Ir Nanoparticles Anchored on Metal-Organic Frameworks for Efficient Overall Water Splitting under pH-Universal Conditions

The construction of high-activity and low-cost electrocatalysts is critical for efficient hydrogen production by water electrolysis. Herein, we developed an advanced electrocatalyst by anchoring well-dispersed Ir nanoparticles on nickel metal-organic framework (MOF) Ni-NDC (NDC: 2,6-naphthalenedicarboxylic) nanosheets. Benefiting from the strong synergy between Ir and MOF through interfacial Ni−O−Ir bonds, the synthesized Ir@Ni-NDC showed exceptional electrocatalytic performance for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting in a wide pH range, superior to commercial benchmarks and most reported electrocatalysts. Theoretical calculations revealed that the charge redistribution of Ni−O−Ir bridge induced the optimization of H₂O, OH* and H* adsorption, thus leading to the accelerated electrochemical kinetics for HER and OER. This work provides a new clue to exploit bifunctional electrocatalysts for pH-universal overall water splitting.     

Air-Stable Mn doped CuCl/CuO Hybrid Triquetrous Nanoarrays as Bifunctional Electrocatalysts for Overall Water Splitting

The development of highly efficient non-precious metal catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is key for large-scale hydrogen evolution through water splitting technology. Here, we report an air-stable Cu-based nanostructure consisting of Mn doped CuCl and CuO (CuCl/CuO(Mn)-NF) as a dual functional electrocatalyst for water splitting. CuCl is identified as the main active component, together with Mn doping and the synergistic effect between CuCl and CuO are found to make responsibility for the excellent OER and HER catalytic activity and stability. The assembled electrolyzes also exhibit decent water splitting performance. This work not only provides a simple method for preparing Cu-based composite catalyst, but also demonstrates the great potential of Cu-based non-noble metal electrocatalysts for water splitting and other renewable energy conversion technologies.     

General π‐Electron‐Assisted Strategy for Ir,Pt, Ru,Pd, Fe,Ni Single‐Atom Electrocatalysts with Bifunctional Active Sites for Highly Efficient Water Splitting

Both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are crucial to water splitting, but require alternative active sites. Now, a general π‐electron‐assisted strategy to anchor single‐atom sites (M=Ir, Pt, Ru, Pd, Fe, Ni) on a heterogeneous support is reported. The M atoms can simultaneously anchor on two distinct domains of the hybrid support, four‐fold N/C atoms (M@NC), and centers of Co octahedra (M@Co), which are expected to serve as bifunctional electrocatalysts towards the HER and the OER. The Ir catalyst exhibits the best water‐splitting performance, showing a low applied potential of 1.603 V to achieve 10 mA cm^?2 in 1.0 m KOH solution with cycling over 5 h. DFT calculations indicate that the Ir@Co (Ir) sites can accelerate the OER, while the Ir@NC₃ sites are responsible for the enhanced HER, clarifying the unprecedented performance of this bifunctional catalyst towards full water splitting.     

General π‐Electron‐Assisted Strategy for Ir,Pt, Ru,Pd, Fe,Ni Single‐Atom Electrocatalysts with Bifunctional Active Sites for Highly Efficient Water Splitting

Both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are crucial to water splitting, but require alternative active sites. Now, a general π‐electron‐assisted strategy to anchor single‐atom sites (M=Ir, Pt, Ru, Pd, Fe, Ni) on a heterogeneous support is reported. The M atoms can simultaneously anchor on two distinct domains of the hybrid support, four‐fold N/C atoms (M@NC), and centers of Co octahedra (M@Co), which are expected to serve as bifunctional electrocatalysts towards the HER and the OER. The Ir catalyst exhibits the best water‐splitting performance, showing a low applied potential of 1.603 V to achieve 10 mA cm^?2 in 1.0 m KOH solution with cycling over 5 h. DFT calculations indicate that the Ir@Co (Ir) sites can accelerate the OER, while the Ir@NC₃ sites are responsible for the enhanced HER, clarifying the unprecedented performance of this bifunctional catalyst towards full water splitting.     

Electrodeposited Cobalt‐Phosphorous‐Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting

One of the challenges to realize large‐scale water splitting is the lack of active and low‐cost electrocatalysts for its two half reactions: H₂ and O₂ evolution reactions (HER and OER). Herein, we report that cobalt‐phosphorous‐derived films (Co‐P) can act as bifunctional catalysts for overall water splitting. The as‐prepared Co‐P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with a current density of 10 mA cm^?2 at overpotentials of ?94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively. They can be employed as catalysts on both anode and cathode for overall water splitting with 100 % Faradaic efficiency, rivalling the integrated performance of Pt and IrO₂. The major composition of the as‐prepared and post‐HER films are metallic cobalt and cobalt phosphide, which partially evolved to cobalt oxide during OER.     

