A durable and pH-universal self-standing MoC-Mo2C heterojunction electrode for efficient hydrogen evolution reaction

Enhancing CO tolerance via molecular trapping effect: Single-atom Pt anchored on Mo2C for efficient alkaline hydrogen oxidation reaction

Developing highly efficient, stable, and CO-tolerant electrocatalysts for hydrogen oxidation reaction (HOR) remains a critical challenge for practical proton/anion exchange membrane fuel cells. Here in, an atomically dispersed platinum (Pt) on Mo2C nanoparticles supported on nitrogen-doped carbon (PtSAMo2C-NC) with a unique yolk-shell structure is presented as a highly efficient and stable catalyst for HOR. The PtSAMo2C-NC catalyst demonstrates remarkable HOR performance, with a high exchange current density of 2.7 mA cm-2 and a mass activity of 2.15 A/mgPt at 50 mV (vs. RHE), which are 1.5 and 18 times greater than those of the 40 % commercial Pt/C catalyst, respectively. Furthermore, the unique PtSAMo2C-NC structure exhibits superior CO tolerance at H2/1,000 ppm CO, significantly outperforming commercial Pt/C catalysts. Density functional theory (DFT) calculations indicate that the introduction of Mo2C forms a strong electronic interaction with Pt, which decreases the electron density around the Pt atoms and shifts the d-band center away from the Fermi level. This results in a reduction of the *H adsorption energy and an optimization of the *OH adsorption energy in PtSAMo2C-NC. In addition, by calculating the CO adsorption energy, it was found that Mo2C exhibits strong CO adsorption ability, which generating a molecular trapping effect, thereby protecting the Pt active sites from poisoning. The strong metal-support electronic interaction significantly enhances the catalytic activity, stability, and CO tolerance of the material, providing a new strategy for developing catalysts with these desirable properties.

Self-activated oxophilic surface of porous molybdenum carbide nanosheets promotes hydrogen evolution activity in alkaline environment

Molybdenum carbides are promising alternatives to Pt-based catalysts for the hydrogen evolution reaction (HER) due to their similar d-band electronic configuration. Notably, MoxC exhibits superior HER kinetics in alkaline media compared to acidic conditions, contrasting with Pt-based catalysts. Herein, we present 3D porous β-Mo2C nanosheets, achieving an overpotential of 111 mV at 10 mA cm-2 in 1 M KOH, significantly lower than in acidic environments. Simulations on pristine Mo2C surface reveal that water dissociation poses a higher energy barrier in alkaline media, suggesting that crystal structure alone does not dictate kinetics. Operando attenuated total reflection surface-enhanced infrared absorption spectroscopy shows that Mo2C activates interfacial water, generating liquid-like and free water, and facilitates hydroxyl species adsorption, reducing activation energy to below 38.43 ± 0.19 kJ/mol. Our findings on the self-activation effect offer insights into the HER mechanism of Mo-based electrocatalysts and guide the design of highly active HER catalysts.

Controllable synthesis of In-Plane Mo/Mo2C heterojunction nanosheets for enhanced hydrogen evolution reaction

Two-dimensional (2D) molybdenum carbide (Mo2C) is a potential electrocatalyst for the hydrogen evolution reaction (HER) due to its Pt-like electronic structure, high electrical conductivity, and abundant active sites. However, its practical application as an HER catalyst is hindered by the sluggish kinetics due to the strong hydrogen absorption. Herein, 2D in-plane Mo/Mo2C heterojunction nanosheets are synthesized from bulk molybdenum disulfide (MoS2) via a molten salt-assisted technique. The ultraviolet photoelectron spectra combined with density-functional theory (DFT) simulations elucidates that electrons are transferred from Mo to Mo2C and fill Mo2C antibonding orbitals, which weakens Mo-H bonding, enhances H2O adsorption and dissociation, optimizes H adsorption/desorption of in-plane Mo/Mo2C heterojunction nanosheets, thus promoting the HER kinetics. The Mo/Mo2C heterojunction nanosheets electrocatalyst demonstrates exceptional HER performance with minimal overpotentials (90 mV in alkaline vs. 96 mV in acidic media at 10 mA cm-2) and favorable Tafel slopes (54.9 vs. 64.2 mV dec-1). Notably, it achieves a 280 mV overpotential at 500 mA cm-2 under alkaline conditions, surpassing that of the commercial Pt/C electrode. The stability is excellent as confirmed by an increase of potential of only about 10 mV after operation for 100 h at 300 mA cm-2. The results reveal a simple and effective strategy to boost the catalytic HER activity boding well for high-efficiency commercial water splitting.

