Bonding interface boosts the intrinsic activity and durability of NiSe@Fe2O3 heterogeneous electrocatalyst for water oxidation

Ru-regulated electronic structure CoNi-MOF nanosheets advance water electrolysis kinetics in alkaline and seawater media

Herein, an ion-exchange strategy is utilized to greatly improve the kinetics of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by Ru-modified CoNi- 1,3,5-Benzenetricarboxylic acid (BTC)-metal organic framework nanosheets (Ru@CoNi-MOF). Due to the higher Ni active sites and lower electron transfer impedance, Ru@CoNi-MOF catalyst requires the overpotential as low as 47 and 279 mV, at a current density of 10 mA/cm2 toward HER and OER, respectively. Significantly, the mass activity of Ru@CoNi-MOF for HER and OER are 25.9 and 10.6 mA mg-1, nearly 15.2 and 8.8 times higher than that of Ni-MOF. In addition, the electrolyzer of Ru@CoNi-MOF demonstrates exceptional electrolytic performance in both KOH and seawater environment, surpasses the commercial Pt/C||IrO2 couple. Theoretical calculations prove that introducing Ru atoms in - CoNi-MOF modulates the electronic structure of Ni, optimizes adsorption energy for H* and reduces energy barrier of metal organic frameworks (MOFs). This modification significantly improves the kinetic rate of the Ru@CoNi-MOF during water splitting. Certainly, this study highlights the utilization of MOF nanosheets as advanced HER/OER electrocatalysts with immense potential, and will paves a way to develop more efficient MOFs for catalytic applications.

Interfacial Electronic Interaction in Amorphous-Crystalline CeOx -Sn Heterostructures for Optimizing CO2 to Formate Conversion

Formate, a crucial chemical raw material, holds significant promise for industrial applications in the context of CO2 electroreduction reaction (CO2 RR). Despite its potential, challenges, such as poor selectivity and low formation rate at high current densities persist, primarily due to the competing hydrogen evolution reaction (HER) and high energy barriers associated with *OCHO intermediate generation. Herein, one-step chemical co-reduction strategy is employed to construct an amorphous-crystalline CeOx -Sn heterostructure, demonstrating remarkable catalytic performance in converting CO2 to formate. The optimized CeOx -Sn heterostructures reach a current density of 265.1 mA cm-2 and a formate Faraday efficiency of 95% at -1.07 V versus RHE. Especially, CeOx -Sn achieves a formate current density of 444.4 mA cm-2 and a formate production rate of 9211.8 µmol h-1  cm-2 at -1.67 V versus RHE, surpassing most previously reported materials. Experimental results, coupled with （density functional theory）DFT calculations confirm that robust interface interaction between CeOx and Sn active center induces electron transfer from crystalline Sn site to amorphous CeOx , some Ce4+ of CeOx get electrons and convert to unsaturated Ce3+ , optimizing the electronic structure of active Sn. This amorphous-crystalline heterostructure promotes electron transfer during CO2 RR, reducing the energy barrier formed by *OCHO intermediates, and thus achieving efficient reduction of CO2 to formate.

Electronic modulation and reaction-pathway optimization on three-dimensional seaweed-like NiSe@NiMn LDH heterostructure to trigger effective oxygen evolution reaction

The development of low-cost and high-efficiency electrocatalysts for the oxygen evolution reaction (OER) is essential to produce high-purity hydrogen in large scale. Herein, a three-dimensional (3D) seaweed-like hierarchical structure was fabricated using two-dimensional (2D) NiMn LDH nanosheets wrapped on one-dimensional (1D) NiSe nanowires with nickel foam (NF) as a substrate (NiSe@NiMn LDH/NF) via hydrothermal and electrodeposition processes. Owing to the strong interfacial synergy, 3D seaweed-like hierarchical structure, higher conductivity, and strong structural stability, the NiSe@NiMn LDH/NF exhibited superior OER performance with an overpotential of 287 mV at 100 mA cm-2, and stably operated for 160 h at large current. Moreover, the overall water splitting system with NiSe@NiMn LDH/NF as the anode and Pt/C/NF as the cathode exhibited a low cell voltage of 1.59/1.64 V to reach 50/100 mA cm-2, and excellent stability for 110 h at 300 mA cm-2. The density function theory (DFT) calculations unveiled that NiSe@NiMn LDH enabled the interfacial synergy, reallocating the electron density at the interface, and further weakening the energy barrier of OH* by strengthening chemical bonds with OH* intermediates to improve the intrinsic OER activity.

