Phosphorized CoNi₂S₄ Yolk-Shell Spheres for Highly Efficient Hydrogen Production via Water and Urea Electrolysis
磷化 CoNi ₂ S ₄ 蛋黄壳球，通过水和尿素电解高效制氢

Introduction 介绍

Renewable energy-driven electrolytic hydrogen generation has been recognized as an eco-friendly manner to alleviate the energy crisis and environmental issues faced by the international community.^1-3 However, the unfavorable thermodynamics and sluggish kinetics of both cathodic reduction and anodic oxidation reactions severely restrict the hydrogen production rate and energy efficiency.^3-7 Noble-metal-based electrocatalysts are known as the most active catalysts for electrolytic hydrogen production, but their large-scale application is largely hindered by the prohibitive costs, low reserves, and unsatisfactory stability.^8-10 Recent years have witnessed the rapid development of earth-abundant electrocatalysts, especially towards hydrogen/oxygen evolution reactions (HER/OER) for alkaline water splitting.^{11, 12} But most of them do not possess excellent performance for both HER and OER in the same electrolyte due to their incompatible activity over different pH ranges.¹⁰ In addition, the high thermodynamic equilibrium potential (1.23 V versus reversible hydrogen electrode (vs. RHE)) of OER undoubtedly increases the energy consumption of hydrogen production.^{7, 9} Therefore, to reduce the overall cell voltage and manufacturing cost, exploring more efficient low-cost multifunctional electrocatalysts and hybrid water electrolysis systems coupled with some diligent anodic oxidations is highly desired.
可再生能源驱动的电解制氢已被公认为一种环保方式，可以缓解国际社会面临的能源危机和环境问题。 ^1-3 然而，阴极还原和阳极氧化反应的不利热力学和迟缓动力学严重限制了制氢速率和能源效率。 ^3-7 贵金属基电催化剂被认为是电解制氢最活跃的催化剂，但其大规模应用在很大程度上受到成本高、储量低和稳定性不理想的阻碍。 ^8-10 近年来，地球丰富的电催化剂迅速发展，特别是用于碱性水分解的氢/氧析出反应（HER/OER）。 ^{11, 12} 但是，由于它们在不同pH值范围内的活性不相容，因此它们中的大多数在同一电解质中对HER和OER都不具有出色的性能。 ¹⁰ 此外，OER的高热力学平衡电位（1.23 V与可逆氢电极（相对于RHE））无疑增加了制氢的能耗。 ^{7, 9} 因此，为了降低电池的整体电压和制造成本，非常需要探索更高效的低成本多功能电催化剂和混合水电解系统，并结合一些精细的阳极氧化。

Urea oxidation reaction (UOR) has been reported as an ideal alternative to OER due to its much lower thermodynamic equilibrium potential (0.37 V vs. RHE), enabling the urea electrolysis a promising electrochemical approach for energy-saving and high-efficiency hydrogen production.^13-16 Moreover, urea-rich wastewater is generated from industrial synthesis and sanitary sewage, thus making urea electrolysis attractive for mitigating the problem of urea-rich water pollution.¹⁵ However, the complicated six-electron transfer process of UOR (CO(NH₂)₂ + 6 OH⁻ → N₂ + 5 H₂O + CO₂ + 6 e⁻) leads to the sluggish kinetics and higher practical cell voltage of urea electrolysis.¹⁶ Among all the transition metal-based electrocatalysts, nickel (Ni) is the most widely used element for energy and environmental applications.¹⁷ For example, Raney Ni and Ni alloys have been widely used in conventional alkaline water electrolyzers due to their low cost and high efficiency.¹⁸ Recently, Ni and Ni-based compounds have received great attention for the UOR to accelerate the kinetics, during which they undergo an oxidation process to generate active Ni³⁺ (NiOOH) for catalyzing UOR.^{13, 15, 19} Nevertheless, there are few reports on trifunctional Ni-based electrocatalysts for highly efficient hydrogen production via both water and urea electrolysis.
据报道，尿素氧化反应 （UOR） 是 OER 的理想替代品，因为它的热力学平衡电位要低得多（0.37 V 与 RHE），使尿素电解成为一种很有前途的节能和高效制氢的电化学方法。 ^13-16 此外，富含尿素的废水是由工业合成和生活污水产生的，因此尿素电解对缓解富含尿素的水污染问题具有吸引力。 ¹⁵ 然而，UOR （CO（NH ₂ ） ₂ + 6 OH ⁻ → N ₂ + 5 H ₂ O + CO ₂ + 6 e ⁻ ） 复杂的六电子转移过程导致尿素电解动力学缓慢，实际电池电压较高。 ¹⁶ 在所有过渡金属基电催化剂中，镍（Ni）是能源和环境应用中使用最广泛的元素。 ¹⁷ 例如，雷尼镍合金和镍合金因其成本低、效率高而广泛应用于常规碱性水电解槽中。 ¹⁸ 近年来，Ni 和 Ni 基化合物因 UOR 加速动力学而受到高度关注，在此过程中它们经历氧化过程生成活性 Ni ³⁺ （NiOOH） 以催化 UOR。 ^{13, 15, 19} 然而，关于三官能镍基电催化剂通过水和尿素电解高效制氢的报道很少。