Two-Dimensional NiIr@N-Doped Carbon Nanocomposites Supported on Ni Foam for Electrocatalytic Overall Water Splitting

Electrochemical water splitting can provide a promising avenue for sustainable hydrogen production. Highly efficient electrocatalysts toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are extremely important for the practical application of water splitting technology. Herein, a one-step annealing strategy is reported for the fabrication of a metal–organic framework-derived bifunctional self-supported electrocatalyst, which is composed of two-dimensional N-doped carbon-wrapped Ir-doped Ni nanoparticle composites supported on Ni foam (NiIr@N-C/NF). The resultant NiIr@N-C/NF displays excellent electrocatalytic performance in 1.0 m KOH, with low overpotentials of 32 mV at 10 mA cm⁻² for the HER and 329 mV at 50 mA cm⁻² for the OER. Particularly, the HER-OER bifunctional NiIr@N-C/NF needs only 1.50 V to yield 10 mA cm⁻² for overall water splitting.     

Single-atom rhodium anchored on S-doped black phosphorene as a promising bifunctional electrocatalyst for overall water splitting

Superior bifunctional electrocatalysts with ultra-high stability and excellent efficiency are crucial to boost the oxygen evolution reaction (OER) and the hydrogen evolution reduction (HER) in the overall water splitting (OWS) for the sustainable production of clean fuels. Herein, comprehensive density functional theory (DFT) computations were performed to explore the potential of several single transition metal (TM) atoms anchored on various S-doped black phosphorenes (TM/S_nx-BP) for bifunctional OWS electrocatalysis. The results revealed that these candidates display good stability, excellent electrical conductivity, and diverse spin moments. Furthermore, the Rh/S₁₂-BP catalyst was identified as an eligible bifunctional catalyst for OWS process due to the low overpotentials for OER (0.43 V) and HER (0.02 V), in which Rh and its adjacent P atoms were identified as the active sites. Based on the computed Gibbs free energies of OH*, O*, OOH* and H*, the corresponding volcano plots for OER and HER were established. Interestingly, the spin moments and the charge distribution of the active sites determine the catalytic trends of OER and HER. Our findings not only propose a promising bifunctional catalyst for OWS, but also widen the potential application of BP in electrocatalysis.     

三维百合花状的CoNi2S4作为先进的双功能电催化剂用于氢析出和氧析出反应

为简化电解水催化剂的合成过程和优化电解水操作系统, 双功能电解水催化剂的研究, 特别是在碱性条件下同时具有优异催化氢析出和氧析出反应性能的双功能电催化剂的研究尤为重要. 其中, 过渡金属硫化物, 特别是 CoNi 硫化物, 被报道有与氢化酶类似的催化活性中心, 从而具有优异的催化氢析出和催化氧析出反应性能. 虽然有关对过渡金属硫化物的研究很多, 但主要集中在具有一维纳米线和二维纳米片形貌结构的过渡金属硫化物. 不幸的是, 这些形貌结构的过渡金属硫化物在电催化过程中容易聚集和受限于电荷传输能力. 三维纳米结构的材料具有较大的比表面积以分布更多的活性位点和拥有良好的电子传输能力, 所以, 开发三维纳米结构的过渡金属硫化物材料可能是改进其催化电解水性能的一个好途径. 本文采用简单的两步水热法, 通过硫化合成的 CoNi 前体得到了长于泡沫镍上的三维百合花状的 CoNi2S4(Co-Ni2S4/Ni). 它只需要 54 mV 的过电位即可获得 10 mA cm-2的催化氢析出反应电流, 是最好的碱性催化氢析出反应电极材料之一. 它在驱动 100 mA cm-2的催化氧析出反应电流时也只需要 328 mV 的过电位. 另外, 把 CoNi2S4/Ni 分别作为阴极和阳极组装成双电极碱性水电解槽时, 它只需要 1.56 V 的电压即可获取 10 mA cm-2的催化全电解水电流并具有良好的催化全电解水稳定性.扫描电子显微镜、透射电子显微镜和 N2吸脱附曲线测试结果表明, 该三维百合花状的 CoNi2S4/Ni 的表面粗糙度高和拥有多孔特性. 多孔结构的 CoNi2S4/Ni 可提供更多可接触的催化活性位点, 也有利于催化过程中的电解质和生成的气体的扩散与传递. 交流阻抗图谱测试结果表明, CoNi2S4/Ni 具有良好的电子传输能力. 另外, 不同于前期对尖晶石结构的硫化物 AB2S4的研究结果, XPS 结果表明, CoNi2S4/Ni 中含有 Niб+和 Sб-活性物种, 表明 CoNi2S4具有与活性氢化酶类似的活 性中心. Niδ+和 Sδ-可分别作为氢氧根和质子的接收体, 协助促进吸附的水分子的分离, 从而提高材料的催化性能. 所以, Niδ+和 Sδ-活性物种的出现, 大比表面积的三维百合花状多孔结构和良好的电荷传输能力等特性集合于 CoNi2S4/Ni 上使得CoNi2S4/Ni 具有优异的催化氢析出和催化氧析出反应性能.     

