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.

Metal-Organic Frameworks as Electrocatalysts

Transition metal complexes are well-known homogeneous electrocatalysts. In this regard, metal-organic frameworks (MOFs) can be considered as an ensemble of transition metal complexes ordered in a periodic arrangement. In addition, MOFs have several additional positive structural features that make them suitable for electrocatalysis, including large surface area, high porosity, and high content of accessible transition metal with exchangeable coordination positions. The present review describes the current state in the use of MOFs as electrocatalysts, both as host of electroactive guests and their direct electrocatalytic activity, particularly in the case of bimetallic MOFs. The field of MOF-derived materials is purposely not covered, focusing on the direct use of MOFs or its composites as electrocatalysts. Special attention has been paid to present strategies to overcome their poor electrical conductivity and limited stability.

Regulating the Synergistic Effect in Bimetallic Two-Dimensional Polymer Oxygen Evolution Reaction Catalysts by Adjusting the Coupling Strength Between Metal Centers

Bimetallic electrocatalysts with its superior performance has a broad application prospect in oxygen evolution reaction (OER), but the fundamental understanding of the mechanism of synergistic effect is still limited since there lacks a practical way to decouple the influence factors on the intrinsic activity of active sites from others. Herein, a series of bimetallic Co-Ni two-dimensional polymer (2DP) model OER catalysts with well-defined architecture, monolayer characteristic, were designed and synthesized to explore the influence of the coupling strength between metal centers on OER performance. The coupling strength was regulated by adjusting the spacing between metal centers or the conjugation degree of bridge skeleton. Among the examined 2DPs, CoTAPP-Ni-MF-2DP, which has the strongest coupling strength between metal centers exhibited the best OER performance. These model systems can help to explore the precise structure-performance relationships, which is important for the rational catalyst design at the atomic/molecular levels.

Recent Advances and Future Perspectives of Metal-Based Electrocatalysts for Overall Electrochemical Water Splitting

Recently, the growing demand for a renewable and sustainable fuel alternative is contingent on fuel cell technologies. Even though it is regarded as an environmentally sustainable method of generating fuel for immediate concerns, it must be enhanced to make it extraordinarily affordable, and environmentally sustainable. Hydrogen (H₂ ) synthesis by electrochemical water splitting (ECWS) is considered one of the foremost potential prospective methods for renewable energy output and H₂ society implementation. Existing massive H₂ output is mostly reliant on the steaming reformation of carbon fuels that yield CO₂ together with H₂ and is a finite resource. ECWS is a viable, efficient, and contamination-free method for H₂ evolution. Consequently, developing reliable and cost-effective technology for ECWS was a top priority for scientists around the globe. Utilizing renewable technologies to decrease total fuel utilization is crucial for H₂ evolution. Capturing and transforming the fuel from the ambient through various renewable solutions for water splitting (WS) could effectively reduce the need for additional electricity. ECWS is among the foremost potential prospective methods for renewable energy output and the achievement of a H₂ -based economy. For the overall water splitting (OWS), several transition-metal-based polyfunctional metal catalysts for both cathode and anode have been synthesized. Furthermore, the essential to the widespread adoption of such technology is the development of reduced-price, super functional electrocatalysts to substitute those, depending on metals. Many metal-premised electrocatalysts for both the anode and cathode have been designed for the WS process. The attributes of H₂ and oxygen (O₂ ) dynamics interactions on the electrodes of water electrolysis cells and the fundamental techniques for evaluating the achievement of electrocatalysts are outlined in this paper. Special emphasis is paid to their fabrication, electrocatalytic performance, durability, and measures for enhancing their efficiency. In addition, prospective ideas on metal-based WS electrocatalysts based on existing problems are presented. It is anticipated that this review will offer a straight direction toward the engineering and construction of novel polyfunctional electrocatalysts encompassing superior efficiency in a suitable WS technique.

Direct Visualization of Atomic Structure in Multivariate Metal-Organic Frameworks (MOFs) for Guiding Electrocatalysts Design

The direct utilization of metal-organic frameworks (MOFs) for electrocatalytic oxygen evolution reaction (OER) has attracted increasing interests. Herein, we employ the low-dose integrated differential phase contrast-scanning transmission electron microscopy (iDPC-STEM) technique to visualize the atomic structure of multivariate MOFs (MTV-MOFs) for guiding the structural design of bulk MOFs for efficient OER. The iDPC-STEM images revealed that incorporating Fe³⁺ or 2-aminoterephthalate (ATA) into Ni-BDC (BDC: benzenedicarboxylate) can introduce inhomogeneous lattice strain that weaken the coordination bonds, which can be selectively cleaved via a mild heat treatment to simultaneously generate coordinatively unsaturated metal sites, conductive Ni@C and hierarchical porous structure. Thus, excellent OER activity with current densities of 10 and 100 mA cm^-2 are achieved over the defective MOFs at small overpotentials of 286 mV and 365 mV, respectively, which is superior to the commercial RuO₂ catalyst and most of the bulk MOFs.

