Engineering Self-Reconstruction via Flexible Components in Layered Double Hydroxides for Superior-Evolving Performance

Boosting oxygen evolution reaction by FeNi hydroxide-organic framework electrocatalyst toward alkaline water electrolyzer

The oxygen evolution reaction plays a vital role in modern energy conversion and storage, and developing cost-efficient oxygen evolution reaction catalysts with industrially relevant activity and durability is highly desired but still challenging. Here, we report an efficient and durable FeNi hydroxide organic framework nanosheet array catalyst that competently affords long-term oxygen evolution reaction at industrial-grade current densities in alkaline electrolyte. The desirable high-intensity performance is attributed to three aspects as follows. First, two-dimensional nanosheet porous arrays with maximum specific surface facilitate mass/charge transfer to accommodate high-current-density catalysis. Second, in situ derived FeNi hydroxide motifs offer bimetallic synergistic catalysis centers with high intrinsic activity. Third, carboxyl ligands alleviate metal oxidation favorable for charge tolerability against peroxidation dissolution under strong polarization. As a result, this catalyst requires an overpotential of only 280 mV to deliver high current density up to 1 A/cm2 with long durability over 1000 h. Moreover, an alkaline water electrolyzer with this catalyst alternative demonstrates an increased economic effectiveness compared to commercial levels at present.

Voltage activation induced MoO42- dissolution to enhance performance of iron doped nickel molybdate for oxygen evolution reaction

Transition metal-based precatalysts are typically voltage-activated before electrochemical testing in the condition of alkaline oxygen evolution reaction. Nevertheless, the impact of voltage on the catalyst and the anion dissolution is frequently disregarded. In this study, Fe-doped NiMoO4 (Fe-NiMoO4) was synthesized as a precursor through a straightforward hydrothermal method, and MoFe-modified Ni (oxygen) hydroxide (MoFe-NiOxHy) was obtained via cyclic voltammetry (CV) activation. The effects of voltage on Fe-NiMoO4 and the dissolved inactive MoO42- ions in the process were examined in relation to OER performance. It has demonstrated that the crystallinity of the catalyst is reduced by voltage, thereby enhancing its electrocatalytic activity. The electron distribution state can be adjusted during the application of voltage, leading to the generation of additional active sites and an acceleration in the reaction rate. Additionally, MoO42- exhibits potential dependence during its dissolution. In the OER process, the dissolution of MoO42- enhances the reconstruction degree of Fe-NiMoO4 into the active substance and expedites the formation of active Ni(Fe)OOH. Hence, the optimized MoFe-NiOxHy exhibited exceptional electrocatalytic performance, with a current density of 100 mA cm-2 achieved at an overpotential of only 256 mV. This discovery contributes to a more comprehensive understanding of alkaline OER performance under the influence of applied voltage and the presence of inactive oxygen ions, offering a promising avenue for the development of efficient electrocatalysts.

Nickel molybdate/cobalt iron carbonate hydroxide heterojunction with oxygen vacancy enables interfacial synergism to trigger oxygen evolution reaction

The development of electrocatalysts with excellent performance toward oxygen evolution reaction (OER) for the production of hydrogen is of great significance to alleviate energy crisis and environmental pollution. Herein, the heterostructure (NMO/FCHC-0.4) was fabricated by the coupling growth of NiMoO4 (NMO) and cobalt iron carbonate hydroxide (FCHC) on nickel foam as an electrocatalyst for OER. The interfacial synergy on NMO/FCHC-0.4 heterojunction can promote the interfacial electron redistribution, affect the center position of d band, optimize the adsorption of intermediate, and improve the conductivity. Beyond, oxygen defect sites are conducive to the adsorption of intermediates, and increase the number of active sites. Real-time OER kinetic simulation revealed that the interfacial synergism and molybdate could reduce the adsorption of hydroxide, promote the deprotonation step of M-OH, and facilitate the formation of M-OOH (M represents the metal active site). As a result, NMO/FCHC-0.4 displays excellent OER electrocatalytic performance with an overpotential of 250/280 mV at the current density 100/200 mA cm-2 and robust stability at 100 mA cm-2 for 100 h. This work provides deep insights into the roles of interfacial electronic modulation and oxygen vacancy to design high-efficiency electrocatalysts for OER.

