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Article

Electrochemically Induced Cu-NiOOH/Cu2O/Cu Mesh Heteroarchitecture with Cu-Ni Dual Active Sites as Efficient Bifunctional Electrocatalyst for Urea-Assisted Energy-Saving Hydrogen Production in Alkaline Electrolyte

by
Kaige Zhao
†,
Xinhao Xu
†,
Manli Wang
,
Tao Chen
,
Qianlin Ju
,
Lulu Hao
,
Zelin Chen
*,
Xiaolong Yu
* and
Changjiu Li
*
State Key Laboratory of Marine Resource Utilization in South China Sea, School of Materials Science and Engineering, Hainan University, Haikou 570228, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2024, 14(8), 496; https://doi.org/10.3390/catal14080496
Submission received: 30 June 2024 / Revised: 16 July 2024 / Accepted: 20 July 2024 / Published: 1 August 2024
(This article belongs to the Section Electrocatalysis)
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Abstract

:
The electrocatalytic oxidation of urea combined with wastewater splitting is considered a promising approach for sustainable hydrogen production, characterized by minimal energy consumption. However, its evolution is greatly hindered by the shortage of efficient and easily accessible electrocatalytic materials. Here, a facile electrochemical activation strategy was conceived and proposed to construct a Cu-doped NiOOH nanolayer encapsulated on Cu2O nanodendrites on Cu mesh substrate (Cu-NiOOH/Cu2O/CM) from the electrodeposited Ni/Cu2O/CM heterostructured precatalyst. It was verified that the incorporation of Cu not only facilitates the rapid formation of Ni(III) species but also contributes to the formation of Cu-Ni(III) bifunctional electrocatalytic active sites. Benefiting from the accessible Cu-Ni(III) dual active sites, high active surface area, good hydrophilic and aerophobic surface properties and superior electrical conductivity of the Cu mesh substrate, the as-prepared Cu-NiOOH/Cu2O/CM exhibits enhanced bifunctional electrocatalytic abilities for electrocatalytic urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). Particularly, for the Cu-NiOOH/Cu2O/CM||Cu-NiOOH/Cu2O/CM configuration toward the UOR||HER coupled system, a significantly reduced cell voltage of 1.43 V vs. RHE @ 10 mA·cm−2 was obtained. The observed cell voltage for the conventional overall water splitting is approximately 190 mV higher than that observed for overall urea splitting. This study proposes a viable approach to achieve and optimize the bifunctional UOR/HER performance of NiOOH active species, which holds significant importance for efficient and stable hydrogen generation from urea-contaminated substandard water.

1. Introduction

Recent studies have shown that electrochemically converting urea, originating from biological urine and industrial effluents, into sustainable green hydrogen is a potential solution to effectively address eutrophication caused by urea pollution in water bodies and energy shortages [1,2,3]. The UOR, with a thermodynamic potential of 0.37 V, exhibits a significantly lower energy requirement compared to the oxygen evolution reaction (OER), which necessitates 1.23 V [4]. This leads to the suggestion of adopting UOR in place of OER as the anodic oxidation reaction in water electrolysis can notably elevate hydrogen production efficiency [5]. Nevertheless, the efficiency of urea electrolysis is impeded by the sluggish kinetics of anodic UOR and cathodic HER in alkaline media [6]. The UOR process, which involves a six-electron transfer mechanism (CO(NH2)2 + 6OH− → N2 + CO2 + 5H2O + 6e−), necessitates the meticulous design of highly efficient electrocatalysts with rational nanostructures and compositions, as well as possessing bifunctional electrocatalytic properties for both UOR and HER.
Throughout the last few decades, electrocatalysts based on noble metals (Pt, Pd, Ir, and Ru) have been extensively employed in water electrolysis for hydrogen production, owing to their excellent electrocatalytic activities for UOR, OER, and HER [7,8,9]. Despite this, the scarcity, high cost, and mono-functional nature of noble metals sharply curtail their broad application [10,11]. Therefore, the exploration of high-performance non-metallic bifunctional catalysts for overall urea splitting is crucial in replacing noble metal-based alternatives, enabling efficient and energy-efficient hydrogen production from wastewater containing urea [12]. Previous investigations have demonstrated that Ni-based catalysts possess significant potential as practical bifunctional electrocatalysts for UOR and HER [13,14,15]. The presence of eight electrons in the 3D orbital endows Ni with its exceptional electrocatalytic properties. In an alkaline electrolyte, the UOR mechanism involving Ni(II) active sites invariably leads to self-oxidation, resulting in the formation of trivalent Ni(III) species on the surface (typically at ~1.36 V) [4]. These species serve as the veritable active sites for UOR. The potential reaction pathway is 6NiOOH + CO(NH2)2 + 6OH− → 6Ni(OH)2 + N2 + CO2 + 5H2O + 6e−.
Nevertheless, the direct synthesis of NiOOH electrocatalysts results in relatively low electrical conductivity, potentially limiting their catalytic performance and making it difficult to achieve bifunctional catalytic properties [16,17]. Therefore, various nickel-based compound nanomaterials with different morphologies, structures, and crystalline forms, such as Ni2S3 [18,19], Ni3S4 [20,21], NiTe [22], and NiSe [23,24] catalysts, have been designed and synthesized. These materials demonstrate variable valence states on their surface component and rapid in situ electrochemical reconstruction during the electrocatalytic process [25]. For instance, Tu et al. [26] applied an activation technique to change the original nickel foam into a fluffy nanosheet structure (I-NiS). In an alkaline electrolyte, the I-NiS electrode exhibited exceptional UOR performance, achieving a 100 mA cm−2 current density at only 1.44 V vs. RHE. The NiOOH species formed during the catalytic process were verified in the veritable catalytic active site according to the in situ Raman spectroscopy characterization. Yang et al. [27] presented a synthetic approach for fabricating ultra-thin 2D nanomaterial Ni(OH)2, and the investigation unveiled that the edges of Ni(OH)2 exhibit enhanced reactivity in generating NiOOH species and adsorbing urea molecules compared to the basal plane, thereby accelerating the catalytic kinetics of UOR. It exhibits strong catalytic activity in electrochemical tests, and its activity increased by 5.7 times at a potential relative to Ag/AgCl of 0.6 V. Nonetheless, the design of such Ni-based electrocatalysts was intricate, the synthesis procedures were relatively complex, and the NiOOH species formed via electrochemical activation still function as single catalytic active sites in the reaction.
In view of the aforementioned challenges, here, we report a Cu-NiOOH/Cu2O/CM heterostructured nanocatalyst with Cu-Ni(III) dual active sites, which was obtained via a facile electrochemical activation treatment of the electrodeposited Ni/Cu2O/CM precatalyst. Structural characterization combined with surface valence state analysis demonstrates that the NiOOH species formed during electrochemical activation feature an amorphous structure. Additionally, the electronic structure of Cu on the surface was also modified due to the generation of Ni(III), yielding highly intrinsically active Cu-Ni(III) dual active sites. According to electrochemical characterization, to obtain the UOR current densities of 10 mA·cm−2 and 100 mA·cm−2, the Cu-NiOOH/Cu2O/CM catalyst requires the potentials of only 1.32 V and 1.38 V vs. RHE, respectively. Furthermore, the Cu-NiOOH/Cu2O/CM catalyst shows a Pt-like HER activity in a urea-rich electrolyte, requiring the overpotentials of 71 mV and 196 mV to attain the HER current densities of 10 mA·cm−2 and 100 mA·cm−2, respectively. The application of the Cu-NiOOH/Cu2O/CM||Cu-NiOOH/Cu2O/CM configuration in a two-electrode electrolyzer was also investigated, highlighting excellent activity and durability for overall urea electrolysis. Overall, the as-prepared Cu-NiOOH/Cu2O/CM catalyst exhibits excellent UOR/HER bifunctional electrocatalytic activity and durability, ascribed to the well-defined architecture of the Cu2O nanodendrites, and most importantly, the high intrinsic activity of the Cu-Ni(III) dual active sites.