Comprehensive and High-throughput Electrolysis of Water and Urea by 3-5 nm Nickel and Copper Coordination Polymers

Metal-organic coordination polymers (CP) have attracted the scientific attention for electrochemical water oxidation as it has the similar coordination structure like natural photosynthetic coordinated complex. However, the harsh synthesis conditions and bulky nature pose a major challenge in the field of catalysis. Herein, 3-5 nm CP particles synthesized at room temperature using aqueous solutions of Ni²⁺ /Cu²⁺ and 2,5-dihydroxyterepthalic acid as precursor were applied for alkaline water and urea electrolysis. The overpotential required is only 300 mV at 10 mA cm^-2 by Nano-Ni CP for water oxidation, with turnover frequency (TOF) of 21.4 s^-1 which is around 8 times higher than its bulk-counterpart. Overall water and urea splitting were achieved with Nano-Cu (-) ∥ Nano-Ni (+) couple on Ni foam at 1.69 and 1.52 V to achieve 10 mA cm^-2 , respectively. High electrochemical surface area (ECSA), high TOF, and enhanced mass diffusion are found to be the key parameters responsible for the state-of-the-art water and urea splitting performances of nano-CPs as compared to their bulk counterparts.

Enhancing One-Dimensional Charge Transport in Metal-organic Framework Hexagonal Nanorods for Electrocatalytic Oxygen Evolution

Metal-organic frameworks (MOFs) have exhibited huge potential in electrocatalytic fields. However, the intrinsic low conductivity and the blockage of metal active sites by organic linkers still seriously hinder their large-scale application. In this study, as a proof of principle, constructing cofacial π-π stacking in the terminal ligand (4,4'-bipyridine) of a Ni/Fe-chain-based MOF to fabricate strong π-π interaction, in combination with unique hexagonal nanorod (HXR) structure, is found to be an effective strategy to enhance one-dimensional charge carrier efficiency and thus achieve excellent activity in the oxygen evolution reaction (OER). The approach yields a high turnover frequency (4.54 s^-1 ) in well-designed bimetallic chain-based MOFs (NiFe-HXR) at an overpotential of 350 mV, which is about 8.7 and 34.9 times higher than those in Ni-HXR (0.52 s^-1 ) and IrO₂ (0.13 s^-1 ), respectively. This work effectively combines "through-bond" channel in chain-based structure of NiFe-HXR and "through-space" transport between face-to-face terminal ligands, thus resulting in outstanding OER activity. This strategy of modulating the structure chemistry and morphology of MOFs to promote the OER may open a new perspective to synthesize MOFs for energy-relevant electrochemical reactions.

Overall Oxygen Electrocatalysis on Nitrogen-Modified Carbon Catalysts: Identification of Active Sites and In Situ Observation of Reactive Intermediates

The recent mechanistic understanding of active sites, adsorbed intermediate products, and rate-determining steps (RDS) of nitrogen (N)-modified carbon catalysts in electrocatalytic oxygen reduction (ORR) and oxygen evolution reaction (OER) are still rife with controversy because of the inevitable coexistence of diverse N configurations and the technical limitations for the observation of formed intermediates. Herein, seven kinds of aromatic molecules with designated single N species are used as model structures to investigate the explicit role of each common N group in both ORR and OER. Specifically, dynamic evolution of active sites and key adsorbed intermediate products including O2 (ads), superoxide anion O2 - *, and OOH* are monitored with in situ spectroscopy. We propose that the formation of *OOH species from O2 - * (O2 - *+H2 O→OOH*+OH- ) is a possible RDS during the ORR process, whereas the generation of O2 from OOH* species is the most likely RDS during the OER process.

Artificial Heterointerfaces Achieve Delicate Reaction Kinetics towards Hydrogen Evolution and Hydrazine Oxidation Catalysis

Electrochemical water splitting for H₂ production is limited by the sluggish anode oxygen evolution reaction (OER), thus using hydrazine oxidation reaction (HzOR) to replace OER has received great attention. Here we report the hierarchical porous nanosheet arrays with abundant Ni₃ N-Co₃ N heterointerfaces on Ni foam with superior hydrogen evolution reaction (HER) and HzOR activity, realizing working potentials of -43 and -88 mV for 10 mA cm^-2 , respectively, and achieving an industry-level 1000 mA cm^-2 at 200 mV for HzOR. The two-electrode overall hydrazine splitting (OHzS) electrolyzer requires the cell voltages of 0.071 and 0.76 V for 10 and 400 mA cm^-2 , respectively. The H₂ production powered by a direct hydrazine fuel cell (DHzFC) and a commercial solar cell are investigated to inspire future practical applications. DFT calculations decipher that heterointerfaces simultaneously optimize the hydrogen adsorption free energy (ΔG_H* ) and promote the hydrazine dehydrogenation kinetics. This work provides a rationale for advanced bifunctional electrocatalysts, and propels the practical energy-saving H₂ generation techniques.

