Cu2Se nanowires shelled with NiFe layered double hydroxide nanosheets for overall water-splitting

Delamination of NiFe layered double hydroxides into perforated monolayers for efficient water splitting

The introduction of vacancies can significantly change the coordination and valence states of the catalytic active sites, thereby modulating the electronic structure to promote the oxygen evolution reaction (OER). However, atomic-level vacancy engineering on low-dimensional layered double hydroxides (LDHs) has not been achieved, which could be due to the significant structural damage and/or carbonate (CO32-) contamination occurring during the vacancy creating process. In this study, atomic-scale cation vacancies were generated in LDHs without apparent structure damage and carbonate contamination. Perforated monolayer nanosheets with an utmost exposure of active sites were successfully obtained through a subsequent exfoliation in formamide. Compared to bulk LDHs, the flocculated vacancy-containing nanosheets exhibit a small overpotential of 245 mV at a current density of 10 mA cm-2 and maintain excellent stability at a high current density of 500 mA cm-2. Density functional theory (DFT) calculations indicate that introducing cation vacancies on monolayer NiFe-LDH nanosheets and creating unsaturated Ni-Fe sites can effectively reduce the Gibbs free energy of the OER process. The two-electrode electrolyzer assembled with commercial Pt/C for overall water splitting can operate at a cell voltage as low as 1.50 V to yield a current density of 10 mA cm-2. It also demonstrates long-term stability of 50 h at a large current density of 500 mA cm-2. The current strategy of atomic cation vacancy engineering on monolayer LDHs provides important insights into the design of low-cost LDH-based catalysts toward efficient alkaline water electrolysis and other energy-related applications.

Modulating the structure of Cu in Cu2X/CNTs hollow tetrakaidecahedron to enhance high-efficiency H2O2 production

Regulation of active sites of electrocatalysts is critical in adjusting electronic structure and catalytic selectivity towards oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2). Herein, the Cu2X/CNTs (X = Se, SSe, S) hollow tetrakaidecahedron catalysts were synthesized to facilitate the electrocatalytic reduction of O2 to H2O2. The introduction of S resulted in a shift from four-electron pathway on Cu2Se/CNTs to two-electron process on Cu2S/CNTs, ultimately leading to an enhancement in H2O2 productivity. Importantly, the addition of extra S species can modulate the chemical environment of active sites, and electrochemical tests demonstrate that the Cu2S/CNTs catalyst exhibits an enhanced selectivity (over 91 %), production rate (360 mmol gcat-1 h-1), and durability for H2O2 undergoing a two-electron process by an H-type electrolytic cell. The in-situ Raman spectroscopy result confirms that the structural stability of Cu2S/CNTs during the reaction, and the accumulation of H2O2 increased with the extension of reaction time. Various experimental results and density functional theory (DFT) reveal that the S atoms can optimize the adsorption strength of the active sites to reaction intermediates, thereby creating an appropriate energy barrier for the formation of the determinant intermediate OOH* in H2O2 production, while maintain a high energy barrier for OO bond breaking of OOH* towards H2O formation. This study proves insights into strategies for controlling H2O2 production and guiding the optimization of catalysts for H2O2 electrosynthesis.

Ag nanoparticles induced abundant Cuδ+ sites in Cu2Se nanoflower rods to promote efficient carbon dioxide electroreduction to ethanol

Electrocatalytic carbon dioxide reduction reaction (eCO2RR) is one of the attractive approaches to CO2 utilization. Nevertheless, it remains a challenge to prepare highly efficient and selective electrocatalysts to realize deep conversion of multi-carbon products. The absence of active sites due to the reconfiguration of copper-based catalysts leads to migration deactivation during catalysis, rendering low catalytic efficiency. In this work, Cu2Se nanowires (NWs) were obtained from the Cu foam. After further electroreduction for 90 s, Ag nanoparticles (NPs) were modified on the flower rod-shaped Cu2Se NWs (Ag2.69%/Cu2Se NWs). This interfacial modification strategy, balanced the Cuδ+ active valence state on the surface of the Ag2.69%/Cu2Se NWs, led to a high Faradaic efficiency (FE) of ethanol (∼70 %) at -0.52 V (vs. reversible hydrogen electrode (RHE)). Besides, the Ag2.69%/Cu2Se NWs achieved a high partial current (∼13.9 mA) for ethanol, 18.5 μmol/h ethanol yield and an energy efficiency of 47.1 % in the H-type electrolytic cell. The reaction mechanism was investigated at the molecular level through density functional theory (DFT) calculation. The interface-modified Ag NPs provided stable *COOH intermediates during the reaction process, effectively achieving surface *CO enrichment. Besides, the presence of Ag NPs resulted in the generation of abundant Cuδ+ active species. The *CO migrated to the Cuδ+ active sites to accelerate the asymmetric coupling of *CO and *CHO, which significantly improved the FE of CO2 electroreduction to ethanol. Our work offers a promising approach for designing Cu2Se-based nanowires electrocatalysts for CO2 reduction with excellent activity and selectivity.

