Universal Surface Engineering of Transition Metals for Superior Electrocatalytic Hydrogen Evolution in Neutral Water

Heterogeneous Interfaces of Ni3Se4 Nanoclusters Decorated on a Ni3N Surface Enhance Efficient and Durable Hydrogen Evolution Reactions in Alkaline Electrolyte

Transition metal selenides (TMSes) have been identified as cost-efficient alternatives to platinum (Pt) for the alkaline hydrogen evolution reaction (HER) owing to their distinct electronic properties and excellent conductivity. However, they encounter challenges such as sluggish water dissociation and severe oxidative degradation, requiring further optimizations. In this study, we developed a dual-site heterogeneous catalyst, Ni3Se4-Ni3N, by decorating Ni3Se4 nanoclusters on a Ni3N substrate. This catalyst design promoted significant interfacial electronic interactions, modulated electronic structures, and enhanced the adsorption of the intermediates. Various spectroscopic analyses and theoretical calculations revealed that the nitride surfaces improved water adsorption and dissociation, enriching the surface with adsorbed hydrogen (H*) atoms, while the Se sites facilitated hydrogen coupling and subsequent release of H2. Following a hydrogen spillover mechanism, the surface-adsorbed hydrogen atoms were transferred to nearby electron-dense selenide sites for H2 formation and release. Consequently, the optimized catalyst demonstrated improved HER activity, requiring only an ∼60 mV overpotential at 10 mA cm-2 current density and maintained stability under higher potential conditions.

Hydrazine-assisted water electrolysis system: performance enhancement and application expansion

Powered by renewable energy sources, water electrolysis has emerged as a highly promising technology for energy conversion, attracting significant attention in recent years, but it faces severe challenges, especially at the anode. Accordingly, hydrazine-assisted water electrolysis, incorporating the electro-oxidation of hydrazine at the anode, holds great promise for greatly reducing the input voltage and optimizing the system by application expansion. In this review, we present an in-depth overview of hydrazine-assisted water electrolysis, introducing its reaction mechanisms, basic parameters, specific advantages compared with conventional water electrolysis and other hybrid water electrolysis systems, strategies for developing efficient electrocatalysts with enhanced electrocatalytic performances, and especially its potential application expansion. An analysis of its technical and economic aspects, feasibility studies, mechanistic investigations, and relevant comparisons are also presented for providing a deeper insight into hydrazine-assisted water electrolysis. Finally, the potential avenues and opportunities for future research on hydrazine-assisted water electrolysis are discussed.

Interface Charge Transfer of Heteroatom Boron Doping Cobalt and Cobalt Nitride for Boosting Water Oxidation

Designing high-performance transition-metal electrocatalysts with controlled active heterointerfacial sites for catalyzing the electrochemical oxygen evolution reaction (OER) is very desirable but remains a great challenge. Here, a facile strategy for the synthesis of transition-metal nitride-based interfacial electrocatalysts boron-doped cobalt/cobalt nitride (B-Co/Co2N) is demonstrated with optimal heterointerfaces between Co and Co2N electrocatalysts by introducing boron as a dopant to the former. Benefiting from the unique electronegativity of B, the obtained B-Co/Co2N electrocatalysts show excellent OER performance with overpotential inputs of as low as 262 and 310 mV for 10 and 100 mA cm-2, which are 1.4 and 6.6 times higher than those of Co/Co2N with the same potential input, respectively. The experimental and theoretical results demonstrate the role of the B dopant in inducing charge redistribution of Co active sites in the Co/Co2N interfacial region, which results in a downshift of the Co 3d band center, the optimal oxidation state of active sites for *OOH formation, and lower energy barriers. Furthermore, the assembled electrolyzer can steadily produce an industrial-grade current density of 1000 mA cm-2 at a cell voltage input of only 1.81 V for at least 100 h with a Faradaic efficiency near 100%. This study provides a promising strategy for heteroatom-doped interfacial electrocatalysts with high performance for energy and environmental applications.

Non-metallic surface-modified X-Cu (X = F, Cl, Br) metal catalysts for all-pH hydrogen evolution reaction with high performance

Producing hydrogen through water electrolysis represents a clean and sustainable solution that is crucial in addressing the energy crisis. Nonetheless, the slow process of water electrolysis leads to large different kinetics for hydrogen evolution reaction (HER) under different pH solutions. Here, we designed surface modified metallic X-Cu catalysts (X = F, Cl, Br) with different non-metallic elements on nickel foam (NF) using an electrochemical deposition method, which realizes high performance for all-pH range. In 1.0 M KOH, 1.0 M phosphate-buffered saline (PBS), and 0.5 M H2SO4 media, F-Cu catalyst reaches 10 mA cm-2 with overpotentials of 56 mV, 110 mV, and 197 mV, respectively. Theoretical calculations disclose that the surface modification of F atom leads to redistribution of electrons, causing an upward shift of Cu's d-band center and enhanced adsorption ability for H2O and H intermediates (H*). This work offers novel perspectives for designing Cu-based catalysts with high HER performance, making them applicable across all-pH conditions.

Tailoring Lewis Acidity of Metal Oxides on Nickel to Boost Electrocatalytic Hydrogen Evolution in Neutral Electrolyte

Neutral-pH water splitting for hydrogen production features a benign environment that could alleviate catalyst and electrolyzer corrosion but calls for the corresponding high-efficiency and earth-abundant hydrogen evolution reaction (HER) catalysts. Herein, we first designed a series of metal oxides decorated on Ni as the model catalysts and found a volcano-shaped relationship between the Lewis acidity of Ni/metal oxides and HER activity in neutral media. The Ni/ZnO with the optimum Lewis acidity could balance water dissociation and hydroxyl desorption, thereby greatly boosting the HER. On the basis of this finding, we further in situ grew the Ni/ZnO heterostructure on a three-dimensional conductive support. The resulting catalyst requires overpotentials of merely 34 and 194 mV to deliver the current densities of 10 and 200 mA cm-2, respectively, and can stably operate at these current densities for 2000 h in 1 M phosphate buffer solution (pH 7), representing the most active and durable HER catalyst in neutral electrolyte reported thus far. Our work provides an effective design scheme for low-cost and high-performance neutral HER catalysts.

