Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

IrOx·nH2O with lattice water-assisted oxygen exchange for high-performance proton exchange membrane water electrolyzers

The trade-off between activity and stability of oxygen evolution reaction (OER) catalysts in proton exchange membrane water electrolyzer (PEMWE) is challenging. Crystalline IrO₂ displays good stability but exhibits poor activity; amorphous IrO_x exhibits outstanding activity while sacrificing stability. Here, we combine the advantages of these two materials via a lattice water-incorporated iridium oxide (IrO_x·nH₂O) that has short-range ordered structure of hollandite-like framework. We confirm that IrO_x·nH₂O exhibits boosted activity and ultrahigh stability of >5700 hours (~8 months) with a record-high stability number of 1.9 × 10⁷ n_oxygen n_Ir^-1. We evidence that lattice water is active oxygen species in sustainable and rapid oxygen exchange. The lattice water-assisted modified OER mechanism contributes to improved activity and concurrent stability with no apparent structural degradation, which is different to the conventional adsorbate evolution mechanism and lattice oxygen mechanism. We demonstrate that a high-performance PEMWE with IrO_x·nH₂O as anode electrocatalyst delivers a cell voltage of 1.77 V at 1 A cm^-2 for 600 hours (60°C).

Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis

Active and nonprecious-metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital components of clean energy conversion devices such as regenerative fuel cells and rechargeable metal-air batteries. Porous manganese oxides (MnO_x) are promising electrocatalyst candidates because of their high surface area and the abundance of Mn. MnO_x catalysts exhibit various oxidation states and crystal structures, which critically affect their electrocatalytic activity. These effects remain elusive mainly because the synthesis of oxidation-state-controlled porous MnO_x with similar structural properties is challenging. In this work, four different mesoporous manganese oxides (m-MnO_x) were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity toward oxygen electrocatalysis. The following activity trends were observed: m-Mn₂O₃ > m-MnO₂ > m-MnO > m-Mn₃O₄ for the ORR and m-MnO₂ > m-Mn₂O₃ > m-MnO ≈ m-Mn₃O₄ for the OER. These activity trends suggest that high-valent Mn species (Mn(III) and Mn(IV)) with disordered atomic arrangements induced by nanostructuring significantly influence electrocatalysis. In situ X-ray absorption spectroscopy was used to analyze the changes in the oxidation states under the ORR and OER conditions, which showed the surface phase transformation and generation of active species during electrocatalysis.

Engineering Single-Atom Electrocatalysts for Enhancing Kinetics of Acidic Volmer Reaction

The design of active and low-cost electrocatalyst for hydrogen evolution reaction (HER) is the key to achieving a clean hydrogen energy infrastructure. The most successful design principle of hydrogen electrocatalyst is the activity volcano plot, which is based on Sabatier principle and has been used to understand the exceptional activity of noble metal and design of metal alloy catalysts. However, this application of volcano plot in designing single-atom electrocatalysts (SAEs) on nitrogen doped graphene (TM/N₄C catalysts) for HER has been less successful due to the nonmetallic nature of the single metal atom site. Herein, by performing ab initio molecular dynamics simulations and free energy calculations on a series of SAEs systems (TM/N₄C with TM = 3d, 4d, or 5d metals), we find that the strong charge-dipole interaction between the negatively charged *H intermediate and the interfacial H₂O molecules could alter the transition path of the acidic Volmer reaction and dramatically raise its kinetic barrier, despite its favorable adsorption free energy. Such kinetic hindrance is also experimentally confirmed by electrochemical measurements. By combining the hydrogen adsorption free energy and the physics of competing interfacial interactions, we propose a unifying design principle for engineering the SAEs used for hydrogen energy conversion, which incorporates both thermodynamic and kinetic considerations and allows going beyond the activity volcano model.

Ni-Activated and Ni-P-Activated Porous Titanium Carbide Ceramic Electrodes as Efficient Electrocatalysts for Overall Water Splitting

Developing electrocatalysts based on transition-metal carbides that can be utilized for commercial water splitting is a challenging endeavor. To address this challenge, we employed a novel approach that merged phase-inversion tape-casting and sintering, subsequently implementing a simple and efficient electrodeposition process, to synthesize Ni-activated and Ni-P-activated titanium carbide (Ni/TiC, Ni-P/TiC) ceramic electrodes as water splitting catalytic cathodes and anodes. These self-supported Ni/TiC and Ni-P/TiC electrodes are binder-free with abundant finger-like pores, which contribute to the formation of highly conductive skeleton and more exposed active sites for direct water splitting. These catalysts exhibit outstanding performance, superior to many reported bifunctional nickel-based catalysts supported on other substrates. Moreover, the exceptional electrocatalytic performance of the Ni/TiC and Ni-P/TiC catalysts is attributed to the synergistic effect between Ni oxides (phosphides) and TiC, as revealed by density functional theory (DFT) calculations. This self-template strategy paves the way for fabricating industrially applicable electrodes to generate hydrogen and oxygen through water splitting.

Colossal Dielectric Perovskites of Calcium Copper Titanate (CaCu3 Ti4 O12 ) with Low-Iridium Dopants Enables Ultrahigh Mass Activity for the Acidic Oxygen Evolution Reaction

Oxygen evolution reaction (OER) under acidic conditions becomes of significant importance for the practical use of a proton exchange membrane (PEM) water electrolyzer. In particular, maximizing the mass activity of iridium (Ir) is one of the maiden issues. Herein, the authors discover that the Ir-doped calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO) perovskite exhibits ultrahigh mass activity up to 1000 A g_Ir ^-1 for the acidic OER, which is 66 times higher than that of the benchmark catalyst, IrO₂ . By substituting Ti with Ir in CCTO, metal-oxygen (M-O) covalency can be significantly increased leading to the reduced energy barrier for charge transfer. Further, highly polarizable CCTO perovskite referred to as "colossal dielectric", possesses low defect formation energy for oxygen vacancy inducing a high number of oxygen vacancies in Ir-doped CCTO (Ir-CCTO). Electron transfer occurs from the oxygen vacancies and Ti to the substituted Ir consequentially resulting in the electron-rich Ir and -deficient Ti sites. Thus, favorable adsorptions of oxygen intermediates can take place at Ti sites while the Ir ensures efficient charge supplies during OER, taking a top position of the volcano plot. Simultaneously, the introduced Ir dopants form nanoclusters at the surface of Ir-CCTO, which can boost catalytic activity for the acidic OER.

Cu-Doped Heterointerfaced Ru/RuSe2 Nanosheets with Optimized H and H2 O Adsorption Boost Hydrogen Evolution Catalysis

Ruthenium chalcogenide is a highly promising catalytic system as a Pt alternative for hydrogen evolution reaction (HER). However, well-studied ruthenium selenide (RuSe₂ ) still exhibits sluggish HER kinetics in alkaline media due to the inappropriate adsorption strength of H and H₂ O. Herein, xx report a new design of Cu-doped Ru/RuSe₂ heterogeneous nanosheets (NSs) with optimized H and H₂ O adsorption strength for highly efficient HER catalysis in alkaline media. Theoretical calculations reveal that the superior HER performance is attributed to a synergistic effect of the unique heterointerfaced structure and Cu doping, which not only optimizes the electronic structure with a suitable d-band center to suppress proton overbinding but also alleviates the energy barrier with enhanced H₂ O adsorption. As a result, Cu-doped heterogeneous Ru/RuSe₂ NSs exhibit a small overpotential of 23 mV at 10 mA cm^-2 , a low Tafel slope of 58.5 mV dec^-1 and a high turnover frequency (TOF) value of 0.88 s^-1 at 100 mV for HER in alkaline media, which is among the best catalysts in noble metal-based electrocatalysts toward HER. The present Cu-doped Ru/RuSe₂ NSs interface catalyst is very stable for HER by showing no activity decay after 5000-cycle potential sweeps. This work heralds that heterogeneous interface modulation opens up a new strategy for the designing of more efficient electrocatalysts.

