Oxygen Evolution Reaction Process Research Articles

High-Valence Cu Induced by Photoelectric Reconstruction for Dynamically Stable Oxygen Evolution Sites.

Oxygen vacancies are generally considered to play a crucial role in the oxygen evolution reaction (OER). However, the generation of active sites created by oxygen vacancies is inevitably restricted by their condensation and elimination reactions. To overcome this limitation, here, we demonstrate a novel photoelectric reconstruction strategy to incorporate atomically dispersed Cu into ultrathin (about 2-3 molecular) amorphous oxyhydroxide (a-CuM, M = Co, Ni, Fe, or Zn), facilitating deprotonation of the reconstructed oxyhydroxide to generate high-valence Cu. The in situ XAFS results and first-principles calculations reveal that Cu atoms are stabilized at high valence during the OER process due to Jahn-Teller distortion, resulting in para-type double oxygen vacancies as dynamically stable catalytic sites. The optimal a-CuCo catalyst exhibits a record-high mass activity of 3404.7 A g-1 at an overpotential of 300 mV, superior to the benchmarking hydroxide and oxide catalysts. The developed photoelectric reconstruction strategy opens up a new pathway to construct in situ stable oxygen vacancies by high-valence Cu single sites, which extends the design rules for creating dynamically stable active sites.

Raney nickel induced interface modulation of active NiFe-hydroxide as efficient and robust electrocatalyst towards oxygen evolution reaction

Preparation of active and durable low-cost catalysts towards oxygen evolution reaction (OER) is essential for alkaline water splitting. Herein, porous Raney-Ni supported NiFe-LDH (NiFe-LDH/Raney-Ni) electrocatalyst was developed to achieve efficient and robust OER performance. Raney-Ni modulates the NiFe-LDH electronic structure by interfacial interaction. In-situ infrared shows the enhanced OH adsorption behavior, and in-situ Raman suggests that Raney-Ni promotes the formation of active NiFeOOH species on NiFe-LDH/Raney-Ni. Theoretical calculations found that the Fe atoms in NiFe-LDH/Raney-Ni enhances the charge transfer to the NiOOH species, which facilitates the formation of O* intermediates accelerating the OER process. The NiFe-LDH/Raney-Ni catalyst exhibits a low OER overpotential of 219 mV at 10 mA cm−2 in 1 M KOH with excellent stability for 200 h at 200 mA cm−2. Excellent performance is also achieved in alkaline electrolyzer test. This work presents a facile approach for the design of affordable and efficient alkaline OER electrocatalysts.

Exploring Structural Evolution Behaviors of Ligand‐Defect‐Rich Ferrocene‐Based Metal‐Organic Frameworks for Electrochemical Oxygen Evolution via Operando X‐Ray Absorption Spectroscopy

AbstractMetal‐organic frameworks (MOFs) have exhibited encouraging catalytic activity for the oxygen evolution reaction (OER), a crucial process for water electrolysis to produce green hydrogen. Nonetheless, distinguishing the source of catalytic activity and establishing the structure‐composition‐property relationships of MOFs during OER processes remain challenging. Here, for the first time, operando X‐ray absorption spectroscopy (XAS) is utilized to monitor the structural evolution and identify the active components of ferrocene‐based MOFs (Ni‐Fc) for OER. Ligand‐defect‐rich Ni‐Fc is synthesized via the co‐deposition method. After electrochemical activation, Ni‐Fc exhibits superior electrocatalytic activity (228 mV at 10 mA cm−2 in 0.1 m KOH), which is highly competitive compared with state‐of‐the‐art electrocatalysts. Operando XAS analysis and ex‐situ characterizations reveal the structural reconstruction of Ni‐Fc into amorphous NiFe‐catalysts (a‐NiFe) during the activation process, and further into real catalytic phases (a‐NiFe‐C) under catalytic potential greater than 1.45 V (vs RHE). In catalytic phases, in‐situ formed deprotonated and oxygen‐defected Ni oxyhydroxide analogues act as catalytic sites, while Fe hydroxide analogues derived from ligands optimize the electronic structure of Ni sites for improving OER activity. Density functional theory (DFT) analysis indicates a reduced energy barrier in a‐NiFe‐C compared to pristine MOFs, supporting the improved catalytic activity of the latter.