Catalyzing Sustainable Water Splitting with Single Atom Catalysts: Recent Advances

Electrochemical water splitting for sustainable hydrogen and oxygen production have shown enormous potentials. However, this method needs low-cost and highly active catalysts. Traditional nano catalysts, while effective, have limits since their active sites are mostly restricted to the surface and edges, leaving interior surfaces unexposed in redox reactions. Single atom catalysts (SACs), which take advantage of high atom utilization and quantum size effects, have recently become appealing electrocatalysts. Strong interaction between active sites and support in SACs have considerably improved the catalytic efficiency and long-term stability, outperforming their nano-counterparts. This review‘s first section examines the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER). In the next section, SACs are categorized as noble metal, non-noble metal, and bimetallic synergistic SACs. In addition, this review emphasizes developing methodologies for effective SAC design, such as mass loading optimization, electrical structure modulation, and the critical role of support materials. Finally, Carbon-based materials and metal oxides are being explored as possible supports for SACs. Importantly, for the first time, this review opens a discussion on waste-derived supports for single atom catalysts used in electrochemical reactions, providing a cost-effective dimension to this vibrant research field. The well-known design techniques discussed here may help in development of electrocatalysts for effective water splitting.     

A comparison study on single metal atoms (Fe,Co, Ni) within nitrogen-doped graphene for oxygen electrocatalysis and rechargeable Zn-air batteries

Single atom catalysts (SACs) with atomically dispersed transition metals on nitrogen-doped carbon supports have recently emerged as highly active non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), showing great application potential in Zn-air batteries. However, because of the complex structure-performance relationships of carbon-based SACs in the oxygen electrocatalytic reactions, the contribution of different metal atoms to the catalytic activity of SACs in Zn-air batteries still remains ambiguous. In this study, SACs with atomically dispersed transition metals on nitrogen-doped graphene sheets (M-N@Gs, M = Co, Fe and Ni), featured with similar physicochemical properties and M-N@C configurations, are obtained. By comparing the on-set potentials and the maximum current, we observed that the ORR activity is in the order of Co-N@G > Fe-N@G > Ni-N@G, while the OER activity is in the order of Co-N@G > Ni-N@G > Fe-N@G. The Zn-air batteries with Co-N@G as the air cathode catalysts outperform those with the Fe-N@G and Ni-N@G. This is due to the accelerated charge transfer between Co-N@C active sites and the oxygen-containing reactants. This study could improve our understanding of the design of more efficient bifunctional electrocatalysts for Zn-air batteries at the atomic level.     

Channel‐Rich RuCu Nanosheets for pH‐Universal Overall Water Splitting Electrocatalysis

Channel‐rich RuCu snowflake‐like nanosheets (NSs) composed of crystallized Ru and amorphous Cu were used as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting in pH‐universal electrolytes. The optimized RuCu NSs/C‐350 °C and RuCu NSs/C‐250 °C show attractive activities of OER and HER with low overpotentials and small Tafel slopes, respectively. When applied to overall water splitting, the optimized RuCu NSs/C can reach 10 mA cm^?2 at cell voltages of only 1.49, 1.55, 1.49 and 1.50 V in 1 m KOH, 0.1 m KOH, 0.5 m H₂SO₄ and 0.05 m H₂SO₄, respectively, much lower than those of commercial Ir/C∥Pt/C. The optimized electrolyzer exhibits superior durability with small potential change after up to 45 h in 1 m KOH, showing a class of efficient functional electrocatalysts for overall water splitting.     