Tailoring Pore Size and Catalytic Activity in Cobalt Iron Layered Double Hydroxides and Spinels by Microemulsion-Assisted pH-Controlled Co-Precipitation

Cobalt iron containing layered double hydroxides (LDHs) and spinels are promising catalysts for the electrochemical oxygen evolution reaction (OER). Towards development of better performing catalysts, the precise tuning of mesostructural features such as pore size is desirable, but often hard to achieve. Herein, a computer-controlled microemulsion-assisted co-precipitation (MACP) method at constant pH is established and compared to conventional co-precipitation. With MACP, the particle growth is limited and through variation of the constant pH during synthesis the pore size of the as-prepared catalysts is controlled, generating materials for the systematic investigation of confinement effects during OER. At a threshold pore size, overpotential increased significantly. Electrochemical impedance spectroscopy (EIS) indicated a change in OER mechanism, involving the oxygen release step. It is assumed that in smaller pores the critical radius for gas bubble formation is not met and therefore a smaller charge-transfer resistance is observed for medium frequencies.

CoMoO4-CoP/NC heterostructure anchored on hollow polyhedral N-doped carbon skeleton for efficient water splitting

We report the synthesis and electrocatalytic properties of a CoMoO₄-CoP heterostructure anchored on a hollow polyhedral N-doped carbon skeleton (CoMoO₄-CoP/NC) for water-splitting applications. The preparation involved the anion exchange of MoO₄^2- to the organic ligand of ZIF-67, the self-hydrolysis of MoO₄^2-, and NaH₂PO₂ phosphating annealing. CoMoO₄ was found to enhance thermal stability and prevent active site agglomeration during annealing, while the hollow structure of CoMoO₄-CoP/NC provided a large specific surface area and high porosity that facilitated mass transport and charge transfer. The interfacial electron transfer from Co to Mo and P sites promoted the generation of electron-deficient Co sites and electron-enriched P sites, which accelerated water dissociation. CoMoO₄-CoP/NC exhibited excellent electrocatalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH solution, with overpotentials of 122 mV and 280 mV at 10 mA cm^-2, respectively. The CoMoO₄-CoP/NC‖CoMoO₄-CoP/NC two-electrode system only required an overall water splitting (OWS) cell voltage of 1.62 V to achieve 10 mA cm^-2 in an alkaline electrolytic cell. In addition, the material showed comparable activity to 20% Pt/C‖RuO₂ in a pure water home-made membrane electrode device, demonstrating potential for practical applications in proton exchange membrane (PEM) electrolyzers. Our results suggest that CoMoO₄-CoP/NC is a promising electrocatalyst for efficient and cost-effective water splitting.

Rational design of bifunctional hydroxide/sulfide heterostructured catalyst for efficient electrochemical seawater splitting

Heterostructure engineering is one of the most promising strategies for efficient water splitting by electrocatalysts. However, it remains challenging to design heterostructured catalysts to achieve the desired goals in both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in seawater splitting. Here, particulate heterostructures of FeCoNi hydroxide/sulfide supported on nickel foams were prepared by hydrothermal methods to achieve a high-performance bifunctional catalyst. The synthesized FeCoNi hydroxide/sulfide exhibited excellent electrocatalytic performance, requiring an overpotential of 195 mV for OER and 76 mV for HER to achieve a current density of 10 mA cm^-2 while showing excellent stability. The catalyst maintains its excellent performance even in artificial or natural seawater with high salinity, which is a harsh environment. When applied directly to a water splitting system, the catalyst achieves a current density of 10 mA cm^-2 at only 1.5 V (1.57 V in alkaline seawater). The FeCoNi hydroxide/sulfide heterostructure is an excellent electrocatalytic bifunctional catalyst due to compositional modulation, systematic charge transfer optimization, improved intermediates adsorption, and increased electrocatalytic active sites and the synergistic effect of the heterostructure.

Optimizing the Electronic Structure of Atomically Dispersed Ru Sites with CoP for Highly Efficient Hydrogen Evolution in both Alkaline and Acidic Media

Developing efficient and stable electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range and industrial large-scale hydrogen production is critical and challenging. Here, a tailoring strategy is developed to fabricate an outstanding HER catalyst in both acidic and alkaline electrolytes containing high-density atomically dispersed Ru sites anchored in the CoP nanoparticles supported on carbon spheres (NC@Ru_SA -CoP). The obtained NC@Ru_SA -CoP catalyst exhibits excellent HER performance with overpotentials of only 15 and 13 mV at 10 mA cm^-2 in 1 m KOH and 0.5 m H₂ SO₄ , respectively. The experimental results and theoretical calculations indicate that the strong interaction between the Ru site and the CoP can effectively optimize the electronic structure of Ru sites to reduce the hydrogen binding energy and the water dissociation energy barrier. The constructed alkaline anion exchange membrane water electrolyze (AAEMWE) demonstrates remarkable durability and an industrial-level current density of 1560 mA cm^-2 at 1.8 V. This strategy provides a new perspective on the design of Ru-based electrocatalysts with suitable intermediate adsorption strengths and paves the way for the development of highly active electrocatalysts for industrial-scale hydrogen production.

In Situ Exsolution of Ba3(VO4)2 Nanoparticles on a V-Doped BaCoO3-δ Perovskite Oxide with Enhanced Activity for Electrocatalytic Hydrogen Evolution

The successful preparation of a perovskite-based heterostructure is important for broadening the applications of perovskites in the field of electrocatalysis, especially in a hydrogen evolution reaction (HER). Nevertheless, the limited active sites of perovskites severely hindered the HER properties. Herein, an in situ exsolution method was used to construct a nanocomposite based on V-doped BaCoO_3-δ decorated with Ba₃(VO₄)₂ (BVCO19) for alkaline HER. The exsolved Ba₃(VO₄)₂ can induce more Co⁴⁺ ions on BaCoO_3-δ, which serves as active sites for the release of H₂. Meanwhile, by regulating the valency of V and Co species, the catalyst can reach a charge balance by generating more oxygen vacancies, which greatly facilitates the adsorption and dissociation of H₂O molecules. The synergistic effect between the oxygen vacancies and high-valence Co⁴⁺ leads to an enhanced HER performance of BVCO19. The as-obtained catalyst delivers a low overpotential of 194 mV at 10 mA cm^-2 as well as impressive stability for 100 h in alkaline media, which outperforms pristine BaCoO_3-δ and most of the nonprecious-based perovskite oxides. This work provides new insights into the preparation of perovskite-based heterostructure for boosting HER.