Preparation of MOF Film/Aerogel Composite Catalysts via Substrate-Seeding Secondary-Growth for the Oxygen Evolution Reaction and CO2 Cycloaddition

Substrate-supported metal-organic frameworks (MOFs) films are desired to realize their potential in practical applications. Herein, a novel substrate-seeding secondary-growth strategy is developed to prepare composites of uniform MOFs films on aerogel walls. Briefly, the organic ligand is "pre-seeded" onto the aerogel walls, and then a small amount of metal-ion solution is sprayed onto the prepared aerogel. The sprayed solution diffuses along the aerogel walls to form a continuous thin layer, which confines the nucleation reaction, promoting the formation of uniform MOFs films on the aerogel walls. The whole process is simple in operation, highly efficient, and eco-friendly. The resulting hierarchical MOFs/aerogel composites have abundant accessible active sites and enable excellent mass transfer, which endows the composite with outstanding catalytic activity and stability in both liquid-phase CO₂ cycloaddition and electrochemical oxygen evolution reaction (OER) process.

Synthesis and Optimization of Zeolitic Imidazolate Frameworks for the Oxygen Evolution Reaction

Metal-organic frameworks/zeolitic imidazolate frameworks (MOFs/ZIFs) and their post-synthesis modified nanostructures, such as oxides, hydroxides, and carbons have generated significant interest for electrocatalytic reactions. In this work, a high and durable oxygen evolution reaction (OER) performance directly from bimetallic Zn_100-x Co_x -ZIF samples is reported, without carrying out high-temperature calcination and/or carbonization. ZIFs can be reproducibly and readily synthesized in large scale at ambient conditions. The bimetallic ZIFs show a systematic and gradually improved OER activity with increasing cobalt concentration. A further increase in OER activity is evidenced in ZIF-67 polyhedrons with controlled particle size of <200 nm among samples of different sizes between 50 nm and 2 μm. Building on this, a significantly enhanced, >50 %, OER activity is obtained with ZIF-67/carbon black, which shows a low overpotential of approximately 320 mV in 1.0 m KOH electrolyte. Such activity is comparable to or better than numerous MOF/ZIF-derived electrocatalysts. The optimized ZIF-67 sample also exhibits increased activity and durability over 24 h, which is attributed to an in situ developed active cobalt oxide/oxyhydroxide related nanophase.

Salen-Based Conjugated Microporous Polymers for Efficient Oxygen Evolution Reaction

Exploring high-performance electrocatalysts, especially non-noble metal electrocatalysts, for the oxygen evolution reaction (OER) is critical to energy storage and conversion. Herein, we report for the first time that conjugated microporous polymers (CMPs) incorporating salen can be used as OER electrocatalysts with outstanding performances. The best OER electrocatalyst (salen-CMP-Fe-3) exhibits a low Tafel slope of 63 mV dec^-1 and an overpotential of 238 mV at 10 mA cm^-2 . DFT and Grand Canonical Monte Carlo calculations confirmed that the significantly improved electrocatalytic properties can be attributed to the intrinsic catalytic activity of the salen moiety and the enrichment effect of the pore structures. This work demonstrates that salen-based conjugated polymers are a type of metal-coordinated porous polymer that show excellent catalyst performance.

Rational Design of a N,S Co-Doped Supermicroporous CoFe-Organic Framework Platform for Water Oxidation

It remains a challenge to rational design of a new metal-organic framework (MOF) as highly efficient direct electrocatalysts for the oxygen evolution reaction (OER). Herein, we developed a simple and effective method to explore a new pillared-layered MOF with syringic acid as a promising OER electrocatalyst. The isostructural mono-, heterobimetallic MOF and N,S co-doped MOF by mixing thiourea were quickly synthesized in a high yield under solvothermal condition. Moreover, the optimized N,S co-doped MOF exhibits the lowest overpotential of 254 mV at 10 mA cm^-2 on a glass carbon electrode and a small Tafel slope of 50 mV dec^-1 , especially, this catalyst also possesses long-term electrochemical durability for at least 16 h. According to the characterization, the incorporation of N and S atoms into this heterobimetallic CoFe-based MOF could modify its pore structure, tune the electronic structure, accordingly, improve the mass and electron transportation, and facilitate the formation of active species, as a consequence, the improved activity of this new N,S co-doped MOF for OER should be mainly be ascribed to higher electrochemical activation toward the active species via in situ surface modification during the OER process.