Among all the Ni-based electrocatalysts, sulfides have shown promising trifunctional electrocatalytic performance towards HER, OER, and UOR, and various strategies have been applied to improve their performance. One appealing idea is composition manipulation by heteroatom doping or substitution, which can modulate the adsorption energies of intermediates by optimizing the electronic structure of catalysts. For example, Yin et al. prepared different metal-doped NiS₂ (M-NiS₂, M=Fe, Co, Cu) nanosheets, in which Co-NiS₂ achieved prominent HER performance in alkaline media due to the optimal e_g¹ electron configuration.²⁰ Besides, cation doping can also significantly increase the content of high-valence Ni³⁺ (NiOOH) species, which have been identified as the catalytic active sites for anodic OER and UOR.^{14, 19, 21, 22} Recently, phosphorization has also been widely reported to improve the HER intrinsic activity and conductivity by tuning the electronic structure and distorting the lattices of the parent sulfides.^23-26 Morphological and structural engineering is another efficient approach for enhancing the catalytic performance through exposing more active sites. In particular, porous yolk-shell structures are favored because of their high specific surface area, large void volume, and reduced ion-diffusion path.^27-29
在所有镍基电催化剂中，硫化物对HER、OER和UOR表现出良好的三官能团电催化性能，并应用了多种策略来改善其性能。一个吸引人的想法是通过杂原子掺杂或取代来操纵成分，它可以通过优化催化剂的电子结构来调节中间体的吸附能。例如，Yin等人制备了不同的金属掺杂NiS ₂ （M-NiS，M ₂ =Fe，Co，Cu）纳米片，其中Co-NiS ₂ 由于最佳的e _g ¹ 电子构型而在碱性介质中取得了突出的HER性能。 ²⁰ 此外，阳离子掺杂还可以显著增加高价Ni ³⁺ （NiOOH）物质的含量，这些物质已被确定为阳极OER和UOR的催化活性位点。 ^{14, 19, 21, 22} 最近，磷化也被广泛报道通过调整电子结构和扭曲母硫化物的晶格来改善 HER 的内在活性和电导率。 ^23-26 形态和结构工程是另一种通过暴露更多活性位点来提高催化性能的有效方法。特别是，多孔卵黄壳结构因其高比表面积、大空隙体积和减少离子扩散路径而受到青睐。 ^27-29

Inspired by these advancements in multielement Ni-based sulfide electrocatalysts, we rationally design and synthesize porous phosphorus substituted CoNi₂S₄ yolk-shell spheres (P-CoNi₂S₄ YSSs) via a facial hydrothermal sulfidation and subsequent gas-phase phosphorization strategy (Figure 1). Benefiting from the advantageous features of bimetallic elements, high electronic conductivity and enriched Ni³⁺ content induced by phosphorization, as well as unique hollow structure, the obtained P-CoNi₂S₄ YSSs electrocatalyst exhibits superior activity and stability towards HER, OER, and UOR in alkaline media, yielding low potentials of −0.135 V, 1.512 V, and 1.306 V vs. RHE at 10 mA cm⁻², respectively. Consequently, the P-CoNi₂S₄∥P-CoNi₂S₄ catalyst couple for water electrolysis only needs a low cell voltage of 1.544 V to steadily drive a benchmark current density of 10 mA cm⁻² for 100 h, much better than that of commercial Pt/C∥RuO₂ couple. Remarkably, the cell voltage can be further reduced to 1.402 V when 0.5 M urea is added, almost rivaling the state-of-the-art urea electrolysis. This work highlights the effectiveness of Ni-based bimetallic sulfides for water and urea electrolysis and strengthens the significance of the nickel valence for performance improvement, hence enabling cost-effective and energy-saving electrochemical hydrogen production.
受多元素镍基硫化物电催化剂这些进展的启发，我们通过表面水热硫化和随后的气相磷化策略，合理地设计和合成了多孔磷取代的CoNi ₂ S ₄ 卵黄壳球（P-CoNi ₂ S ₄ YSS）（图1）。得益于双金属元素的优越特性、磷化诱导的高电子电导率和富集的Ni ³⁺ 含量以及独特的中空结构，所制备的P-CoNi ₂ S ₄ YSSs电催化剂在碱性介质中对HER、OER和UOR表现出优异的活性和稳定性，在10 mA cm ⁻² 时，与RHE相比，电位为−0.135 V、1.512 V和1.306 V。 分别。因此，用于水电解的 P-CoNi ₂ S ₄ ∥P-CoNi ₂ S ₄ 催化剂偶只需要 1.544 V 的低电池电压即可在 100 小时内稳定驱动 10 mA cm ⁻² 的基准电流密度，远优于商业 Pt/C∥RuO ₂ 偶。值得注意的是，当添加 0.5 M 尿素时，电池电压可以进一步降低到 1.402 V，几乎可以与最先进的尿素电解相媲美。这项工作强调了镍基双金属硫化物在水和尿素电解中的有效性，并加强了镍价对性能改进的重要性，从而实现了具有成本效益和节能的电化学制氢。