Modulation of the structure and morphology of NiCo2S4 by varying the anion types of nickel and cobalt salts to achieve high-rate supercapacitive performance

In this work, the structure and morphology of NiCo2S4 (NCS) were modulated by varying the anion types (Cl-, CH3COO-, and NO3-) of nickel and cobalt salts. Extensive material characterization and analyses revealed that the grain size of the obtained NCSs was determined by different solvation free energies, capping effects, and steric hindrance during the crystal growth process. Among these three anions, Cl-, with the smallest ionic size, exhibited the lowest capping effect, steric hindrance, and solvation free energy, leading to the largest average grain size of 15.34 nm for Cl--based NiCo2S4 (NCS-C). Moreover, the sea urchin-like morphology of NCS-C provided a high reaction interface for electrochemical energy storage. As a result, the specific capacitance of NCS-C could reach 1112.4 F/g at a current density of 6 A/g, retaining 692 F/g even at a high current density of 16 A/g. The assembled asymmetric supercapacitor could also deliver a high energy density of 23.4 Wh kg-1. This work highlights the significant influence of anion type on the structural and morphological evolution of NCS materials, providing new insights for the development of high-rate NCS-based electrode materials.

Oxide Heterostructure Engineering Drives Stable Lattice Oxygen Evolution for Highly Efficient and Robust Water Electrolysis

Achieving a highly active and stable oxygen evolution reaction (OER) is critical for the implementation of water electrolysis in green hydrogen production but remains challenging. Steering the OER pathway from an adsorbate evolution mechanism (AEM), where a metal site serves as the active site, to the lattice oxygen mechanism (LOM) has been found to enhance OER activity; however, it suffers from low stability. In this work, we propose to construct CuOx/Co3O4 heterointerface, which enables the realization of a stable LOM pathway. The lattice oxygen characteristics are modulated near the heterointerface, resulting in a shift in the reaction pathway from AEM to LOM. In situ X-ray Absorption Fine Structure results further reveal that the valence state of cobalt is stabilized during the OER process, which alleviates corrosion of cobalt and maintains LOM stability. Consequently, the obtained CuOx/Co3O4 exhibits outstanding activity and stability for overall water electrolysis in freshwater, natural seawater, and high-salt wastewater, with a low overpotential of 308 mV at 100 mA cm-2 and stable overall water electrolysis at 500 mA cm-2 for 100 h. Our work demonstrates interface engineering as an effective strategy to activate and stabilize lattice oxygen, advancing the design of high-performance electrocatalysts for energy and environmental applications.

Atomic-Scale Confined Synthesis of Ultrathin W2C Nanowires in Single-Wall Carbon Nanotubes for the High-Performance Hydrogen Evolution Reaction

Phase-pure ultrafine W2C nanostructures are promising electrocatalysts but face synthesis challenges due to unclear formation mechanisms and harsh thermodynamics. Here, we reveal the formation mechanism of ultrathin W2C nanowires (NWs) confined in the cavity of single-wall carbon nanotubes (SWCNTs) at the atomic scale by combined in situ transmission electron microscopy and density functional theory calculations. It was found that the hollow core of SWCNTs can control the phase, axial orientation, and diameter of W2C NWs. Leveraging this mechanism, we synthesized SWCNT-encapsulated W2C NWs, WS2-W2C heterostructures, and WS2 NWs (1D@1D), which assembled into free-standing hybrid films. The integrated W2C NWs@SWCNT membrane was primarily tested, exhibiting a low overpotential of 44 mV to reach a current density of 10 mA cm-2 and outstanding durability (500 h at a high current density of 250 mA cm-2 in acidic conditions).