2. Results and Discussion

Figure 1a depicts the preparation pathway employed for the Cu-NiOOH/Cu2O/CM catalyst. Initially, a facile corrosion and oxidation approach was utilized to enable the in situ growth of abundant branched nanostructures composed of Cu(OH)2 and Cu2O adorned on the surface of Cu mesh (referred to as Cu(OH)2/CM and Cu2O/CM). Subsequently, Ni nanoparticles were deposited onto the surface of Cu2O/CM through an electrochemical reduction strategy in a nickel-containing electrolyte, resulting in the formation of the heterostructural Ni/Cu2O/CM catalyst. By adjusting the electrodeposition time (120, 240, and 360 s), the Ni loading on the Cu2O nanodendrites surface was conveniently controlled, with the morphologies of the samples presented in Figure S3. It was observed that the amount of Ni particles deposited increased rapidly with prolonged deposition time. The sample with 240 s of electrodeposition demonstrated the most uniform deposition of Ni nanoparticles. However, after 360 s of electrodeposition, there was substantial agglomeration of Ni particles on the Cu2O nanodendrites surface, which would evidently impact the subsequent electrochemical activation process. Consequently, the sample with 240 s of electrodeposition was chosen for subsequent electrochemical activation treatment as the Ni/Cu2O/CM precatalyst. Ultimately, through a straightforward electrochemical activation step (see Section 3 and Figure S2 for details), the Cu-NiOOH/Cu2O/CM heteroarchitectures were successfully synthesized. Correspondingly, the surface morphologies of the pristine CM, Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM, and Cu-NiOOH/Cu2O/CM electrodes are displayed in Figure 1b,k and Figure S4. The process of Cu(OH)2 converting to Cu2O was validated by XRD characterization, as demonstrated in Figure S5. Evidently, the porous structure of the Cu mesh substrate remains consistent during the preparation process, whereas well-defined nanodendrites, about 100 nm in diameter, uniformly grow on the Cu mesh surface, becoming progressively rougher and denser. According to previous studies, nanodendrites exhibit an expanded electrochemical active surface area (ECSA), enhanced active site efficiency, and accelerated mass and charge transfer, highlighting the suitability of Cu-NiOOH/Cu2O/CM catalyst for electrocatalysis in UOR and HER [28,29,30]. According to the Brunauer–Emmett–Teller (BET) surface area analysis (Figure S6), the surface areas of the pristine CM, Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM, and Cu-NiOOH/Cu2O/CM samples are measured to be 12.79, 13.88, 15.09, 20.60, and 13.49 cm2·mg−1, respectively. This observation reveals that the deposition of Ni nanostructures onto the Cu2O surface enhances the surface area, while the subsequent electrochemical activation slightly reduces the surface area of Cu-NiOOH/Cu2O/CM. Subsequent electrochemical performance tests have confirmed the improved UOR and HER activity of Cu-NiOOH/Cu2O/CM, indirectly highlighting the enhanced intrinsic activity of the Cu-NiOOH dual active sites [31,32].
To elucidate the intricate morphology and structural composition of the Ni/Cu2O/CM pre-catalyst and Cu-NiOOH/Cu2O/CM catalysts, Transmission electron microscopy (TEM) and corresponding selected-area electron diffraction (SAED) techniques were employed. As illustrated in Figure 1c, the Ni nanoparticles are visibly clear and evenly distributed on the surface of Cu2O nanorods, averaging a particle size of approximately 20.67 nm (Figure S7). The high-resolution TEM (HRTEM) image depicted in Figure 1d confirms that Ni nanoparticles are decorated on both the surface and internal sections of Cu2O nanorods in the Ni/Cu2O/CM sample, revealing distinct lattice fringes with interplanar distances of around 0.207 nm and 0.214 nm, which can be attributed to the Ni(111) and Cu2O(200) planes [33]. In the SAED image (Figure 1e), several bright spots can be observed forming concentric rings, which indicate the polycrystalline nature of the Ni/Cu2O/CM, with three types of diffraction rings corresponding to Ni, Cu2O, and Cu species. The corresponding high-angle annular dark-field (HAADF) image accompanied by element mapping images (Figure 1f–j) demonstrates that the nanorods comprise Ni, Cu, and O elements, with Ni elements conspicuously distributed on the periphery of Cu, in agreement with the TEM findings.
Moreover, for the Cu-NiOOH/Cu2O/CM catalyst obtained by electrochemical activation, the TEM image depicted in Figure 1l reveals that the surface Ni nanoparticles have disappeared substantially post-activation, with a thin, low-crystallinity layer forming on the nanodendrites. The HRTEM characterization (Figure 1m) results demonstrate that the low-crystallinity species in the outer layer have a thickness of about 1–2 nm, potentially indicating a NiOOH layer, which requires further characterization to confirm. The lattice spacing of the internal Cu2O remains unchanged at 0.214 nm, consistent with the results shown in Figure 1d. The SAED image (Figure 1n) demonstrates that the Cu-NiOOH/Cu2O/CM sample exhibits diffraction rings solely corresponding to Cu and Cu2O, and no diffraction rings associated with NiOOH species were observed, suggesting the NiOOH layer formed possesses low crystallinity, which is in accordance with the HRTEM results. The associated HAADF image combined with the element mapping images (Figure 1o–s) indicates that after electrochemical activation treatment, Cu distributes within the surface range of Ni, accompanied by a concentration of O on the surface, suggesting that Cu has been incorporated into the NiOOH layer. On the whole, the successful fabrication of Cu-NiOOH/Cu2O heteroarchitectures on CM substrate is strongly supported by all these remarkable results.