Rhenium-Based Electrocatalysts for Water Splitting

Due to the contamination and global warming problems, it is necessary to search for alternative environmentally friendly energy sources. In this area, hydrogen is a promising alternative. Hydrogen is even more promising, when it is obtained through water electrolysis operated with renewable energy sources. Among the possible devices to perform electrolysis, proton exchange membrane (PEM) electrolyzers appear as the most promising commercial systems for hydrogen production in the coming years. However, their massification is affected by the noble metals used as electrocatalysts in their electrodes, with high commercial value: Pt at the cathode where the hydrogen evolution reaction occurs (HER) and Ru/Ir at the anode where the oxygen evolution reaction (OER) happens. Therefore, to take full advantage of the PEM technology for green H2 production and build up a mature PEM market, it is imperative to search for more abundant, cheaper, and stable catalysts, reaching the highest possible activities at the lowest overpotential with the longest stability under the harsh acidic conditions of a PEM. In the search for new electrocatalysts and considering the predictions of a Trasatti volcano plot, rhenium appears to be a promising candidate for HER in acidic media. At the same time, recent studies provide evidence of its potential as an OER catalyst. However, some of these reports have focused on chemical and photochemical water splitting and have not always considered acidic media. This review summarizes rhenium-based electrocatalysts for water splitting under acidic conditions: i.e., potential candidates as cathode materials. In the various sections, we review the mechanism concepts of electrocatalysis, evaluation methods, and the different rhenium-based materials applied for the HER in acidic media. As rhenium is less common for the OER, we included a section about its use in chemical and photochemical water oxidation and as an electrocatalyst under basic conditions. Finally, concluding remarks and perspectives are given about rhenium for water splitting.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Progress and prospects of dealloying methods for energy-conversion electrocatalysis

Developing hydrogen production and utilization technologies is a promising way to achieve large-scale applications of renewable energy. For both water electrolysis and fuel cell electrode reactions, electrocatalysts are critical to their energy conversion efficiencies. Among the various strategies for improving the performance of electrocatalysts, dealloying has been developed as a commonly used effective post-processing method. It originated from anti-corrosion science and can form metal materials with porous or "skin" nanostructures by selectively dissolving the active components in alloys. There are generally two types of dealloying methods: electrochemical dealloying and chemical dealloying. Electrochemical dealloying is more controllable, while chemical dealloying is simpler and less expensive. In this review, the fundamentals, histories, and progress of dealloying methods for energy conversion electrocatalysis are systematically summarized. Furthermore, current problems and prospects are proposed.

Rhenium Suppresses Iridium (IV) Oxide Crystallization and Enables Efficient, Stable Electrochemical Water Oxidation

IrO₂ as benchmark electrocatalyst for acidic oxygen evolution reaction (OER) suffers from its low activity and poor stability. Modulating the coordination environment of IrO₂ by chemical doping is a methodology to suppress Ir dissolution and tailor adsorption behavior of active oxygen intermediates on interfacial Ir sites. Herein, the Re-doped IrO₂ with low crystallinity is rationally designed as highly active and robust electrocatalysts for acidic OER. Theoretical calculations suggest that the similar ionic sizes of Ir and Re impart large spontaneous substitution energy and successfully incorporate Re into the IrO₂ lattice. Re-doped IrO₂ exhibits a much larger migration energy from IrO₂ surface (0.96 eV) than other dopants (Ni, Cu, and Zn), indicating strong confinement of Re within the IrO₂ lattice for suppressing Ir dissolution. The optimal catalysts (Re: 10 at%) exhibit a low overpotential of 255 mV at 10 mA cm^-2 and a high stability of 170 h for acidic OER. The comprehensive mechanism investigations demonstrate that the unique structural arrangement of the Ir active sites with Re-dopant imparts high performance of catalysts by minimizing Ir dissolution, facilitating *OH adsorption and *OOH deprotonation, and lowering kinetic barrier during OER. This study provides a methodology for designing highly-performed catalysts for energy conversion.

Nano Si-Doped Ruthenium Oxide Particles from Caged Precursors for High-Performance Acidic Oxygen Evolution

RuO₂ is well known as the benchmark acidic oxygen evolution reaction (OER) catalyst, but its practical application has been impeded by its limited durability. Herein, it is presented that the stability of ruthenium oxide can be significantly improved by pretrapping RuCl₃ precursors within a cage compound possessing 72 aromatic rings, which leads to well carbon-coated RuO_x particles (Si-RuO_x @C) after calcination. The catalyst survives in 0.5 M H₂ SO₄ for an unprecedented period of 100 hours at 10 mA cm^-2 with minimal overpotential change during OER. In contrast, RuO_x prepared from similar non-tied compounds doesn't exhibit such catalytic activity, highlighting the importance of the preorganization of Ru precursors within the cage prior to calcination. In addition, the overpotential at 10 mA cm^-2 in acid solution is only 220 mV, much less than that of commercial RuO₂ . X-ray absorption fine structure (FT-EXAFS) reveals the Si doping through unusual Ru-Si bond, and density functional theory (DFT) calculation reveals the importance of the Ru-Si bond in enhancing both the activity and stability of the catalyst.

Atomically Dispersed Dual-Metal Sites Showing Unique Reactivity and Dynamism for Electrocatalysis

The real structure and in situ evolution of catalysts under working conditions are of paramount importance, especially for bifunctional electrocatalysis. Here, we report asymmetric structural evolution and dynamic hydrogen-bonding promotion mechanism of an atomically dispersed electrocatalyst. Pyrolysis of Co/Ni-doped MAF-4/ZIF-8 yielded nitrogen-doped porous carbons functionalized by atomically dispersed Co-Ni dual-metal sites with an unprecedented N8V4 structure, which can serve as an efficient bifunctional electrocatalyst for overall water splitting. More importantly, the electrocatalyst showed remarkable activation behavior due to the in situ oxidation of the carbon substrate to form C-OH groups. Density functional theory calculations suggested that the flexible C-OH groups can form reversible hydrogen bonds with the oxygen evolution reaction intermediates, giving a bridge between elementary reactions to break the conventional scaling relationship.

Morphology-Engineering Construction of Anti-Aggregated Co/N-Doped Hollow Carbon from Metal-Organic Frameworks for Efficient Biomass Upgrading

The controlled pyrolysis of metal/carbon-containing precursors is commonly used for fabricating multifunctional metal/carbon-based catalysts, nevertheless, the inevitable agglomeration of these precursors in pyrolysis is extremely negative for efficient catalysis. This study reports the first example of suppressing the interfacial fusion and agglomeration of metal/carbon-based catalyst in its pyrolysis-involved fabrication process by developing a facile morphology-engineering strategy. Metal-organic framework precursors are chosen as a proof of concept and five Co/N-doped hollow carbons with different morphologies (rhombic dodecahedron, cube, plate, interpenetration twin, and rod) are synthesized via the pyrolysis of their corresponding core-shell ZIF-8@ZIF-67 precursors. It is demonstrated that the interpenetration twin precursor shows the minimum interfacial contact of interparticles due to its partly-concave morphology with abundant facets, which endows it with the best resistibility from interfacial fusion and thus aggregation of interparticles during pyrolysis. Benefiting from its unique anti-aggregated structure with high specific surface area, abundant fully-exposed active sites, and good dispersibility, the resultant 36-facet Co/N-doped hollow carbon exhibit remarkably improved catalytic property for biomass upgrading as compared with its aggregated counterparts. This study highlights the crucial role of engineering morphology to prevent metal/carbon-containing precursors from detrimental agglomeration during pyrolysis, demonstrating a new approach to constructing anti-aggregated metal/carbon-based catalysts.