Results and Discussion 结果与讨论

Uniform CoNi-glycerate solid spheres (CoNi-G SSs) are prepared by a previously reported method with slight modification (experimental details in Supporting Information).²⁸ Field-emission scanning electrode microscopy (FESEM) and transmission electron microscopy (TEM) images demonstrate the smooth surface and solid nature of CoNi-G SSs with an average diameter of 490 nm (Figure 2 a,d; Supporting Information, Figure S1a,b). X-ray diffraction (XRD) pattern (Figure S1c) and energy-dispersive X-ray (EDX) spectrum (Figure S1d) indicate the amorphous phase of metal alkoxides and the chemical composition of a Co/Ni atomic ratio around 1/2, respectively.³⁰ CoNi₂S₄ YSSs are obtained via the chemical etching/anion exchange reactions between CoNi-G SSs and thioacetamide solution under hydrothermal conditions (see the experimental details in the Supporting Information). FESEM and TEM images show the rough surface and void space between the interior solid core and the outer porous shell of CoNi₂S₄ YSSs with an average diameter of 570 nm (Figure 2 b,e; Figure S2a). Subsequently, P-CoNi₂S₄ YSSs are achieved by a gas-phase phosphorization of CoNi₂S₄ YSSs (see the experimental details in the Supporting Information). The porous yolk-shell structure is well preserved with a slightly reduced diameter of 520 nm (Figure 1 c,f; Figure S2b). A typical TEM image of an individual P-CoNi₂S₄ YSS further demonstrates that the yolk-shell structure with a porous, thin shell (about 35 nm in thickness) is assembled by ultrafine nanoparticles (Figure 2 g), which is beneficial for the exposure of active sites. The high-resolution TEM (HRTEM) and inverse fast Fourier transform (IFFT) images (Figure 2 h) clearly show two sets of distinct lattice fringes with interplanar spacings of 0.24 and 0.28 nm, which are readily assigned to the (400) and (311) planes of CoNi₂S₄ (JCPDS card No. 24-0334), respectively. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding elemental mapping images (Figure 2 i) show the homogenous distribution of Co, Ni, S, and P elements in P-CoNi₂S₄ YSSs.
均匀的辅酶-甘油酸固体球体（CoNi-G SSs）是通过先前报道的方法制备的，略有修改（实验细节在支持信息中）。 ²⁸ 场发射扫描电极显微镜（FESEM）和透射电子显微镜（TEM）图像显示了平均直径为490 nm的CoNi-G SS的光滑表面和固体性质（图2 a，d;支持信息，图S1a，b）。X射线衍射（XRD）图谱（图S1c）和能量色散X射线（EDX）光谱（图S1d）分别表明金属醇盐的无定形相和Co/Ni原子比约为1/2的化学成分。 ³⁰ CoNi ₂ S ₄ YSSs是在水热条件下通过CoNi-G SSs与硫代乙酰胺溶液之间的化学蚀刻/阴离子交换反应获得的（参见支持信息中的实验细节）。FESEM和TEM图像显示了平均直径为570 nm的CoNi ₂ S ₄ YSSs内部实心和外部多孔壳之间的粗糙表面和空隙空间（图2 b，e;图S2a）。随后，通过CoNi ₂ S ₄ YSS的气相磷化实现P-CoNi ₂ S ₄ YSS（参见支持信息中的实验细节）。多孔卵黄壳结构保存完好，直径略微减小至520nm（图1 c，f;图S2b）。单个P-CoNi ₂ S ₄ YSS的典型TEM图像进一步表明，具有多孔薄壳（厚度约35nm）的蛋黄壳结构是由超细纳米颗粒组装而成的（图2g），这有利于活性位点的暴露。 高分辨率透射电镜（HRTEM）和逆快速傅里叶变换（IFFT）图像（图2 h）清楚地显示了两组不同的晶格条纹，其面间间距分别为0.24和0.28 nm，它们很容易分别分配给CoNi ₂ S ₄ （JCPDS卡号24-0334）的（400）和（311）平面。高角度环形暗场扫描透射电子显微镜（HAADF-STEM）和相应的元素映射图像（图2 i）显示了P-CoNi ₂ S ₄ YSS中Co、Ni、S和P元素的均匀分布。