X-ray diffraction (XRD) was also carried out to assess the crystalline phase of pristine CM, Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM, and Cu-NiOOH/Cu2O/CM samples, and the results are illustrated in Figure 2a. For the Cu-NiOOH/Cu2O/CM sample, the diffraction peaks located at 2θ = 36.4°, 42.3°, 50.5°, 61.4° and 73.6° were discerned, indexing to the (111), (200), (211), (220) and (310) planes of Cu2O (JCPDS card No. 65-3288), respectively [34]. As presented in the enlarged XRD pattern of Cu-NiOOH/Cu2O/CM depicted in Figure S8, the diffraction peaks, positioned at 2θ = 42.1° and 43.4°, were observed to overlap with the diffraction peaks of Cu2O and Cu, which can be attributed to the (111) and (210) crystallographic planes of NiOOH (JCPDS card No. 27-0956) [35]. Therefore, supplementary characterization is essential to confirm the occurrence of NiOOH species in the Cu-NiOOH/Cu2O/CM material. Other three pronounced diffraction peaks centered at 2θ = 43.3°, 50.7° and 74.2° were observed as well, which can be attributed to the (111), (200) and (220) planes of the Cu mesh substrate (JCPDS card No. 04-0836) [36]. Moreover, the as-prepared Ni/Cu2O/CM, and Cu2O/CM materials also exhibited similar XRD patterns. Specifically, there is no distinct diffraction peak attributed to Ni species except for a weaker diffraction peak centered at 2θ = 44.9° observed in the Ni/Cu2O/CM sample, which may be attributed to either a low Ni content or the poor crystallinity.
To further investigate the structural transformation of Ni and Cu species during the electrochemical activation process, Raman spectroscopy was performed at room temperature on Cu2O/CM, Ni/Cu2O/CM, and Cu-NiOOH/Cu2O/CM samples in the range of 100–900 cm−1, as illustrated in Figure 2b. As shown, the Cu2O and CuO species exhibited characteristic Raman bands at 148, 217, 280 and 631 cm−1 and in 324 cm−1, respectively, in the Cu2O/CM catalyst. After the electrochemical activation process, the similar-looking Raman spectra can be observed in Ni/Cu2O/CM sample [37]. Moreover, the electrodeposited Ni nanoparticles in Ni/Cu2O/CM exhibited no distinguishable characteristics, indicating the absence of any Raman allowed bands in metallic Ni, which is in agreement with the HRTEM observations (Figure 1c,d). Specially, the Cu-NiOOH/Cu2O/CM sample showed a broad peak centered at approximately 522 cm−1, which could be attributed to Ni–O and Cu–O bands, indicating the formation of Cu-NiOOH [38].
X-ray photoelectron spectroscopy (XPS) analysis was executed to capture the surface elemental compositions and valence states of the as-synthesized Ni/Cu2O/CM pre-catalyst and Cu-NiOOH/Cu2O/CM catalyst. Figure S9 shows that the survey XPS spectra of Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM samples reflect the presence of Cu, Ni, and O elements, in line with the EDS mapping results. Figure 2c shows the Ni 2p XPS spectra for Cu-NiOOH/Cu2O/CM, Ni/Cu2O/CM samples and pristine Ni mesh for comparison. As displayed, two additional sharp peaks at 857.2 and 876.1 eV can be observed in the Ni 2p3/2 and Ni 2p1/2 high-resolution spectrum of the Cu-NiOOH/Cu2O/CM catalyst, which can only be attributed to the trivalent Ni3+ sites formed throughout the electrochemical activation procedure [39]. For comparison, the spectrum of Ni 2p3/2 in the Ni/Cu2O/CM sample and the spectrum of Ni 2p3/2 and Ni 2p1/2 in the Ni mesh sample displayed less pronounced peaks associated with zero-valent Ni0, demonstrating that the electrodeposited Ni nanoparticles were in the metallic state, in agreement with the HRTEM observations (Figure 1c,d). The vanishing of zero-valent Ni0 in the Cu-NiOOH/Cu2O/CM sample results from the thorough oxidation of Ni species during the electrochemical activation phase (Figure S2). Furthermore, a considerable amount of divalent Ni2+ was also detected in all samples. The oxidation of Ni in the air, which is inevitable, can be attributed to this phenomenon [40]. Additionally, the Cu 2p XPS spectra for the Cu-NiOOH/Cu2O/CM and Ni/Cu2O/CM samples are presented in Figure 2d. The Cu2O/CM sample was also investigated for comparative purposes. As illustrated, the high-resolution Cu 2p3/2 and 2p1/2 spectrum for Cu2O/CM shows two major peaks at binding energies of 932.4 eV and 952.2 eV, associated with Cu+ species, along with minor peaks at 934.6 eV and 954.6 eV, corresponding to Cu2+ species [41]. A strong electronic interaction between the surface-deposited Ni species and Cu2O can be inferred from the 1.3 eV shift to higher binding energies of the Cu+ 2p3/2 and 2p1/2 peaks upon Ni surface electrodeposition [42]. Additionally, compared to the Ni/Cu2O/CM samples, the Cu+ 2p peaks in the Cu-NiOOH/Cu2O/CM samples shift by 0.3 eV towards lower binding energies, suggesting that during the electrochemical activation process, the Cu sites on the material’s surface acquire partial electrons, facilitating the formation of Ni(III) [31]. Similar phenomena have been observed in recent research. For instance, Luo and colleagues [43] demonstrated that the introduction of Cu heteroatoms not only promotes the formation of Ni3+ in the Ni2P-Ni12P5 heterojunction but also accelerates electron transfer between Cu and NiOOH, thereby exhibiting enhanced performance for UOR.
To accurately disclose the contribution of Cu incorporation into NiOOH sites to improving OER performance, CV measurements were employed and conducted in a three-electrode cell system with a 1.0 M KOH electrolyte at room temperature, and the half-cycle CV curves from the negative potential sweep were depicted in Figure S10a. The pristine CM, Cu(OH)2/CM, Cu2O/CM and IrO2 catalysts were also examined for comparison. As shown, a relatively higher OER activity is exhibited by the Cu-NiOOH/Cu2O/CM catalyst among these catalysts. To reach a HER current density of 10 mA·cm−2, the Cu-NiOOH/Cu2O/CM catalyst requires an overpotential of 290 mV, which is more advantageous than those of IrO2 (330 mV), Cu2O/CM (420 mV), Cu(OH)2/CM (470 mV), and pristine CM (530 mV) samples (Figure S10c). Moreover, Tafel plots were applied to evaluate the OER electrocatalytic reaction kinetics for each catalyst. As illustrated in Figure S10b, the Cu-NiOOH/Cu2O/CM catalyst demonstrated a relatively lower Tafel slope of 44.9 mV·dec−1, in comparison with those of IrO2 (67.9 mV·dec−1), Cu2O/CM (78.5 mV·dec−1), Cu(OH)2/CM (143.0 mV·dec−1), and pristine CM (165.0 mV·dec−1) samples, revealing the enhanced OER kinetics on Cu-NiOOH/Cu2O/CM with Cu-NiOOH dual active sites.