Fe and Cu co-encapsulated by nitrogen-doped carbon for zinc-air battery applications

Zinc-air batteries (ZABs) have attracted attention for their environmental friendliness, durability, safety and high energy density. The superior performance of rechargeable ZABs hinges on the activity of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here, we successfully develop a Fe, Cu co-doped ZIF-derived N-doped carbon (FeCux-NC) catalyst, featuring a lamellar “red blood cell” structure. In the ORR process, the optimized FeCux-NC catalyst achieves a half-wave potential of 0.925 V under alkaline conditions, 70 mV higher than commercial Pt/C catalysts, and exhibits good stability. In the OER process, at 10 mA cm−2, the overpotential is measured at 510 mV and the Tafel slope value is 55.2 mV dec−1. When utilizing optimized FeCux-NC catalyst without RuO2 at a catalyst loading of 2 mg cm−1, the assembled ZABs demonstrate a power density of 85 mW cm−2 at 140 mA cm−2, while maintaining excellent cycling stability (with only a 0.11 V increase in the voltage interval between charging and discharging over 300 cycles), thereby realizing the dual function of ORR/OER.

Nitrogen-metal-oxygen moieties enriched surface reconstruction in CoFe electrocatalysts for stabilizing high-valence Co sites enable water oxidation

Despite the potential of heterostructure construction and surface reconstruction strategies to significantly enhance the oxygen evolution reaction (OER) performance of transition metal compounds, integrating these approaches into a single material system remains a significant challenge. In this work, we report an efficient CoFe-LDH catalyst with multi-metal nitrides on the surface through simple and tunable N2-plasma treatment (PL CoFe-LDH). Structural characterization and theoretical calculations indicate a high concentration of defective sites in PL CoFe-LDH, which facilitates the exposure of a greater number of active NFeO and NCoO centers. During the initial stage of the OER process, there is a rapid phase transition of Fe and Co, leading to the formation of metal (oxy)hydroxides. Meanwhile, Co3+ species gradually accumulate on the surfaces, which is responsible for the excellent OER catalytic activity. Then, the neighboring N atoms function as electron reservoirs, donating electrons to the Co sites to prevent overoxidation and dissolution of Co in the electrolyte, thus significantly enhancing the stability of the OER. The catalysts enable the achievement of ultralow OER overpotential, measuring 161.1 and 205.5 mV at current densities of 10 mA cm−2 and 200 mA cm−2, respectively, while exhibiting exceptional stability.

One-step electrodeposition of bifunctional MnCoPi electrocatalysts with wrinkled globular-flowers-like structure for highly efficient electrocatalytic water splitting

The development and exploration of electrocatalysts with the high reactive and abundant availability is still extremely crucial in electrocatalytic overall water splitting. Herein, a novel globular-flowers-like MnO2/Co3(PO4)2 (denoted as MnCoPi) electrocatalyst on nickel foam was successfully prepared through a simple one-step electrodeposition method. The MnCoPi electrocatalyst simultaneously delivers remarkable electrocatalytic activities for hydrogen evolution reaction (HER) with overpotentials (η10) reaching 102 mV and oxygen evolution reaction (OER) with overpotentials (η10) up to 225 mV in 1 M KOH solution. Furthermore, the assembled electrolyzer cell utilizing MnCoPi as the cathode and anode only requires a low voltage of 1.55 V to achieve a current density of 10 mA cm−2. Moreover, the developed MnCoPi electrocatalyst shows excellent stability during continuous operation for 48 h in OER, HER and overall water splitting process. Compared with the pristine MnO2, the significant electrocatalytic properties of MnCoPi can mainly be attributed to the improved physicochemical properties such as distinctive globular-flowers-like morphology, huge specific surface areas and abundant porosity structure, low electrochemical resistance, especially for the formed heterojunction between MnO2 and Co3(PO4)2, which can provide abundant reactive sites and accelerated electron transfer, etc. Consequently, this work provides a new avenue for the development of efficient and stable bifunctional electrocatalysts for overall water splitting.