Highly selective CO2 electroreduction in an exsolution-induced flow cell using a hierarchical monolithic nano-Ag foam electrode

Electrochemical CO2 reduction driven by renewable energy offers a promising route to carbon neutrality. The flow-through induced dynamic triple-phase boundary cell (FTDT cell) addresses the key challenges of conventional gas diffusion electrodes (GDEs), including salt precipitation and electrode flooding, by enabling direct electrolysis of CO2-saturated solutions. However, the Ag catalyst with carbon cloth as a substrate in the FTDT cell exhibits the shortcomings of a few active sites and poor structural stability. Here, we reported a monolithic nano-Ag foam electrode featuring well-developed pores and a hierarchical nanostructure with a high electrochemically active surface area (ECSA), which is ten times that of the Ag NPs electrode, enhancing the CO2 electroreduction performance of the FTDT cell significantly. Classical nucleation theory (CNT) clarified that the nanostructures accelerate bubble nucleation, and visualization experiments confirmed that the periodically connected pore structure provides abundant dynamic gas-liquid-solid triple-phase boundaries (TPBs). At an industrial current density of 200 mA cm-2 and a cell voltage of 2.34 V, the nano-Ag foam electrode achieves a CO faradaic efficiency of 93% and an overall energy efficiency of 51.34%, presenting a promising approach for the commercialization of CO2 electrolysis.

CNT-Supported RuNi Composites Enable High Round-Trip Efficiency in Regenerative Fuel Cells

Regenerative fuel cells hold significant potential for efficient, large-scale energy storage by reversibly converting electrical energy into hydrogen and vice versa, making them essential for leveraging intermittent renewable energy sources. However, their practical implementation is hindered by the unsatisfactory efficiency. Addressing this challenge requires the development of cost-effective electrocatalysts. In this study, a carbon nanotube (CNT)-supported RuNi composite with low Ru loading is developed as an efficient and stable catalyst for alkaline hydrogen and oxygen electrocatalysis, including hydrogen evolution, oxygen evolution, hydrogen oxidation, and oxygen reduction reaction. Furthermore, a regenerative fuel cell using this catalyst composite is assembled and evaluated under practical relevant conditions. As anticipated, the system exhibits outstanding performance in both the electrolyzer and fuel cell modes. Specifically, it achieves a low cell voltage of 1.64 V to achieve a current density of 1 A cm- 2 for the electrolyzer mode and delivers a high output voltage of 0.52 V at the same current density in fuel cell mode, resulting in a round-trip efficiency (RTE) of 31.6% without further optimization. The multifunctionality, high activity, and impressive RTE resulted by using the RuNi catalyst composites underscore its potential as a single catalyst for regenerative fuel cells.

Potassium-stabilized metastable carbides and chalcogenides via surface chemical potential modulation

Metastable carbides and chalcogenides are attractive candidates for wide and promising applications. However, their inherent instability leads to synthetic difficulty and poor durability. Thus, the development of facile strategies for the controllable synthesis and stabilization of metastable carbides is still a great challenge. Here, taking metastable ɛ-Fe2C as a case study, potassium ions (K+) are theoretically predicted and experimentally reported to control the synthesis of metastable ɛ-Fe2C from an Fe2N precursor by increasing the surface carbon chemical potential (μC). The controllable synthesis and improved stability are attributed to the better-matched denitriding and carburizing rates and the impeded spillover of carbon atoms in metastable ɛ-Fe2C with high carbon contents due to the enhanced surface μC. In addition, this strategy is suitable for synthesizing metastable γ'-MoC, MoN, 1T-MoS2, 1T-MoSe2, 1T-MoSe2xTe2(1-x), and 1T-Mo1-xWxSe2, highlighting the universality of the methodology. Impressively, gram-level scalable metastable ɛ-Fe2C remains stable for more than 398 days in air. Furthermore, ɛ-Fe2C exhibits remarkable olefin selectivity and durability for more than 36 h of continuous testing. This work not only demonstrates a facile, easily scalable, and general strategy for accessing various metastable carbides and chalcogenides but also addresses the synthetic difficulty and poor durability challenge of metastable materials.