Boosting urea-assisted water splitting over P-MoO2@CoNiP through Mo leaching/reabsorption coupling CoNiP reconstruction

Combining the urea oxidation reaction (UOR) with the hydrogen evolution reaction (HER) is an effective technology for energy-saving hydrogen production. Herein, a bifunctional electrocatalyst with CoNiP nanosheet coating on P-doped MoO2 nanorods (P-MoO2@CoNiP) is obtained via a two-step hydrothermal followed a phosphorization process. The catalyst demonstrates exceptional alkaline HER performance due to the formation of MoO2 and the dissolution/absorption of Mo. Meanwhile, the inclusion of Co and P in the P-MoO2@CoNiP catalyst facilitated the formation of NiOOH, enhancing UOR performance. Density functional theory calculations reveal that the hydrogen adsorption Gibbs free energy (ΔGH*) of P-MoO2@CoNiP is closer to 0 eV than CoNiP, favoring the HER. The catalyst only needs -0.08 and 1.38 V to reach 100 mA cm-2 for catalyzing the HER and UOR, respectively. The full urea electrolysis system driven by P-MoO2@CoNiP requires 1.51 V to achieve 100 mA cm-2, 120 mV lower than the traditional water electrolysis.

Nickel-Iron Layered Double Hydroxides/Nickel Sulfide Heterostructured Electrocatalysts on Surface-Modified Ti Foam for the Oxygen Evolution Reaction

Electrochemical approaches for generating hydrogen from water splitting can be more promising if the challenges in the anodic oxygen evolution reaction (OER) can be harnessed. The interface heterostructure materials offer strong electronic coupling and appropriate charge transport at the interface regions, promoting accessible active sites to prompt kinetics and optimize the adsorption-desorption of active species. Herein, we have designed an efficient multi-interface-engineered Ni3Fe1 LDH/Ni3S2/TW heterostructure on in situ generated titanate web layers from the titanium foam. The synergistic effects of the multi-interface heterostructure caused the exposure of rich interfacial electronic coupling, fast reaction kinetics, and enhanced accessible site activity and site populations. The as-prepared electrocatalyst demonstrates outstanding OER activity, demanding a low overpotential of 220 mV at a high current density of 100 mA cm-2. Similarly, the designed Ni3Fe1 LDH/Ni3S2/TW electrocatalyst exhibits a low Tafel slope of 43.2 mV dec-1 and excellent stability for 100 h of operation, suggesting rapid kinetics and good structural stability. Also, the electrocatalyst shows a low overpotential of 260 mV at 100 mA cm-2 for HER activity. Moreover, the integrated electrocatalyst exhibits an incredible OER activity in simulated seawater with an overpotential of 370 mV at 100 mA cm-2 and stability for 100 h of operation, indicating good OER selectivity. This work might benefit further fabricating effective and stable self-sustained electrocatalysts for water splitting in large-scale applications.

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.

Stability of Ni-Fe-Layered Double Hydroxide Under Long-Term Operation in AEM Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) is an attractive method for green hydrogen production. It allows the use of non-platinum group metal catalysts and can achieve performance comparable to proton exchange membrane water electrolyzers due to recent technological advances. While current systems already show high performances with available materials, research gaps remain in understanding electrode durability and degradation behavior. In this study, the performance and degradation tracking of a Ni3Fe-LDH-based single-cell is implemented and investigated through the correlation of electrochemical data using chemical and physical characterization methods. A performance stability of 1000 h, with a degradation rate of 84 µV h-1 at 1 A cm-2 is achieved, presenting the Ni3Fe-LDH-based cell as a stable and cost-attractive AEMWE system. The results show that the conductivity of the formed Ni-Fe-phase is one key to obtaining high electrolyzer performance and that, despite Fe leaching, change in anion-conducting binder compound, and morphological changes inside the catalyst bulk, the Ni3Fe-LDH-based single-cells demonstrate high performance and durability. The work reveals the importance of longer stability tests and presents a holistic approach of electrochemical tracking and post-mortem analysis that offers a guideline for investigating electrode degradation behavior over extended measurement periods.