Transforming Adsorbate Surface Dynamics in Aqueous Electrocatalysis: Pathways to Unconstrained Performance

Developing highly efficient catalysts to accelerate sluggish electrode reactions is critical for the deployment of sustainable aqueous electrochemical technologies, yet remains a great challenge. Rationally integrating functional components to tailor surface adsorption behaviors and adsorbate dynamics would divert reaction pathways and alleviate energy barriers, eliminating conventional thermodynamic constraints and ultimately optimizing energy flow within electrochemical systems. This approach has, therefore, garnered significant interest, presenting substantial potential for developing highly efficient catalysts that simultaneously enhance activity, selectivity, and stability. The immense promise and rapid evolution of this design strategy, however, do not overshadow the substantial challenges and ambiguities that persist, impeding the realization of significant breakthroughs in electrocatalyst development. This review explores the latest insights into the principles guiding the design of catalytic surfaces that enable favorable adsorbate dynamics within the contexts of hydrogen and oxygen electrochemistry. Innovative approaches for tailoring adsorbate-surface interactions are discussed, delving into underlying principles that govern these dynamics. Additionally, perspectives on the prevailing challenges are presented and future research directions are proposed. By evaluating the core principles and identifying critical research gaps, this review seeks to inspire rational electrocatalyst design, the discovery of novel reaction mechanisms and concepts, and ultimately, advance the large-scale implementation of electroconversion technologies.

Recent Developments in Membrane-Free Hybrid Water Electrolysis for Low-Cost Hydrogen Production Along with Value-Added Products

Water electrolysis using renewable energy is considered as a promising technique for sustainable and green hydrogen production. Conventional water electrolysis has two components - hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occurring at the cathode and anode respectively. However, electrolysis of water suffers from high overpotential due to the slow kinetics of OER. To overcome this hybrid water electrolysis has been developed by replacing conventional anode oxidation producing oxygen with oxidation of cost-effective materials producing value-added chemicals. This review summarizes recent advances in organic oxidative reactions such as alcohols, urea, hydrazine, and biomass at the anode instead of OER. Furthermore, the review also highlights the use of membrane-free hybrid water electrolysis as a method to overcome the cost and complexity associated with conventional membrane-based electrolyzer thereby improving overall efficiency. This approach holds promise for scalable and cost-effective large-scale hydrogen production along with value-added products. Finally, current challenges and future perspectives are discussed for further development in membrane-free hybrid water electrolysis.

Heteroatom Engineering in Earth-Abundant Cobalt Electrocatalyst for Energy-Saving Hydrogen Evolution Coupling with Urea Oxidation

The development of multifunctional electrocatalysts with high performance for electrocatalyzing urea oxidation-assisted water splitting is of great significance for energy-saving hydrogen production. In this work, we demonstrate a novel heteroatom engineering strategy for development of B-doped Co as a multifunctional electrocatalyst for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and urea oxidation reaction (UOR). Density functional theory (DFT) results suggest that a B dopant can efficiently adjust the electron reconstruction of the exposure of Co sites nearby and facilitate electron transfer, resulting in an optimal d-band center along with a lower Gibbs free energy barrier. Ultimately, the obtained B-Co exhibits pH-universal HER properties in various electrolytes. A highly efficient HER performance with overpotentials as low as 27, 163, and 430 mV to -10, -100, and -500 mA cm-2 in 1.0 M KOH, respectively, is observed for the B-Co electrode. More importantly, the UOR-assisted electrolyzer only requires a voltage input of 1.55 V to produce the current densities of 50 mA cm-2, resulting in a 200 mV saving-energy potential compared to water electrolysis, demonstrating its high efficiency of hydrogen production in industrial applications.

Energy-Efficient Co-production of Benzoquinone and H2 Using Waste Phenol in a Hybrid Alkali/Acid Flow Cell

In both the manufacturing and chemical industries, benzoquinone is a crucial chemical product. A perfect and economical method for making benzoquinone is the electrochemical oxidation of phenol, thanks to the traditional thermal catalytic oxidation of phenol process requires high cost, serious pollution and harsh reaction conditions. Here, a unique heterostructure electrocatalyst on nickel foam (NF) consisting of nickel sulfide and nickel oxide (Ni9S8-Ni15O16/NF) was produced, and this catalyst exhibited a low overpotential (1.35 V vs. RHE) and prominent selectivity (99 %) for electrochemical phenol oxidation reaction (EOP). Ni9S8-Ni15O16/NF is beneficial for lowering the reaction energy barrier and boosting reactivity in the EOP process according to density functional theory (DFT) calculations. Additionally, an alkali/acid hybrid flow cell was successfully established by connecting Ni9S8-Ni15O16/NF and commercial RuIr/Ti in series to catalyze phenol oxidation in an alkaline medium and hydrogen evolution in an acid medium, respectively. A cell voltage of only 0.60 V was applied to produce a current density of 10 mA cm-2. Meanwhile, the system continued to operate at 0.90 V for 12 days, showing remarkable long-term stability. The unique configuration of the acid-base hybrid flow cell electrolyzer provides valuable guidance for the efficient and environmentally friendly electrooxidation of phenol to benzoquinone.

Improving NiFe Electrocatalysts through Fluorination-Driven Rearrangements for Neutral Water Electrolysis

Neutral electrolysis to produce hydrogen is prime challenging owing to the sluggish kinetics of water dissociation for the electrochemical reduction of water to molecular hydrogen. An ion-enriched electrode/electrolyte interface for electrocatalytic reactions can efficiently obtain a stable electrolysis system. Herein, we found that interfacial accumulated fluoride ions and the anchored Pt single atoms/nanoparticles in catalysts can improve hydrogen evolution reaction (HER) activity of NiFe-based hydroxide catalysts, prolonging the operating stability at high current density in neutral conditions. NiFe hydroxide electrode obtains an outstanding performance of 1000 mA cm-2 at low overpotential of 218 mV with 1000 h operation at 100 mA cm-2. Electrochemical experiments and theoretical calculations have demonstrated that the interfacial fluoride contributes to promote the adsorption of Pt to proton for sustaining a large current density at low potential, while the Pt single atoms/nanoparticles provide H adsorption sites. The synergy effect of F and Pt species promotes the formation of Pt─H and F─H bonds, which accelerate the adsorption and dissociation process of H2O and promote the HER reaction with a long-term durability in neutral conditions.

