Modular Design of Noble‐Metal‐Free Mixed Metal Oxide Electrocatalysts for Complete Water Splitting

Catalytic Water Electrolysis by Co-Cu-W Mixed Metal Oxides: Insights from X-ray Absorption Spectroelectrochemistry

Mixed metal oxides (MMOs) are a promising class of electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Despite their importance for sustainable energy schemes, our understanding of relevant reaction pathways, catalytically active sites, and synergistic effects is rather limited. Here, we applied synchrotron-based X-ray absorption spectroscopy (XAS) to explore the evolution of the amorphous Co-Cu-W MMO electrocatalyst, shown previously to be an efficient bifunctional OER and HER catalyst for water splitting. Ex situ XAS measurements provided structural environments and the oxidation state of the metals involved, revealing Co2+ (octahedral), Cu+/2+ (tetrahedral/square-planar), and W6+ (octahedral) centers. Operando XAS investigations, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), elucidated the dynamic structural transformations of Co, Cu, and W metal centers during the OER and HER. The experimental results indicate that Co3+ and Cu0 are the active catalytic sites involved in the OER and HER, respectively, while Cu2+ and W6+ play crucial roles as structure stabilizers, suggesting strong synergistic interactions within the Co-Cu-W MMO system. These results, combined with the Tafel slope analysis, revealed that the bottleneck intermediate during the OER is Co3+ hydroperoxide, whose formation is accompanied by changes in the Cu-O bond lengths, pointing to a possible synergistic effect between Co and Cu ions. Our study reveals important structural effects taking place during MMO-driven OER/HER electrocatalysis and provides essential experimental insights into the complex catalytic mechanism of emerging noble-metal-free MMO electrocatalysts for full water splitting.

Dopant-Induced Surface Self-Etching of Cobalt Carbonate Hydroxide Boosts Efficient Water Splitting

Herein, vanadium-doped cobalt carbonate hydroxide, V-CoCH, was synthesized as efficient catalyst for water splitting. Vanadium species were partially dissolved in the early stages of the oxygen-evolution reaction (OER), inducing self-etching of the catalyst surface, which is helpful for catalyst surface reconstruction and resulted in a higher number of active sites and oxygen vacancies. The synergy between V-doping and oxygen vacancies improved the catalytic activity: V-CoCH showed an exceptional OER catalytic performance with an overpotential of 183 mV at 10 mA cm^-2 . The water-splitting cell consisting of V-CoCH only required 1.52 V to reach 10 mA cm^-2 . Theoretical calculations revealed that vanadium in V-CoCH played an important role in electron regulation of active sites. The oxygen vacancies had an important effect on improvement of the OER performance through not only the exposure of more active sites but also through modulation of the electronic structure. This work provides an effective strategy for constructing high-performance electrocatalysts.

POM@ZIF Derived Mixed Metal Oxide Catalysts for Sustained Electrocatalytic Oxygen Evolution

The design of efficient and stable oxygen evolution reaction (OER) catalysts based on noble-metal-free materials is crucial for energy conversion and storage. In this work, it was demonstrated how polyoxometalate (POM)-doped ZIF-67 can be converted into a stable oxygen evolution electrocatalyst by chemical etching, cation exchange, and thermal annealing steps. Characterization by X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy indicate that POM-doped ZIF-67 derived carbon-supported metal oxides were synthesized. The resulting composite shows structural and compositional advantages which lead to low overpotential (306 mV at j=10 mA ⋅ cm^-2 ) and long-term stability under harsh OER conditions (1.0 M aqueous KOH).

Manipulating the Water Dissociation Electrocatalytic Sites of Bimetallic Nickel-Based Alloys for Highly Efficient Alkaline Hydrogen Evolution

Transition-metal alloys are currently drawing increasing attention as promising electrocatalysts for the alkaline hydrogen evolution reaction (HER). However, traditional density-functional-theory-derived d-band theory fails to describe the hydrogen adsorption energy (ΔG_H ) on hollow sites. Herein, by studying the ΔG_H for a series of Ni-M (M=Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, W) bimetallic alloys, an improved d-band center was provided and a potential NiCu electrocatalyst with a near-optimal ΔG_H was discovered. Moreover, oxygen atoms were introduced into Ni-M (O-NiM) to balance the adsorption/desorption of hydroxyl species. The tailored electrocatalytic sites for water dissociation can synergistically accelerate the multi-step alkaline HER. The prepared O-NiCu shows the optimum HER activity with a low overpotential of 23 mV at 10 mA cm^-2 . This work not only broadens the applicability of d-band theory, but also provides crucial understanding for designing efficient HER electrocatalysts.

Rapid Surface Reconstruction of Amorphous Co(OH)2 /WOx with Rich Oxygen Vacancies to Promote Oxygen Evolution

Herein, a transition metal dissolution-oxygen vacancy strategy, based on dissolution of highly oxidized transition metal species in alkaline electrolyte, was suggested to construct a high-performance amorphous Co(OH)₂ /WO_x (a-CoW) catalyst for the oxygen evolution reaction (OER). The surface reconstruction of a-CoW and its evolution were described by regulating oxygen vacancies. With continuous dissolution of W species, oxygen vacancies on the surface were generated rapidly, the surface reconstruction was promoted, and the OER performance was improved significantly. During the surface reconstruction, W species also played a role in electronic modulation for Co. Due to its rapid surface reconstruction, a-CoW exhibited excellent OER performance in alkaline electrolyte with an overpotential of 208 mV at 10 mA cm^-2 and had long-term stability for at least 120 h. This work shows that the transition metal dissolution-oxygen vacancy strategy is effective for preparation of high-performance catalysts.