XRD analysis (Figure 3 a) indicates the similar diffraction patterns in CoNi₂S₄ YSSs and P-CoNi₂S₄ YSSs, which correspond well with the standard JCPDS card of No. 24-0334, manifesting that phosphorization would not destroy the crystal structure, which may be due to the similar atomic radius of S and P.^{31, 32} The average atomic ratio of P/S is determined to be about 0.9 from the EDX spectra (Figure 3 b), giving about 47 % substitution of the S sites. Further insights into the changes of surface chemical states after P doping could be acquired from the X-ray photoelectron spectroscopy (XPS) spectra. The peak-fitting analysis of Co 2p spectra (Figure 3 c) suggests that two chemical states exist in both CoNi₂S₄ and P-CoNi₂S₄, corresponding to the spin-orbital characteristics of Co³⁺ (778.56 eV for Co 2p_3/2 and 793.55 eV for Co 2p_1/2) and Co²⁺ (781.50 eV for Co 2p_3/2 and 798.01 eV for Co 2p_1/2), accompanied with a pair of broad satellite peaks.^{33, 34} Similarly, the fitting peaks of Ni 2p spectra (Figure 3 d) show two pairs of main peaks (853.27 eV for Ni²⁺ 2p_3/2 and 870.43 eV for Ni²⁺ 2p_1/2; 856.74 eV for Ni³⁺ 2p_3/2 and 874.64 eV for Ni³⁺ 2p_1/2) and two broad satellites.^{34, 35} Notably, the higher ratios of both Co²⁺/Co³⁺ and Ni³⁺/Ni²⁺ calculated by integrating the peak areas indicate much higher crystalline purity of CoNi₂S₄ in P-CoNi₂S₄, thereby being conducive to stronger redox reaction and enhancing the catalytic performance.^{14, 34, 35} For the S 2p spectra in Figure 3 e, both of them can be fitted with three distinct doublets of S 2p_3/2 and S 2p_1/2 with an area ratio of 2:1, including the first one (161.48 eV, 162.59 eV) that arises from a sulfide species, the second one (163.50 eV, 164.66 eV) indicative of sulfur in a thiolate-type environment, and the third one (168.81 eV, 169.97 eV) ascribed to surface oxidation.^{31, 32, 36} Furthermore, the reduction in the area ratio of the fitting peaks belonging to oxides and sulfides in P-CoNi₂S₄ suggests phosphorization could suppress the oxidation of sulfides in air.^{24, 32, 34} The P 2p spectrum of P-CoNi₂S₄ (Figure 3 f) only displays one doublet (133.52 eV, 134.39 eV) assigned to the P-bonded with Co/Ni, no oxidized P species are observed.^{31, 37} This fully confirms the substitution characteristics of P, which will induce more electronic interactions for enriched Ni³⁺ content, thereby promoting electrocatalytic performance.
XRD分析（图3a）表明CoNi ₂ S ₄ YSS和P-CoNi ₂ S ₄ YSS的衍射图谱相似，与标准JCPDS卡编号24-0334非常吻合，表明磷化不会破坏晶体结构，这可能是由于S和P的原子半径相似。 ^{31, 32} 从EDX光谱中确定P / S的平均原子比约为0.9（图3 b），S位点的取代率约为47%。从X射线光电子能谱（XPS）光谱中可以进一步了解磷掺杂后表面化学状态的变化。Co 2p光谱的峰拟合分析（图3 c）表明，CoNi ₂ S ₄ 和P-CoNi ₂ S ₄ 中存在两种化学态，对应于Co ³⁺ （Co 2p _3/2 为778.56 eV，Co 2p _1/2 为793.55 eV）和Co ²⁺ （Co 2p _3/2 为781.50 eV，Co 2p _1/2 为798.01 eV）的自旋轨道特性，并伴有一对宽卫星峰。 ^{33, 34} 同样，Ni 2p光谱的拟合峰（图3 d）显示了两对主峰（Ni ²⁺ 2p _3/2 为853.27 eV，Ni ²⁺ 2p _1/2 为870.43 eV;Ni ³⁺ 2p _3/2 为856.74 eV，Ni ³⁺ 2p _1/2 为874.64 eV）和两颗宽卫星。 ^{34, 35} 值得注意的是，通过积分峰面积 ²⁺ 计算出的Co ²⁺ /Co ³⁺ 和Ni ³⁺ /Ni的较高比率表明P-CoNi ₂ S ₄ 中CoNi ₂ S ₄ 的结晶纯度更高，从而有利于更强的氧化还原反应和提高催化性能。 ^{14, 34, 35} 对于图3 e中的S 2p光谱，它们都可以拟合三个不同的双峰，即S 2p _3/2 和S 2p _1/2 ，面积比为2：1，包括第一个（161.48 eV，162.59 eV）来自硫化物物种，第二个（163.50 eV，164.66 eV）表示硫酸盐型环境中的硫， 第三个（168.81 eV，169.97 eV）归因于表面氧化。 ^{31, 32, 36} 此外，P-CoNi ₂ S ₄ 中属于氧化物和硫化物的拟合峰的面积比降低表明磷化可以抑制空气中硫化物的氧化。 ^{24, 32, 34} P-CoNi ₂ S ₄ 的 P 2p 光谱（图 3 f）仅显示一个分配给与 Co/Ni 键合的 P 的双峰 （133.52 eV， 134.39 eV），未观察到氧化的 P 物种。 ^{31, 37} 这充分证实了P的取代特性，这将诱导更多的电子相互作用来丰富Ni ³⁺ 含量，从而促进电催化性能。