Highly efficient electrocatalytic water oxidation based on non-precious metal oxides/sulfides derived from a FeCoNi-metal organic framework

One of the main challenges for oxygen evolution reaction (OER) is to develop electrocatalysts with high performance, suitable stability, and low cost. First-row transition metal compounds have received attention due to their high electrocatalytic activity and diverse electronic structure. Meanwhile, metal-organic frameworks (MOFs) have emerged as promising alternative for OER due to their porous structure and unsaturated metal sites. In this study, FeCoNi-MOF based on MIL-88B was synthesized by a hydrothermal method. Then, the performance of the catalyst was investigated and improved by calcination and sulfidation at three temperatures of 250 ℃, 350 ℃, and 450 ℃. The phase development and crystal structure parameters of the synthesized samples were confirmed through X-ray diffraction. Additionally, the intensity of the peaks increased with the rise in calcination temperature. Field emission scanning electron microscopy revealed well-defined and uniformly distributed nanorods and nanospheres particles of calcinated and sulfides samples. The weight percentage (wt%) of the elements in the samples were verified to align with their stoichiometric ratios. The synthesized catalysts were loaded onto a nickel foam by a one-step method, and their electrocatalytic properties were examined in the OER process in 1.0 M KOH alkaline solution. The (FeCoNi)-O250 ℃ electrocatalyst, maintaining its nanorod morphology, exhibited an overpotential of 313 mV at a current density of 10 mA.cm-² and a small, stable Tafel slope of 31.5 mV.dec−1 and good charge transfer of 0.73 mF. Cm−2. However, the (FeCoNi)-S250 ℃ electrocatalyst showed better performance in the OER process with an overpotential of 298 mV at a current density of 10 mA.cm-² and a small and stable Tafel slope of 29 mV.dec−1 and good charge transfer of 0.82 mF. Cm−2. In other words, a higher Cdl increased reaction rates or lower overpotentials required for the reaction to occur. Furthermore, the activity of this catalyst remained constant at a fixed current density of 10 mA.cm-² for 18 hours. Based on the electron microscopy observations, and electrochemical evaluations, this superior performance can be attributed to the morphological change from nanorods to porous nanospheres due to sulfidation, resulting in increased electrochemically active sites and decreased charge transfer resistance.

The interfacial synergy of hierarchical FeCoNiP@FeNi-LDH heterojunction for efficient alkaline water splitting

In response to the growing demand for clean, green, and sustainable energy sources, the development of cost-effective and durable high-activity overall water splitting electrocatalysts is urgently needed. In this study, the heterogeneous structure formed by the combination of FeCoNiP and FeNi-LDH was homogeneously dispersed onto CuO nanowires generated by in-situ oxidation of copper foam as a substrate using an electrodeposition method. This multilevel structure exhibits excellent bifunctional properties as an electrode material in alkaline solutions, for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) only 206 mV and 147 mV overpotentials are needed to achieve a current density of 100 mA cm−2 respectively. Full water electrolysis is thus enabled to take place at such a low cell voltage as 1.64 V to reach the current density of 100 mA cm−2, which exhibits a long-term stability of 30 h. These improved electrocatalytic performances stem from the construction of multilevel structures. The X-ray photoelectron spectroscopy suggests that strong electron transfer occurs between heterogeneous structures, thus facilitating the OER and HER process. The dispersion of CuO nanowires not only increases the electrochemically active surface areas but also improves the overall hydrophilic and aerophobic properties. This work highlights the positive effect of multilevel structure in the design of more efficient electrocatalysts and provides a reference for the preparation of other low-cost, high-activity bifunctional electrocatalysts.

Manganese-decorated CoP@CoFe2O4 nanorod arrays for high-efficiency alkaline water oxidation

Regulating the intrinsic activity of electrocatalysts is an effective strategy towards outstanding behaviour for oxygen evolution reaction (OER). Herein, the rational design of a Mn–CoP@CoFe2O4 core-shell heterojunction using nickel foam (NF) as a supporting material with special interface engineering has been investigated by a facile hydrothermal technique combined with phosphorization and electrodeposition method. The physical characterization demonstrates Mn–CoP@CoFe2O4 core-shell heterostructure with Mn–CoP nanoneedles as core and CoFe2O4 nanosheets as shell has been successfully acquired. The electrochemical measurements of the as-prepared Mn–CoP@CoFe2O4/NF core-shell heterojunction nanoarray electrode exhibit the superior electrocatalytic behaviour for OER (Tafel slope of 35.6 mV dec−1 and overpotential of 209.6 mV@10 mA cm−2). The remarkable OER behaviour may be ascribed to the sufficient active sites and the enhanced intrinsic activity due to the introduction of CoFe2O4 and Mn element, which can regulate the electron structure and increase the turnover frequency. Furthermore, the Mn–CoP@CoFe2O4/NF electrode appears an excellent long-run stability during the OER process at 100 mA cm−2 for 100 h. Therefore, the construction of metal doping transition metal-based core-shell heterostructure electrocatalysts may be a promising way for the remarkable OER electrocatalytic performance towards water splitting.