Cobalt Single Atom Incorporated in Ruthenium Oxide Sphere: A Robust Bifunctional Electrocatalyst for HER and OER

The development of highly active and stable bifunctional noble-metal-based electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is a crucial goal for clean and renewable energy, which still remains challenging. Herein, we report an efficient and stable catalyst comprising a Co single atom incorporated in an RuO₂ sphere for HER and OER, in which the Co single atom in the RuO₂ sphere was confirmed by XAS, AC-STEM, and DFT. This tailoring strategy uses a Co single atom to modify the electronic structures of the surrounding Ru atoms and thereby remarkably elevates the electrocatalytic activities. The catalyst requires ultralow overpotentials, 45 mV for HER and 200 mV for OER, to deliver a current density of 10 mA cm^-2 . The theoretical calculations reveal that the energy barriers for HER and OER are lowered after incorporation of a cobalt single atom.

Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction

Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large‐scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half‐reaction. Co‐, Ni‐, and Fe‐based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co‐, Ni‐, and Fe‐based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis.

Synchronous Electrocatalytic Design of Architectural and Electronic Structure Based on Bifunctional LDH-Co3 O4 /NF toward Water Splitting

The rational design of highly efficient bifunctional electrocatalysts for water splitting is extremely urgent for application in sustainable energy conversion processes to alleviate the energy crisis and environmental pollution. In this work, through simple deposition of layered double hydroxides (LDH) on Co₃ O₄ /NF (NF=nickel foam) nanosheets arrays, hierarchical Co³⁺ -rich materials based on LDH-Co₃ O₄ /NF are prepared as highly active and stable electrocatalysts for water splitting. The NiFe-LDH-Co₃ O₄ /NF demonstrates excellent electrochemical activity with an overpotential of 214 mV for the OER and an overpotential of 162 mV for the HER at 10 mA cm^-2 . Such a performance is attributed to the optimized electronic states with a high concentration of Co³⁺ , which improves the intrinsic activity, and the sheet-on-sheet hierarchical structure, which increases the number of active sites. The unique synchronous design of both the architectural and electronic structure of nanomaterials can simultaneously accelerate the reaction kinetics and provide a more convenient charge transfer path. Therefore, the strategy reported herein may open a new pathway for the design of excellent electrocatalysts for water splitting.

Boosting Electrocatalytic Water Oxidation by Creating Defects and Lattice-Oxygen Active Sites on Ni-Fe Nanosheets

Layered nickel-iron oxide/hydroxide nanosheets have proven to be the most efficient catalyst for the water oxidation reaction. Introducing structural defects to the nanosheets is a particularly attractive method for increasing the number of active sites and tailoring the intrinsic electronic properties. Herein, defects were introduced on Ni-Fe nanosheets through sequentially electrodoping and dedoping the surface of the material with tetramethylammonium ions. The as-prepared defect-rich Ni-Fe nanosheets showed an enhanced catalytic performance for the oxygen evolution reaction (OER) compared with conventional NiFe layered double hydroxides (LDHs), exhibiting an overpotential of only 172 mV at the current density of 10 mA cm^-2 . The relationship between pH and OER activity indicated that the lattice oxygens participated in the catalytic OER process as active sites. This work provides new insights into the understanding of the structure-activity relationship of layered materials and helps to develop new methods to implement defects on such frameworks aided by organic molecules.

Bottom-up Design of Bimetallic Cobalt-Molybdenum Carbides/Oxides for Overall Water Splitting

Earth-abundant transition-metal-based catalysts for electrochemical water splitting are critical for sustainable energy schemes. In this work, we use a rational design method for the synthesis of ultrasmall and highly dispersed bimetallic CoMo carbide/oxide particles deposited on graphene oxide. Thermal conversion of the molecular precursors [H3 PMo12 O40 ], Co(OAc)2 ⋅4 H2 O and melamine in the presence of graphene oxide gives the mixed carbide/oxide (Co6 Mo6 C2 /Co2 Mo3 O8 ) nanoparticle composite deposited on highly dispersed, N,P-doped carbon. The resulting composite shows outstanding electrocatalytic water-splitting activity for both the oxygen evolution and hydrogen evolution reaction, and superior performance to reference samples including commercial 20 % Pt/C & IrO2 . Electrochemical and other materials analyses indicate that Co6 Mo6 C2 is the main active phase in the composite, and the N,P-doping of the carbon matrix increases the catalytic activity. The facile design could in principle be extended to multiple bimetallic catalyst classes by tuning of the molecular metal oxide precursor.

Devisable POM/Ni Foam Composite: Precisely Control Synthesis toward Enhanced Hydrogen Evolution Reaction at High pH

Polyoxometalates (POMs) are promising catalysts for the electrochemical hydrogen production from water owing to their high intrinsic catalytic activity and chemical tunability. However, poor electrical conductivity and easy detachment of the POMs from the electrode cause significant challenges under operating condition. Herein, a simple one-step hydrothermal method is reported to synthesize a series of Dexter-Silverton POM/Ni foam composites (denoted as NiM-POM/Ni; M=Co, Zn, Mn), in which the stable linkage between the POM catalysts and the Ni foam electrodes lead to high activity for the hydrogen evolution reaction (HER). Among them, the highest HER performance can be observed in the NiCo-POM/Ni, featuring an overpotential of 64 mV (at 10 mA cm-2 , vs. reversible hydrogen electrode), and a Tafel slope of 75 mV dec-1 in 1.0 m aqueous KOH. Moreover, the NiCo-POM/Ni catalyst showed a high faradaic efficiency ≈97 % for HER. Post-catalytic of NiCo-POM/Ni analyses showed virtually no mechanical or chemical degradation. The findings propose a facile and inexpensive method to design stable and effective POM-based catalysts for HER in alkaline water electrolysis.