Direct Synthesis of Dual Mo-Based Functional Materials Derived from Molybdenite by Molten Salt Paired Electrolysis

A facile synthesis process that facilitates the industrial-scale production of catalysts is the prerequisite of the hydrogen evolution reaction (HER) industry. Molybdenum-based catalysts are ideal alternatives for precious-metal-based HER materials; however, they remain challenging in scale-up preparation due to the costly and complex Mo sources. Herein, we propose a molten salt paired electrolysis approach to synthesize transition-metal-doped Mo2C catalysts directly from molybdenite (mainly consisting of micrometer-scale MoS2 bulks), an earth-abundant natural Mo ore. Unlike Fe-doped Mo2C in which those transition metal dopants are inclined to diffuse into inner planes of Mo2C, this unique synthesis approach leverages the differences in transition metal diffusion energy barriers, ultimately leading to the development of Ni-doped Mo2C catalytic materials with specific Ni-enriched interfaces. Owing to the unique design and structures of the catalyst, the interfacially Ni-enriched Ni-doped Mo2C exhibited promising HER performance and long-term stability. Very interestingly, MoS2 nanoflakes, as side products, can be collected at the anode side while interfacially Ni-enriched Mo2C was concurrently obtained at the cathode side during the electrolytic synthesis process. This work not only deepens our knowledge on constructing active-site-enriched interfaces that are beneficial to HER but also gives clues to upcycling raw materials.

Ultrafast Carbothermal Shock Synthesis of Intermetallic Silicides with Anion-Cation Double Active Sites for Efficient Hydrogen Evolution

The exploration and elucidation of the active site of catalysts is crucial for advancing the comprehension of the catalytic mechanism and propelling the development of exceptional catalysts. Herein, it is unveiled that anionic Si and cationic Pt in platinum silicide (PtSi) intermetallic compounds, obtained by ultrafast Joule heating (PtSi JH), simultaneously function as dual active sites for the hydrogen evolution reaction (HER). Density functional theory calculations reveal that, when both Pt and Si simultaneously serve as the active sites, the Gibbs free energy of hydrogen adsorption is 0.70 eV, significantly lower than that of either Pt (1.14 eV) or Si (0.90 eV) alone. Furthermore, both Pt-H and Si-H species are monitored by in situ Raman during the HER process. Consequently, PtSi JH exhibits ultralow overpotentials of 14, 30, and 51 mV at current densities of 10, 50, and 100 mA cm-2, respectively, outperorming commercial Pt/C and Si powder. More importantly, the Joule heating method represents a versatile approach for synthesizing a range of metal silicides including RhSi, RuSix, and Pd2Si. Therefore, this work opens a new avenue for the identification of genuine active sites and explores promising metal silicide for HER electrocatalysis and beyond.

Triggering Intrinsic Electrochemical Hydrogen Evolution Activity within Heusler Alloys via Electronegativity-Induced Charge Rearrangement

Heusler alloys have gradually attracted extensive research interests in the electrochemical hydrogen evolution reaction (HER) due to their unique structural features. However, the inherent half-metallic characteristics of them impede their application. Therefore, reasonable tuning the electronic structure of Heusler alloy catalysts is crucial for enhancing their catalytic activity. Here, we modulated the Fe2MnSi Heusler alloy by precisely substituting Mn with Ru using arc melting and prepared Fe2MnxRu1- xSi materials with enhanced HER properties. The incorporation of highly electronegative Ru introduces substantial electronegativity gradients among the metals, prompting a pronounced Ru-induced electron enrichment effect that facilitates significant charge redistribution. This electronic modulation enhances conductivity and fine-tunes the d-band center, ultimately optimizing the hydrogen adsorption energy barrier at the Si active site. As a result, the overpotential at a current density of 10 mA cm-2 is significantly reduced from 525 mV to 19 mV under acidic conditions. This work provides insights into the design of highly efficient HER electrocatalysts and expands the potential applications of Heusler alloys in electrocatalysis.