MXene-mediated reconfiguration induces robust nickel-iron catalysts for industrial-grade water oxidation

Nickel-iron oxy/hydroxides (NiFeOxHy) emerge as an attractive type of electrocatalysts for alkaline water oxidation reaction (WOR), but which encounter a huge challenge in stability, especially at industrial-grade large current density due to uncontrollable Fe leakage. Here, we tailor the Fe coordination by a MXene-mediated reconfiguration strategy for the resultant NiFeOxHy catalyst to alleviate Fe leakage and thus reinforce the WOR stability. The introduction of ultrafine MXene with surface dangling bonds in the electrochemical reconfiguration over Ni-Fe Prussian blue analogue induces the covalent hybridization of NiFeOxHy/MXene, which not only accelerates WOR kinetics but also improves Fe oxidation resistance against segregation. As a result, the NiFeOxHy coupled with MXene exhibits an extraordinary durability at ampere-level current density over 1,000 h for alkaline WOR with an ultralow overpotential of only 307 mV. This work provides a broad avenue and mechanistic insights for the development of nickel-iron catalysts toward industrial applications.

La and S Co-Doping Induced the Synergism of Multiphase Nickel-Iron Nanosheets with Rich Oxygen Vacancies to Trigger Large-Current-Density Oxygen Evolution and Urea Oxidation Reactions

The development of cost-effective electrocatalysts for oxygen evolution reaction (OER) and urea oxidation reaction (UOR) is of great significance for hydrogen production. Herein, La and S co-doped multiphase electrocatalyst (LSFN-63) is fabricated by metal-corrosion process. FeOOH can reduce the formation energy of NiOOH, and enhance the stability of NiOOH as active sites for OER/UOR. The rich oxygen vacancies can increase the number of active sites, optimize the adsorption of intermediates, and improve electrical conductivity. Beyond, La and S co-doping can also regulate the electronic structure of FeOOH. As a result, LSFN-63 presents a low overpotential of 210/450 mV at 100/1000 mA cm-2 , small Tafel slope (32 mV dec-1 ), and outstanding stability under 1000 mA cm-2 @60 h, and can also display excellent OER activity with 180 mV at 250 mA cm-2 and long-term catalytic durability at 250 mA cm-2 @135 h in 30 wt% KOH under 60 °C. Moreover, LSFN-63 demonstrates remarkable UOR performance in 1 m KOH + 0.5 m urea, which just requires an ultra-small overpotential of 140 mV at 100 mA cm-2 , and maintain long-term durability over 120 h. This work opens up a promising avenue for the development of high-efficiency electrocatalysts by a facile metal-corrosion strategy.

Machine learning-assisted optimization of multi-metal hydroxide electrocatalysts for overall water splitting

Green hydrogen produced via electrochemical water splitting is a suitable candidate to replace emission-intensive fuels. However, the successful widespread adoption of green hydrogen is contingent on the development of low-cost, earth-abundant catalysts. Herein, machine learning models built on experimental data were used to optimize the precursor ratios of hydroxide-based electrocatalysts, with the objective of improving the product's electrocatalytic performance for overall water splitting. The Neural Network-based models were found to be the most effective in predicting and minimizing the overpotentials of the catalysts, reaching a minimum in two iterations. The relatively mild reaction conditions of the synthesis procedure, coupled with its scalability demonstrated herein, renders the optimized catalyst relevant for industrial implementation in the future. The optimized catalyst, characterized to be a molybdate-intercalated CoFe LDH, demonstrated overpotentials of 266 and 272 mV at 10 mA cm-2 for oxygen and hydrogen evolution reactions respectively in alkaline electrolyte, alongside unwavering stability for overall water splitting over 50 h. Overall, our results reflect the efficacy and advantages of machine learning strategies to alleviate the time and labour-intensive nature of experimental optimizations, which can greatly accelerate electrocatalysts research.

A Unique Etching-Doping Route to Fe/Mo Co-Doped Ni Oxyhydroxide Catalyst for Enhanced Oxygen Evolution Reaction

Fe-doped Ni (oxy)hydroxide shows intriguing activity toward oxygen evolution reaction (OER) in alkaline solution, yet it remains challenging to further boost its performance. In this work, a ferric/molybdate (Fe3+ /MoO4 2- ) co-doping strategy is reported to promote the OER activity of Ni oxyhydroxide. The reinforced Fe/Mo-doped Ni oxyhydroxide catalyst supported by nickel foam (p-NiFeMo/NF) is synthesized via a unique oxygen plasma etching-electrochemical doping route, in which precursor Ni(OH)2 nanosheets are first etched by oxygen plasma to form defect-rich amorphous nanosheets, followed by electrochemical cycling to trigger simultaneously Fe3+ /MoO4 2- co-doping and phase transition. This p-NiFeMo/NF catalyst requires an overpotential of only 274 mV to reach 100 mA cm-2 in alkaline media, exhibiting significantly enhanced OER activity compared to NiFe layered double hydroxide (LDH) catalyst and other analogs. Its activity does not fade even after 72 h uninterrupted operation. In situ Raman analysis reveals that the intercalation of MoO4 2- is able to prevent the over-oxidation of NiOOH matrix from β to γ phase, thus keeping the Fe-doped NiOOH at the most active state.