An asymmetrically coordinated Zn-Co diatomic site catalyst for efficient hydrogen evolution reactions

We propose an innovative preparation method, namely, a two-step pyrolysis process, to synthesize Zn-Co bimetallic catalysts with excellent hydrogen evolution performance. In the synthesized Zn1Co1-SNC catalyst, there exists a strong interaction between Zn and Co, along with synergistic effects with S/N atoms, collectively promoting the stability of the catalyst structure. Experimental results demonstrate that the overpotential of this catalyst at 10 mA cm-2 current density is only 49 mV, and it maintains excellent hydrogen evolution performance even after 5000 cycles.

Electronic redistribution through the interface of MnCo2O4-Ni3N nano-urchins prompts rapid In situ phase transformation for enhanced oxygen evolution reaction

One of the most coveted objectives in the realm of energy conversion technologies is the development of highly efficient and economically viable electrocatalysts for the oxygen evolution reaction. The commercialization of such techniques has thus far been impeded by their slow response kinetics. One of the many ways to develop highly effective electrocatalysts is to judiciously choose a coupling interface that maximizes catalyst performance. In this study, the in situ electrochemical phase transformation of MnCo2O4-Ni3N into MnCo2O4-NiOOH is described. The catalyst has an exceptional overpotential of 224 mV to drive a current density of 10 mA cm-2. Strong interfacial contact is seen in the MnCo2O4-Ni3N catalyst, leading to a considerable electronic redistribution between the MnCo2O4 and Ni3N phases. This causes an increase in the valence state of Ni, which makes it an active site for the adsorption of *OH, O*, and *OOH (intermediates). This charge transfer facilitates the rapid phase transformation to form NiOOH from Ni3N. At a higher current density of 300 mA cm-2, the catalyst remained stable for a period of 140 h. DFT studies also revealed that the in situ-formed NiOOH on the MnCo2O4 surface results in superior OER kinetics compared to that of NiOOH alone.

Enhancing Electrocatalytic CO2-to-CO Conversion by Weakening CO Binding through Nitrogen Integration in the Metallic Fe Catalyst

Metallic iron (Fe) typically demonstrates the unfavorable catalytic activity for the CO2 reduction reaction (CO2RR), mainly attributed to the excessively strong binding of CO products on Fe sites. Toward this end, we employed an effective approach involving electronic structure modulation through nitrogen (N) integration to enhance the performance of the CO2RR. Here, an efficient catalyst has been developed, composed of N-doped metallic iron (Fe) nanoparticles encapsulated in a porous N-doped carbon framework. Notably, this N-integrated Fe catalyst displays significantly enhanced performance in the electrocatalytic reduction of CO2, yielding the highest CO Faradaic efficiency of 97.5% with a current density of 6.68 mA cm-2 at -0.7 V versus the reversible hydrogen electrode. The theoretical calculations, combined with the in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy study, reveal that N integration modulates the electron density around Fe, resulting in the weakening of the binding strength between the Fe active sites and *CO intermediates, consequently promoting the desorption of CO and the overall CO2RR process.

Hydrogen Evolution in Neutral Media by Differential Intermediate Binding at Charge-Modulated Sites of a Bimetallic Alloy Electrocatalyst

The energy barrier to dissociate neutral water has been lowered by the differential intermediate binding on the charge-modulated metal centers of Co85Mo15 sheets supported on Ni-foam (NF), where the overpotential for hydrogen evolution reaction (HER) in 1 M phosphate buffer solution (PBS) is only 50±9 mV at -10 mA cm-2. It has a turnover frequency (TOF) of 0.18 s-1, mass activity of 13.2 A g-1 at -200 mV vs. reversible hydrogen electrode (RHE), and produces 16 ml H2 h-1 at -300 mV vs. RHE, more than double that of 20 % Pt/C. The Moδ+ and Coδ- sites adsorb OH*, and H*, respectively, and the electron injection from Co to H-O-H cleaves the O-H bond to form the Mo-OH* intermediate. Operando spectral analyses indicate a weak H-bonded network for facilitating the H2O*/OH* transport, and a potential-induced reversal of the charge density from Co to the more electronegative Mo, because of the electron withdrawing Co-H* and Mo-OH* species. Co85Mo15/NF can also drive the complete electrolysis of neutral water at only 1.73 V (10 mA cm-2). In alkaline, and acidic media, it demonstrates a Pt-like HER activity, accomplishing -1000 mA cm-2 at overpotentials of 161±7, and 175±22 mV, respectively.

Nickel Nanoparticles Protruding from Molybdenum Carbide Micropillars with Carbon Layer-Protected Biphasic 0D/1D Heterostructures for Efficient Water Splitting

It remains a tremendous challenge to achieve high-efficiency bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for hydrogen production by water splitting. Herein, a novel hybrid of 0D nickel nanoparticles dispersed on the one-dimensional (1D) molybdenum carbide micropillars embedded in the carbon layers (Ni/Mo2C@C) was successfully prepared on nickel foam by a facile pyrolysis strategy. During the synthesis process, the nickel nanoparticles and molybdenum carbide were simultaneously generated under H2 and C2H2 mixed atmospheres and conformally encapsulated in the carbon layers. Benefiting from the distinctive 0D/1D heterostructure and the synergistic effect of the biphasic Mo2C and Ni together with the protective effect of the carbon layer, the reduced activation energy barriers and fast catalytic reaction kinetics can be achieved, resulting in a small overpotential of 96 mV for the HER and 266 mV for the OER at the current density of 10 mA cm-2 together with excellent durability in 1.0 M KOH electrolyte. In addition, using the developed Ni/Mo2C@C as both the cathode and anode, the constructed electrolyzer exhibits a small voltage of 1.55 V for the overall water splitting. The novel designed Ni/Mo2C@C may give inspiration for the development of efficient bifunctional catalysts with low-cost transition metal elements for water splitting.