Interfacial Electronic Interaction in In2O3/Poly(3,4-ethylenedioxythiophene)-Modified Carbon Heterostructures for Enhanced Electroreduction of CO2 to Formate

Formate, as an important chemical raw material, is considered to be one of the most promising products for industrialization among CO₂ electroreduction reaction (CO₂RR) products, but it still suffers from poor selectivity and a low formation rate at a high current density on account of the competitory hydrogen evolution reaction. Herein, the heterogeneous nanostructure was constructed by anchoring In₂O₃ nanoparticles on poly(3,4-ethylenedioxythiophene) (PEDOT)-modified carbon black (In₂O₃/PC), in which the PEDOT polymer interface layer could immobilize In₂O₃ nanoparticles and obtain a notable reduction in electron transfer resistance among the In₂O₃ particles, showing a 27% increase in the total electron transfer rate. The optimized In₂O₃/PC with rich heterogeneous interfaces selectively reduced CO₂ to formate with a high FE of 95.4% and a current density of 251.4 mA cm^-2 under -1.18 V vs RHE. Also, the formate production rate for In₂O₃/PC was up to 7025.1 μmol h^-1 cm^-2, surpassing most previously reported CO₂RR catalysts. The in situ XRD results revealed that In₂O₃ particles were reduced to metallic indium (In) as catalytic active sites during CO₂RR. DFT calculations verified that a strong interface interaction between In sites and PC induced electron transfer from In sites to PC, which could optimize the charge distribution of active sites, accelerate electron transfer, and elevate the p-band center of In sites toward the Fermi level, thereby lowering the adsorption energy of *OCHO intermediates for CO₂ conversion to formate.

Selenite-Decorated Polycrystalline NiO Nanosheets Generated from Cathodic Reconstruction for Electrocatalytic Hydrogen Production

Precatalyst reconstruction in alkaline hydrogen evolution reaction (HER) usually leads to changes in the morphology, composition, and structure, thus improving the catalytic activity, which recently receives intensive attention. However, the design strategies of cathodic reconstruction and the structural features of reconstruction products have not achieved a profound understanding. Here, from the point of thermodynamic stability, metastable nickel selenite dihydrate (NiSeO₃·2H₂O) is deliberately fabricated as a precatalyst to comprehensively study the reconstruction dynamics in alkaline HER. Multiple in/ex situ techniques capture the geometric, component, and phase evolutions, proving that NiSeO₃·2H₂O can be transformed into SeO₃^2--decorated polycrystalline NiO nanosheets with rich active sites and good conductivity under alkaline HER conditions, which act as a real catalytic active species. Density functional theory calculations demonstrate that the adsorption of SeO₃^2- can further promote the HER activity of NiO due to the optimized free energy of water activation and hydrogen adsorption. As a result, the SeO₃^2--NiO catalyst exhibits a low overpotential at -10 mA cm^-2 (90 mV) and long-term stability (>100 h). This work highlights the targeted design of precatalyst to trigger and utilize cathodic reconstruction and provides an available method for the development of adsorption-modulated efficient electrocatalysts.

Synergistic Effect of P Doping and Mo-Ni-Based Heterostructure Electrocatalyst for Overall Water Splitting

Heterostructure construction and heteroatom doping are powerful strategies for enhancing the electrolytic efficiency of electrocatalysts for overall water splitting. Herein, we present a P-doped MoS₂/Ni₃S₂ electrocatalyst on nickel foam (NF) prepared using a one-step hydrothermal method. The optimized P_[0.9mM]-MoS₂/Ni₃S₂@NF exhibits a cluster nanoflower-like morphology, which promotes the synergistic electrocatalytic effect of the heterostructures with abundant active centers, resulting in high catalytic activity for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte. The electrode exhibits low overpotentials and Tafel slopes for the HER and OER. In addition, the catalyst electrode used in a two-electrode system for overall water splitting requires an ultralow voltage of 1.42 V at 10 mA·cm^-2 and shows no obvious increase in current within 35 h, indicating excellent stability. Therefore, the combination of P doping and the heterostructure suggests a novel path to formulate high-performance electrocatalysts for overall water splitting.