The Nyquist plots in Figure S10d were obtained from EIS characterization and fitted with the appropriate equivalent circuit, with Rs signifying the electrolyte resistance and Rct signifying the charge transfer resistance at the electrode–electrolyte interface [44,45]. As illustrated, the NiOOH/Cu2O/CM catalyst demonstrates lower Rs (1.30 Ω) and Rct (2.83 Ω) values relative to other as-prepared catalysts, indicating a higher charge transfer rate during the OER process, which is likely caused by the synergistic effect between the Ni(III) active sites and the doping of Cu in Cu-NiOOH/Cu2O/CM catalyst. Additionally, the calculation of the double-layer capacitance (Cdl) from CV curves was utilized to determine the electrochemically active surface area (ECSA) of different Cu-NiOOH/Cu2O/CM, Cu2O/CM and Cu(OH)2/CM electrodes, as depicted in Figure S10e. It should be highlighted that the Cu-NiOOH/Cu2O/CM catalyst has a noteworthy calculated Cdl value of 20.3 mF·cm−2, smaller than those of Cu2O/CM (31.7 mF·cm−2) and Cu(OH)2/CM (58.9 mF·cm−2). This implies that the ECSA of the Cu-NiOOH/Cu2O/CM catalyst has unexpectedly decreased after electrochemical activation. Considering the improved OER performance, one can infer that the intrinsic activity of the Cu-NiOOH dual active sites in Cu-NiOOH/Cu2O/CM is much higher than that of the single Cu active site in Cu2O/CM and Cu(OH)2/CM catalysts. Comparable results were also observed in preceding studies [46]. Furthermore, the assessment of long-term durability toward OER was conducted through chronopotentiometry measurement, as depicted in Figure S10f. It is worth mentioning that the Cu-NiOOH/Cu2O/CM catalyst sustained a stable overpotential at OER current densities of 10 and 100 mA·cm−2 over a 16 h test period, and the CV curves displayed negligible variation before and after the long-term chronopotentiometry test.
The UOR electrocatalytic activities of the Cu-NiOOH/Cu2O/CM sample were primarily evaluated in a 1.0 M KOH solution with and without 0.5 M urea. According to Figure 3a, the potentials needed for the Cu-NiOOH/Cu2O/CM catalyst to attain current densities of 10 and 100 mA·cm−2 in UOR are 1.32 and 1.38 V vs. RHE, respectively, which are much lower compared to those for OER (1.53 and 1.62 V vs. RHE to achieve current densities of 10 and 100 mA·cm−2, respectively). The observed reduction in potential underscores the thermodynamic advantages of UOR over OER, and implies that utilizing UOR instead of anodic OER can facilitate energy-efficient hydrogen generation at lower potentials [47]. For comparation, the UOR performance of the as-prepared pristine CM, Cu(OH)2/CM, Cu2O/CM, IrO2, and Cu-NiOOH/Cu2O/CM samples was also determined in a 1.0 M KOH with 0.5 M urea electrolyte and the UOR curves are displayed in Figure 3b. As shown, Cu-NiOOH/Cu2O/CM requires the lowest potential to deliver 10 and 100 mA·cm−2 (Figure 3c) compared to that of IrO2 (1.37 and 1.48 V vs. RHE), Cu2O/CM (1.50 and 1.69 V vs. RHE), Cu(OH)2/CM (1.45 and 1.73 V vs. RHE) and pristine CM (1.50 and 1.90 V vs. RHE). The UOR activity of Cu-NiOOH/Cu2O/CM is also higher than that of the majority of catalysts reported in recent publications (as shown in Figure 3g and Table S1), indicating that Cu-NiOOH/Cu2O/CM catalysts with Cu-NiOOH dual-active sites induced by an electrochemical activation process exhibit high-efficiency catalysis of urea decomposition. To uncover the UOR dynamics on different catalysts, the corresponding Tafel slopes were plotted in Figure 3d. As shown, the as-synthesized Cu-NiOOH/Cu2O/CM catalyst exhibits a relatively minimal Tafel slope of 17 mV·dec−1, significantly smaller than that of IrO2 (34 mV·dec−1), Cu2O/CM (102 mV·dec−1), Cu(OH)2/CM (117 mV·dec−1), and pristine CM (171 mV·dec−1). The observed smallest Tafel slope from Cu-NiOOH/Cu2O/CM indicates its accelerated UOR reaction kinetics, which is likely caused by the synergistic effect between the Ni(III) active sites and the doping of Cu in Cu-NiOOH/Cu2O/CM catalyst and the unique electronic configuration among the components (Figure 3c,d) [48]. As exhibited in Figure S11, the determined Cdl values of the as-synthesized Cu-NiOOH/Cu2O/CM, Cu2O/CM and Cu(OH)2/CM catalysts were 17.2, 37.3 and 48.2 mF·cm−2, suggesting that the electrochemical activated Cu-NiOOH/Cu2O/CM catalyst possesses a relatively lower ECSA value. This also indicates that the intrinsic activity of the Cu-NiOOH dual active sites in Cu-NiOOH/Cu2O/CM substantially exceeds that of the single Cu active site in Cu2O/CM and Cu(OH)2/CM catalysts toward UOR [49]. Moreover, to further probe the electron-transfer kinetics, an EIS analysis was also conducted and the spectra were recorded at 1.40 V vs. RHE. The Nyquist plots illustrated that the Cu-NiOOH/Cu2O/CM sample exhibited an Rs value of 1.30 Ω and a smaller Rct value of 0.90 Ω (Figure 3e) compared with that of IrO2 (Rs = 1.60 Ω, Rct = 2.70 Ω), Cu2O/CM (Rs = 1.50 Ω, Rct = 5.45 Ω), Cu(OH)2/CM (Rs = 1.50 Ω, Rct = 8.16 Ω) and pristine CM (Rs = 1.80 Ω, Rct = 9.25 Ω) electrode, indicating a higher charge transfer rate during the UOR process for Cu-NiOOH dual active sites in Cu-NiOOH/Cu2O/CM. Furthermore, chronopotentiometry was employed to assess the long-term durability of UOR, as elaborated in Figure 3f. It was observed that the Cu-NiOOH/Cu2O/CM catalyst sustained a stable overpotential at UOR current densities of 10 and 100 mA·cm−2 over a 16 h test period, and the CV curves displayed negligible variation before and after the long-term chronopotentiometry test.
To uncover the bifunctional catalytic characteristics of the Cu-NiOOH/Cu2O/CM catalyst in urea-containing water, CVs were primarily performed in a 1.0 M KOH electrolyte both with and without the presence of 0.5 M urea. As presented in Figure 4a, the HER activity of Cu-NiOOH/Cu2O/CM catalyst was similar regardless of the urea presence in the electrolyte, implying that the HER efficiency is well preserved during urea-containing wastewater treatment [50]. For comparison, the HER activities of the as-prepared Cu-NiOOH/Cu2O/CM, Ni/Cu2O/CM, Cu2O/CM, Cu(OH)2/CM, pristine CM samples and the commercial Pt/C were also determined in a 1.0 M KOH with 0.5 M urea electrolyte and the HER polarization curves are displayed in Figure 4b. As shown, Cu-NiOOH/Cu2O/CM requires a Pt-like overpotential of 71 and 196 mV to deliver the HER current density of 10 and 100 mA·cm−2 in urea-rich electrolyte (1.0 M KOH + 0.5 M urea), respectively, smaller compared to that of Ni/Cu2O/CM (210 and 450 mV), Cu2O/CM (392 and 565 mV), Cu(OH)2/CM (431 and 613 mV) and pristine CM (486 and 781 mV) (Figure 4c). Among recently reported catalysts, the HER activity of the electrochemically activated Cu-NiOOH/Cu2O/CM catalyst is notably higher (Figure 4g and Table S2), highlighting the HER electrocatalytic potential of the Cu-NiOOH dual active sites. To uncover the HER dynamics on different catalysts, the corresponding Tafel slopes were plotted in Figure 4d. A shown, the as-synthesized Cu-NiOOH/Cu2O/CM catalyst exhibits a relatively minimal Tafel slope of 73 mV·dec−1, slightly lower than that of Pt/C (54 mV·dec−1) but significantly smaller than that of Ni/Cu2O/CM (126 mV·dec−1), Cu2O/CM (211 