Water activating fresh NiMo foam for enhanced urea electrolysis

Recently, production of hydrogen (H2) through the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) has acquired great attention because it is more environmentally friendly and energy-saving. Herein, an approach of water activation was developed for in situ growth of NiMo LDH nanosheet arrays on NiMo foam without using any binder or pressurizing or heating steps. The obtained NiMo foam electrodes showed exceptional catalytic activity and durability for both the UOR and HER. This work offers a new standpoint on designing electrodes with high activation for efficient and sustainable hydrogen production coupled with urea organic oxidation.

Roles of Heterojunction and Cu Vacancies in the Au@Cu2-xSe for the Enhancement of Electrochemical Nitrogen Reduction Performance

The utilization of hydrogen (H2) as a fuel source is hindered by the limited infrastructure and storage requirements. In contrast, ammonia (NH3) offers a promising solution as a hydrogen carrier due to its high energy density, liquid storage capacity, low cost, and sustainable manufacturing. NH3 has garnered significant attention as a key component in the development of next-generation refueling stations, aligning with the goal of a carbon-free economy. The electrochemical nitrogen reduction reaction (ENRR) enables the production of NH3 from nitrogen (N2) under ambient conditions. However, the low efficiency of the ENRR is limited by challenges such as the electron-stealing hydrogen evolution reaction (HER) and the breaking of the stable N2 triple bond. To address these limitations and enhance ENRR performance, we prepared Au@Cu2-xSe electrocatalysts with a core@shell structure using a seed-mediated growth method and a facile hot-injection method. The catalytic activity was evaluated using both an aqueous electrolyte of KOH solution and a nonaqueous electrolyte consisting of tetrahydrofuran (THF) solvent with lithium perchlorate and ethanol as proton donors. ENRR in both aqueous and nonaqueous electrolytes was facilitated by the synergistic interaction between Au and Cu2-xSe (copper selenide), forming an Ohmic junction between the metal and p-type semiconductor that effectively suppressed the HER. Furthermore, in nonaqueous conditions, the Cu vacancies in the Cu2-xSe layer of Au@Cu2-xSe promoted the formation of lithium nitride (Li3N), leading to improved NH3 production. The synergistic effect of Ohmic junctions and Cu vacancies in Au@Cu2-xSe led to significantly higher ammonia yield and faradaic efficiency (FE) in nonaqueous systems compared to those in aqueous conditions. The maximum NH3 yields were approximately 1.10 and 3.64 μg h-1 cm-2, with the corresponding FE of 2.24 and 67.52% for aqueous and nonaqueous electrolytes, respectively. This study demonstrates an attractive strategy for designing catalysts with increased ENRR activity by effectively engineering vacancies and heterojunctions in Cu-based electrocatalysts in both aqueous and nonaqueous media.

Alkali treatment of layered double hydroxide nanosheets as highly efficient bifunctional electrocatalysts for overall water splitting

Efficient and economic bifunctional electrocatalyst for water splitting to produce hydrogen is urgently required. The layered double hydroxides (LDHs) have shown superior activity for oxygen evolution reaction (OER) for water electrolysis, while their hydrogen evolution reaction (HER) activity remains challenging. Herein, we report an alkali hydrothermal-treatment strategy to enhance the HER as well as OER performance of NiCo-LDH. This method can create metal vacancies and newly formed Ni/Co(OH)₂ phase over NiCo-LDH, tune the electronic structure, and improve the electrical conductivity, thereby improving the electrochemical activity. The NiCo-LDH-OH catalyst delivers a current density of 10 mA cm^-2 at an overpotential of 180 mV for HER and an overpotential of 317 mV for OER, which is greatly reduced compared to the pristine NiCo-LDH (295 mV for HER and 336 mV for OER). When assembled into an electrolyzer both as a cathode and anode, it demonstrates superior activity for overall water splitting with no obvious decay after 20 h. This work paves a new path for fabricating efficient LDHs-based HER/OER bifunctional catalysts.