Facile top-down fabrication of integrated amorphous NiFe-based electrocatalytic electrodes for high current and long-life oxygen evolution

Developing an industrially relevant electrode with high catalytic activity, stability, and tunable composition/size for large-scale water electrolysis is a significant challenge. We have created an integrated electrode (NFM30-N) for the oxygen evolution reaction (OER) using a facile top-down approach that combines arc melting with dealloying-oxidation. Due to the dealloying-oxidation effect, the as-derived porous amorphous M–O, M–OH, and M–OOH (M = Ni, Fe) nanocones cover the basic NiFeMn alloy. This integrated design enables NFM30-N to exhibit outstanding OER performance at high current densities, requiring low overpotentials of only 282 and 323 mV to achieve large current densities of 100 and 500 mA cm−2, respectively. It also displays a small Tafel slope of 44.1 mV dec−1 and remarkable stability for over 100 h at 100 and 500 mA cm−2. When used as an anode, a two-electrode electrolyzer cell with NFM30-N at 500 mA cm−2 only requires a cell voltage of 1.619 V and exhibits excellent stability, with almost no performance degradation after continuous chronopotentiometry test for each 100 h at 500 and 100 mA cm−2. This exceptional OER electrocatalytic performance is attributed to the integrated structure providing high electrical conductivity and stability, the presence of numerous active sites due to dealloying and the amorphous structure, and the promotion of the OER process by M–O, M–OH, and M–OOH species. This work offers a novel idea for fabricating integrated, industrially relevant electrocatalytic electrodes through traditional metallurgy combined with dealloying-oxidation.

Ultrathin Ni-Fe MOF Nanosheets: Efficient and Durable Water Oxidation at High Current Densities.

Efficient, durable, and economical electrocatalysts are crucial for advancing energy technology by facilitating the oxygen evolution reaction (OER). Here, ultrathin Ni-Fe metal-organic skeleton (MOF) nanosheets were created in situ on nickel foam (NiFe-UMNs/NF). The catalyst exhibited excellent OER catalytical abilities, with only 269 mV overpotentials at 250 mA cm-2. Besides, when integrated with Pt/C/NF, NiFe-UMNs/NF held the potential for application in industrial alkaline water electrolysis with an initial voltage retention of approximately 86% following a continuous operation of 100 h at a current density of 250 mA cm-2. The super performance of the NiFe-UMNs/NF catalyst was attributed to ultrathin morphology, super hydrophilicity, and synergistic effects between Ni and Fe within the MOF. In situ Raman showed that NiFe-UMNs were converted to NiFeOOH as the active species in the OER process. Density functional theory revealed that iron doping accelerated the rate-determining step and reduced the OER reaction energy barrier. This work elucidated a promising electrocatalyst for OER and enriched the practical implementation of MOF materials.

A Universal Coulombic Efficiency Compensation Strategy for Zinc-Based Flow Batteries.

Alkaline zinc-iron flow batteries (AZIFBs) are well suited for energy storage because of their good safety, high cell voltage, and low cost. However, the occurrence of irreversible anodic parasitic reactions results in a diminished coulombic efficiency (CE), unbalanced charge state of catholyte/anolyte and subsequently, a poor cycling performance. Here, a universal CE compensation strategy centered around the oxygen evolution reaction (OER) on the cathodic side, is reported. This strategy aims to equalize the charge state of the [Fe(CN)6]3-/4--based catholyte and counteract pH fluctuations. The OER process can be implemented either directly on the electrode through electrochemical reaction or in an external catalytic reactor column via a redox-mediated process. This innovative approach effectively mitigates the gradual accumulation of [Fe(CN)6]3- in discharged catholyte and [Zn(OH)4]2- in charged anolyte by consuming the extra OH- during a continuous cycling process. As a result, AZIFBs demonstrate exceptional cycling performance with an extremely low capacity fading rate of 0.0128%/day (or 0.0005%/cycle) over 600 cycles at 80% state of charge (SOC). The proposed CE compensation strategy not only provides an effective way to address the CE loss issue for AZIFBs, but also can be applied to diverse battery technologies encountering CE loss caused by water/oxygen-induced parasitic reactions.