Integrating ceria with cobalt sulfide as high-performance electrocatalysts for overall water splitting

The development of bifunctional electrocatalysts for overall water splitting is highly desired for converting electricity into chemical energy. However, the synthesis of high-performance bifunctional electrocatalysts remains a pressing challenge. Here, we found that both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance of the Co3S4 electrode can be significantly improved by integration with CeO2. Specifically, as-prepared 5% Ce-Co3S4 and 1% Ce-Co3S4 delivered low overpotentials of 290 and 257 mV to achieve 10 mA cm-2 for the OER and HER in 1.0 M KOH, respectively. The crucial role of CeO2 originated from its unique surface with abundant oxygen vacancies, which were beneficial for the stabilization of Co2+ sites with high OER activity and both the adsorption and dissociation of water molecules in the HER process. This work is expected to provide a general approach to prepare a wide range of high-performance electrode materials for energy-related applications.

A facile strategy to prepare FeNx decorated PtFe intermetallic with excellent acidic oxygen reduction reaction activity and stability

The construction of low-Pt-content intermetallic on carbon supports has been verified as a promising method to promote the activity of the oxygen reduction reaction (ORR). In this study, we have developed a simple and effective strategy to obtain a well-designed CNT-PtFe-PPy precursor. This precursor contains modulated Pt- and Fe-based content dispersed in polypyrrole (PPy) chain segments, which are in-situ generated on the templates of carbon nanotubes (CNTs). Subsequent pyrolysis of the CNT-PtFe-PPy precursor produces a CNT-PtFe@FeNC catalyst, which contains both Fe-N_x and PtFe intermetallic active sites. Due to the highly efficient dispersion of active species, the CNT-PtFe@FeNC electrocatalyst displays a 9.5 times higher specific activity (SA) and 8.5 times higher mass activity (MA) than those of a commercial Pt/C catalyst in a 0.1 M HClO₄ solution. Additionally, these results, combined with excellent durability (the SA and MA maintained 94 % and 91 % of initial activity after a 10-k cycle accelerated durability test), represent among the best performance achieved so far for Pt-based ORR electrocatalysts. Furthermore, density functional theory (DFT) calculations revealed that the presence of Fe-N₄ species reduces the adsorption energy between the PtFe intermetallic compound and OH*, accelerating the ORR process.

Progress in electrocatalytic hydrogen evolution of transition metal alloys: synthesis, structure, and mechanism analysis

At present, the problems of high energy consumption and low efficiency in electrocatalytic hydrogen production have limited the large-scale industrial application of this technology. Constructing effective catalysts has become the way to solve these problems. Transition metal alloys have been proved to be very promising materials in hydrogen evaluation reaction (HER). In this study, the related theories and characterization methods of electrocatalysis are summarized, and the latest progress in the application of binary, ternary, and high entropy alloys to HER in recent years is analyzed and studied. The synthesis methods and optimization strategies of transition metal alloys, including composition regulation, hybrid engineering, phase engineering, and morphological engineering were emphatically discussed, and the principles and performance mechanism analysis of these strategies were discussed in detail. Although great progress has been made in alloy catalysts, there is still considerable room for applications. Finally, the challenges, prospects, and research directions of transition metal alloys in the future were predicted.

NaCl sealing Strategy-Assisted synthesis CoO-Co heterojunctions as efficient oxygen electrocatalysts for Zn air batteries

Developing highly efficient, low-cost, and stable bifunctional oxygen electrocatalysts is essential for the wide popularization of rechargeable Zn-air batteries. Combining zero-dimensional metal nanoparticles with two-dimensional metal oxide nanosheets is an appealing strategy to balance performance and cost. However, the precise construction of these composites remains a great challenge, and their interaction mechanisms lack thorough study. Herein, a cobalt-oxide-based bifunctional oxygen electrocatalyst comprising a rich Co-CoO heterointerface (CoO/Co@NG) was synthesized via a NaCl sealing-assisted pyrolysis strategy. The NaCl crystals played the role of a closed nanoreactor, which facilitated the formation of a CoO-Co heterojunction. Experimental results and theoretical calculations confirmed that the ingeniously constructed heterojunction expedited the oxygen reduction reaction and oxygen evolution reaction kinetics, which is superior to Pt/C. When serving as the air electrode in an assembled liquid-state Zn-air battery, the battery shows high power density (215 mW cm^-2), specific capacity (710 mAh g_zn^-1), and outstanding durability (720 h at 10 mA cm^-2). This work provides an innovative avenue to design high-performance heterojunction electrocatalysts for perdurable Zn-air batteries.

Stepwise Monomicelle Assembly for Highly Ordered Mesoporous TiO2 Membranes with Precisely Tailored Mesophase and Porosity

Mesoporous materials with crystalline frameworks have been acknowledged as very attractive materials in various applications. Nevertheless, due to the cracking issue during crystallization and incompatible hydrolysis and assembly, the precise control for crystalline mesoscale membranes is quite infertile. Herein, we presented an ingenious stepwise monomicelle assembly route for the syntheses of highly ordered mesoporous crystalline TiO₂ membranes with delicately controlled mesophase, mesoporosity, and thickness. Such a process involves the preparation of monomicelle hydrogels and follows self-assembly by stepwise solvent evaporation, which enables the sensitive hydrolysis of TiO₂ oligomers and dilatory micelle assembly to be united. In consequence, the fabricated mesoporous TiO₂ membranes exhibit a broad flexibility, including tunable ordered mesophases (worm-like, hexagonal p6mm to body-centered cubic Im3̅m), controlled mesopore sizes (3.0-8.0 nm), and anatase grain sizes (2.3-8.4 nm). Besides, such mesostructured crystalline TiO₂ membranes can be extended to diverse substrates (Ti, Ag, Si, FTO) with tailored thickness. The great mesoporosity of the in situ fabricated mesoscopic membranes also affords excellent pseudocapacitive behavior for sodium ion storage. This study underscores a novel pathway for balancing the interaction of precursors and micelles, which could have implications for synthesizing crystalline mesostructures in higher controllability.

Unconventional Self-Reconstructed Trimer-like Metal Zigzag Edge of 1T-Phase Transition Metal Dichalcogenides

Undercoordination-induced slight bond contraction of the pristine edge is a conventional edge self-reconstructed pattern in two-dimensional materials that generally cannot drive the edge into its ground state. Despite reports of unconventional edge self-reconstructed patterns of 1H-phase transition metal dichalcogenides (TMDCs), there have been no reports of sister 1T-phase TMDCs. Here, we predict an unconventional (2 × 1) edge self-reconstructed pattern for 1T-TMDCs by considering 1T-TiTe₂. A novel self-reconstructed trimer-like metal zigzag edge (TMZ edge) with one-dimensional metal atomic chains and Ti₃ trimers is uncovered. Its metal triatomic 3d orbital coupling leads to Ti₃ trimerization. This TMZ edge occurs in group IV, V, and X 1T-TMDCs and has an energetic advantage far beyond conventional bond contraction. The unique triatomic synergistic effect results in better catalysis of the hydrogen evolution reaction (HER) for the 1T-TMDCs than with commercial platinum-based catalysts. This study provides a new strategy for maximizing the HER catalytic efficiency of 1T-TMDCs by using atomic edge engineering.