Doping Mo into NiFe LDH/NiSe Heterostructure to Enhance Oxygen Evolution Activity by Synergistically Facilitating Electronic Modulation and Surface Reconstruction

It is of great significance to design highly efficient electrocatalysts with abundant earth elements instead of precious metals for water splitting. Herein, Mo-doped NiFe-layered double hydroxides/NiSe heterostructure (Mo-NiFe LDH/NiSe) was fabricated by coupling Mo-doped NiFe LDH and NiSe on nickel foam (NF). The heterostructure electrocatalyst showed ultra-low overpotential (250 mV) and remarkable durability for oxygen evolution reaction (OER) at 150 mA cm^-2 . Both theoretical and experimental results confirmed that Mo doping and interfacial synergism induced the interfacial charge redistribution and the lifted d-band center to weaken the energy barrier (EB) of the formation of OOH^* . Mo doping also facilitated the surface reconstruction of NiFe LDH into Ni(Fe)OOH as the active sites under electro-oxidation process. This work provides a facile strategy for electronic modulation and surface reconstruction of OER electrocatalyst by transition metal doping and heterostructure generation.

Niobium-Incorporated CoSe2 Nanothorns with Electronic Structural Alterations for Efficient Alkaline Oxygen Evolution Reaction at High Current Density

Developing cost-effective, highly active, and robust electrocatalysts for oxygen evolution reaction (OER) at high current density is a critical challenge in water electrolysis since the sluggish kinetics of the OER significantly impedes the energy conversion efficiency of overall water splitting. Here, a 1D nanothorn-like Nb-CoSe₂ /CC (CC=carbon cloth) structure was developed as an efficient OER catalyst. The optimized Nb-CoSe₂ /CC catalyst exhibited remarkable OER performance with the low overpotentials of 220 mV at 10 mA cm^-2 and 297 mV 200 mA cm^-2 and a small Tafel slope (54.1 mV dec^-1 ) in 1.0 m KOH electrolyte. More importantly, the Nb-CoSe₂ /CC electrode displayed superior stability after 60 h of continuous operation. In addition, cell voltages of 1.52 and 1.93 V were required to achieve 10 and 500 mA cm^-2 for the electrolyzer made of Nb-CoSe₂ /CC (anode) and the Pt/C (cathode). Density functional theory (DFT) calculations combined with experimental results revealed that incorporating niobium into the CoSe₂ could optimize the adsorption free energy of the reaction intermediates and enhance the conductivity to improve the catalytic activity further. Additionally, the super-hydrophilicity of Nb-CoSe₂ /CC resulting from the surface defects increased the surface wettability and facilitated reaction kinetics. These results indicate that Nb-CoSe₂ /CC intrinsically enhances OER performance and possesses potential practical water electrolysis applications.

[Ru(tmphen)3]2[Fe(CN)6] and [Ru(phen)3][Fe(CN)5(NO)] complexes and formation of a heterostructured RuO2-Fe2O3 nanocomposite as an efficient alkaline HER and OER electrocatalyst

Water electrolysis is one of the most capable processes for supplying clean fuel. Herein, two novel ionic Ru(II)-Fe(II) complexes, [Ru(tmphen)₃]₂[Fe(CN)₆] and [Ru(phen)₃][Fe(CN)₅(NO)], where tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline and phen = 1,10-phenanthroline, were synthesized and characterized by UV-Vis spectroscopy, elemental analysis, FT-IR, and single-crystal X-ray structural analysis. By thermally decomposing the [Ru(tmphen)₃]₂[Fe(CN)₆] complex at 600 °C for 4 h, a heterostructured RuO₂-Fe₂O₃ nanocomposite was fabricated through a facile one-pot treatment and then characterized by FT-IR, XRD, FT-Raman, UV-Vis (DRS), ICP-OES, FE-SEM, TEM, TGA/DTG, BET, and XPS analyses, which revealed the formation of highly crystalline RuO₂-Fe₂O₃ nanoparticles with an average size of 8-12 nm. The prepared nanocomposite was an efficient heterostructured electrocatalyst for performing water-splitting redox reaction processes, including hydrogen and oxygen evolution reactions (HER and OER) in alkaline solutions. In this regard, RuO₂ and Fe₂O₃ samples were also prepared through thermal decomposition of [Ru(tmphen)₃](NO₃)₂ and K₄[Fe(CN)₆] precursors, respectively, as control experiments to compare their HER and OER electrocatalytic activity with that of the RuO₂-Fe₂O₃ nanocomposite. Specifically, the RuO₂-Fe₂O₃ nanocomposite exhibited significant electrocatalytic performance, generating 10 mA cm^-2 current density at -148 and 292 mV overpotentials, and the Tafel slope results from fitting the LSV curves to the Tafel equation were -43 and 56.08 mV dec^-1 for the HER and OER, respectively. Therefore, the heterostructured RuO₂-Fe₂O₃ nanocomposite can be viewed as a bi-functional electrocatalyst for HER and OER because it exploits the synergistic effects of heterostructures and active sites at its interface.