Stable Iron Hydroxide Nanosheets@Cobalt-Metal-Organic-Framework Heterostructure for Efficient Electrocatalytic Oxygen Evolution

Most studies are devoted to the use of metal-organic frameworks (MOFs) as templates to construct desirable electrocatalysts in situ by high-temperature pyrolysis. The emergence of heterostructures invokes new opportunities to use the full potential of pristine MOFs as efficient catalysts in the oxygen evolution reaction (OER). Here, a MOF surface-reaction strategy is developed to synthesize MOF-based heterostructures without pyrolysis. Uniform Fe(OH)₃ nanosheets are grown controllably on the Co-MOF-74 surface by a fast "phenol-Fe" reaction that takes advantage of the hydroxyl sites in Co-MOF-74. The resulting Fe(OH)₃ @Co-MOF-74 heterostructure delivers an excellent performance in the OER with a low overpotential of 292 mV at 10 mA cm^-2 . Notably, the introduction of Fe can improve the intrinsic activity of the original Co atom significantly. The turnover frequency in Fe(OH)₃ @Co-MOF-74 (1.209 s^-1 ) is more than 25 times higher than that in Co-MOF-74 (0.048 s^-1 ). This work presents a fresh concept for the fundamental design of advanced pure-MOF-based heterostructures and, thereby, provides a new avenue for the fabrication of other energy-conversion and -storage materials.

Oxygen Evolution Reaction at Carbon Edge Sites: Investigation of Activity Evolution and Structure-Function Relationships with Polycyclic Aromatic Hydrocarbons

The abundance of available surface chemical information and edge structures of carbon materials have attracted tremendous interest in catalysis. For the oxygen evolution reaction (OER), the edge effects of carbon materials have rarely been studied in detail because of the complexity of various coexisting edge configurations and the controversy between carbon corrosion and carbon catalysis. Herein, the exact roles of common carbon active edge sites in the OER were interrogated using polycyclic aromatic hydrocarbons (PAHs) with designated configurations (zigzag and armchair) as model probe molecules, with a focus on structure-function relationships. Zigzag configurations of PAHs showed high activity for the OER while also showing a good stability at a reasonable potential. They show a TOF value of 0.276 s^-1 in 0.1 m KOH. The catalytic activity of carbon edge sites was further effectively regulated by extending the π conjugation structure at a molecular level.

Non-3d Metal Modulation of a Cobalt Imidazolate Framework for Excellent Electrocatalytic Oxygen Evolution in Neutral Media

Cobalt imidazolate frameworks are classical electrocatalysts for the oxygen evolution reaction (OER) but suffer from the relatively low activity. Here, a non-3d metal modulation strategy is presented for enhancing the OER activity of cobalt imidazolate frameworks. Two isomorphous frameworks [Co₄ (MO₄ )(eim)₆ ] (M=Mo or W, Heim=2-ethylimidazole) having Co(eim)₃ (MO₄ ) units and high water stabilities were designed and synthesized. In different neutral media, the Mo-modulated framework coated on a glassy carbon electrode shows the best OER performances (1 mA cm^-2 at an overpotential of 210 mV in CO₂ -saturated 0.5 m KHCO₃ electrolyte and 2/10/22 mA cm^-2 at overpotential of 388/490/570 mV in phosphate buffer solution) among non-precious metal catalysts and even outperforms RuO₂ . Spectroscopic measurements and computational simulations revealed that the non-3d metals modulate the electronic structure of Co for optimum reactant/product adsorption and tailor the energy of rate-determining step to a more moderate value.

Template-Directed Growth of Bimetallic Prussian Blue-Analogue Nanosheet Arrays and Their Derived Porous Metal Oxides for Oxygen Evolution Reaction

Coordination polymers (CPs) are ideal precursors for synthesizing porous catalysts. However, the direct thermolysis of CPs is prone to generate agglomerates, greatly reducing the electrical conductivity and active sites of their derived catalysts. The construction of well-ordered CP nanostructures is a promising strategy for alleviating the above issue, but it remains challenging. Here, a facile chemical etching approach is developed for the fabrication of well-aligned three-dimensional (3D) bimetallic Prussian blue-analogue nanosheet arrays. Impressively, the derived porous metal oxide (Fe-NiO) acts as a remarkable oxygen evolution reaction (OER) catalyst, which merely requires overpotentials as low as 218 and 270 mV to achieve 10 and 100 mA cm^-2 in 1.0 m KOH aqueous solution, respectively. The excellent electrocatalytic performance of Fe-NiO is ascribed to the 3D porous nanosheet array architecture, which endows the bimetallic catalyst with abundant electrocatalytic active sites, enhanced surface permeability, and high electronic conductivity. It is expected that the proposed strategy can pave a new way for fabricating highly efficient electrocatalysts for energy storage and conversion.