The electrocatalytic performance of P-CoNi₂S₄ YSSs for water splitting was first evaluated using a three-electrode configuration cell in alkaline media (1.0 M KOH, pH 14) at room temperature. Figure 4 a shows the polarization curves of CoNi₂S₄ and P-CoNi₂S₄ towards HER, which are corrected for ohmic potential drop (iR) loss. The P-CoNi₂S₄ catalyst exhibits a smaller onset overpotential (20 mV, Figure S3) than CoNi₂S₄, beyond which the cathodic current density rises rapidly under more negative potentials. Specifically, the P-CoNi₂S₄ catalyst requires a small overpotential of 135 mV to reach the current density of 10 mA cm⁻², which is a common criterion for evaluating the activity of water splitting.^{38, 39} Moreover, the P-CoNi₂S₄ catalyst displays a rapid rise in current density and constantly higher current density throughout the applied potential range, which is further evidenced by the analysis of Tafel slopes (Figure 4 b). The reduction in Tafel slope from 81 mV dec⁻¹ for CoNi₂S₄ to 65 mV dec⁻¹ for P-CoNi₂S₄, together with the smaller overpotential of P-CoNi₂S₄ at the same current density, fully confirms the promotional effect of P substitution on the activity and kinetics of alkaline HER. This enhancement may be due to the electronic modulation effect of P on metal and/or S sites to facilitate the water dissociation, and in addition the P sites could also serve as the active sites for HER.^{10, 40-42} Moreover, the P-CoNi₂S₄ catalyst shows superior HER activity and kinetics when compared to some previously reported sulfide-based electrocatalysts (Supporting Information, Table S1). The 40 h chronopotentiometry test (Figure S4) also shows the superior catalytic stability of P-CoNi₂S₄.
首先在室温下使用碱性介质（1.0 M KOH，pH 14）中的三电极构型池评估了P-CoNi ₂ S ₄ YSS用于水分解的电催化性能。图 4 a 显示了 CoNi ₂ S ₄ 和 P-CoNi ₂ S ₄ 朝向 HER 的极化曲线，并针对欧姆电位压降 （iR） 损耗进行了校正。P-CoNi ₂ S ₄ 催化剂的起始过电位（20 mV，图S3）比CoNi ₂ S ₄ 小，超过该过电位，阴极电流密度在更大的负电位下迅速上升。具体来说，P-CoNi ₂ S ₄ 催化剂需要135 mV的小过电位才能达到10 mA cm ⁻² 的电流密度，这是评估水分解活性的常用标准。 ^{38, 39} 此外，P-CoNi ₂ S ₄ 催化剂在施加的电位范围内表现出电流密度的快速上升和不断升高的电流密度，Tafel斜率的分析进一步证明了这一点（图4b）。CoNi S 的 Tafel 斜率从 81 mV dec ⁻¹ 降低到 P-CoNi ₂ S ₄ 的 65 mV dec ⁻¹ ，以及 P-CoNi ₂ S ₄ 在相同电流密度下较小的过电位，充分证实了 P 取代对碱性 HER 活性和动力学的促进作用。 _{4 2} 这种增强可能是由于P对金属和/或S位点的电子调制作用，以促进水的解离，此外，P位点也可以作为HER的活性位点。 ^{10, 40-42} 此外，与先前报道的一些硫化物基电催化剂相比，P-CoNi ₂ S ₄ 催化剂显示出优异的HER活性和动力学（支持信息，表S1）。40 h计时电位试验（图S4）也显示了P-CoNi ₂ S ₄ 的卓越催化稳定性。

The iR-compensated cyclic voltammetry (CV) curves of these two samples (Figure S5) both show the pre-oxidation process to generate catalytically active high-valence species.^{22, 43-45} To avoid the effect of oxidation peak, we used the reverse sweep branch of the CV curve to evaluate the OER activity and kinetics. As shown in Figure 4 c,d, the OER activity and kinetics of CoNi₂S₄ have been greatly improved after P substitution, suggesting positive effects of P on OER. To reach a current density of 10 and 100 mA cm⁻², the P-CoNi₂S₄ catalyst requires overpotentials of 282 and 327 mV, respectively, which are lower than that of CoNi₂S₄ (288 and 349 mV). The smaller Tafel slope of 40 mV dec⁻¹ for P-CoNi₂S₄ than that of CoNi₂S₄ (53 mV dec⁻¹) shows the faster OER kinetics after P substitution. Such superior activity and kinetics of P-CoNi₂S₄ compare favorably with that of many sulfide-based OER electrocatalysts (Table S2). In addition, the 60 h chronopotentiometry test (Figure S6) also shows the superior catalytic stability of P-CoNi₂S₄ towards OER.
这两个样品的iR补偿循环伏安（CV）曲线（图S5）都显示了产生催化活性高价物质的预氧化过程。 ^{22, 43-45} 为了避免氧化峰的影响，我们使用CV曲线的反向扫描分支来评估OER活性和动力学。如图4 c，d所示，磷取代后CoNi ₂ S ₄ 的OER活性和动力学大大提高，表明磷对OER有积极影响。为了达到 10 和 100 mA cm ⁻² 的电流密度，P-CoNi ₂ S ₄ 催化剂需要的过电位分别为 282 和 327 mV，低于 CoNi ₂ S ₄ （288 和 349 mV）。P-CoNi S 的 Tafel 斜率为 40 mV dec ⁻¹ ，比 CoNi ₂ S ₄ 的 Tafel 斜率小 （53 mV dec ⁻¹ ） 表明 P 取代后的 OER 动力学更快。 _{4 2} P-CoNi ₂ S ₄ 的这种优越的活性和动力学性能优于许多硫化物基OER电催化剂（表S2）。此外，60 h计时电位试验（图S6）也显示了P-CoNi ₂ S ₄ 对OER的卓越催化稳定性。