Triclosan and related compounds in the environment: Recent updates on sources, fates, distribution, analytical extraction, analysis, and removal techniques

Triclosan (TCS) has been widely used in daily life because of its broad-spectrum antibacterial activities. The residue of TCS and related compounds in the environment is one of the critical environmental safety problems, and the pandemic of COVID-19 aggravates the accumulation of TCS and related compounds in the environment. Therefore, detecting TCS and related compound residues in the environment is of great significance to human health and environmental safety. The distribution of TCS and related compounds are slightly different worldwide, and the removal methods also have advantages and disadvantages. This paper summarized the research progress on the source, distribution, degradation, analytical extraction, detection, and removal techniques of TCS and related compounds in different environmental samples. The commonly used analytical extraction methods for TCS and related compounds include solid-phase extraction, liquid-liquid extraction, solid-phase microextraction, liquid-phase microextraction, and so on. The determination methods include liquid chromatography coupled with different detectors, gas chromatography and related methods, sensors, electrochemical method, capillary electrophoresis. The removal techniques in various environmental samples mainly include biodegradation, advanced oxidation, and adsorption methods. Besides, both the pros and cons of different techniques have been compared and summarized, and the development and prospect of each technique have been given.

Boron-Doped Activated Carbon Supports for Cobalt-Catalyzed Oxygen Evolution in Alkaline Electrolyte

Activated carbons (ACs) are the most widely used and attractive support materials for electrocatalytic applications because of their significant surface areas, high electrical conductivities, and moderate affinities toward supported metal catalysts. However, the corrosive behavior of ACs at oxidative potentials causes an inevitable reduction in the active surface area of supported catalysts, resulting in the continuous deterioration of their electrocatalytic performance. Therefore, the introduction of corrosion-resistant durable catalyst supports is essential for sustainable and efficient electrocatalysis. Here, we modified ACs to obtain different boron (B)-doped structures via doping-temperature controls to investigate the corrosion resistance of B-doped ACs. With increasing doping temperature, the B-doped ACs exhibited a decreased defect density and enhanced crystallinity owing to the accelerating dopant-induced graphitization. We found that the substitution of B atoms into the carbon lattice improved the structural integrity of the carbon structure, and cyclic voltammetry (CV) tests suggested that the highly B-substituted structures caused electrochemical surface passivation against carbon corrosion. Moreover, B-doped ACs significantly contributed to the increase in loading mass of cobalt (Co)-based catalyst on them and the electrochemical durability toward the oxygen evolution reaction as catalyst-support hybrid. The B22 (B-doped AC obtained at a 2200 °C B-doping temperature)-supported Co catalyst with the lowest oxidation current exhibited a voltage change of 32 mV at a current density of 10 mA/cm² (ΔE_j=10) after 10,000 cycles, which was a factor of ∼7 higher cycle durability and stability than that of the conventional IrO₂ catalyst (ΔE_j=10 = 205 mV). Here, we propose that surface engineering by B-doping to improve the structural integrity of ACs is an attractive method for designing durable electrocatalytic support materials.

Highly enhanced electrocatalytic OER activity of water-coordinated copper complexes: effect of lattice water and bridging ligand

The use of metal-organic compounds as electrocatalysts for water splitting reactions has gained increased attention; however, a fundamental understanding of the structural requirement for effective catalytic activity is still limited. Herein, we synthesized water-coordinated mono and bimetallic copper complexes (CuPz-H₂O·H₂O, CuPz-H₂O, CuBipy-H₂O·H₂O, and CuMorph-H₂O) with varied intermetallic spacing (pyrazine/4,4'-bipyridine) and explored the structure-dependent oxygen evolution reaction (OER) activity in alkaline medium. Single crystal structural studies revealed water-coordinated monometallic complexes (CuMorph-H₂O) and bimetallic complexes (CuPz-H₂O·H₂O, CuPz-H₂O, CuBipy-H₂O·H₂O). Further, CuPz-H₂O·H₂O and CuBipy-H₂O·H₂O contained lattice water along with coordinated water. Interestingly, the bimetallic copper complex with lattice water and shorter interspacing between the metal centres (CuPz-H₂O·H₂O) showed strong OER activity and required an overpotential of 228 mV to produce a benchmark current density of 10 mA cm^-2. Bimetallic copper complex (CuPz-H₂O) without lattice water but the same intermetallic spacing and bimetallic complex with increased interspacing but with lattice water (CuBipy-H₂O·H₂O) exhibited relatively lower OER activity. CuPz-H₂O and CuBipy-H₂O·H₂O required an overpotential of 236 and 256 mA cm^-2, respectively. Monometallic CuMorph-H₂O showed the lowest OER activity (overpotential 271 mV) compared to bimetallic complexes. The low Tafel slope and charge transfer resistance of CuPz-H₂O·H₂O facilitated faster charge transfer kinetics at the electrode surface and supported the enhanced OER activity. The chronoamperometric studies indicated good stability of the catalyst. Overall, the present structure-electrocatalytic activity studies of copper complexes might provide structural insight for designing new efficient electrocatalysts based on metal coordination compounds.

Hierarchical NiOx nanotube arrays/CoP nanosheets heterostructure enables robust alkaline hydrogen evolution reaction

Rational design of low-cost and high-efficiency electrocatalysts for hydrogen evolution reaction (HER) is critical for scalable and sustainable hydrogen production from economical water-alkali splitting. Herein, density functional theory (DFT) calculations reveal that coupling NiO_x and CoP could effectively boost overall HER kinetics through lowing the H₂O dissociation barrier, accelerating the OH* transfer process, and providing the rapid H* migration kinetics as well as the appropriate H* energetics. Based on these findings, we successfully prepared a three-dimensional (3D) self-supported electrode of ultrathin CoP nanosheets directly grown on the surface-oxidized Ni nanotube arrays via a simple and scalable electrochemical synthesis method. As expected, such a heterostructure electrode exhibits superior alkaline HER performance with low overpotentials of 51 and 164 mV to drive the current densities of 10 and 500 mA cm^-2, respectively, outperforming most of the efficient alkaline HER electrocatalysts.

Fabrication of Ru/WO3-W2N/N-doped carbon sheets for hydrogen evolution reaction

Recent experimental analysis indicates WO₃-based nanostructures exhibit poor hydrogen evolution reactivity, particularly in alkaline medium, arising from the low electron transfer rate. It is imperative to tune the composition and structure of WO₃ to boost the cleavage of H-OH bond. Here, we construct Ru/WO₃-W₂N/N-doped carbon sheets (Ru/WO₃-W₂N/NC) using m-WO₃ nanosheets as precursors with the aid of RuCl₃, Tris (hydroxymethyl) aminomethane, and dopamine. Structural investigation reveals the formation of N-doped carbon sheets, Ru nanoparticles, and WO₃-W₂N. As a result, hydrogen evolution reactivity is greatly improved on Ru/WO₃-W₂N/N-doped carbon sheets with 64 mV at 10 mA/cm² in 1 mol/L (M) KOH, outperforming most of WO₃-based electrocatalysts in previous literatures. Meanwhile, it facilitates the generation of H₂ in 0.5 M H₂SO₄ with the excellent activity of 110 mV at 10 mA/cm². Our work provides an efficient strategy to tailor the electronic structure of WO₃ to catalyze acidic and alkaline hydrogen evolution reaction.