Metal nitrides for seawater electrolysis

Electrocatalytic high-throughput seawater electrolysis for hydrogen production is a promising green energy technology that offers possibilities for environmental and energy sustainability. However, large-scale application is limited by the complex composition of seawater, high concentration of Cl- leading to competing reaction, and severe corrosion of electrode materials. In recent years, extensive research has been conducted to address these challenges. Metal nitrides (MNs) with excellent chemical stability and catalytic properties have emerged as ideal electrocatalyst candidates. This review presents the electrode reactions and basic parameters of the seawater splitting process, and summarizes the types and selection principles of conductive substrates with critical analysis of the design principles for seawater electrocatalysts. The focus is on discussing the properties, synthesis, and design strategies of MN-based electrocatalysts. Finally, we provide an outlook for the future development of MNs in the high-throughput seawater electrolysis field and highlight key issues that require further research and optimization.

Efficient and selective electrosynthesis of adiponitrile by electrohydrodimerization of acrylonitrile over a bismuth nanosheet modified electrode

The developed bismuth nanosheet electrode was explored for the first time, to exhibit high-performance toward the electrosynthesis of ADN, making it a promising alternative to the toxic electrode materials currently used in industrial ADN production.

Enhanced Hydrogen Storage Capacity in Two-Dimensional Fullerene Networks

In contrast to isolated C60 molecular dispersion in solvents, the monolayer C60 networks synthesized by Hou et al. (Nature 2022, 606, 507-510) feature compact nanocages, serving as natural containers for hydrogen storage. The anisotropic lattice and intrinsic local strains induce delocalization of conjugated π orbitals within C60, enabling hydrogen chemisorption without an additional chemical modification. Through first-principles calculations and molecular dynamics simulations, we reveal the correspondence between chemisorption sites and orbital distributions, determining the orientation of polyhedrons formed by physiosorbed hydrogen molecules. The combination of chemisorption and physisorption processes significantly enhances hydrogen storage capacity in monolayer C60 networks while maintaining the thermodynamic stability of the nanocage structures. Numerical results indicate a maximum internal hydrogen pressure exceeding 116 GPa at room temperature and atmospheric pressure. These findings suggest that monolayer C60 networks are promising solid-state candidates for highly efficient hydrogen storage.

Precise Interstitial Built-In Electric Field Tuning for Hydrogen Evolution Electrocatalysis

The built-in electric field (BEF) has become an effective means of adjusting the electronic structure and hydrogen spillover to influence the adsorption of intermediates. However, the previously reported BEF cannot be tuned continuously and precisely. Herein, a series of nanocatalysts with interstitial BEF were successfully synthesized, and the effect of precisely tuned interstitial BEF on the intermediate's adsorption and hydrogen spillover was systematically investigated using changing the insertion of interstitial B. Three catalysts with different BEF strengths were obtained by changing the interstitial content (B0.22-Cu/NC, B0.30-Cu/NC, B0.41-Cu/NC), and it was demonstrated that B0.30-Cu/NC gave the best catalytic performance for hydrogen evolution reactions (HERs). The turnover frequency (TOF) value is shown to reach 0.36 s-1 at just -0.1 V vs. RHE, which is about 3 times that of Cu (0.12 s-1). For the HER, it is one of the best Cu-based catalysts reported to date (Table S3). Besides, when the catalyst was applied to the cathode of the PEM water electrolyzer, B0.30-Cu/NC exhibited long-time stability at a water-splitting current density of 500 mA cm-2. Density functional theory and in situ Raman spectroscopy suggest that a suitable interstitial BEF can not only optimize the intermediate's adsorption but also promote hydrogen spillover.

Aluminum-incorporation activates vanadium carbide with electron-rich carbon sites for efficient pH-universal hydrogen evolution reaction

Vanadium carbide (VC) is the greatest potential hydrogen evolution reaction (HER) catalyst because of its platinum-like property and abundant earth reserves. However, it exhibits insufficient catalytic performance due to the unfavorable interaction of reaction intermediates with catalysts. In this work, using NH4VO3 as the main raw material, the flow ratio of CH4 to Ar was accurately controlled, and a non-transition metal Al-doped into VC (100) nano-flowers with carbon hybrids on nickel foams (Al-VC@C/NF) was prepared for the first time as a high-efficiency HER catalyst by chemical vapor carbonization. The overpotential of Al-VC@C/NF catalysts in 0.5 M H2SO4 and 1 M KOH at a current density of 10 mA cm-2 are only 58 mV and 97 mV, respectively, which are the best HER performance among non-noble metal vanadium carbide based catalysts. Simultaneously, Al-VC@C/NF exhibits small Tafel slope (45 mV dec-1 and 73 mV dec-1) and excellent stability in acidic and alkaline media. Theoretical calculations demonstrate that doped Al atoms can induce electron redistribution on the vanadium carbide surface to form electron-rich carbon sites, which significantly reduces the energy barrier during the HER process. This work provides a new tactic to modulate vanadium-based carbons as efficient HER catalysts through non-transition metal doping.

Emerging Electrocatalysts in Urea Production

Urea synthesis from abundant CO2 and N-feedstocks via renewable electricity has attracted increasing interests, offering a promising alternative to the industrial-applied Haber-Meiser process. However, the studies toward electrochemical urea production remain scarce and appeal for more research. Herein, in this perspective, an up-to-date overview on the urea electrosynthesis is highlighted and summarized. Firstly, the reaction pathways of urea formation through various feedstocks are comprehensively discussed. Then, we focus on the strategies of materials design to improve C-N coupling efficiency by identifying the descriptor and understanding the reaction mechanism. Finally, the current challenges and disadvantages in this field are reviewed and some future development directions of electrocatalytic urea synthesis are also prospected. This Minireview aims to promote future investigations of the electrochemical urea synthesis.

Single-Atom Pt Embedded in Defective Layered Double Hydroxide for Efficient and Durable Hydrogen Evolution

Electrocatalysis in neutral conditions is appealing for hydrogen production by utilizing abundant wastewater or seawater resources. Single-atom catalysts (SACs) immobilized on supports are considered one of the most promising strategies for electrocatalysis research. While they have principally exhibited breakthrough activity and selectivity for the hydrogen evolution reaction (HER) electrocatalysis in alkaline or acidic conditions, few SACs were reported for HER in neutral media. Herein, we report a facile strategy to tailor the water dissociation active sites on the NiFe LDH by inducing Mo species and an ultralow single atomic Pt loading. The defected NiFeMo LDH (V-NiFeMo LDH) shows HER activity with an overpotential of 89 mV at 10 mA cm-2 in 1 M phosphate buffer solutions. The induced Mo species and the transformed NiO/Ni phases after etching significantly increase the electron conductivity and the catalytic active sites. A further enhancement can be achieved by modulating the ultralow single atom Pt anchored on the V-NiFeMo LDH by potentiostatic polarization. A potential as low as 37 mV is obtained at 10 mA cm-2 with a pronounced long-term durability over 110 h, surpassing its crystalline LDH materials and most of the HER catalysts in neutral medium. Experimental and density functional theory calculation results have demonstrated that the synergistic effects of Mo/SAs Pt and phase transformation into NiFe LDH reduce the kinetic energy barrier of the water dissociation process and promote the H* conversion for accelerating the neutral HER.