Interface strong-coupled 3D Mo-NiS@Ni-Fe LDH flower-cluster as exceptionally efficient electrocatalyst for water splitting in wide pH range

It is crucial to create a bifunctional catalyst with high efficiency and low cost for electrochemical water splitting under alkaline and neutral pH conditions. This study investigated the in-situ creation of ultrafine Mo-NiS and NiFe LDH nanosheets as an effective and stable electrocatalyst with a three-dimensional (3D) flower-cluster hierarchical structure (Mo-NiS@NiFe LDH). The strong interfacial connection between Mo-NiS and NiFe LDH enhances the formation of metal higher chemical states in the material, optimizes the electronic structure, increases OH^- adsorption capacity improves electron transfer/mass diffusion, and promotes O₂/H₂ gas release. As a result, at 10 mA cm^-2, Mo-NiS@NiFe LDH/NF demonstrates the outstanding bifunctional electrocatalytic activity of just 107 mV (HER, hydrogen evolution reaction) and 184 mV (hydrogen evolution reaction) (OER, oxygen evolution reaction). The catalytic performance is remarkably stable after 72 h of continuous operation in 1 M KOH at high current densities (300 mA cm^-2). More interestingly, in the overall water splitting system, the cell voltages for anode and cathode in both alkaline and neutral electrolytes for Mo-NiS@NiFe LDH/NF are only 1.54 V (alkaline) and 2.06 V (neutral) at 10 mA cm^-2. These results demonstrated that the bifunctional electrocatalyst design concept is a viable solution for water splitting in both alkaline and neutral systems.

Recent Advances of Modified Ni (Co, Fe)-Based LDH 2D Materials for Water Splitting

Water splitting technology is an efficient approach to produce hydrogen (H₂) as an energy carrier, which can address the problems of environmental deterioration and energy shortage well, as well as establishment of a clean and sustainable hydrogen economy powered by renewable energy sources due to the green reaction of H₂ with O₂. The efficiency of H₂ production by water splitting technology is intimately related with the reactions on the electrode. Nowadays, the efficient electrocatalysts in water splitting reactions are the precious metal-based materials, i.e., Pt/C, RuO₂, and IrO₂. Ni (Co, Fe)-based layered double hydroxides (LDH) two-dimensional (2D) materials are the typical non-precious metal-based materials in water splitting with their advantages including low cost, excellent electrocatalytic performance, and simple preparation methods. They exhibit great potential for the substitution of precious metal-based materials. This review summarizes the recent progress of Ni (Co, Fe)-based LDH 2D materials for water splitting, and mainly focuses on discussing and analyzing the different strategies for modifying LDH materials towards high electrocatalytic performance. We also discuss recent achievements, including their electronic structure, electrocatalytic performance, catalytic center, preparation process, and catalytic mechanism. Furthermore, the characterization progress in revealing the electronic structure and catalytic mechanism of LDH is highlighted in this review. Finally, we put forward some future perspectives relating to design and explore advanced LDH catalysts in water splitting.

Electrodeposition of NiFe-layered double hydroxide layer on sulfur-modified nickel molybdate nanorods for highly efficient seawater splitting

Developing high-efficiency and earth-abundant electrocatalysts for electrochemical seawater-splitting is of great significance but remains a grand challenge due to the presence of high-concentration chloride. This work presents the synthesis of a three-dimensional core-shell nanostructure with an amorphous and crystalline NiFe-layered double hydroxide (NiFe-LDH) layer on sulfur-modified nickel molybdate nanorods supported by porous Ni foam (S-NiMoO₄@NiFe-LDH/NF) through hydrothermal and electrodeposition. Benefiting from high intrinsic activity, plentiful active sites, and accelerated electron transfer, S-NiMoO₄@NiFe-LDH/NF displays an outstanding bifunctional catalytic activity toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in both simulated alkaline seawater and natural seawater electrolytes. To reach a current density of 100 mA cm^-2, this catalyst only requires overpotentials of 273 and 315 mV for OER and 170 and 220 mV for HER in 1 M KOH + 0.5 M NaCl freshwater and 1 M KOH + seawater electrolytes, respectively. Using S-NiMoO₄@NiFe-LDH as both anode and cathode, the electrolyzer shows superb overall seawater-splitting activity, and respectively needs low voltages of 1.68 and 1.73 V to achieve a current density of 100 mA cm^-2 in simulated alkaline seawater and alkaline natural seawater electrolytes with good Cl^- resistance and satisfactory durability. The electrolyzer outperforms the benchmark IrO₂||Pt/C pair and many other reported bifunctional catalysts and exhibits great potential for realistic seawater electrolysis.