High Entropy Spinel Oxide (AlCrCoNiFe2)O as Highly Active Oxygen Evolution Reaction Catalysts.

The advancement of water electrolyzer technologies and the production of sustainable hydrogen fuel heavily rely on the development of efficient and cost-effective electrocatalysts for the oxygen evolution reaction (OER). High entropy ceramics, characterized by their unique properties, such as lattice distortion and high configurational entropy, hold significant promise for catalytic applications. In this study, we utilized the sol-gel autocombustion method to synthesize high entropy ceramics containing a combination of 3d transition metals and aluminum ((AlCrCoNiFe2)O). We then compared their electrocatalytic performance with other series of synthesized multimetal and monometallic oxides for the OER under alkaline conditions. Our electrochemical analysis revealed that the high entropy ceramics exhibited excellent performance and the lowest charge transfer resistance, Tafel slope (29 mV·dec-1), and overpotential (η10 = 230 mV). These remarkable results can be primarily attributed to the high entropy effect induced by the addition of Al, Cr, Co, Ni, and Fe, which introduces increased disorder and complexity into the material's structure. This, in turn, facilitates more efficient OER catalysis by providing diverse active sites and promoting optimal electronic configurations for the reaction. Furthermore, the strong electronic interactions among the constituent elements in the metallic spinels further enhance their catalytic activity in the initiation of the OER process. Combined with the reduced charge transfer resistance, these factors collectively play pivotal roles in enhancing the OER performance of the electrocatalysts. Overall, our study provides valuable insights into the design and development of high-performance electrocatalysts for sustainable energy applications. By harnessing the high entropy effect and leveraging strong electronic interactions, electrocatalytic materials can be tailored to improve efficiency and stability, thus advancing the progress of clean energy technologies.

Charge engineering in black phosphorene with tunable electronic structures as efficient oxygen evolution electrocatalyst

The unique electronic structure of layered black phosphorus (BP) makes it an ideal candidate material for electrocatalytic oxygen evolution reaction (OER). Charge doping effectively improves the environmental stability and catalytic activity of BP by providing electron transfer channels and reducing charge transfer barrier. Therefore, based on the first principles calculation, this paper theoretically discusses how the intrinsic charge doping without introducing impurities changes the electronic structure and improves the catalytic activity of BP. It is found that charge engineering can effectively regulate and change the electronic structure and work function by stimulating the hybridization between different P-p orbitals, while maintaining the direct bandgap semiconductor characteristics of monolayer BP system. More importantly, in the catalytic process of OER, electrons and hole doping as free charges provide different donor and acceptor energy levels for the system depending on the doping concentration, and affect the adsorption capacity of monolayer BP to different reaction intermediates. At a certain doping concentration, the carrier mobility increases significantly, and the optimal Gibbs free energy and overpotential can be achieved in monolayer BP. These results provide new opportunities and possibilities for designing charge-engineered BP catalysts with adjustable electronic structure and excellent OER activity.

Polyaniline layered N-doped carbon-coated iron oxide nanocapsules for extremely active Li-ion battery anode and oxygen evolution reaction