Dynamic Electrodeposition on Bubbles: An Effective Strategy toward Porous Electrocatalysts for Green Hydrogen Cycling

ConspectusClosed-loop cycling of green hydrogen is a promising alternative to the current hydrocarbon economy for mitigating the energy crisis and environmental pollution. It stores energy from renewable energy sources like solar, wind, and hydropower into the chemical bond of dihydrogen (H₂) via (photo)electrochemical water splitting, and then the stored energy can be released on demand through the reverse reactions in H₂-O₂ fuel cells. The sluggish kinetics of the involved half-reactions like hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and oxygen reduction reaction (ORR) limit its realization. Moreover, considering the local gas-liquid-solid triphase microenvironments during H₂ generation and utilization, rapid mass transport and gas diffusion are critical as well. Accordingly, developing cost-effective and active electrocatalysts featuring three-dimensional hierarchically porous structures are highly desirable to promote the energy conversion efficiency. Traditionally, the synthetic approaches of porous materials include soft/hard templating, sol-gel, 3D printing, dealloying, and freeze-drying, which often need tedious procedures, high temperature, expensive equipment, and/or harsh physiochemical conditions. In contrast, dynamic electrodeposition on bubbles using the in situ formed bubbles as templates can be conducted at ambient conditions with an electrochemical workstation. Moreover, the whole preparation process can be finished within minutes/hours, and the resulting porous materials can be employed as catalytic electrodes directly, avoiding the use of polymeric binders like Nafion and the consequent issues like limited catalyst loading, reduced conductivity, and inhibited mass transport.In this Account, we summarize our contributions to the dynamic electrodeposition on bubbles toward advanced porous electrocatalysts for green hydrogen cycling. These dynamic electrosynthesis strategies include potentiodynamic electrodeposition that linearly scans the applied potentials, galvanostatic electrodeposition that fixes the applied currents, and electroshock which quickly switches the applied potentials. The resulting porous electrocatalysts range from transition metals to alloys, nitrides, sulfides, phosphides, and their hybrids. We mainly focus on the 3D porosity design of the electrocatalysts by tuning the electrosynthesis parameters to tailor the behaviors of bubble co-generation and thus the reaction interface. Then, their electrocatalytic applications for HER, OER, overall water splitting (OWS), biomass oxidation (to replace OER), and HOR are introduced, with a special emphasis on the porosity-promoted activity. Finally, the remaining challenges and future perspective are also discussed. We hope this Account will encourage more efforts into this attractive research field of dynamic electrodeposition on bubbles for various energy catalytic reactions like carbon dioxide/monoxide reduction, nitrate reduction, methane oxidation, chlorine evolution, and others.

Ensemble Effect of Ruthenium Single-Atom and Nanoparticle Catalysts for Efficient Hydrogen Evolution in Neutral Media

Hydrogen evolution reaction (HER) plays a key role in electrochemical water splitting, which is a sustainable way for hydrogen production. The kinetics of HER is sluggish in neutral media that requires noble metal catalysts to alleviate energy consumption during the HER process. Here, we present a catalyst comprising a ruthenium single atom (Ru₁) and nanoparticle (Ru_n) loaded on the nitrogen-doped carbon substrate (Ru₁-Ru_n/CN), which exhibits excellent activity and superior durability for neutral HER. Benefiting from the synergistic effect between single atoms and nanoparticles in the Ru₁-Ru_n/CN, the catalyst exhibits a very low overpotential down to 32 mV at a current density of 10 mA cm^-2 while maintaining excellent stability up to 700 h at a current density of 20 mA cm^-2 during the long-term test. Computational calculations reveal that, in the Ru₁-Ru_n/CN catalyst, the existence of Ru nanoparticles affects the interactions between Ru single-atom sites and reactants and thus improves the catalytic activity of HER. This work highlights the ensemble effect of electrocatalysts for HER and could shed light on the rational design of efficient catalysts for other multistep electrochemical reactions.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

An inclusive review and perspective on Cu-based materials for electrochemical water splitting

In recent years, there has been a resurgence of interest in developing green and renewable alternate energy sources as a solution to the energy and environmental problems produced by conventional fossil fuel use. As a very effective energy transporter, hydrogen (H2) is a possible candidate for the future energy supply. Hydrogen production by water splitting is a promising new energy option. Strong, efficient, and abundant catalysts are required for increasing the efficiency of the water splitting process. Cu-based materials as an electrocatalyst have shown promising results for application in the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in water splitting. In this review, our aim is to cover the latest developments in the synthesis, characterisation, and electrochemical behaviour of Cu-based materials as a HER, and OER electrocatalyst, highlighting the impact that these advances have had on the field. It is intended that this review article will serve as a roadmap for developing novel, cost-effective electrocatalysts for electrochemical water splitting based on nanostructured materials with particular emphasis on Cu-based materials for electrocatalytic water splitting.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Selective Electrochemical Conversion of N2 to NH3 in Neutral Media Using B, N-Containing Carbon with a Nanotubular Morphology

Electrochemical dinitrogen reduction (NRR) has riveted substantial attention as a greener method to synthesize ammonia (NH₃) under ambient conditions. Here, B, N-containing carbon catalysts with a discrete morphology were synthesized from the metal-organic framework-ionic liquid (MOF-IL) composite for NRR in a neutral electrolyte medium (pH = 7). Morphology-dependent activity is witnessed, wherein C-BN@600 with a nanotubular morphology is able to achieve a high NH₃ yield rate of 204 μg h^-1 mg_cat^-1 and an F.E. of 16.7% with a TOF value of 0.2 h^-1 at -0.2 V vs RHE. Further, a rigorous protocol is put forward for true NH₃ estimation by tracing/eliminating any source of contamination in catalysts, electrolytes, or gas supply via ultraviolet-visible (UV-vis) spectroscopy, gas-purification methods, and isotope labeling experiments. Density functional theory predicts BN to be the favorable active site for N₂ adsorption with a reduced energy barrier in the first reduction step and sequential stabilization of the B-N bond by an adjacent carbon atom.

