Hierarchical Metal Sulfides Heterostructure as Superior Bifunctional Electrode for Overall Water Splitting

Dual-metal heterogeneous electrode enabling efficient co-electrosynthesis of adipic acid and hydrogen

The electrochemical oxidation of cyclohexanone to produce adipic acid (AA), coupled with hydrogen (H2) production, represents a promising strategy. However, the development of low-cost and high-performance electrodes remains a significant challenge. Herein, we present Ni@Cu dual-metal heterogeneous material as a proof of concept, demonstrating its potential for efficient co-electrosynthesis of adipic acid and H2. The Ni@Cu material, featuring abundant heterogeneous interfaces, is grown on copper foam (CF) through a straightforward electrochemical reconstitution strategy. This approach enhances the exposure of catalytic active sites, improves interfacial charge transfer, and accelerates reaction kinetics during electrolysis. As a result, the Ni@Cu/CF electrode achieves low potentials of -172 mV vs. RHE and 1.55 V vs. RHE at 100 mA cm-2 for the hydrogen evolution reaction (HER) and cyclohexanone oxidation reaction (COR), respectively. The assembled HER||COR electrolyzer delivers a high adipic acid yield (1.15 mmol h-1 at 250 mA cm-2) and a maximum Faradaic efficiency (FE) of 88 % at 100 mA cm-2. It also achieves a high FE for H2 (over 96 % at 250 mA cm-2) and demonstrates excellent co-electrolysis stability for over 100 h. In-situ spectroscopy confirms that the formation of heterogeneous Ni@Cu facilitates the generation of active species and accelerates their kinetic transformation into adipic acid.

Modulation of Charge Redistribution in Heterogeneous CoSe-Ni0.95Se Coupling with Ti3C2Tx MXene for Hydrazine-Assisted Water Splitting

Integrating abundant dual sites of hydrazine oxidation reaction (HzOR) and hydrogen evolution reaction (HER) into one catalyst is extremely urgent toward energy-saving H2 production. Herein, CoSe-Ni0.95Se heterostructure coupling with Ti3C2Tx MXene (CoSe-Ni0.95Se/MXene) is fabricated on nickel foam (NF) to enhance the catalytic performance. The heterogeneous CoSe-Ni0.95Se and MXene coupling effect can change the coordination of Ni and Co, resulting in adjusted interfacial electronic field and enhanced electron transfer from Ni0.95Se to CoSe especially near MXene surface. Also, the appearance of MXene can anchor more active sites, thereby abundant nucleophilic CoSe and electrophilic Ni0.95Se are formed induced by the charge redistribution, which can tailor d-band center, moderate *H adsorption free energy (∆GH *) and facilitate adsorption/desorption for hydrazine intermediates, contributing to much enhanced HER and HzOR performance. For example, the low potentials of -160.8 and 116.1 mV at 400 mA cm-2 are seen for HER and HzOR with long-term stability of 7 days. When assembled as overall hydrazine splitting (OHzS), a small cell voltage of 0.35 V to drive 100 mA cm-2 is obtained. Such concept of integrating abundant nucleophilic and electrophilic dual sites and regulating their d-band centers can offer in-depth understandings to design efficient bifunctional HER and HzOR electrocatalysts.

Plasma-Engraved Lattice-Matched NiO/NiFe2O4 Heterostructure with Ample Oxygen Vacancies for Efficient Water Electrolysis and Zn-Air Batteries

Heterogeneous interface and defect engineering offer effective pathways to accelerate oxygen evolution reaction (OER) charge transfer kinetics and motivate optimal intrinsic catalytic activity. Herein, we report the lattice-matched NiO/NiFe2O4 heterostructure with ample oxygen vacancies (Vo-NiO/NiFe2O4) induced by a feasible hydrothermal followed by calcination and plasma-engraving assistant technique, which shows the unique porous microflower arrangement of intertwined nanosheets. Benefitting from the synergetic effects between lattice-matched heterointerface and oxygen vacancies induce the strong electronic coupling, optimized OH-/O2 diffusion pathway and ample active sites, thus-prepared Vo-NiO/NiFe2O4 presents a favorable OER performance with a low overpotential (261 mV @ 10 mA cm-2) and small Tafel slope (39.4 mV dec-1), even surpassing commercial RuO2 catalyst. Additionally, the two-electrode configuration water electrolyzer and rechargeable zinc-air battery assembled by Vo-NiO/NiFe2O4 catalyst show the potential practical application directions. This work provides an innovative avenue for strengthening OER performance toward water electrolysis and Zn-air batteries via the interface and vacancy engineering strategy.