We report the effective synthesis of polyaniline (PANi)-layered nitrogen-doped carbon-coated Fe3O4 (FNC@PANi) nanocapsules (NCs) for electrocatalysis of oxygen evolution reaction (OER) as well as anode material for lithium (Li)-ion batteries (LIBs) via a simple hydrothermal method and oxidative polymerization technique. The prepared FNC@PANi NCs revealed a dual core-shell structure, in which an intermediate carbon layer allowed excellent electrical conduction between the Fe3O4 NCs and PANi. The dual core-shell structure also allowed the Fe3O4 NCs to expand freely during the Li-ion insertion/extraction and OER processes without breaking the outer layer, thus providing a high surface area (147.85 m2 g−1) and enhanced electrical conductivity. These properties facilitate the application of the dual core-shell FNC@PANi NCs as an advanced electrode material for LIBs, delivering a high reversible specific capacity of 1556.48 mAh g−1 at 0.1 A g−1, excellent rate performance (896.96 mAh g−1 at 1 A g−1), and durable cycling life (680.12 mAh g−1 at 5 A g−1 for 3000 cycles). The dual core-shell FNC@PANi NCs exhibited high electrocatalytic activity in the OER, with a small Tafel slope of 108.7 mV dec−1 owing to synergistic effects between the copious active sites of the Fe3O4 NCs and the carbon core-shell structure and a modest overpotential of 219 mV at 10 mA cm−2. The electrodes showed excellent stability over 10 h, as determined by chronopotentiometry at 10 mA cm−1. The resultant dual-core-shell FNC@PANi NCs are efficient iron-oxide-based electrode materials for LIBs and OER electrocatalysts.

Unleashing unprecedented activation of high-valent Ni and Fe charge dynamics in CeF3-NiFe layered double hydroxide heterostructure: Demonstrating oxygen evolution reaction at an extremely high current density

The efficacy of any electrochemical reaction hinges on the extent of interaction achievable between reactive intermediates and the electrocatalytic active site. Any weak adsorption of these intermediates on the metal’s active site results in low oxygen evolution reaction (OER) rates, mainly when catalysed by the Ni-based layered double hydroxide. To tackle this challenge, a heterojunction consisting of nickel-iron layered double hydroxide (NiFe-LDH) and cerium trifluoride (CeF3) is synthesized. Both phases were developed in-situ to have an abundance of heterointerfaces. The charge transfer amid the NiFe-LDH and CeF3 phases is brought about via these heterointerfaces. As a result, the overall charge dynamics associated with nickel (Ni) and iron (Fe) atoms are somewhat increased, and an enhanced positive charge on the metal site makes it more active in grabbing the reactive species, thereby making the entire OER process faster. The CeF3-NiFeLDH catalyst reaches a current density of 1000 mA cm−2 at an overpotential of 340 mV. Such a high current density is highly significant for the industrial-scale production of the products. The catalyst demonstrated impressive durability, maintaining stable performance for 90 h while operating at 500 mA cm−2. The charge dynamics between both phases were thoroughly examined using X-ray photoelectron spectroscopy (XPS).

Self-protecting CoFeAl-layered double hydroxides enable stable and efficient brine oxidation at 2 A cm−2

Low-energy consumption seawater electrolysis at high current density is an effective way for hydrogen production, however the continuous feeding of seawater may result in the accumulation of Cl−, leading to severe anode poisoning and corrosion, thereby compromising the activity and stability. Herein, CoFeAl layered double hydroxide anodes with excellent oxygen evolution reaction activity are synthesized and delivered stable catalytic performance for 350 hours at 2 A cm−2 in the presence of 6-fold concentrated seawater. Comprehensive analysis reveals that the Al3+ ions in electrode are etched off by OH− during oxygen evolution reaction process, resulting in M3+ vacancies that boost oxygen evolution reaction activity. Additionally, the self-originated Al(OH)n− is found to adsorb on the anode surface to improve stability. An electrode assembly based on a micropore membrane and CoFeAl layered double hydroxide electrodes operates continuously for 500 hours at 1 A cm−2, demonstrating their feasibility in brine electrolysis.

Metal vacancies and self-reconstruction of high entropy metal borates to boost the oxygen evolution reaction