Heterostructure of Semiconductors on Self-Supported Cuprous Phosphide Nanowires for Enhanced Overall Water Splitting

Rational design, controllable synthesis, and an in-depth mechanism study of Cu-based bifunctional semiconductor heterostructures toward overall water splitting (OWS) are imperative but still face challenges. Herein, n-type iron oxide and p-type nickel phosphide and cobalt phosphide are respectively coupled with p-type cuprous phosphide nanowires on Cu foams via a general growth-phosphorization strategy. These self-supported semiconductor heterojunctions with different built-in potentials (E_BI) are used as binder-free electrodes for OWS and exhibit significantly improved electrocatalytic activities compared to their counterparts. Among them, the heterostructure with the largest E_BI of 1.57 V attains the smallest overpotential of 97 mV at 10 mA cm^-2 for the hydrogen evolution reaction and 243 mV at 50 mA cm^-2 for the oxygen evolution reaction in 1 M KOH. The corresponding two-electrode electrolyzer requires a cell voltage of 1.685 V at 50 mA cm^-2 and shows admirable long-term stability at 100 mA cm^-2 with a Faraday efficiency of around 98%. These promoted electrocatalytic performances originate from the enhanced active site, accelerated charge transfer, enlarged electrochemical active surface area, and synergy between different components at the heterointerface. This work represents a promising avenue to construct cost-efficient semiconductor heterostructures as bifunctional electrocatalysts applied to the sustainable energy industry.

PEO-PPO-PEO induced holey NiFe-LDH nanosheets on Ni foam for efficient overall water-splitting and urea electrolysis

Exploring the transition-metal-based bifunctional electrocatalysts with high performance for efficient water-splitting and urea electrolysis is significant but challenging. This work presents the in situ preparation of holey NiFe-LDH nanosheets on Ni foam (H-NiFe-LDH/NF) via a one-step hydrothermal method in the presence of PEO-PPO-PEO as the soft template. The holey NiFe-LDH nanosheets provide a high electrochemical surface area, more edge catalytic sites, and abundant oxygen vacancies. Consequently, H-NiFe-LDH/NF exhibits excellent catalytic activity to oxygen evolution, urea oxidation, and hydrogen evolution reactions (OER, UOR, and HER) with good stability in alkaline electrolytes. This electrode requires an overpotential of 261 mV for the OER, a potential of 1.480 V for the UOR to achieve a current density of 100 mA cm^-2 in alkaline solutions. By employing the self-supported electrode as both the anode and cathode, this electrolysis cell (H-NiFe-LDH/NF||H-NiFe-LDH/NF) gains current densities of 10 and 100 mA cm^-2 at low cell voltages of 1.575 and 1.933 V in the 1.0 M KOH solution. After adding 0.33 M urea, the voltages to deliver 10 and 100 mA cm^-2 respectively decrease to 1.418 and 1.691 V. The H-NiFe-LDH/NF electrode also shows excellent stability for water-splitting and urea electrolysis. This work not only contributes to developing a low-cost, high-efficiency, bifunctional electrocatalyst but also provides a practically feasible approach for urea-rich wastewater electrolysis.

Heterogeneous interface-induced electrocatalytic efficiency boosting of bimetallic Cu/Zn selenides for stable water oxidation and oxygen reduction reactions

Low overpotential and high stability of heterostructured bimetallic copper and zinc selenides for OER and ORR.

Self-driven Ru-modified NiFe MOF nanosheet as multifunctional electrocatalyst for boosting water and urea electrolysis

Urea electro-oxidation reaction (UOR) has been a promising strategy to replace oxygen evolution reaction (OER) by urea-mediated water splitting for hydrogen production. Naturally, rational design of high-efficiency and multifunctional electrocatalyst towards UOR and hydrogen evolution reaction (HER) is of vital significance, but still a grand challenge. Herein, an innovative 3D Ru-modified NiFe metal-organic framework (MOF) nanoflake array on Ni foam (Ru-NiFe-x/NF) was elaborately designed via spontaneous galvanic replacement reaction (GRR). Notably, the adsorption capability of intermediate species (H*) of catalyst is significantly optimized by Ru modification. Meanwhile, rich high-valence Ni active species can be acquired by self-driven electronic reconstruction in the interface, then dramatically accelerating the electrolysis of water and urea. Remarkably, the optimized Ru-NiFe-③/NF (1.6 at% of Ru) only requires the overpotential of 90 and 310 mV to attain 100 mA cm^-2 toward HER and OER in alkaline electrolyte, respectively. Impressively, an ultralow voltage of 1.47 V is required for Ru-NiFe-③/NF to deliver a current density of 100 mA cm^-2 in urea-assisted electrolysis cell with superior stability, which is 190 mV lower than that of Pt/C-NF||RuO₂/NF couple. This work is desired to explore a facile way to exploit environmentally-friendly energy by coupling hydrogen evolution with urea-rich sewage disposal.