Iron-Locked Hydr(oxy)oxide Catalysts via Ion-Compensatory Reconstruction Boost Large-Current-Density Water Oxidation

Nickel-iron based hydr(oxy)oxides have been well recognized as one of the best oxygen-evolving catalysts in alkaline water electrolysis. A crucial problem, however, is that iron leakage during prolonged operation would lead to the oxygen evolution reaction (OER) deactivation over time, especially under large current densities. Here, the NiFe-based Prussian blue analogue (PBA) is designed as a structure-flexible precursor for navigating an electrochemical self-reconstruction (ECSR) with Fe cation compensation to fabricate a highly active hydr(oxy)oxide (NiFeO_x H_y ) catalyst stabilized with NiFe synergic active sites. The generated NiFeO_x H_y catalyst exhibits the low overpotentials of 302 and 313 mV required to afford large current densities of 500 and 1000 mA cm^-2 , respectively. Moreover, its robust stability over 500 h at 500 mA cm^-2 stands out among the NiFe-based OER catalysts reported previously. Various in/ex situ studies indicate that the Fe fixation by dynamic reconstruction process can reinforce the Fe-activated effect on the OER amenable to the industrial-level large current conditions against the Fe leakage. This work opens up a feasible strategy to design highly active and durable catalysts via thermodynamically self-adaptive reconstruction engineering.

Reconstructured Electrocatalysts during Oxygen Evolution Reaction under Alkaline Electrolytes

The development of electrocatalysts with high-efficiency and clear structure-activity relationship towards the sluggish oxygen evolution reaction (OER) is essential for the wide application of water electrolyzers. Recently, the dynamic reconstruction phenomenon of the catalysts' surface structures during the OER process has been discovered. With the help of various advanced ex situ and in situ characterization, it is demonstrated that such surface reconstruction could yield actual active species to catalyze the water oxidation process. However, the attention and studies of potential interaction between reconstructed species and substrate are lacking. This review summarizes the recent development of typical reconstructed electrocatalysts and the substrate effect. First, the advanced characterization for electrocatalytic reconstruction is briefly discussed. Then, typical reconstructed electrocatalysts are comprehensively summarized and the key role of substrate effects during the OER process is emphasized. Finally, the future challenges and perspectives of surface reconstructed catalysts for water electrolysis are discussed.

Selective H2O2 Electrosynthesis over Defective Carbon from Electrochemical Etching of Molybdenum Carbide

The controllable synthesis of specific defective carbon catalysts is crucial for two-electron oxygen reduction reaction (2e^- ORR) to generate H₂O₂ due to the great potential applications. Herein, the defective carbon catalysts (Mo-CDC-ns) were prepared by an electrochemical activation (ECA) method with Mo₂C/C as a parent. Electrochemical cyclic voltammetry curves, X-ray photoelectron spectroscopy, inductively coupled plasma-mass spectrometry, scanning electron microscopy, and high-resolution transmission electron microscopy confirm the evolution process of a defective carbon structure from the Mo₂C phase in which Mo species are first oxidized to Mo⁶⁺ species and then the latter are dissolved into the solution and defective carbon is simultaneously formed. Raman and electron paramagnetic resonance spectra reveal that the defect types in Mo-CDC-ns are the edge defect and vacancy defect sites. Compared with the parent Mo₂C/C, Mo-CDC-ns exhibit gradually increased kinetic current density and selectivity for H₂O₂ generation with an extension of activation cycles from 10 (Mo-CDC-10) to 30 (Mo-CDC-30). Over Mo-CDC-30, a kinetic current density of 19.4 mA cm^-2 and a selectivity close to 90% in 0.1 M KOH solution were achieved, as well as good stability for H₂O₂ production in an extended test up to 12 h in an H-cell. Graphene planes and Stone Wales 5757-carbon were constructed as basic models for density functional theory calculations. It revealed that the obtained defective structure after the removal of Mo atoms contains the double vacancy at the edge of graphene (Edge-DVC) and the topological defect on the plane of 5757-carbon (5757C-D), which show more moderate reaction free energy for forming *OOH and smaller energy barrier of 2e^- ORR.

Self-sacrificial reconstruction of MoO42- intercalated NiFe LDH/Co2P heterostructures enabling interfacial synergies and oxygen vacancies for triggering oxygen evolution reaction