Interstitial Hydrogen Atom to Boost Intrinsic Catalytic Activity of Tungsten Oxide for Hydrogen Evolution Reaction

Tungsten oxide (WO₃ ) is an appealing electrocatalyst for the hydrogen evolution reaction (HER) owing to its cost-effectiveness and structural adjustability. However, the WO₃ electrocatalyst displays undesirable intrinsic activity for the HER, which originates from the strong hydrogen adsorption energy. Herein, for effective defect engineering, a hydrogen atom inserted into the interstitial lattice site of tungsten oxide (H_0.23 WO₃ ) is proposed to enhance the catalytic activity by adjusting the surface electronic structure and weakening the hydrogen adsorption energy. Experimentally, the H_0.23 WO₃ electrocatalyst is successfully prepared on reduced graphene oxide. It exhibits significantly improved electrocatalytic activity for HER, with a low overpotential of 33 mV to drive a current density of 10 mA cm^-2 and ultra-long catalytic stability at high-throughput hydrogen output (200 000 s, 90 mA cm^-2 ) in acidic media. Theoretically, density functional theory calculations indicate that strong interactions between interstitial hydrogen and lattice oxygen lower the electron density distributions of the d-orbitals of the active tungsten (W) centers to weaken the adsorption of hydrogen intermediates on W-sites, thereby sufficiently promoting fast desorption from the catalyst surface. This work enriches defect engineering to modulate the electron structure and provides a new pathway for the rational design of efficient catalysts for HER.

Tuning the Electronic Structure of a Novel 3D Architectured Co-N-C Aerogel to Enhance Oxygen Evolution Reaction Activity

Hydrogen generation through water electrolysis is an efficient technique for hydrogen production, but the expensive price and scarcity of noble metal electrocatalysts hinder its large-scale application. Herein, cobalt-anchored nitrogen-doped graphene aerogel electrocatalysts (Co-N-C) for oxygen evolution reaction (OER) are prepared by simple chemical reduction and vacuum freeze-drying. The Co (0.5 wt%)-N (1 wt%)-C aerogel electrocatalyst has an optimal overpotential (0.383 V at 10 mA/cm²), which is significantly superior to that of a series of M-N-C aerogel electrocatalysts prepared by a similar route (M = Mn, Fe, Ni, Pt, Au, etc.) and other Co-N-C electrocatalysts that have been reported. In addition, the Co-N-C aerogel electrocatalyst has a small Tafel slope (95 mV/dec), a large electrochemical surface area (9.52 cm²), and excellent stability. Notably, the overpotential of Co-N-C aerogel electrocatalyst at a current density of 20 mA/cm² is even superior to that of the commercial RuO₂. In addition, density functional theory (DFT) confirms that the metal activity trend is Co-N-C > Fe-N-C > Ni-N-C, which is consistent with the OER activity results. The resulting Co-N-C aerogels can be considered one of the most promising electrocatalysts for energy storage and energy saving due to their simple preparation route, abundant raw materials, and superior electrocatalytic performance.

Recent Advances in Manganese-Based Materials for Electrolytic Water Splitting

Developing earth-abundant and highly effective electrocatalysts for electrocatalytic water splitting is a prerequisite for the upcoming hydrogen energy society. Recently, manganese-based materials have been one of the most promising candidates to replace noble metal catalysts due to their natural abundance, low cost, adjustable electronic properties, and excellent chemical stability. Although some achievements have been made in the past decades, their performance is still far lower than that of Pt. Therefore, further research is needed to improve the performance of manganese-based catalytic materials. In this review, we summarize the research progress on the application of manganese-based materials as catalysts for electrolytic water splitting. We first introduce the mechanism of electrocatalytic water decomposition using a manganese-based electrocatalyst. We then thoroughly discuss the optimization strategy used to enhance the catalytic activity of manganese-based electrocatalysts, including doping and defect engineering, interface engineering, and phase engineering. Finally, we present several future design opportunities for highly efficient manganese-based electrocatalysts.

Ni electrodes with 3D-ordered surface structures for boosting bubble releasing toward high current density alkaline water splitting

The performance of alkaline water electrolysis (AWE) at high current densities is limited by gas bubble generation on the surface of electrodes, which covers active sites and blocks mass transfer, resulting in lower AWE efficiency. Here, we utilize electro-etching to construct Ni electrodes with hydrophilic and aerophobic surfaces to improve the efficiency of AWE. Ni atoms on the Ni surface can be exfoliated orderly along the crystal planes by electro-etching, forming micro-nano-scale rough surfaces with multiple crystal planes exposed. The 3D-ordered surface structures increase the exposure of active sites and promote the removal of bubbles on the surface of the electrode during the AWE process. In addition, experimental results from high-speed camera reveal that rapidly released bubbles can improve the local circulation of electrolyte. Lastly, the accelerated durability test based on practical working condition demonstrates that the 3D-ordered surface structures are robust and durable during the AWE process.

Ambient Electrosynthesis toward Single-Atom Sites for Electrocatalytic Green Hydrogen Cycling

With the ultimate atomic utilization, well-defined configuration of active sites and unique electronic properties, catalysts with single-atom sites (SASs) exhibit appealing performance for electrocatalytic green hydrogen generation from water splitting and further utilization via hydrogen-oxygen fuel cells, such that a vast majority of synthetic strategies toward SAS-based catalysts (SASCs) are exploited. In particular, room-temperature electrosynthesis under atmospheric pressure offers a novel, safe, and effective route to access SASs. Herein, the recent progress in ambient electrosynthesis toward SASs for electrocatalytic sustainable hydrogen generation and utilization, and future opportunities are discussed. A systematic summary is started on three kinds of ambient electrochemically synthetic routes for SASs, including electrochemical etching (ECE), direct electrodeposition (DED), and electrochemical leaching-redeposition (ELR), associated with advanced characterization techniques. Next, their electrocatalytic applications for hydrogen energy conversion including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, and oxygen reduction reaction are reviewed. Finally, a brief conclusion and remarks on future challenges regarding further development of ambient electrosynthesis of high-performance and cost-effective SASCs for many other electrocatalytic applications are presented.

Understanding the Copper-Iridium Nanocrystals as Highly Effective Bifunctional pH-universal Electrocatalysts for Water Splitting

It is pivotal to develop an economical, effective, and stable catalyst to promote the oxygen/hydrogen evolution reaction (OER/HER) throughout pH electrolytes, as the demand for hydrogen energy will increase greatly with the future development. Herein, a series of Ir-Cu nanoparticle composite carbon (Ir_xCu_y/C) catalysts are successfully synthesized using ethylene glycol reduction. In addition, the structure, morphology and composition of the electrocatalysts were systematically characterized, and the OER/HER performance of the catalysts was also tested under different pH conditions. According to experimental findings, amorphous Ir₃Cu/C has superior competent performance to catalyze oxygen (O₂) production in alkaline and acidic environments. The comparatively low overpotentials required are 222 mV and 304 mV, respectively, while generating a current density of 10 mA cm^-2. The reduced amount of precious metal and the further improvement in activity and durability make Ir₃Cu/C an excellent noble metal-based electrocatalyst. Meanwhile, IrCu/C has significant electrocatalytic performance for the HER in acidic media.