Rational Design of Molybdenum-Doped Cobalt Nitride Nanowire Arrays for Robust Overall Water Splitting

Rational design of efficient electrocatalysts is highly imperative but still a challenge for overall water splitting. Herein, we construct self-supported Co₃ N nanowire arrays with different Mo doping contents by hydrothermal and nitridation processes that serve as robust electrocatalysts for overall water splitting. The optimal Co₃ N-Mo_0.2 /Ni foam (NF) electrode delivers a low overpotential of 97 mV at a current density of 50 mA cm^-2 as well as a highly stable hydrogen evolution reaction (HER). Density functional theory (DFT) calculations prove that Mo doping can effectively modulate the electronic structure and surface adsorption energies of H₂ O and hydrogen intermediates on Co₃ N, leading to improved reaction kinetics with high catalytic activity. Further modification with FeOOH species on the surface of Co₃ N-Mo_0.2 /NF improves the oxygen evolution reaction (OER) performance benefiting from the synergistic effect of dual Co-Fe catalytic centers. As a result, the Co₃ N-Mo_0.2 @FeOOH/NF catalysts display outstanding OER catalytic performance with a low overpotential of 250 mV at 50 1 mA cm^-2 . The constructed Co₃ N-Mo_0.2 /NF||Co₃ N-Mo_0.2 @FeOOH/NF water electrolyzer exhibits a small voltage of 1.48 V to achieve a high current density of 50 mA cm^-2 at 80 °C, which is superior to most of the reported electrocatalysts. This work provides a new approach to developing robust electrode materials for electrocatalytic water splitting.

Coupled MoO3-x@CoP heterostructure as a pH-universal electrode for hydrogen generation at a high current density

Developing high-performance and low-cost self-supporting electrodes as pH-universal electrocatalysts for the hydrogen-evolution reaction (HER) and realizing high-quality hydrogen production at a high current density are highly desirable, but are hugely challenging. We created a self-supporting electrode with a coupled hierarchical heterostructure by simple electrodeposition followed by sulfurization. It comprised oxygen-deficient molybdenum oxide (MoO_3-x) and cobalt phosphide (CoP) on nickel foam (NF), which represented a highly active pH-universal electrocatalyst for the HER at a high current density. Benefiting from a plethora of catalytic active sites, improved interfacial charge transfer, and strong electronic interaction, this type of MoO_3-x@CoP/NF electrode delivered a superior catalytic performance. Overpotentials of only 100 mV, 135 mV, and 400 mV were needed to realize a high current density of 1 A cm^-2 in alkaline, acid and neutral media, respectively, which were superior to those of most other well-developed materials based on non-noble metals. Our experimental work demonstrates the synergistic advantages of a MoO_3-x@CoP heterostructure for improving the intrinsic catalytic performance but also paves a new path for the rational design of advanced electrodes for hydrogen generation in a wide range of pH conditions.

Engineering Coupled NiSx -WO2.9 Heterostructure as pH-Universal Electrocatalyst for Hydrogen Evolution Reaction

Exploiting highly active and low-cost materials as pH-universal electrocatalysts for the hydrogen evolution reaction (HER) and achieving high-purity hydrogen fuel is highly desirable but remains challenging. Herein, a novel type of coupled heterostructure was designed by simple electrodeposition followed by a sulfurization treatment. This hierarchical structure was composed of nickel sulfides (NiS, NiS₂ , denoted as NiS_x ) and oxygen-deficient tungsten oxide (WO_2.9 ), which was directly grown on nickel foam (NF) as self-supporting electrodes (NiS_x -WO_2.9 /NF) for HER over a wide pH range. The systematic experimental characterizations confirmed that the material had abundant catalytic active sites, fast interfacial electron transfer ability, and strong electronic interaction, resulting in the optimized reaction kinetics for HER. Consequently, the NiS_x -WO_2.9 /NF catalyst required low overpotentials of 96 and 117 mV to reach current densities of 50 and 100 mA cm^-2 in an alkaline medium, outperforming most of the reported non-noble metal-based materials. Moreover, this self-supported electrode exhibited impressive performance over a wide pH range, only requiring 220 and 304 mV overpotential at 100 mA cm^-2 in 0.5 m H₂ SO₄ and 1 m phosphate-buffered saline electrolytes. This work may offer a new approach to the development of advanced pH-universal electrodes for hydrogen production.

