) The SEM images of S, Pt-CoNi/NF sample; Figure S7. (a,b) The TEM images of S-CoNi/NF sample; Figure S8. The XPS survey spectrum of S-CoNi/NF sample; Figure S9. The XPS survey spectrum of S, Pt-CoNi/NF sample; Figure S10. (a) The polarization curves and (b) Tafel slopes of NF, Pt-NF, S, Pt-NF and S, PtCoNi/NF samples; Figure S11. (a) The polarization curves and (b) Tafel slopes of Pt-CoNi/NF and S, Pt-CoNi/NF samples; Figure S12. The mass activity of Pt/C and S, Pt-CoNi/NF samples for HER in 1 M KOH; Figure S13. Polarization curves before and after 3000 cycles of S, Pt-CoNi/NF for HER in 1 M KOH; Figure S14. The cyclic voltammetry curves of (a) S-Co/NF, (b) S-Ni/NF, (c) S-CoNi/NF and (d) S, Pt-CoNi/NF electrodes in 1.0 M KOH recorded at different scan rates from 20 to 100 mV s−1; Figure S15. The XRD pattern of S, Pt-CoNi/CP sample after HER test; Figure S16. (a-d) The SEM images of S, Pt-CoNi/NF sample after HER test; Figure S17. High-resolution XPS spectra of (a) Co 2p, (b) Ni 2p, (c) Pt 4f and (d) S 2p for S, PtCoNi/NF after HER test; Table S1. The summary of synthesis conditions for the samples.

Author Contributions

Conceptualization, Y.T.; investigation, Y.Y., G.Z. and K.L.; writing—review and editing, Y.T. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by Zhejiang Sci-Tech University (ZSTU) under Grant 22062312-Y.

Data Availability Statement

Data will be made available upon reasonable request.

Acknowledgments

The authors would like to thank the Shiyanjia Lab (www.shiyanjia.com) for the XPS measurements.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 01064 sch001 550
Scheme 1. The schematic representation for the synthesis of S, Pt-CoNi/NF electrodes.
Scheme 1. The schematic representation for the synthesis of S, Pt-CoNi/NF electrodes.
Catalysts 13 01064 sch001
Catalysts 13 01064 g001 550
Figure 1. (a–c) TEM images; (d) HRTEM image; (e) elemental mapping images of S, Pt-CoNi sample.
Figure 1. (a–c) TEM images; (d) HRTEM image; (e) elemental mapping images of S, Pt-CoNi sample.
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Catalysts 13 01064 g002 550
Figure 2. High-resolution XPS spectra of (a) Co 2p, (b) Ni 2p, (c) Pt 4f, and (d) S 2p for obtained samples.
Figure 2. High-resolution XPS spectra of (a) Co 2p, (b) Ni 2p, (c) Pt 4f, and (d) S 2p for obtained samples.
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Catalysts 13 01064 g003 550
Figure 3. (a) IR-corrected HER polarization curves; (b) Tafel plots; (c) Nyquist plots for the commercial Pt/C (20 wt%), S-Co/NF, S-Ni/NF, S-CoNi/NF, S, and Pt-CoNi/NF samples in 1 M KOH; (d) stability test of S, Pt-CoNi/NF for HER.
Figure 3. (a) IR-corrected HER polarization curves; (b) Tafel plots; (c) Nyquist plots for the commercial Pt/C (20 wt%), S-Co/NF, S-Ni/NF, S-CoNi/NF, S, and Pt-CoNi/NF samples in 1 M KOH; (d) stability test of S, Pt-CoNi/NF for HER.
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Catalysts 13 01064 g004 550
Figure 4. (a) The Cdl linear fitting result, and (b) corresponding ECSA values of the prepared samples. (c) The ECSA normalized polarization curves, and (d) the performance comparison of obtained samples.
Figure 4. (a) The Cdl linear fitting result, and (b) corresponding ECSA values of the prepared samples. (c) The ECSA normalized polarization curves, and (d) the performance comparison of obtained samples.
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Catalysts 13 01064 g005 550
Figure 5. (a) Diagram of two-electrode system; (b) LSV curve; (c) chronopotentiometry test at a voltage of 1.58 V of S, Pt-CoNi/NF||FeOOH/NF electrolyzer for overall water splitting in 1 M KOH; (d) The generated volume and Faradaic efficiency of H2 by using S, Pt-CoNi/NF as cathode for water splitting.
Figure 5. (a) Diagram of two-electrode system; (b) LSV curve; (c) chronopotentiometry test at a voltage of 1.58 V of S, Pt-CoNi/NF||FeOOH/NF electrolyzer for overall water splitting in 1 M KOH; (d) The generated volume and Faradaic efficiency of H2 by using S, Pt-CoNi/NF as cathode for water splitting.
Catalysts 13 01064 g005
Table
Table 1. The summary of catalytic performance for the prepared catalysts.
Table 1. The summary of catalytic performance for the prepared catalysts.
CatalystsTafel Slope
(mV dec−1)		Cdl
(mF cm−1)		ECSARct (Ω)η50 (mV)
S-Co/NF2033.0510.4281
S-Ni/NF1145.0354178
S-CoNi/NF896.068143
S, Pt-CoNi/NF756.26.24.2116
Pt/C49//2.272
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Ye, Y.; Zhou, G.; Li, K.; Tong, Y. Dual-Modification Engineering of CoNi Alloy Realizing Robust Performance for Electrocatalytic Hydrogen Production. Catalysts 2023, 13, 1064. https://doi.org/10.3390/catal13071064

AMA Style

Ye Y, Zhou G, Li K, Tong Y. Dual-Modification Engineering of CoNi Alloy Realizing Robust Performance for Electrocatalytic Hydrogen Production. Catalysts. 2023; 13(7):1064. https://doi.org/10.3390/catal13071064

Chicago/Turabian Style

Ye, Yutong, Guorong Zhou, Kaixun Li, and Yun Tong. 2023. "Dual-Modification Engineering of CoNi Alloy Realizing Robust Performance for Electrocatalytic Hydrogen Production" Catalysts 13, no. 7: 1064. https://doi.org/10.3390/catal13071064

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