Pt nanoclusters on GaN nanowires for solar-asssisted seawater hydrogen evolution

Seawater electrolysis provides a viable method to produce clean hydrogen fuel. To date, however, the realization of high performance photocathodes for seawater hydrogen evolution reaction has remained challenging. Here, we introduce n⁺-p Si photocathodes with dramatically improved activity and stability for hydrogen evolution reaction in seawater, modified by Pt nanoclusters anchored on GaN nanowires. We find that Pt-Ga sites at the Pt/GaN interface promote the dissociation of water molecules and spilling H* over to neighboring Pt atoms for efficient H₂ production. Pt/GaN/Si photocathodes achieve a current density of -10 mA/cm² at 0.15 and 0.39 V vs. RHE and high applied bias photon-to-current efficiency of 1.7% and 7.9% in seawater (pH = 8.2) and phosphate-buffered seawater (pH = 7.4), respectively. We further demonstrate a record-high photocurrent density of ~169 mA/cm² under concentrated solar light (9 suns). Moreover, Pt/GaN/Si can continuously produce H₂ even under dark conditions by simply switching the electrical contact. This work provides valuable guidelines to design an efficient, stable, and energy-saving electrode for H₂ generation by seawater splitting.

Tunable Structured MXenes With Modulated Atomic Environments: A Powerful New Platform for Electrocatalytic Energy Conversion

Owing to their rich surface chemistry, high conductivity, tunable bandgap, and thermal stability, structured 2D transition-metal carbides, nitrides, and carbonitrides (MXenes) with modulated atomic environments have emerged as efficient electrochemical energy conversion systems in the past decade. Herein, the most recent advances in the engineering of tunable structured MXenes as a powerful new platform for electrocatalytic energy conversion are comprehensively summarized. First, the state-of-the-art synthetic and processing methods, tunable nanostructures, electronic properties, and modulation principles of engineering MXene-derived nanoarchitectures are focused on. The current breakthroughs in the design of catalytic centers, atomic environments, and the corresponding structure-performance correlations, including termination engineering, heteroatom doping, defect engineering, heterojunctions, and alloying, are discussed. Furthermore, representative electrocatalytic applications of structured MXenes in energy conversion systems are also summarized. Finally, the challenges in and prospects for constructing MXene-based electrocatalytic materials are also discussed. This review provides a leading-edge understanding of the engineering of various MXene-based electrocatalysts and offers theoretical and experimental guidance for prospective studies, thereby promoting the practical applications of tunable structured MXenes in electrocatalytic energy conversion systems.

Reversible hydrogen spillover in Ru-WO3-x enhances hydrogen evolution activity in neutral pH water splitting

Noble metal electrocatalysts (e.g., Pt, Ru, etc.) suffer from sluggish kinetics of water dissociation for the electrochemical reduction of water to molecular hydrogen in alkaline and neutral pH environments. Herein, we found that an integration of Ru nanoparticles (NPs) on oxygen-deficient WO_3-x manifested a 24.0-fold increase in hydrogen evolution reaction (HER) activity compared with commercial Ru/C electrocatalyst in neutral electrolyte. Oxygen-deficient WO_3-x is shown to possess large capacity for storing protons, which could be transferred to the Ru NPs under cathodic potential. This significantly increases the hydrogen coverage on the surface of Ru NPs in HER and thus changes the rate-determining step of HER on Ru from water dissociation to hydrogen recombination.

While water splitting electrolysis offers an appealing means to produce H₂ fuel, catalysts show sluggish reaction rate in neutral media. Here, authors utilize hydrogen spillover from oxygen-deficient tungsten oxides to Ru nanoparticles to boost the neutral-pH H₂ evolution performances.

Ionization Energies and Redox Potentials of Hydrated Transition Metal Ions: Evaluation of Domain-Based Local Pair Natural Orbital Coupled Cluster Approaches

Hydrated transition metal ions are prototypical systems that can be used to model properties of transition metals in complex chemical environments. These seemingly simple systems present challenges for computational chemistry and are thus crucial in evaluations of quantum chemical methods for spin-state and redox energetics. In this work, we explore the applicability of the domain-based pair natural orbital implementation of coupled cluster (DLPNO-CC) theory to the calculation of ionization energies and redox potentials for hydrated ions of all first transition row (3d) metals in the 2+/3+ oxidation states, in connection with various solvation approaches. In terms of model definition, we investigate the construction of a minimally explicitly hydrated quantum cluster with a first and second hydration layer. We report on the convergence with respect to the coupled cluster expansion and the PNO space, as well as on the role of perturbative triple excitations. A recent implementation of the conductor-like polarizable continuum model (CPCM) for the DLPNO-CC approach is employed to determine self-consistent redox potentials at the coupled cluster level. Our results establish conditions for the convergence of DLPNO-CCSD(T) energetics and stress the absolute necessity to explicitly consider the second solvation sphere even when CPCM is used. The achievable accuracy for redox potentials of a practical DLPNO-based approach is, on average, 0.13 V. Furthermore, multilayer approaches that combine a higher-level DLPNO-CCSD(T) description of the first solvation sphere with a lower-level description of the second solvation layer are investigated. The present work establishes optimal and transferable methodological choices for employing DLPNO-based coupled cluster theory, the associated CPCM implementation, and cost-efficient multilayer derivatives of the approach for open-shell transition metal systems in complex environments.

Recent Advances in Seawater Electrolysis

Hydrogen energy, as a clean and renewable energy, has attracted much attention in recent years. Water electrolysis via the hydrogen evolution reaction at the cathode coupled with the oxygen evolution reaction at the anode is a promising method to produce hydrogen. Given the shortage of freshwater resources on the planet, the direct use of seawater as an electrolyte for hydrogen production has become a hot research topic. Direct use of seawater as the electrolyte for water electrolysis can reduce the cost of hydrogen production due to the great abundance and wide availability. In recent years, various high-efficiency electrocatalysts have made great progress in seawater splitting and have shown great potential. This review introduces the mechanisms and challenges of seawater splitting and summarizes the recent progress of various electrocatalysts used for hydrogen and oxygen evolution reaction in seawater electrolysis in recent years. Finally, the challenges and future opportunities of seawater electrolysis for hydrogen and oxygen production are presented.