Exploring highly efficient oxygen evolution reaction (OER) electrocatalysts is important for industrial water electrolysis. Herein, a new high entropy material was reported, i.e., an amorphous high entropy metal borate (CrMnCoNiFe)0.2BOx was synthesized by a new simple solvothermal method. The key of this new synthesis method was to obtain (CrMnCoNiFe)0.2BOx with a high specific surface area (446 m2/g) and porous structure, being expected to promote OER reaction. (CrMnCoNiFe)0.2BOx provided a low overpotential of 236 mV at 10 mA cm−2, a reduced Tafel slope of 64 mV dec–1, and good stability. The outstanding OER performance of (CrMnCoNiFe)0.2BOx was attributed to the high entropy structure, generation of metal vacancies, and self-reconstruction during OER. The Cr vacancies generated during the OER process have been demonstrated by Cr 2P XPS and EPR experiments. Various in situ and Ex-situ analyses such as In situ Raman, XPS, and TEM were used to investigate the self-reconstruction of the (CrMnCoNiFe)0.2BOx during the OER. The resulting metal (oxy)hydroxide was believed to be the true active center for OER. The DFT calculation showed the high entropy structure can reduce the adsorption and conversion energy barriers of oxygen-containing intermediates, matching well with the catalytic performance results. This study provided a new simple strategy to construct high entropy metal borate (CrMnCoNiFe)0.2BOx and would inspire the design of high-performance OER catalysts.

Designing triple-layer photocatalytic systems based on direct Z-scheme photocatalytic systems: Towards higher solar-to-hydrogen efficiency

Direct Z-scheme photocatalytic systems, with their strong photocatalytic redox capabilities and efficient separation of photogenerated carriers, are beneficial for enhancing photocatalytic efficiency. However, the low solar-to-hydrogen (STH) efficiency of the entire photocatalytic system is caused by the recombination of half of the photogenerated carriers at the interface. Based on this, we first propose monolayer-MoSeTe/SnSe2 and monolayer-WSeTe/SnSe2 heterostructures as direct Z-scheme photocatalytic systems for overall water splitting, with theoretical estimated STH efficiencies of 10.14% and 9.18%, respectively. To enhance the STH efficiency of the proposed direct Z-scheme systems, triple-layer photocatalytic systems are designed based on the proposed direct Z-scheme systems: bilayer-MoSeTe/SnSe2 and bilayer-WSeTe/SnSe2 heterostructures, with STH efficiencies of 11.94% and 10.78%, respectively. Compared to the monolayer-MoSeTe/SnSe2 and monolayer-WSeTe/SnSe2 heterostructures, the triple-layer photocatalytic systems possess higher efficiencies of carrier utilization and the STH efficiencies increased by 17.75% and 17.43%, respectively. Among them, the STH efficiencies of monolayer-MoSeTe/SnSe2, bilayer-MoSeTe/SnSe2 and bilayer-WSeTe/SnSe2 heterostructures exceed the threshold of 10% required for commercial applications. By introducing Te vacancies on the surfaces of Janus TMDCs, all four proposed van der Waals heterostructures can spontaneously perform hydrogen evolution reaction and oxygen evolution reaction processes in a neutral environment under illumination. Thus, our research provides a new strategy for designing highly-efficient photocatalytic systems.

Ultrathin 2D metal-organic framework nanosheet arrays to boost the overall efficiency of water splitting

Developing cost-effective and highly efficient electrocatalysts for water splitting has posed a significant challenge in recent research. This work presents a facile approach to synthesizing a 2D nickel-based metal-organic framework (TIT-1) using 2,3,5,6-tetracarboxyphenylpyrimidine (TCPP), 4,4′-bipyridine (BPY) and nickel nitrate hexahydrate under solvothermal conditions. The detailed structure and phase purity were characterized by X-ray analysis, X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). To enhance the water electrolysis ability, the TIT-1 nanosheet (TIT-1@NS/NF) was fabricated via ultrasonic treatment of pure TIT-1 for 30 min. The TIT-1@NS/NF electrode was synthesized via an in-situ growth method and employed as an electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at ambient temperature. The results indicate that TIT-1@NS/NF achieved a current density of 10 mA cm−2 at an overpotential of 196 mV for OER and 270 mV for HER under the same conditions, so exhibiting superior performance compared to TIT-1. The electrochemical durability of TIT-1@NS/NF showed a retention rate of 87% for OER and 90% for HER. The superior electrocatalytic performance is primarily attribute to the synergistic impact of the 2D nanosheet architecture, which significantly increased the specific surface area and facilitated enhances electronic conductivity. The presence of partially deprotonated carboxylate groups and open metal sites (OMS) in TIT-1 further boosts the enhanced activity, as they exposed a multitude of unsaturated metal sites, thereby optimizing both the OER and HER process. Additionally, an asymmetric electrolytic cell comprising TIT-1@NS/NF was designed and assembled to assess its effectiveness for overall water splitting in alkaline conditions.