Ni2 Se3 -CuSex Heterostructure as a Highly Efficient Bifunctional Electrocatalyst for Urea-Assisted Hydrogen Generation

Coupling urea oxidation reaction (UOR) with hydrogen evolution reaction (HER) is an attractive alternative anode reaction for electrochemical hydrogen generation with low energy consumption. However, the development of highly efficient bifunctional electrocatalysts is still a challenge. In this work, Ni₂ Se₃ -CuSe_x heterostructure was synthesized on copper foam (Ni₃ Se₂ @CuSe_x /CF) by electrodeposition accompanied by a selenization process. Benefiting from the abundant active sites, faster reaction kinetics, and modulated electronic structure, the self-supporting Ni₃ Se₂ @CuSe_x /CF electrode exhibited superior catalytic performance. Extremely low overpotentials of 120 and 140 mV were achieved at the current density of 100 mA cm^-2 for HER/UOR, respectively. Respectively, in HER||UOR coupled electrolyzer for H₂ generation, the Ni₃ Se₂ @CuSe_x /CF||Ni₃ Se₂ @CuSe_x /CF delivered a low cell voltage of 1.49 V to reach a high current density of 100 mA cm^-2 along with good stability, outperforming most of the other well-developed materials to date. The rational design of coupled heterostructure as bifunctional electrodes is a promising approach for energy-saving H₂ production.

Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production

The development of highly active non-precious metal electrocatalysts is crucial for advancing the practical application of hydrogen evolution reaction (HER). Doping engineering is one of the important strategies to optimize the electrocatalytic activity of electrocatalysts. Herein, we put forward a simple strategy to optimize the catalytic activity of MoO3 material by incorporating the Cu atoms into the interlayer (denoted as Cu-MoO3). The prepared Cu-MoO3 nanosheet has a larger surface area, higher conductivity, and strong electron interactions, which contributes to optimal reaction kinetics of the HER process. As a result, the Cu-MoO3 nanosheet only needs a small overpotential of 106 mV to reach the geometric current density of 10 mA cm−2. In addition, it also delivers a low Tafel slope of 83 mV dec−1, as well as high stability and Faraday efficiency. Notably, when using the Cu-MoO3 as a cathode to construct the water electrolyzer, it only needs 1.55 V to reach the 10 mA cm−2, indicating its promising application in hydrogen generation. This work provides a novel type of design strategy for a highly active electrocatalyst for an energy conversion system.

Heterostructure engineering of iridium species on nickel/molybdenum nitride for highly-efficient anion exchange membrane water electrolyzer

Developing highly active electrocatalysts is a pivotal issue for anion-exchange membrane water electrolyzers (AEMWE). However, realizing the continuous hydrogen generation at a large current density remains challenging. Herein, a novel kind of hybrid electrode is successfully developed by introducing trace iridium (Ir) species onto a hierarchical Ni/Mo₅N₆ heterostructure on Ni foam (Ir-Ni/Mo₅N₆/NF). The synergistic advantages of high conductivity, abundant active sites, and strong electronic interaction endow superior reaction kinetics, presenting a highly-active bifunctional electrocatalyst. Remarkably, the Ir-Ni/Mo₅N₆/NF exhibit extremely low overpotentials of 52 mV and 250 mV at 100 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). By exploiting the Ir-Ni/Mo₅N₆ as both anode/cathode, the constructed AEMWE device delivers superior performance. The current density reaches 2.1 A cm^-2 at a voltage of 2.0 V and 250 mA cm^-2 at 1.8 V in alkaline/neutral media. This work put forward a facile and effective strategy to synthesize advanced bifunctional electrocatalysts for water electrolysis.

Fluorine-anion engineering endows superior bifunctional activity of nickel sulfide/phosphide heterostructure for overall water splitting

Designing advanced transition metal-based materials for electrocatalytic water splitting is of significance, but their wide application is still limited due to the lack of an effective regulation strategy. Herein, a synergistic regulation strategy of surface/interface is developed to optimize the catalytic activity of nickel sulfide (Ni₃S₂). The construction of nickel phosphide with Ni₃S₂ heterostructure by using fluorine (F)-anion modification is successfully developed on nickel foam (F-NiP_x/Ni₃S₂-NF) via a simple fluorination and phosphating treatment. This new kind of electrocatalyst contains plenty of active sites and strong electronic interactions, presenting superior bifunctional activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overpotentials only need 182 mV and 370 mV to reach the current density of 100 mA cm^-2 for HER and OER, respectively. In addition, the F-NiP_x/Ni₃S₂-NF-based electrolyzer delivers promising performance for overall water splitting. A low potential of 1.55 V and 1.7 V can be achieved at the current density of 10 mA cm^-2 and 50 mA cm^-2. This work provides a new surface/interface regulation strategy for high-efficient bifunctional electrocatalysts.

Hierarchically structured nickel/molybdenum nitride heterojunctions as superior bifunctional electrodes for overall water splitting

A 3D hierarchical heterostructure of intermetallic compound heterojunctions is first rationally designed and presented as a highly-active bifunctional electrode for water splitting.