mV·dec−1), Cu(OH)2/CM (216 mV·dec−1), and pristine CM (233 mV·dec−1). As presented in Figure 4e, the Nyquist plots illustrated that the Cu-NiOOH/Cu2O/CM sample exhibited a Rs value of 1.20 Ω and a smaller Rct value of 4.20 Ω, compared with that of Ni/Cu2O/CM (Rs = 1.30 Ω, Rct = 8.25 Ω), Cu2O/CM (Rs = 1.30 Ω, Rct = 9.37 Ω), Cu(OH)2/CM (Rs = 1.30 Ω, Rct = 13.56 Ω) and pristine CM (Rs = 1.20 Ω, Rct = 23.54 Ω) electrode, indicating a higher charge transfer rate during the HER process for Cu-NiOOH dual active sites in Cu-NiOOH/Cu2O/CM. Furthermore, Figure 4f depicts the long-term HER chronopotentiometry testing in a 1.0 M KOH + 0.5 M urea electrolyte for the Cu-NiOOH/Cu2O/CM catalyst. It was observed that the Cu-NiOOH/Cu2O/CM catalyst sustained a stable overpotential at HER current densities of 10 mA·cm−2 over a 16 h test period, and the CV curves displayed negligible variation before and after the long-term chronopotentiometry test in a 1.0 M KOH + 0.5 M urea electrolyte.
It is noteworthy that relevant research has demonstrated that the insufficient and ineffective infiltration of the electrolyte into the electrode’s surface and interior obstructs the full exposure of the catalyst’s active sites, which negatively impacts the electrocatalytic UOR process. In addition, the accumulation of gas bubbles on the electrode surface during the catalytic reaction, if not released in time, leads to a pronounced ‘bubble shielding effect’, obstructing the mass transport of electrolyte ions [51,52,53]. In light of these considerations, contact angle tests were performed to assess the hydrophilic and aerophobic properties of the electrode surfaces, and the results are shown in Figures S12 and S13. As observed, the as-prepared Cu-NiOOH/Cu2O/CM surface exhibits excellent hydrophilicity with a static contact angle (θ) of 15.7°, which is significantly smaller than that of Ni/Cu2O/CM (141.7°), Cu2O/CM (68.6°), and Cu(OH)2/CM (168.6°), demonstrating that the surface’s hydrophilicity, induced by electrochemical activation and featuring Cu-NiOOH dual active sites, can be effectively enhanced. Additionally, the electrochemical activation of Cu-NiOOH/Cu2O/CM significantly boosts its air bubble release capacity compared to the Ni/Cu2O/CM precatalyst, enabling an air bubble to escape from its surface rapidly, within 200 ms. Therefore, it is reasonable to speculate that the bubbles generated on the Cu-NiOOH/Cu2O/CM surface during the catalytic reaction can rapidly detach from the electrode surface, resulting in outstanding bifunctional UOR and HER performance.
Encouraged by the remarkable bifunctional UOR and HER activities of the Cu-NiOOH/Cu2O/CM catalyst in a urea-containing alkaline electrolyte, a two-electrode electrolyzer was utilized, with the target Cu-NiOOH/Cu2O/CM catalyst used as both the anode and cathode in a 1.0 M KOH + 0.5 M urea electrolyte. For the sake of comparison, overall water splitting was performed in a 1.0 M KOH solution, as evidenced by Figure 5a. As demonstrated by the LSV curves (derived from the reverse scan of the corresponding CV curves), the configuration of Cu-NiOOH/Cu2O/CM||Cu-NiOOH/Cu2O/CM required a cell voltage of 1.43 V vs. RHE to obtain a current density of 10 mA·cm−2 for overall urea splitting, which is much lower than the potential needed for overall water splitting (1.62 V vs. RHE) and surpasses several efficient reported catalysts (as shown in Table 1). This demonstrates the promising application potential of the Cu-NiOOH/Cu2O/CM electrode for urea oxidation-assisted hydrogen evolution [54]. Moreover, the chronopotentiometry measurement indicated the robust long-term stability of the Cu-NiOOH/Cu2O/CM||Cu-NiOOH/Cu2O/CM configuration at 30 mA·cm−2. The morphological characterization of Cu-NiOOH/Cu2O/CM revealed significant morphological changes during the long-term stability test (Figure S14), revealing that the material’s superior UOR/HER catalytic activity is primarily maintained by the Cu-NiOOH dual active sites, with the macroscopic morphology of the sample having minimal influence on performance. XRD was also employed to evaluate the crystal structure of the Cu-NiOOH/Cu2O/CM catalyst after stability testing. As shown in Figure S15, compared to Figure 2a, the target sample still predominantly consists of Cu2O after long-term stability testing. However, new diffraction peaks appear at 2θ = 35.3°, 38.5°, 48.7°, and 61.5°, corresponding to the crystal structure of CuO (JCPDS card No. 44-0706), indicating that an unavoidable and slight oxidation occurred during urea electrocatalysis. Furthermore, XPS was employed to characterize the surface valence state changes in the target sample on the anode side after stability testing. As illustrated in Figure S16, after prolonged urea electrocatalysis, the surface of the target material remains dominated by Ni and Cu. Compared to Figure 2c, more trivalent Ni3+ and divalent Cu2+ species are observed (consistent with the XRD results shown in Figure S15). These changes in valence states may be the reason for the performance degradation of the material after long-term stability testing.
Additionally, the Faradaic efficiency of the Cu-NiOOH/Cu2O/CM catalyst was further evaluated through a water drainage strategy. The Faradaic efficiency of hydrogen evolution at the cathode catalyst during urea electrolysis is defined as the ratio of the H2 amount produced during the catalytic process to the theoretically expected hydrogen amount [55]. Figure 5c demonstrates the collection of hydrogen produced during urea electrolysis while operating at a constant current density of 200 mA·cm−2 using a 50 mL graduated cylinder, and the Faradaic efficiency for hydrogen generation was estimated to be approximately 100% (Figure 5d). The above suggests that this study is likely to pave the way for industrial green hydrogen production with lower energy consumption in the near future, and to simplify the fabrication of the self-supporting integrated transition metal electrode materials.
Table 1. Comparison of urea electrolysis of Cu-NiOOH/Cu2O/CM with several reported catalysts.
Table 1. Comparison of urea electrolysis of Cu-NiOOH/Cu2O/CM with several reported catalysts.
ElectrocatalystUrea Concentration in 1.0 M KOH ElectrolyteCell Voltage at
@10 mA cm−2		Reference
Cu-NiOOH/Cu2O/CM0.5 M1.43 VThis work
Ni/NiO@CrOx0.33 M1.44 V[56]
Ni/NiMoN0.5 M1.42 V[57]
NiO/Ni2P/NF0.33 M1.45 V[58]
CoSx/Ni(OH)20.5 M1.48 V[59]
Mo-NiS0.5 M1.51 V[60]
NiFeSbP0.5 M1.54 V[61]
Fe2V-MOF 0.5 M1.63 V[62]