CV curves at different scan rates (Figure S7) are measured to evaluate the electrochemically active surface area (ECSA).^{9, 38} As shown in Figure 4 e, the P-CoNi₂S₄ catalyst shows a much higher value (4.7 mF cm⁻²) than CoNi₂S₄ (2.6 mF cm⁻²), indicating more exposed active sites in the former. The increased ECSA may be attributed to the formation of additional active sites that originated from the incorporation of P into CoNi₂S₄ by adjusting the catalytic properties of adjacent metal sites in the CoNi₂S₄,^{46, 47} which can be seen from the difference in the CV curves (Figure S5). In addition, the incorporated P sites may serve as new active sites for water dissociation.^{40, 41} Electrochemical impedance spectroscopy (EIS) measurements are also performed to evaluate the charge-transfer resistance (R_ct), which is associated with electrocatalytic kinetics.^{9, 48, 49} The smaller diameter of the semicircle in the high frequency region for P-CoNi₂S₄ suggests faster charge transfer and electrocatalytic kinetics (Figure 4 f; Figure S8), which may be due to the increase in electron delocalization induced by lone-pair electrons in 3p orbitals and empty 3d orbitals of phosphorus.^{50, 51} Previously reported density functional theory calculation results also confirm that P-doped sulfides display a much lower band gap than undoped sulfides due to the generation of new electronic states in the conduction band by P doping.^{46, 52} This effect of P substitution on increasing ECSA and decreasing R_ct is expected from previous studies, in which heteroatom doping or substitution has been reported to be a versatile strategy for modulating the electronic and/or surface structures of the host materials.^{26, 31}
测量不同扫描速率下的CV曲线（图S7）以评估电化学活性表面积（ECSA）。 ^{9, 38} 如图4 e所示，P-CoNi ₂ S ₄ 催化剂的值（4.7 mF cm ⁻² ）比CoNi ₂ S ₄ （2.6 mF cm ⁻² ）高得多，表明前者的活性位点暴露更多。ECSA的增加可归因于通过调整CoNi ₂ S ₄ 中相邻金属位点的催化性能，将P掺入CoNi ₂ S ₄ 中而形成的额外活性位点， ^{46, 47} 这可以从CV曲线的差异中看出（图S5）。此外，掺入的 P 位点可作为水解离的新活性位点。 ^{40, 41} 电化学阻抗谱（EIS）测量也用于评估电荷转移电阻（R _ct ），这与电催化动力学有关。 ^{9, 48, 49} P-CoNi ₂ S ₄ 在高频区域的半圆直径较小，表明电荷转移和电催化动力学更快（图4 f;图S8），这可能是由于磷的3p轨道和空3d轨道中的孤对电子诱导的电子离域增加。 ^{50, 51} 先前报道的密度泛函理论计算结果也证实，由于P掺杂在导带中产生新的电子态，P掺杂硫化物的带隙比未掺杂的硫化物低得多。 ^{46, 52} P取代对增加ECSA和降低R _ct 的这种影响在以前的研究中是可以预期的，其中杂原子掺杂或取代已被报道为调节主体材料的电子和/或表面结构的通用策略。 ^{26, 31}

In view of the distinguished electrocatalytic performance of the as-prepared P-CoNi₂S₄ catalyst towards both HER and OER, a two-electrode electrolyzer was fabricated with P-CoNi₂S₄ as both the cathodic and anodic catalysts and 1.0 M KOH solution as the medium. The polarization curve for water electrolysis (Figure 4 g) indicates the superior overall water splitting performance of the P-CoNi₂S₄∥P-CoNi₂S₄ couple, which requires a low cell voltage of 1.544 V to afford a current density of 10 mA cm⁻² compared to 1.533 V for a Pt/C∥RuO₂ couple using commercial catalysts. More importantly, the advantage of this P-CoNi₂S₄∥P-CoNi₂S₄ couple is much more significant at higher current densities. One can see that this low cell voltage of 1.544 V for P-CoNi₂S₄ is comparable or superior to that of the most efficient bifunctional electrocatalysts reported (Table S3), including some recently reported values, such as 1.46 V for Mo-doped Ni₃S₂/Ni_xP_y hollow nanorods,⁵³ 1.58 V for CoFeO@black phosphorus,⁵⁴ 1.59 V for Fe₂CoPS₃,⁵⁵ and 1.62 V for Fe-Ni₂P@P-doped carbon/Cu_xS arrays.⁵⁶ Besides, the chronopotentiometry curves recorded at 10 mA cm⁻² (Figure 4 h) show that the P-CoNi₂S₄ catalyst exhibits outstanding overall water splitting durability over 100 h, much better than that of the Pt/C∥RuO₂ couple. These results suggest that the as-prepared P-CoNi₂S₄ sample is a promising catalyst to replace noble metal catalysts for efficient and long-lasting hydrogen production in the practical water electrolysis device.
鉴于所制备的P-CoNi ₂ S ₄ 催化剂对HER和OER具有出色的电催化性能，以P-CoNi ₂ S ₄ 为阴极和阳极催化剂，以1.0 M KOH溶液为介质，制备了一种双电极电解槽。水电解的极化曲线（图4 g）表明P-CoNi ₂ S ₄ ∥P-CoNi ₂ S ₄ 偶具有优异的整体水分解性能，该对需要1.544 V的低电池电压才能提供10 mA cm ⁻² 的电流密度，而使用商业催化剂的Pt/C∥RuO ₂ 偶为1.533 V。更重要的是，这种P-CoNi ₂ S ₄ ∥P-CoNi ₂ S ₄ 耦合的优势在更高的电流密度下更为显著。可以看出，P-CoNi ₂ S ₄ 的 1.544 V 低电池电压与报道的最有效的双功能电催化剂相当或优于（表 S3），包括一些最近报道的值，例如掺钼 Ni ₃ S ₂ /Ni _x P _y 空心纳米棒为 1.46 V，CoFeO@black ⁵³ 磷为 1.58 V，Fe ⁵⁴ ₂ CoPS 为 1.59 V ₃ ， ⁵⁵ Fe-Ni ₂ P@P掺杂碳/Cu _x S 阵列为 1.62 V。 ⁵⁶ 此外，在10 mA cm ⁻² 处记录的计时电位曲线（图4 h）表明，P-CoNi ₂ S ₄ 催化剂在100 h内表现出出色的整体水分解耐久性，远优于Pt/C∥RuO ₂ 对。 这些结果表明，制备的P-CoNi ₂ S ₄ 样品是一种很有前途的催化剂，可以替代贵金属催化剂，在实用的水电解装置中高效、持久地制氢。