Interface Engineering of MoS2/Ni3S2 Heterostructures for Highly Enhanced Electrochemical Overall‐Water‐Splitting Activity

To achieve sustainable production of H₂ fuel through water splitting, low‐cost electrocatalysts for the hydrogen‐evolution reaction (HER) and the oxygen‐evolution reaction (OER) are required to replace Pt and IrO₂ catalysts. Herein, for the first time, we present the interface engineering of novel MoS₂/Ni₃S₂ heterostructures, in which abundant interfaces are formed. For OER, such MoS₂/Ni₃S₂ heterostructures show an extremely low overpotential of ca. 218 mV at 10 mA cm^?2, which is superior to that of the state‐of‐the‐art OER electrocatalysts. Using MoS₂/Ni₃S₂ heterostructures as bifunctional electrocatalysts, an alkali electrolyzer delivers a current density of 10 mA cm^?2 at a very low cell voltage of ca. 1.56 V. In combination with DFT calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygen‐containing intermediates, thus accelerating the overall electrochemical water splitting.     

Ru−FeNi Alloy Heterojunctions on Lignin-derived Carbon as Bifunctional Electrocatalysts for Efficient Overall Water Splitting

Rational design of efficient, stable, and inexpensive bifunctional electrocatalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) is a key challenge to realize green hydrogen production via electrolytic water splitting. Herein, Ru nanoparticles and FeNi alloy heterojunction catalyst (Ru−FeNi@NLC) encapsulated via lignin-derived carbon was prepared by self-assembly precipitation and in situ pyrolysis. The designed catalyst displays excellent performance at 10 mA cm⁻² with low overpotentials of 36 mV for HER and 198 mV for OER, and only needs 1.48 V for overall water splitting. Results and DFT calculations show the unique N-doped lignin-derived carbon layer and Ru−FeNi heterojunction contribute to optimized electronic structure for enhancing electron transfer, balanced free energy of reactants and intermediates in the sorption/desorption process, and significantly reduced reaction energy barrier for the HER and OER rate-determining steps, thus improved reaction kinetics. This work provides a new in situ pyrolysis doping strategy based on renewable biomass for the construction of highly active, stable and cost-effective catalysts.     

Molybdenum Carbide‐Embedded Multichannel Hollow Carbon Nanofibers as Bifunctional Catalysts for Water Splitting

With the environmental pollution and non‐renewable fossil fuels, it is imperative to develop eco‐friendly, renewable, and highly efficient electrocatalysts for sustainable energy. Herein, a simple electrospinning process used to synthesis Mo₂C‐embedded multichannel hollow carbon nanofibers (Mo₂C‐MCNFs) and followed by the pyrolysis process. As prepared lotus root‐like nanoarchitecture could offer rich porosity and facilitate the electrolyte infiltration, the Mo₂C‐MCNFs delivered favourable catalytic activity for HER and OER. The resultant catalysts exhibit low overpotentials of 114 mV and 320 mV at a current density of 10 mA cm^?2 for HER and OER, respectively. Furthermore, using the Mo₂C‐MCNFs catalysts as a bifunctional electrode toward overall water splitting, which only needs a small cell voltage of 1.68 V to afford a current density of 10 mA cm^?2 in the home‐made alkaline electrolyzer. This interesting work presents a simple and effective strategy to further fabricating tunable nanostructures for energy‐related applications.     

Folded nanosheet-like Co<Subscript>0.85</Subscript>Se array for overall water splitting

Active, stable, and earth-abundant bifunctional electrocatalyst for overall water splitting is pivotal to actualize large-scale water splitting via electrolysis. In this work, the hierarchical folded nanosheet-like Co_0.85Se array on Ni foam is constructed by liquid-phase chemical conversion with cobalt precursor nanorod array. It can serve as an efficient bifunctional electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte, with a current density of 10 mA cm^?2 at overpotential of 232 mV for OER and 129 mV for HER and Tafel slope of 78.9 mV dec^?1 for OER and 95.0 mV dec^?1 for HER, respectively. The two-electrode alkaline water electrolyzer utilizing this folded nanosheet-like Co_0.85Se array as both anode and cathode toward overall water splitting offered a current of 10 mA cm^?2 at a cell voltage of 1.60 V. This work explores an efficient and low-cost electrocatalyst for overall water splitting application in alkaline electrolytes.     