3. Experimental Section

3.1. Chemicals and Materials

Nickel sulfate hexahydrate (NiSO4·6H2O, ACS reagent, ≥98.0%), sodium sulfate (Na2SO4, BioXtra, ≥99.0%), boric acid (H3BO3, ACS reagent, ≥99.5%), hydrochloric (HCl, ACS reagent, 37%), potassium hydroxide (KOH, ACS reagent, ≥85.0%), ammonium peroxodisulfate ((NH4)2S2O8, ACS reagent, ≥98.0%), and sodium hydroxide (NaOH, ACS reagent, ≥97.0%) were purchased from Sigma-Aldrich (Shanghai, China). Absolute ethanol was obtained from Decon Labs, Lnc. (Shanghai, China). The IrO2 and commercial Pt/C (20 wt%) catalysts were purchased from Sigma-Aldrich (Shanghai, China). The Nafion perfluorinated resin solution (P400486, 5.0–5.4 wt%) was obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The copper mesh substrate was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). No additional purification was made for the reagents, and all aqueous solutions were made with ultrapure deionized water (Milli-Q® Reference Pure Water System, Shanghai, China ≥ 18.2 MΩ@25 °C).

3.2. Synthesis Methods

Preparation of Ni/Cu2O/CM. The Cu mesh (CM) was cut into small pieces of 1.0 × 1.5 cm2, soaked in 2.0 M HCl solution, and ultrasonicated for 10 min to remove oxides from the electrode surface. The cleaned CM electrode was immersed in 30 mL of an aqueous solution containing 0.3 M (NH4)2S2O8 and 4.0 M NaOH, etched for 5 min to form Cu(OH)2/CM, thoroughly rinsed with abundant quantities of deionized water, and subsequently air-dried at ambient temperature. Subsequently, the Cu(OH)2/CM electrode was annealed at 450 °C for 2 h in 50 sccm of Ar gas to obtain the Cu2O/CM electrode. Then, the Ni/Cu2O/CM catalyst was prepared by electrodeposition. Using Cu2O/CM as the cathode and a graphite rod as the anode, the electrolyte consisted of 0.5 M NiSO4, 0.2 M Na2SO4, and 0.5 M H3BO3. Electrodeposition was performed at a current density of 50 mA·cm−2 for varying deposition times (120, 240, and 360 s). The resulting Ni/Cu2O/CM electrodes were rinsed several times with deionized water and anhydrous ethanol, and air-dried for subsequent use. The morphological and color changes in the pristine CM, Cu(OH)2/CM, Cu2O/CM, and Cu-NiOOH/Cu2O/CM electrodes are shown in Figure S1.
Preparation of Cu-NiOOH/Cu2O/CM. To fabricate the Cu-NiOOH/Cu2O/CM catalyst, the Ni/Cu2O/CM catalyst was electrochemically activated using cyclic voltammetry (CV), which was conducted in a 1.0 M KOH electrolyte with a potential sweep range of 0–1.0 V (vs. SCE), a scan rate of 50 mV·s−1, and a total of 3 cycles. As shown in Figure S2, the electrochemically activated Cu-NiOOH/Cu2O/CM catalyst delivered a significantly enhanced HER electrochemical activity.
Preparation of Pt/C and IrO2 electrode. The Pt/C electrode was prepared by a drop-casting method. Specifically, 10 mg of commercial Pt/C (20 wt%) powder was dispersed in a solution containing 50 μL of Nafion solution and 950 μL of absolute ethanol. The mixed solution was ultrasonicated for 60 min to form a homogeneous catalyst ink. The black dispersion was then drop-cast onto the CM and left to dry in air overnight at room temperature for further measurements. The loading of Pt/C catalyst was ~1.5 mg·cm−2. The IrO2 electrode was also prepared by the abovementioned drop-casting method. The loading of IrO2 catalyst was ~1.5 mg·cm−2.

3.3. Physical Characterizations and Electrochemical Tests

For a detailed description of the physical characterizations and electrochemical tests covered in this paper, please refer to the ‘Supporting Information’.