Surface Reconstruction Enabled Efficient Hydrogen Generation on a Cobalt-Iron Phosphate Electrocatalyst in Neutral Water

Electrolytic hydrogen evolution reaction (HER) that can be performed efficiently in neutral conditions enables the direct splitting of seawater. However, the sluggish water dissociation kinetics in neutral media severely limits the practical deployment of this technology. Herein, we present a simple strategy to rationally design oxophilic and nucleophilic moieties through the in situ reconstruction of a free-standing bimetallic cobalt-iron phosphate electrode. Through an electrochemical reduction step, the electrode surface undergoes self-reconstruction to generate a thin (oxy)hydroxide layer, enabling a significantly improved HER activity in both buffered electrolyte and natural seawater. Our mechanistic investigations reveal the essential role of oxophilic (oxy)hydroxide species in improving the HER activity of nucleophilic bimetallic phosphate sites. In a buffer electrolyte (pH = 7), the resultant electrocatalyst only requires overpotentials of 97 and 198 mV to deliver a current density of 10 and 100 mA cm^-2, respectively, which outperforms that of the Pt benchmark. The in situ reconstruction strategy of active sites within such electrodes brings significant opportunity in developing active electrocatalysts that are capable of direct seawater splitting.

Single-Atom Co Doped in Ultrathin WO3 Arrays for the Enhanced Hydrogen Evolution Reaction in a Wide pH Range

Owing to the scarcity of Pt, low-cost, stable, and efficient nonprecious metal-based electrocatalysts that can be applied in a wide pH range for the hydrogen evolution reaction (HER) are urgently required. Herein, a highly efficient and robust HER catalyst that is applicable at all pH values is fabricated, containing isolated Co-single atomic sites anchored in the self-supported WO₃ arrays grown on Cu foam. At a current density of 10 mA cm^-2, the HER overpotentials are 117, 105, and 149 mV at pH values of 0, 7, and 14, respectively, which are significantly lower than those of the undoped WO₃, suggesting superior electrocatalytic H₂-evolution activity at all pH values. The catalyst also exhibits long-term stability over a wide pH range, particularly in an acidic medium over 24 h, owing to the excellent anticorrosion properties of WO₃. Density functional theory calculations prove that the enhanced HER activity is attributed to the isolated Co sites because these optimize the adsorption energy of H* species on WO₃. Moreover, the high electrical conductivity of Co-doped WO₃ and the three-dimensional array structure supported on the porous metal support afford a catalyst with suitable HER kinetics to enhance the catalytic performance.

On the evaluation of hydrogen evolution reaction performance of metal-nitrogen-doped carbon electrocatalysts using machine learning technique

Single-atom catalysts (SACs) introduce as a promising category of electrocatalysts, especially in the water-splitting process. Recent studies have exhibited that nitrogen-doped carbon-based SACs can act as a great HER electrocatalyst. In this regard, Adaptive Neuro-Fuzzy Inference optimized by Gray Wolf Optimization (GWO) method was used to predict hydrogen adsorption energy (ΔG) obtained from density functional theory (DFT) for single transition-metal atoms including Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, and Au embedded in N-doped carbon of different sizes. Various descriptors such as the covalent radius, Zunger radius of the atomic d-orbital, the formation energy of the single-atom site, ionization energy, electronegativity, the d-band center from - 6 to 6 eV, number of valence electrons, Bader charge, number of occupied d states from 0 to - 2 eV, and number of unoccupied d states from 0 to 2 eV were chosen as input parameters based on sensitivity analysis. The R-squared and MSE of the developed model were 0.967 and 0.029, respectively, confirming its great accuracy in determining hydrogen adsorption energy of metal/NC electrocatalysts.

Tuning the Electronic Structure of CoO Nanowire Arrays by N-Doping for Efficient Hydrogen Evolution in Alkaline Solutions

Electrochemical hydrogen evolution reactions (HER) have drawn tremendous interest for the scalable and sustainable conversion of renewable electricity to clear hydrogen fuel. However, the sluggish kinetics of the water dissociation step severely restricts the high production of hydrogen in alkaline media. Tuning the electronic structure by doping is an effective method to boost water dissociation in alkaline solutions. In this study, N-doped CoO nanowire arrays (N-CoO) were designed and prepared using a simple method. X-ray diffraction (XRD), element mappings and X-ray photoelectron spectroscopy (XPS) demonstrated that N was successfully incorporated into the lattice of CoO. The XPS of Co 2p and O 1s suggested that the electronic structure of CoO was obviously modulated after the incorporation of N, which improved the adsorption and activation of water molecules. The energy barriers obtained from the Arrhenius relationship of the current density at different temperatures indicated that the N-CoO nanowire arrays accelerated the water dissociation in the HER process. As a result, the N-CoO nanowire arrays showed an excellent performance of HER in alkaline condition. At a current density of 10 mA cm−1, the N-CoO nanowire arrays needed only a 123 mV potential, which was much lower than that of CoO (285 mV). This simple design strategy provides some new inspiration to promote water dissociation for HER in alkaline solutions at the atomic level.

Defect and Doping Engineered Penta-graphene for Catalysis of Hydrogen Evolution Reaction

Water electrolysis is a sustainable and clean method to produce hydrogen fuel via hydrogen evolution reaction (HER). Using stable, effective and low-cost electrocatalysts for HER to substitute expensive noble metals is highly desired. In this paper, by using first-principles calculation, we designed a defect and N-, S-, P-doped penta-graphene (PG) as a two-dimensional (2D) electrocatalyst for HER, and its stability, electronic properties and catalytic performance were investigated. The Gibbs free energy (ΔG_H), which is the best descriptor for the HER, is calculated and optimized, the calculation results show that the ΔG_H can be 0 eV with C2 vacancies and P doping at C1 active sites, which should be the optimal performance for a HER catalyst. Moreover, we reveal that the larger charge transfer from PG to H, the closer ΔG_H is to zero according to the calculation of the electron charge density differences and Bader charges analysis. Ulteriorly, we demonstrated that the HER performance prefers the Volmer-Heyrovsky mechanism in this study.