Single Cobalt Atoms Immobilized on Palladium-Based Nanosheets as 2D Single-Atom Alloy for Efficient Hydrogen Evolution Reaction

Single-atom alloys (SAAs) display excellent electrocatalytic performance by overcoming the scaling relationships in alloys. However, due to the lack of a unique structure engineering design, it is difficult to obtain SAAs with a high specific surface area to expose more active sites. Herein, single Co atoms are immobilized on Pd metallene (Pdm) support to obtain Co/Pdm through the design of the engineered morphology of Pd, realizing the preparation of ultra-thin 2D SAA. The unsaturated coordination environments combined with the unique geometric and electronic structures realize the modulation of the d-band center and the redistribution of charges, generating highly active electronic states on the surface of Co/Pdm. Benefiting from the synergistic interaction and spillover effect, the Co/Pdm electrocatalyst exhibits outstanding hydrogen evolution reaction (HER) performance in both acid and alkaline solutions, especially with a Tafel slope of 8.2 mV dec^-1 and a low overpotential of 24.7 mV at 10 mA cm^-2 in the acidic medium, which outperforms commercial Pt/C and Pd/C. This work highlights the successful preparation of 2D ultra-thin SAA, which provides a new strategy for the preparation of HER electrocatalyst with high efficiency, activity, and stability.

Lamella-heterostructured nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride electrodes as stable catalysts for oxygen evolution

Developing robust nonprecious-metal electrocatalysts with high activity towards sluggish oxygen-evolution reaction is paramount for large-scale hydrogen production via electrochemical water splitting. Here we report that self-supported laminate composite electrodes composed of alternating nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride (FeCo/CeO_2-xN_x) heterolamellas hold great promise as highly efficient electrocatalysts for alkaline oxygen-evolution reaction. By virtue of three-dimensional nanoporous architecture to offer abundant and accessible electroactive CoFeOOH/CeO_2-xN_x heterostructure interfaces through facilitating electron transfer and mass transport, nanoporous FeCo/CeO_2-xN_x composite electrodes exhibit superior oxygen-evolution electrocatalysis in 1 M KOH, with ultralow Tafel slope of ~33 mV dec^-1. At overpotential of as low as 360 mV, they reach >3900 mA cm^-2 and retain exceptional stability at ~1900 mA cm^-2 for >1000 h, outperforming commercial RuO₂ and some representative oxygen-evolution-reaction catalysts recently reported. These electrochemical properties make them attractive candidates as oxygen-evolution-reaction electrocatalysts in electrolysis of water for large-scale hydrogen generation.

A corpus of CO2 electrocatalytic reduction process extracted from the scientific literature

The electrocatalytic CO₂ reduction process has gained enormous attention for both environmental protection and chemicals production. Thereinto, the design of new electrocatalysts with high activity and selectivity can draw inspiration from the abundant scientific literature. An annotated and verified corpus made from massive literature can assist the development of natural language processing (NLP) models, which can offer insight to help guide the understanding of these underlying mechanisms. To facilitate data mining in this direction, we present a benchmark corpus of 6,086 records manually extracted from 835 electrocatalytic publications, along with an extended corpus with 145,179 records in this article. In this corpus, nine types of knowledge such as material, regulation method, product, faradaic efficiency, cell setup, electrolyte, synthesis method, current density, and voltage are provided by either annotating or extracting. Machine learning algorithms can be applied to the corpus to help scientists find new and effective electrocatalysts. Furthermore, researchers familiar with NLP can use this corpus to design domain-specific named entity recognition (NER) models.

Defects in Carbon-Based Materials for Electrocatalysis: Synthesis, Recognition, and Advances

ConspectusOwing to climate change and over-reliance on fossil fuels, the study and development of sustainable energy is of essential importance in the next few decades. In recent years, rapid advances have been witnessed in various power to gas electrocatalysis technologies including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) for realizing the target of blue planet with carbon neutrality. Nevertheless, practical applications with superior performance and affordable cost are largely limited by the electrode materials because the reactions are regularly driven by precious metals such as platinum (Pt) or iridium (Ir) based catalysts. Therefore, it is of significance to develop novel electrocatalysts with high electroactivity and limited cost for boosting the commercialization of green hydrogen technology.Since nitrogen-doped carbon nanotubes were first reported for enhanced ORR performance in 2009, the exploitation of carbon-based metal-free catalysts (CMFCs) as potential replacements for the precious metal electrocatalysts has become an attractive research field. To date, great progress has been made in developing new dopant strategies for CMFCs; however, the details of the catalytic mechanism and identification of active sites remain unclear, owing to the complexity in controlling the dopants and their homogeneity in carbon-based materials. To tackle this issue, our group has presented a series of works on defects catalyzing electrochemical reactions and proposed a defect catalysis mechanism since 2015. This theory is now widely accepted by the research community and has become a very important area in electrocatalysis worldwide.In this Account, we first present the defect theory for the reasonable design of defective carbon-based materials (DCMs) and subsequently summarize our previous works on the state-of-the-art defect engineering strategies to design DCMs possessing high activity, with the particular emphasis on the conjunction between defect structures and electrochemical performances. We also categorize recent defect modulation approaches on active sites in DCMs as well as showcase the advanced characterization techniques to confirm the types and densities of defects in DCMs. Finally, several perspectives on the challenges and future research opportunities of this exciting field are proposed. Remarkably, rapid advances of DCMs possessing both high electrochemical activities and low cost as a new generation of electrode materials may greatly facilitate the deployment of sustainable energy infrastructures.

Design Strategy of Corrosion-Resistant Electrodes for Seawater Electrolysis

Electrocatalytic water splitting for hydrogen (H₂) production has attracted more and more attention in the context of energy shortages. The use of scarce pure water resources, such as electrolyte, not only increases the cost but also makes application difficult on a large scale. Compared to pure water electrolysis, seawater electrolysis is more competitive in terms of both resource acquisition and economic benefits; however, the complex ionic environment in seawater also brings great challenges to seawater electrolysis technology. Specifically, chloride oxidation-related corrosion and the deposition of insoluble solids on the surface of electrodes during seawater electrolysis make a significant difference to electrocatalytic performance. In response to this issue, design strategies have been proposed to improve the stability of electrodes. Herein, basic principles of seawater electrolysis are first discussed. Then, the design strategy for corrosion-resistant electrodes for seawater electrolysis is recommended. Finally, a development direction for seawater electrolysis in the industrialization process is proposed.

Passivating Oxygen Evolution Activity of NiFe-LDH through Heterostructure Engineering to Realize High-Efficiency Electrocatalytic Formate and Hydrogen Co-Production

An electrocatalytic methanol oxidation reaction (MOR) is proposed to replace oxygen evolution reaction (OER) in water electrolysis owing to the favorable thermodynamics of MOR than OER. However, there is still a competition between the MOR and the OER when the applied potential is in the conventional OER zone. How to inhibit OER while maintaining efficient MOR is an open and challenging question, and there are few reports focusing on this thus far. Herein, by taking NiFe layered double hydroxide (LDH) as a model catalyst due to its intrinsically high catalytic activity for the OER, the perspective of inhibiting OER is shown and thus promoting MOR through a heterogenous engineering of NiFe-LDH. The engineered heterostructure comprising NiFe-LDH and in situ formed NiFe-hexylaminobenzene (NiFe-HAB) coordination polymer exhibits outstanding electrocatalytic capability for methanol oxidation to formic acid (e.g., the Faradaic efficiencies (FEs) of formate product are close to 100% at various current densities, all of which are much larger than those (53-65%) on unmodified NiFe-LDH). Mechanism studies unlock the modification of NiFe-HAB passivates the OER activity of NiFe-LDH through tailoring the free energies for element reaction steps of the OER and increasing the free energy of the rate-determining step, consequently leading to efficient MOR.