4. Conclusions

In conclusion, we proposed an electrochemical activation method for the electrodeposited Ni/Cu2O/CM heterostructured precatalyst, resulting in the Cu-NiOOH/Cu2O/CM catalyst with accessible Cu-Ni(III) dual active sites, high active surface area, good hydrophilic and aerophobic surface properties, and superior electrical conductivity. This catalyst demonstrated a remarkable electrocatalytic performance for UOR (1.32 and 1.38 V vs. RHE at 10 and 100 mA·cm−2) and HER (the overpotential of 71 mV at 10 mA·cm−2). The electrochemically activated NiOOH species exhibited exceptional bifunctional electrocatalytic activity for UOR/HER, benefiting greatly from the incorporation of Cu doping. The introduction of Cu accelerates the formation of trivalent Ni(III), not only increasing the number of active NiOOH sites but also creating bifunctional active sites for UOR/HER. The urea-assisted electrolysis hydrogen production system showed a significantly decreased cell voltage of 1.43 V vs. RHE @ 10 mA·cm−2 in a 1.0 M KOH with 0.5 M urea, which is about 190 mV lower than that of conventional water electrolysis. This approach of electrochemical activation is expected to be applicable to other Ni-based heterostructured nanomaterial systems, serving as a reference for further development of NiOOH catalysts with improved bifunctional UOR/HER catalytic properties.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14080496/s1, Figure S1. Digital photographs illustrating the surface color variations of CM, Cu(OH)2/CM, Cu2O/CM, and Cu-NiOOH/Cu2O/CM electrodes during the preparation process. Figure S2. Electrochemical activation. (a) Activation process, (b) HER performance comparison before and after activation. Figure S3. SEM images of Ni/Cu2O/CM catalyst synthesized at various electrodeposition: (a) 120 s. (b) 240 s. (c) 360 s. Figure S4. SEM images of (a) CM, (b,c) Cu(OH)2/CM and (d) Cu2O/CM catalysts. Figure S5. XRD patterns of (a) Cu(OH)2/CM and (b) Cu2O/CM catalysts, indicating the successful transformation of the crystal structure. Figure S6. N2 adsorption-desorption curves obtained from the BET analysis of the pristine CM, Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM catalysts. Figure S7. The particle size distribution histogram of the electrodeposited Ni nanoparticles decorated on the surface of Cu2O/CM. Figure S8. The magnified segment of the XRD patterns within the 2θ = 40–45° of Cu-NiOOH/Cu2O/CM catalyst. Figure S9. XPS survey spectrum of the as-prepared Ni Mesh, Cu2O/CM, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM catalysts. Figure S10. Electrocatalytic OER properties. (a) Polarization curves of Cu-NiOOH/Cu2O/CM, IrO2, Cu2O/CM, Cu(OH)2/CM and pristine CM catalyst in a 1.0 M KOH electrolyte. (b) Tafel slopes. (c) Summarized overpotential at the OER current density of 10 and 50 mA·cm−2. (d) Nyquist plots recorded at 1.60 V vs. RHE in a 1.0 M KOH electrolyte. (e) The plots of the current density versus the scan rate of Cu-NiOOH/Cu2O/CM, Cu2O/CM and Cu(OH)2/CM catalysts. (f) Long-term chronopotentiometry response recorded at 50 mA·cm−2 for 16 h OER. Figure S11. Typical cyclic voltammograms curves within the non-Faraday potential range for UOR of Cu-NiOOH/Cu2O/CM, Cu2O/CM, Cu(OH)2/CM catalyst at scan rate ranging from 20 to 120 mV·s−1. Figure S12. Water contact angles on the as-prepared Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM catalysts. Figure S13. Air contact angles on the Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM. Figure S14. SEM morphology before and after stability test of Cu-NiOOH/Cu2O/CM catalyst. (a) before stability test, (b) after stability test. Figure S15. XRD pattern of Cu-NiOOH/Cu2O/CM catalyst after stability test. Figure S16. High-resolution XPS of (a) Ni 2p and (b) Cu 2p in Cu-NiOOH/Cu2O/CM catalyst after stability test. Table S1. Comparison of the UOR performance of Cu-NiOOH/Cu2O/CM and other reported electrocatalysts in the past three years. Table S2. Comparison of the HER performance of Cu-NiOOH/Cu2O/CM and other reported electrocatalysts in the past years. References [15,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90] are cited in the supplementary materials.

Author Contributions

K.Z. and X.X. contributed equally to this work. K.Z. and Z.C. designed this work; K.Z., X.X., M.W., T.C., Q.J. and L.H. carried out the synthesis, characterization, and electrochemical experiments; K.Z., M.W. and Z.C. wrote the manuscript. The analysis of the experimental data, discussions of the results, and preparation of the manuscript were all carried out collaboratively by all authors. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 52201246), the Hainan Provincial Natural Science Foundation of China (No. 522QN280), the National Key Research and Development Program of China (No. 2022YFB3705703), and the Collaborative Innovation Center of Marine Science and Technology, Hainan University (XTCX2022HYC02).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no competing financial interests.