Although the P-CoNi₂S₄ catalyst exhibits excellent performance in overall water splitting, the energy efficiency for hydrogen production still needs to be further improved from the industrial application perspective. As mentioned earlier, UOR is a facile anodic reaction to efficiently reduce the energy input for hydrogen generation.^13-16 The UOR performance of P-CoNi₂S₄ is evaluated through a three-electrode configuration in 1.0 M KOH solution containing 0.5 M urea (the concentration screening is shown in Figure S9). To illustrate the electrocatalytic mechanism of UOR, CV curves of P-CoNi₂S₄ in 1.0 M KOH solution with and without urea are plotted in Figure 5 a for comparison. In the bare KOH solution, a typical pre-oxidation process occurs before OER, while when urea is added, the current density in the positive sweep dramatically increases. The potential to reach current densities of 10 and 100 mA cm⁻² is drastically reduced to 1.306 and 1.367 V in 1.0 M KOH containing 0.5 M urea. A small Tafel slope of 55 mV dec⁻¹ (Figure S10) also well supports the observed rapid increment in the current density, indicating the fast kinetics towards UOR. Such superior UOR performance endows the as-prepared P-CoNi₂S₄ as one of the state-of-the-art Ni-based electrocatalysts (Table S4). Moreover, the negligible anodic current for OER compared with that for UOR demonstrates the superior selectivity of P-CoNi₂S₄ towards UOR at low potentials (<1.5 V vs. RHE). The smaller reduction peak in the reverse sweep branch of CV curves also indicates that metal species are mostly involved directly in UOR rather than pre-oxidized to catalyze OER.⁵⁷ Both branches of the CV curves confirm the low onset potential for UOR, which is close to the onset potential of pre-oxidation, proving that the high-valence metal species promotes the catalysis of urea.^{14, 16, 57} The UOR rate capability of P-CoNi₂S₄ is studied by tuning the scan rate from 2 to 100 mV s⁻¹ (Figure 5 b). The current density and square root of the scan rate maintain a good linear relationship at 1.40 V vs. RHE (inset in Figure 5 b), which indicates that UOR is a surface controlled process with highly efficient charge and mass transfer.^{16, 58}
尽管P-CoNi ₂ S ₄ 催化剂在整体水分解中表现出优异的性能，但从工业应用的角度来看，制氢的能效仍有待进一步提高。如前所述，UOR 是一种简单的阳极反应，可有效减少氢气产生的能量输入。 ^13-16 P-CoNi ₂ S ₄ 的UOR性能通过在含有0.5M尿素的1.0M KOH溶液中的三电极配置进行评估（浓度筛选如图S9所示）。为了说明UOR的电催化机理，图5a绘制了P-CoNi ₂ S ₄ 在1.0 M KOH溶液中含尿素和不含尿素的CV曲线进行比较。在裸露的KOH溶液中，典型的预氧化过程发生在OER之前，而当添加尿素时，正扫描中的电流密度急剧增加。在含有 0.5 M 尿素的 1.0 M KOH 中，达到 10 和 100 mA cm ⁻² 电流密度的可能性急剧降低到 1.306 和 1.367 V。55 mV dec 的小 Tafel 斜率 ⁻¹ （图 S10）也很好地支持了观察到的电流密度的快速增量，表明了朝向 UOR 的快速动力学。这种优异的 UOR 性能使制备的 P-CoNi ₂ S ₄ 成为最先进的镍基电催化剂之一（表 S4）。此外，与UOR相比，OER的阳极电流可以忽略不计，这表明P-CoNi ₂ S ₄ 在低电位（<1.5 V与RHE相比）对UOR具有优越的选择性。CV曲线反向扫描分支中较小的还原峰也表明，金属物种大多直接参与UOR，而不是预氧化以催化OER。 ⁵⁷ CV曲线的两个分支都证实了UOR的低起始电位，接近预氧化的起始电位，证明高价金属物种促进了尿素的催化。 ^{14, 16, 57} 通过将扫描速率从2调整到100 mV s ⁻¹ 来研究P-CoNi ₂ S ₄ 的UOR速率能力（图5 b）。在1.40 V电压下，电流密度和平方根与RHE保持良好的线性关系（图5b中的插图），这表明UOR是一种具有高效电荷和传质的表面控制过程。 ^{16, 58}