Exploring high-efficiency electrocatalysts for oxygen evolution reaction (OER) is one of the most important concerns to produce hydrogen in water electrolysis. Herein, the FNM/Co2P-0.4 heterostructure was designed as an electrocatalyst for the OER process by the combination of MoO42- intercalating NiFe LDH and Co2P on nickel foam (NF). The surface reconstruction and MoO42- leaching can induce the conversion of Co2P and NiFe LDH on FNM/Co2P-0.4 to generate Co/NiOOH with more oxygen vacancies. Beyond, CoOOH and NiOOH can also synergize to reduce the energy barrier of OER, optimize conductivity, and improve stability. The surface reconstruction and the formation of OOH⁎ were further unveiled by in-situ UV-vis absorption spectra and Fourier-transformed alternative current voltammetry (FTACV). The integration of interfacial synergies and oxygen vacancies can facilitate the adsorption/desorption of intermediates, regulate the d-band center, and expose more active sites. And as a result, FNM/Co2P-0.4 shows a significant low overpotential (240 mV) at 50 mA cm-2, a small Tafel (74 mV dec-1), low activation energy (Ea) and remarkable durability. This work provides a new pathway to improve the OER performance by using interfacial synergies and rich oxygen vacancies derived from the self-sacrificial reconstruction of heterostructured electrocatalysts.

Chameleon-Like Reconstruction on Redox Catalysts Adaptive to Alkali Water Electrolysis

Pre-catalyst reconstruction in electrochemical processes has recently attracted intensive attention with mechanistic potentials to uncover really active species and catalytic mechanisms and advance targeted catalyst designs. Here, nickel-molybdenum oxysulfide is deliberately fabricated as pre-catalyst to present a comprehensive study on reconstruction dynamics for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkali water electrolysis. Operando Raman spectroscopy together with X-ray photoelectron spectroscopy and electron microscopy capture dynamic reconstruction including geometric, component and phase evolutions, revealing a chameleon-like reconstruction self-adaptive to OER and HER demands under oxidative and reductive conditions, respectively. The in situ generated active NiOOH and Ni species with ultrafine and porous textures exhibit superior OER and HER performance, respectively, and an electrolyzer with such two reconstructed electrodes demonstrates steady overall water splitting with an extraordinary 80% electricity-to-hydrogen (ETH) energy conversion efficiency. This work highlights dynamic reconstruction adaptability to electrochemical conditions and develops an automatic avenue toward the targeted design of advanced catalysts.

Construction of superhydrophilic metal-organic frameworks with hierarchical microstructure for efficient overall water splitting

Metal-organic frameworks (MOFs) display promising potential due to their exquisite structural advantages. Carboxylate-based MOFs, such as MIL-53 structures, attract a lot of attention among MOF families because of their remarkable stability in water and even alkaline condition. Hence, the delicate hierarchical microstructure is constructed by introducing MoO₄^2- into NH₂-MIL-53(NiFe) using a straightforward solvothermal strategy. The NiFeMo-MOF/NF electrode manifests a superior OER performance, producing an overpotential of 239 mV at 50 mA cm^-2 and a decent Tafel slope of 87.0 mV dec^-1. Furthermore, in a typical electrodeposition equipment, NiFeMo-MOF/NF is applied as the working electrode and the composite electrode named as (M) Ni-NiOOH/NF is generated by electrodeposition and electrooxidation process to assess HER performance, producing an overpotential of 119 mV at 50 mA cm^-2 and a decent Tafel slope of 58.3 mV dec^-1. The integrated electrolysis device delivers an extraordinarily low cell voltage of 1.50 V at 10 mA cm^-2 while applying NiFeMo-MOF/NF as the anode, (M)Ni-NiOOH/NF as the cathode for overall water splitting, exceeding the noble RuO₂/NF||Pt-C/NF (1.60 V@10 mA cm^-2). This study provides a promising design strategy for future electrolysis catalysts.

Defect-rich Fe-doped NiS/MoS2 heterostructured ultrathin nanosheets for efficient overall water splitting

With the demand for efficient hydrogen/oxygen evolution reaction (HER/OER) bifunctional electrocatalysts, defect-rich two-dimensional (2D) heterostructured materials attract increasing attention due to abundant active sites and facile mass/charge transfer. However, precise manipulation of lattice defects in a 2D heterostructured material is still a challenge. Herein, through pyrolytic sulfurization of a layered Fe-doped Ni/Mo MOF precursor, a series of defect-rich Fe-doped NiS/MoS₂ ultrathin nanosheets were obtained. For 0.1Fe-NiS/MoS₂, abundant lattice defects induced by Fe atoms provide more water adsorption sites, and intimate interface between NiS and MoS₂ can optimize the adsorption energy of a HER/OER intermediate. As a result, both HER and OER activities are significantly enhanced. The respective overpotential is 120 mV and 297 mV for the HER and OER. Small Tafel slopes of 69.0 mV dec^-1 and 54.7 mV dec^-1 indicate favorable electrochemical reaction kinetics. The catalytic performance of this material can be compared with those of 20% Pt/C and RuO₂ catalysts and top-rated MoS₂-based materials. For overall water splitting, only 1.66 V voltage is required to deliver 10 mA cm^-2. Long-term stability of 0.1Fe-NiS/MoS₂ presents a prospect for its practical application.