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Figure 1. Synthesis, structure and morphology characterizations of Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM catalysts. (a) Schematic illustration of the synthesis process. (b,k) SEM images of Ni/Cu2O/CM and NiOOH/Cu2O/CM catalysts; (c,l) low-resolution TEM (The arrows indicate the locations where Ni nanoparticles are deposited) and (d,m) HRTEM images of Ni/Cu2O/CM and NiOOH/Cu2O/CM catalysts; (e,n) corresponding SAED pattern, (f,o) HAADF-STEM images and (g–j,p–s) EDS elemental mapping of Ni/Cu2O/CM and NiOOH/Cu2O/CM catalysts within the corresponding dashed box regions, respectively.
Figure 1. Synthesis, structure and morphology characterizations of Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM catalysts. (a) Schematic illustration of the synthesis process. (b,k) SEM images of Ni/Cu2O/CM and NiOOH/Cu2O/CM catalysts; (c,l) low-resolution TEM (The arrows indicate the locations where Ni nanoparticles are deposited) and (d,m) HRTEM images of Ni/Cu2O/CM and NiOOH/Cu2O/CM catalysts; (e,n) corresponding SAED pattern, (f,o) HAADF-STEM images and (g–j,p–s) EDS elemental mapping of Ni/Cu2O/CM and NiOOH/Cu2O/CM catalysts within the corresponding dashed box regions, respectively.
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Figure 2. (a) XRD patterns of CM, Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM, and Cu-NiOOH/Cu2O/CM. (b) Raman spectra; high-resolution XPS of (c) Ni 2p in Ni Mesh, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM samples, respectively, and (d) Cu 2p in Cu2O/CM, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM samples, respectively.
Figure 2. (a) XRD patterns of CM, Cu(OH)2/CM, Cu2O/CM, Ni/Cu2O/CM, and Cu-NiOOH/Cu2O/CM. (b) Raman spectra; high-resolution XPS of (c) Ni 2p in Ni Mesh, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM samples, respectively, and (d) Cu 2p in Cu2O/CM, Ni/Cu2O/CM and Cu-NiOOH/Cu2O/CM samples, respectively.
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Figure 3. Electrocatalytic UOR properties. (a) Polarization curves of Cu-NiOOH/Cu2O/CM catalyst in a 1.0 M KOH electrolyte with and without 0.5 M urea. (b) Polarization curves of Cu-NiOOH/Cu2O/CM, IrO2, Cu2O/CM, Cu(OH)2/CM and pristine CM. (c) Summarized potential at the UOR current density of 10 and 100 mA·cm−2. (d) Tafel slopes. (e) Nyquist plots recorded at 1.40 V vs. RHE in a 1.0 M KOH + 0.5 M urea electrolyte. (f) Chronopotentiometry response of Cu-NiOOH/Cu2O/CM catalyst at 10 and 100 mA·cm−2. The inset illustrates the three-electrode setup used for stability testing and the corresponding changes in UOR performance before and after the tests. (g) Comparison of UOR activity with recently reports (The blue bars denote the potential required to achieve a UOR current density of 10 mA·cm−2, and the red bars denote the corresponding Tafel slope).
Figure 3. Electrocatalytic UOR properties. (a) Polarization curves of Cu-NiOOH/Cu2O/CM catalyst in a 1.0 M KOH electrolyte with and without 0.5 M urea. (b) Polarization curves of Cu-NiOOH/Cu2O/CM, IrO2, Cu2O/CM, Cu(OH)2/CM and pristine CM. (c) Summarized potential at the UOR current density of 10 and 100 mA·cm−2. (d) Tafel slopes. (e) Nyquist plots recorded at 1.40 V vs. RHE in a 1.0 M KOH + 0.5 M urea electrolyte. (f) Chronopotentiometry response of Cu-NiOOH/Cu2O/CM catalyst at 10 and 100 mA·cm−2. The inset illustrates the three-electrode setup used for stability testing and the corresponding changes in UOR performance before and after the tests. (g) Comparison of UOR activity with recently reports (The blue bars denote the potential required to achieve a UOR current density of 10 mA·cm−2, and the red bars denote the corresponding Tafel slope).
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Figure 4. Electrocatalytic HER properties. (a) Polarization curves of Cu-NiOOH/Cu2O/CM catalyst in a 1.0 M KOH electrolyte with and without 0.5 M urea. (b) Polarization curves of Pt/C, Cu-NiOOH/Cu2O/CM, Ni/Cu2O/CM, Cu2O/CM, Cu(OH)2/CM and pristine CM. (c) Summarized overpotential at the HER current density of 10 and 100 mA·cm−2. (d) Tafel slopes. (e) Nyquist plots recorded at −0.20 V vs. RHE in a 1.0 M KOH + 0.5 M urea electrolyte. (f) Chronopotentiometry response of Cu-NiOOH/Cu2O/CM catalyst at 10 mA·cm−2. The inset illustrates the corresponding changes in HER performance before and after the tests. (g) Comparison of HER activity with recently reports (The blue bars denote the overpotential required to achieve a HER current density of 10 mA·cm−2, and the red bars denote the corresponding Tafel slope).
Figure 4. Electrocatalytic HER properties. (a) Polarization curves of Cu-NiOOH/Cu2O/CM catalyst in a 1.0 M KOH electrolyte with and without 0.5 M urea. (b) Polarization curves of Pt/C, Cu-NiOOH/Cu2O/CM, Ni/Cu2O/CM, Cu2O/CM, Cu(OH)2/CM and pristine CM. (c) Summarized overpotential at the HER current density of 10 and 100 mA·cm−2. (d) Tafel slopes. (e) Nyquist plots recorded at −0.20 V vs. RHE in a 1.0 M KOH + 0.5 M urea electrolyte. (f) Chronopotentiometry response of Cu-NiOOH/Cu2O/CM catalyst at 10 mA·cm−2. The inset illustrates the corresponding changes in HER performance before and after the tests. (g) Comparison of HER activity with recently reports (The blue bars denote the overpotential required to achieve a HER current density of 10 mA·cm−2, and the red bars denote the corresponding Tafel slope).
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Figure 5. (a) Polarization curves of water electrolysis and urea electrolysis for Cu-NiOOH/Cu2O/CM catalyst. (b) Urea electrolysis chronopotentiometry response recorded at 30 mA·cm−2 for 16 h. (c) Enlarged digital images of the gas collection device of water splitting in a 1.0 M KOH + 0.5 M urea electrolyte and levels of hydrogen gas generated at 0, 180, 360, 540 and 720 s. (d) Faradaic efficiency of H2 production in HER||UOR system upon operation time.
Figure 5. (a) Polarization curves of water electrolysis and urea electrolysis for Cu-NiOOH/Cu2O/CM catalyst. (b) Urea electrolysis chronopotentiometry response recorded at 30 mA·cm−2 for 16 h. (c) Enlarged digital images of the gas collection device of water splitting in a 1.0 M KOH + 0.5 M urea electrolyte and levels of hydrogen gas generated at 0, 180, 360, 540 and 720 s. (d) Faradaic efficiency of H2 production in HER||UOR system upon operation time.
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Zhao, K.; Xu, X.; Wang, M.; Chen, T.; Ju, Q.; Hao, L.; Chen, Z.; Yu, X.; Li, C. Electrochemically Induced Cu-NiOOH/Cu2O/Cu Mesh Heteroarchitecture with Cu-Ni Dual Active Sites as Efficient Bifunctional Electrocatalyst for Urea-Assisted Energy-Saving Hydrogen Production in Alkaline Electrolyte. Catalysts 2024, 14, 496. https://doi.org/10.3390/catal14080496

AMA Style

Zhao K, Xu X, Wang M, Chen T, Ju Q, Hao L, Chen Z, Yu X, Li C. Electrochemically Induced Cu-NiOOH/Cu2O/Cu Mesh Heteroarchitecture with Cu-Ni Dual Active Sites as Efficient Bifunctional Electrocatalyst for Urea-Assisted Energy-Saving Hydrogen Production in Alkaline Electrolyte. Catalysts. 2024; 14(8):496. https://doi.org/10.3390/catal14080496

Chicago/Turabian Style

Zhao, Kaige, Xinhao Xu, Manli Wang, Tao Chen, Qianlin Ju, Lulu Hao, Zelin Chen, Xiaolong Yu, and Changjiu Li. 2024. "Electrochemically Induced Cu-NiOOH/Cu2O/Cu Mesh Heteroarchitecture with Cu-Ni Dual Active Sites as Efficient Bifunctional Electrocatalyst for Urea-Assisted Energy-Saving Hydrogen Production in Alkaline Electrolyte" Catalysts 14, no. 8: 496. https://doi.org/10.3390/catal14080496

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