To verify the feasibility of UOR as an alternative anodic reaction, the HER activity of P-CoNi₂S₄ was also evaluated in a 1.0 M KOH solution containing 0.5 M urea. The almost overlapped LSV curves (Figure S11) indicate the negligible influence of urea on the HER activity, showing the bifunctional activity of P-CoNi₂S₄ towards both HER and UOR. Subsequently, a two-electrode configuration in 1.0 M KOH solution containing 0.5 M urea is fabricated using P-CoNi₂S₄ as both the anode and cathode to perform urea-assisted electrochemical hydrogen production. As shown in Figure 5 c, the cell voltage to achieve the current density of 10 mA cm⁻² is 1.402 V, which is much lower than that required for water splitting (1.544 V), proving the assistance of urea oxidation can lower the energy consumption of hydrogen production. The performance of P-CoNi₂S₄ in overall urea electrolysis is also superior to that of many other electrocatalysts (Table S5). Furthermore, the chronopotentiometry curve recorded at 10 mA cm⁻² (Figure 5 d) shows good durability with only a 26 mV increment in the cell voltage after a continuous operation of 100 h. The excellent catalytic activity and long-term durability indicate that the synthesized P-CoNi₂S₄ YSSs might serve as a low-cost and high-efficiency bifunctional electrocatalyst for feasible urea-assisted electrochemical hydrogen production.
为了验证UOR作为替代阳极反应的可行性，还在含有0.5M尿素的1.0M KOH溶液中评估了P-CoNi ₂ S ₄ 的HER活性。几乎重叠的LSV曲线（图S11）表明尿素对HER活性的影响可以忽略不计，显示了P-CoNi ₂ S ₄ 对HER和UOR的双功能活性。随后，在 _{2 4} 含有0.5 M尿素的1.0 M KOH溶液中制备了含有0.5 M尿素的双电极配置，以进行尿素辅助电化学制氢。如图5 c所示，实现10 mA cm ⁻² 电流密度的电池电压为1.402 V，远低于水分解所需的电压（1.544 V），证明尿素氧化的辅助可以降低制氢的能耗。P-CoNi ₂ S ₄ 在整个尿素电解中的性能也优于许多其他电催化剂（表S5）。此外，在10 mA cm ⁻² 处记录的计时电位曲线（图5 d）显示出良好的耐久性，在连续运行100小时后，电池电压仅增加26 mV。P-CoNi ₂ S ₄ YSSs具有优异的催化活性和长期耐久性，可作为尿素辅助电化学制氢的低成本、高效率双功能电催化剂。

Conclusion 结论

In summary, we have rationally designed and synthesized phosphorus-substituted bimetallic cobalt–nickel sulfide yolk-shell spheres (P-CoNi₂S₄ YSSs) via composition manipulation and nanostructure engineering. Thanks to the desired composition and structure, the obtained P-CoNi₂S₄ YSSs exhibit excellent electrocatalytic performance for HER and OER, giving rise to reduced cell voltage in overall water splitting. Moreover, the enriched Ni³⁺ content induced by P substitution yields P-CoNi₂S₄ YSSs, which are state-of-the-art UOR catalysts with a low potential of 1.306 V vs. RHE at 10 mA cm⁻². When replacing OER with more facile UOR, the urea-mediated electrolysis cell requires a very low potential of 1.402 V to reach 10 mA cm⁻², which is 142 mV less than that required for water splitting. This work provides a feasible and efficient strategy for the design and synthesis of high-performance electrocatalysts based on multifunctional nickel-based sulfides, which might open an avenue towards cost-effective and energy-saving electrochemical hydrogen production as well as promote the advancement of sulfides in related energy and environmental fields.
综上所述，我们通过成分操作和纳米结构工程合理设计合成了磷取代的双金属钴-镍硫化物卵黄壳球（P-CoNi ₂ S ₄ YSSs）。由于所需的成分和结构，所获得的P-CoNi ₂ S ₄ YSSs对HER和OER表现出优异的电催化性能，从而降低了整个水分解中的电池电压。此外，P取代诱导的富集Ni ³⁺ 含量产生了P-CoNi ₂ S ₄ YSSs，这是最先进的UOR催化剂，在10 mA cm ⁻² 时与RHE相比，电位低至1.306 V。当用更简单的UOR代替OER时，尿素介导的电解池需要非常低的1.402 V电位才能达到10 mA cm ⁻² ，这比水分解所需的电位低142 mV。本研究为基于多功能镍基硫化物的高性能电催化剂的设计与合成提供了可行且高效的策略，为经济节能的电化学制氢开辟了一条道路，并促进硫化物在相关能源和环境领域的发展。

Conflict of interest 利益冲突

The authors declare no conflict of interest.
作者声明没有利益冲突。