A general bimetal-ion adsorption strategy to prepare nickel single atom catalysts anchored on graphene for efficient oxygen evolution reaction

Single-atom catalysts (SACs) supported on two-dimensional (2D) materials are highly attractive for maximizing their catalytic activity.However,graphene based SACs are primarily bonded with nitrogen and carbon sites,resulting in poor performance for the oxygen evolution reaction (OER).Herein,we develop a general bimetal-ion adsorption strategy for the synthesis of individually dispersed Ni SACs anchored on the oxygenated sites of ultrathin reduced graphene oxide as efficient OER electrocatalysts.The resultant Ni SACs for OER in alkaline electrolyte exhibit a highly stable overpotential of 328 mV at the current density of 10 mA cm~(-2),and Tafel slope of 84 mV dec~(-1) together with long-term durability and negligible degradation for 50 h,which is greatly outperform its counterparts of nitrogen bonded Ni SACs (564 mV,364 mV dec~(-1)) and Ni(OH)_2 nanoparticles anchored on graphene (450 mV,142 mV dec~(-1)),and most reported Ni based OER electrocatalysts.Furthermore,the extended X-ray absorption fine structure at the Ni K-edge and theoretical simulation reveal that the nickel-oxygen coordination significantly boost OER performance.Therefore,this work will open numerous opportunities for creating novel-type 2D SACs via oxygen-metal bonding as highly robust OER catalysts.     

NixCo3‐xO4 Nanoneedle Arrays Grown on Ni Foam as an Efficient Bifunctional Electrocatalyst for Full Water Splitting

Developing highly active, stable and robust electrocatalysts based on earth‐abundant elements for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is important for many renewable energy conversion processes. Herein, Ni_xCo_3‐xO₄ nanoneedle arrays grown on 3D porous nickel foam (NF) was synthesized as a bifunctional electrocatalyst with OER and HER activity for full water splitting. Benefiting from the advantageous structure, the composite exhibits superior OER activity with an overpotential of 320 mV achieving the current density of 10 mA cm^?2. An exceptional HER activity is also acquired with an overpotential of 170 mV at the current density of 10 mA cm^?2. Furthermore, the catalyst also shows the superior activity and stability for 20 h when used in the overall water splitting cell. Thus, the hierarchical 3D structure composed of the 1D nanoneedle structure in Ni_xCo_3‐xO₄/NF represents an avenue to design and develop highly active and bifunctional electrocatalysts for promising energy conversion.     

Bifunctional Electrocatalysts for Overall Water Splitting from an Iron/Nickel‐Based Bimetallic Metal–Organic Framework/Dicyandiamide Composite

Pyrolysis of a bimetallic metal–organic framework (MIL‐88‐Fe/Ni)‐dicyandiamide composite yield a Fe and Ni containing carbonaceous material, which is an efficient bifunctional electrocatalyst for overall water splitting. FeNi₃ and NiFe₂O₄ are found as metallic and metal oxide compounds closely embedded in an N‐doped carbon–carbon nanotube matrix. This hybrid catalyst (Fe‐Ni@NC‐CNTs) significantly promotes the charge transfer efficiency and restrains the corrosion of the metallic catalysts, which is shown in a high OER and HER activity with an overpotential of 274 and 202 mV, respectively at 10 mA cm^?2 in alkaline solution. When this bifunctional catalyst was further used for H₂ and O₂ production in an electrochemical water‐splitting unit, it can operate in ambient conditions with a competitive gas production rate of 1.15 and 0.57 μL s^?1 for hydrogen and oxygen, respectively, showing its potential for practical applications.     

Highly exposed NiFeOx nanoclusters supported on boron doped carbon nanotubes for electrocatalytic oxygen evolution reaction

The development of efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) is crucial for the overall water splitting. Herein, we prepared a highly exposed NiFeO_x ultra-small nanoclusters supported on boron-doped carbon nonotubes catalyst, which achieves a 10 mA/cm² anodic current density at a low overpotential of 213 mV and the Tafel slope of 52 mV/dec in 1.0 mol/L KOH, superior to the pristine NiFeO_x-CNTs and other state-of-the-art OER catalysts in alkaline media. A combination study (XPS, sXAS and XAFS) verifies that the local atomic structure of Ni and Fe atoms in the nanoclusters are similar to NiO and Fe₂O₃, respectively, and the B atoms which are doped into the crystal lattice of CNTs leads to the optimization of Ni 3d e_g orbitals. Furthermore, in-situ X-ray absorption spectroscopies reveal that the high valence state of Ni atoms are served as the real active sites. This work highlights that the precise control of highly exposed multicomponent nanocluster catalysts paves a new way for designing highly efficient catalysts at the atomic scale.