NiCoP–CeO2 composites for efficient electrochemical oxygen evolution

In this study, a novel NiCoP–CeO₂ composite was constructed on a Ni foam by a simple hydrothermal method and thermal phosphating strategy. In the OER test, NiCoP–CeO₂ exhibited a low overpotential of 217 mV at 10 mA cm⁻², 45 mV dec⁻¹ of Tafel slopes. With the help of theoretical calculations and experimental characterization, the reason for performance improvement was analyzed in depth. The results show that CeO₂ leads to a confinement effect, maintaining the nanosheet morphology of NiCo-LDHs, which contributes to sustaining the catalyst in favourable contact with H₂O and minimizing the OER potential. Furthermore, by loading CeO₂ onto NiCoP, the hydrophilicity of the catalyst is significantly enhanced. Our work provides an ingenious synthesis strategy for the preparation of efficient and inexpensive electrocatalytic materials.

A NiCoP–CeO₂ composite was constructed by a simple hydrothermal method and thermal phosphating strategy. Benefiting from the confinement effect and optimized H₂O adsorption ability, NiCoP–CeO₂ exhibits superior OER performance than NiCoP.

Hierarchical sheet-on-sheet heterojunction array of a β-Ni(OH)2/Fe(OH)3 self-supporting anode for effective overall alkaline water splitting

Rationally designing high-performance non-noble metal electrocatalysts is of essence to improve energy conversion efficiency in water splitting. Herein, a unique 3D hierarchical sheet-on-sheet heterojunction between Fe(OH)₃ and β-Ni(OH)₂ on pretreated Ni foam (NiFe-HD/pre-NF) was fabricated by a two-step strategy involving the interfacial hydrolysis-deposition of Fe²⁺ and electrodeposition of Ni²⁺. The presence of the Ni-O-Fe bridge at the Fe(OH)₃/β-Ni(OH)₂ heterointerface can induce interfacial electronic redistribution to form Ni³⁺ in NiFe-HD/pre-NF, and further strengthen the adsorption of OH^- and weaken the O-H bond to change the rate-determining step (RDS) for accelerating OER kinetics. Benefiting from the sheet-on-sheet architecture and dual-phase synergism on NiFe-HD/pre-NF, the optimal NiFe-HD/pre-NF exhibits excellent OER performance with a lower overpotential of 256 mV at 100 mA cm^-2, a small Tafel slope of 81 mV dec^-1, high intrinsic activity and robust stability. Alkaline water-splitting using NiFe-HD/pre-NF as the anode requires ultralow cell voltages of 1.62 V and 1.83 V at current densities of 100 mA cm^-2 and 400 mA cm^-2, respectively, which are comparable with commercial alkaline water electrolysis, and operates steadily at a current density of 100 mA cm^-2 for 85 h without decay. This work proposes a facile strategy for constructing heterojunctions and modulating electronic interaction to develop electrocatalysts with new architectures.

A nitrogen-doped NiCo2S4/CoO hollow multi-layered heterostructure microsphere for efficient oxygen evolution in Zn-air batteries

Exploring highly effective and low-cost non-noble metal-based electrocatalysts for oxygen evolution reaction (OER) is critical for renewable energy conversion and metal-air batteries. Herein, a novel and high-efficient OER catalyst was reported with nitrogen-doped oxide/sulfide heterostructures (named N-NiCo2S4/CoO microsphere). The N-NiCo2S4/CoO microsphere was synthesized by annealing NiCo-BTC MOF to a multi-layered hollow structure of NiCo2O4 microspheres, followed by the direct vulcanization in the presence of NH4HCO3, resulting in an oxide/sulfide heterojunction. Benefiting from the nitrogen doping, the abundant multi-layered hollow heterostructure and the interfaces between multiple components, the N-NiCo2S4/CoO microsphere exhibited excellent OER activity with a low overpotential of 227 mV at 10 mA cm-2. The Zn-air battery based on the N-NiCo2S4/CoO + Pt/C catalyst displayed excellent cycling stability after 900 cycles at a large current density of 5 mA cm-2, where the commercial RuO2 + Pt/C-based battery exhibited a big drop after only 30 cycles, suggesting its great application prospects as power source devices.