Activating Basal Surface of Palladium by Electronic Modulation via Atomically Dispersed Nitrogen Doping for High-Efficiency Hydrogen Evolution Reaction

Surface doping by atomically dispersed heteroatoms has become one of the most promising strategies for facilitating the catalytic activity of non-noble transition metals to replace platinum-based catalysts in the hydrogen evolution reaction (HER). However, the underlying mechanism for the atomically dispersed heteroatoms to modulate the electronic structure and the HER activity of a metal surface is still ambiguous. Moreover, the active catalytic region is limited by the small fraction of doped atoms, and the remaining basal surface is inactivated. Here, we demonstrate that the nitrogen doping is atomically dispersed on the palladium surface, which can achieve the near-thermoneutral hydrogen adsorption and promote the HER activity of the basal surface. The theoretical modeling reveals that the dispersed nitrogen atoms attract electrons from palladium and downdrift the d-band center for accelerating the hydrogen desorption. Our work offers understandings of atomically dispersed nitrogen doping on the surface of transition metals and paves the way for a further optimization of nonprecious-metal HER electrocatalysts.

Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects

Hydrogen economy has emerged as a very promising alternative to the current hydrocarbon economy, which involves the process of harvesting renewable energy to split water into hydrogen and oxygen and then further utilization of clean hydrogen fuel. The production of hydrogen by water electrolysis is an essential prerequisite of the hydrogen economy with zero carbon emission. Among various water electrolysis technologies, alkaline water splitting has been commercialized for more than 100 years, representing the most mature and economic technology. Here, the historic development of water electrolysis is overviewed, and several critical electrochemical parameters are discussed. After that, advanced nonprecious metal electrocatalysts that emerged recently for negotiating the alkaline oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are discussed, including transition metal oxides, (oxy)hydroxides, chalcogenides, phosphides, and nitrides for the OER, as well as transition metal alloys, chalcogenides, phosphides, and carbides for the HER. In this section, particular attention is paid to the catalyst synthesis, activity and stability challenges, performance improvement, and industry-relevant developments. Some recent works about scaled-up catalyst synthesis, novel electrode designs, and alkaline seawater electrolysis are also spotlighted. Finally, an outlook on future challenges and opportunities for alkaline water splitting is offered, and potential future directions are speculated.

Graphitic nitrogen in carbon catalysts is important for the reduction of nitrite as revealed by naturally abundant 15N NMR spectroscopy

Metal-free nitrogen-doped carbon is considered as a green functional material, but the structural determination of the atomic positions of nitrogen remains challenging. We recently demonstrated that directly-excited solid state 15N NMR (ssNMR) spectroscopy is a powerful tool for the determination of such positions in N-doped carbon at natural 15N isotope abundance. Here we report a green chemistry approach for the synthesis of N-doped carbon using cellulose as a precursor, and a study of the catalytic properties and atomic structures of the related catalyst. N-doped carbon (NH3) was obtained by the oxidation of cellulose with HNO3 followed by ammonolysis at 800 °C. It had a N content of 6.5 wt% and a surface area of 557 m2 g-1, and 15N ssNMR spectroscopy provided evidence for graphitic nitrogen besides regular pyrrolic and pyridinic nitrogen. This structural determination allowed probing the role of graphitic nitrogen in electrocatalytic reactions, such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nitrite reduction reaction. The N-doped carbon catalyst (NH3) showed higher electrocatalytic activities in the OER and HER under alkaline conditions and higher activity for nitrite reduction, as compared with a catalyst prepared by the carbonization of HNO3-treated cellulose in N2. The electrocatalytic selectivity for nitrite reduction of the N-doped carbon catalyst (NH3) was directly related to the graphitic nitrogen functions. Complementary structural analyses by means of 13C and 1H ssNMR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature N2 adsorption were performed and provided support to the findings. The results show that directly-excited 15N ssNMR spectroscopy at natural 15N abundance is generally capable of providing information on N-doped carbon materials if relaxation properties are favorable. It is expected that this approach can be applied to a wide range of solids with an intermediate concentration of N atoms.

Stable and Highly Efficient Hydrogen Evolution from Seawater Enabled by an Unsaturated Nickel Surface Nitride

Electrocatalytic production of hydrogen from seawater provides a route to low-cost and clean energy conversion. However, the hydrogen evolution reaction (HER) using seawater is greatly hindered by the lack of active and stable catalysts. Herein, an unsaturated nickel surface nitride (Ni-SN@C) catalyst that is active and stable for the HER in alkaline seawater is prepared. It achieves a low overpotential of 23 mV at a current density of 10 mA cm^-2 in alkaline seawater electrolyte, which is superior to Pt/C. Compared to conventional transition metal nitrides or metal/metal nitride heterostructures, the Ni-SN@C has no detectable bulk nickel nitride phase. Instead, unsaturated NiN bonding on the surface is present. In situ Raman measurements show that the Ni-SN@C performs like Pt with the ability to generate hydronium ions in a high-pH electrolyte. The catalyst operation is then demonstrated in a two-electrode electrolyzer system, coupling with hydrazine oxidation at the anode. Using this system, a cell voltage of only 0.7 V is required to achieve a current density of 1 A cm^-2 .

Destabilizing Alkaline Water with 3d-Metal (Oxy)(Hydr)Oxides for Improved Hydrogen Evolution

Alkaline water electrolysis enables the use of nonprecious metal-based catalysts and therefore holds great promise for large-scale generation of renewable hydrogen fuel, especially when driven by renewable energy sources such as solar and wind. However, the sluggish kinetics of the water adsorption and dissociation steps in the alkaline hydrogen evolution reaction (HER) lower its energy conversion efficiency. Recent achievements have proved that 3d-metal (oxy)(hydr)oxides can accelerate these two kinetic processes and thereby improve the activity of diverse HER electrocatalysts from experimental and theoretical points of view. Moreover, a positive role of strong coupling between HER catalysts and 3d-metal (oxy)(hydr)oxides has been discovered recently. In this minireview, a compendious introduction to recent progress is provided, including experiments and theory. Remarks on the challenges and perspectives in this rapidly developing field are also provided.