Enhancing the oxygen evolution reaction of cobalt hydroxide by fabricating nanocomposites with fluorine-doped graphene oxide

Fluorine and nitrogen codoped cobalt hydroxide-graphene oxide nanocomposites (N,F-Co(OH)₂/GO) were synthesized by a simple hydrothermal method and demonstrated highly enhanced oxygen evolution activity in an alkaline medium. N,F-Co(OH)₂/GO synthesized under optimized reaction conditions required an overpotential of 228 mV to produce the benchmark current density of 10 mA cm^-2 (scan rate 1 mV s^-1). In contrast, N,F-Co(OH)₂ without GO and Co(OH)₂/GO without fluorine required higher overpotentials (370 (N,F-Co(OH)₂) and 325 mV (Co(OH)₂/GO)) for producing the current density of 10 mA cm^-2. The low Tafel slope (52.6 mV dec^-1) and charge transfer resistance, and high electrochemical double layer capacitance of N,F-Co(OH)₂/GO compared to N,F-Co(OH)₂ indicate faster kinetics at the electrode-catalyst interface. The N,F-Co(OH)₂/GO catalyst showed good stability over 30 h. High-resolution transmission electron microscope (HR-TEM) images showed good dispersion of polycrystalline Co(OH)₂ nanoparticles in the GO matrix. X-ray photoelectron spectroscopic (XPS) analysis revealed the coexistence of Co²⁺/Co³⁺ and the doping of nitrogen and fluorine in N,F-Co(OH)₂/GO. XPS further revealed the presence of F in its ionic state and being covalently attached to GO. The integration of highly electronegative F with GO stabilizes the Co²⁺ active centre along with improving the charge transfer and adsorption process that contributes to improved OER. Thus, the present work reports a facile method for preparing F-doped GO-Co(OH)₂ electrocatalysts with enhanced OER activity under alkaline conditions.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

F and rare V4+ doped cobalt hydroxide hybrid nanostructures: excellent OER activity with ultralow overpotential

Highly efficient and stable Earth abundant transition metal electrocatalysts are in great demand for the oxygen evolution reaction (OER), a bottleneck process involved in the water splitting reaction and metal-air batteries. Herein, we have demonstrated a single step direct fabrication of cobalt hydroxide (Co(OH)₂) nanowires doped with vanadium(V) in a less stable +4 oxidation state and fluoride (F) ions (V-Co(OH)₂) on a carbon cloth electrode that showed highly enhanced OER activity under alkaline conditions. V-Co(OH)₂ nanowires synthesized under the optimized conditions produced excellent OER activity with an ultralow overpotential of 136 mV at 10 mA cm^-2 (scan rate 1 mV s^-1), a small Tafel slope (51.6 mV dec^-1) and good stability over 72 h. To the best of our knowledge, this is the lowest overpotential reported for cobalt-based electrocatalysts to achieve a geometric current density of 10 mA cm^-2. The controlled synthesis and HR-TEM studies revealed the formation of hybrid nanostructures (nanowires along with spherical assembly of nanoparticles) and codoping of V and F ions played an important role in enhancing the OER activity. The detailed chemical composition and oxidation state analysis by X-ray photoelectron spectroscopy (XPS) confirmed the doping of V⁴⁺ and ionic F in V-Co(OH)₂ with mixed valence states of Co²⁺/Co³⁺ and a higher Co²⁺ ratio. The outstanding OER activity of V-Co(OH)₂ is attributed to the formation of a spherical assembly of nanoparticles with nanowires, which provided a high number of catalytically active sites with enhanced charge transport, and doping of higher valence V⁴⁺ and strongly electronegative F in V-Co(OH)₂ with a higher ratio of Co²⁺/Co³⁺ promoted OOH* intermediate generation and significantly boosted the OER activity. Overall, the present work highlights the possibility of achieving highly active Earth abundant OER electrocatalysts by controlling the mixed oxidation state of Co with a judicious choice of dopants along with maintaining optimal nanostructure morphologies.

Research on engineered electrocatalysts for efficient water splitting: a comprehensive review

Water electrolysis plays an interesting role toward hydrogen generation for overcoming global environmental crisis and solving the energy storage problem. However, there is still a deficiency of efficient electrocatalysts to overcome sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Great efforts have been employed to produce potential catalysts with low overpotential, rapid kinetics, and excellent stability for HER and OER. At present, hydrogen economy is driven by electrocatalysts with excellent characteristics; thus, systematic design strategy has become the driving force to exploit earth-abundant transition metal-based electrocatalysts toward H₂ economy. In this review, the recent progress on newer materials including metals, alloys, and transition metal oxides (manganese oxides, cobalt oxides, nickel oxides, PBA-derived metal oxides, and metal complexes) as photocatalysts/electrocatalysts has been overviewed together with some methodologies for efficient water splitting. Metal-organic framework (MOF)-based electrocatalysts have been highly exploited owing to their interesting functionalities. The photovoltaic-electrocatalytic (PV-EC) process focused on harvesting high solar-to-hydrogen efficiency (STH) among various solar energy conversion as well as storage systems. Electrocatalysts/photocatalysts with high efficiency have become an urgent need for overall water splitting. Also, cutting-edge achievements in the fabrication of electrocatalysts along with theoretical consideration have been discussed.

Constructing abundant phase interfaces of the sulfides/metal-organic frameworks p-p heterojunction array for efficient overall water splitting and urea electrolysis

Designing efficient electrocatalysts to improve the overall water splitting and urea electrolysis efficiency for hydrogen generation can greatly solve the dilemma of energy shortage and environmental pollution. In this work, Co₈FeS₈@CoFe-MOF/NF heterojunction arrays were fabricated by embedding sulfides into the surface of metal-organic frameworks (MOFs) nanosheets as multifunctional electrocatalyst. The introduction of sulfide on CoFe-MOF/NF can not only adjust the electronic structure (electron-rich state) and change the surface properties (more hydrophilic), but also increase the active area to enhance the catalytic activity. The in situ Raman shows Co₈FeS₈@CoFe-MOF/NF is more easily to generate active species at low potentials and generates a higher content of active β-MOOH phase than CoFe-MOF/NF. Therefore, the Co₈FeS₈@CoFe-MOF/NF exhibits excellent oxygen evolution reaction (OER) performance with an overpotential of 213 mV at 10 mA cm^-2. Furthermore, when used as a urea oxidation reaction (UOR), only an ultralow potential of 1.311 V at 10 mA cm^-2. More importantly, the assembled two-electrode drives overall water splitting and urea electrolysis with cell voltages of 1.62 V and 1.55 V at 10 mA cm^-2, respectively. This work provides insights into the preparation of electrocatalysts with multifunctional heterostructure arrays for hydrogen production.