Pt/α-MoC Catalyst Boosting pH-Universal Hydrogen Evolution Reaction at High Current Densities

Constructing subnanometric electrocatalysts is an efficient method to synergistically accelerate H2O dissociation and H+ reduction for pH-universal hydrogen evolution reaction (HER) for industrial water electrolysis to produce green hydrogen. Here, we construct a subnanometric Pt/α-MoC catalyst, where the α-MoC component can dissociate water effectively, with the rapid proton release kinetics of Pt species on Pt/α-MoC to obtain a good HER performance at high current densities in all-pH electrolytes. Quasi-in situ X-ray photoelectron spectroscopy analyses and density functional theory calculations confirm the highly efficient water dissociation capability of α-MoC and the thermodynamically favorable desorption process of hydrolytically dissociated protons on Pt sites at the high current density. Consequently, Pt/α-MoC requires only a low overpotential of 125 mV to achieve a current density of 1000 mA cm-2. Moreover, a Pt/α-MoC-based proton exchange membrane water electrolysis device exhibits a low cell voltage (1.65 V) and promising stability over 300 h with no performance degradation at an industrial-level current density of 1 A cm-2. Notably, even at a current of 100 A, the cell voltage remains low at 2.15 V, demonstrating Pt/α-MoC's promising potential as a scalable alternative for industrial hydrogen production. These findings elucidate the synergistic mechanism of α-MoC and atomically dispersed Pt in promoting efficient HER, offering valuable guidance for the design of electrocatalysts in high current density hydrogen.

MoCx/CoP Janus Structure Embedded Carbon Frame for Boosting Hydrazine Oxidation and Hydrogen Evolution Reactions

The integration of hydrazine electrooxidation (HzOR) and hydrogen evolution reaction (HER) presents an efficient pathway for high-purity hydrogen production. However, developing bifunctional catalysts remains challenging for the demands of multiple active-centers and tailored electronic properties. Here, a unique Janus nano-catalysts of MoCx/CoP embedded on carbon frameworks (MoCx/CoP@C) is introduced, featuring dual electronic states (depletion and accumulation)driven by charge redistribution within MoCx/CoP, acting as dual active-sites (DAS) for both HER and HzOR. Theoretical analysis reveals these independent DAS in MoCx/CoP significantly enhance catalytic activity for both HER and HzOR. Specifically, accumulated electrons at MoCx/CoP interfaces weaken the bonding strength of N-H in N2H4, thereby decreasing dehydrogenation energy barrier while electronic-deficient Mo sites within MoCx accelerate H* desorption, thus promoting HER kinetics. This catalyst exhibits ultra-low potential of -73 mV at 10 mA cm-2 for anodic HzOR, comparable to noble catalysts and low overpotential of 95 mV at 10 mA cm-2 for cathodic HER. When employed in an overall hydrazine splitting (OHzS) system, MoCx/CoP@C shows promising commercial potential, with low energy consumption (0.16 V), high Faradaic efficiency (95.4%) and long-term stability. This study underscores the feasibility of designing independent DAS catalysts and elucidates the mechanistic origins of bifunctional activities.

Regulating Carbon Vacancies and Undercoordinated Mo Sites in Mo2C Catalysts Toward Photo-Thermal Catalytic Conversion of Biomass Into H2 Fuel

The conversion of biomass into chemical fuels is exciting but quite challenging in the development of an effective conversion strategy to generate easily-separated products without energy consumption. Herein, a lignocellulosic biomass-to-H2 conversion system via photo-thermal catalysis over Mo2C hierarchical nanotube catalysts in an acidic solution, in which the lignocellulose is hydrolyzed to small organic molecules (such as glucose, etc) by dilute H2SO4, and then the resulting glucose is oxidized by Mo2C catalyst to generate H2 are reported. During the photo-thermal catalytic processes, the carbon vacancy in Mo2C catalysts results in the generation of undercoordinated Mo sites, which act as active sites for both biomass oxidation and H2 generation reactions. Thus, the successful photo-thermal catalytic conversion of common agricultural and forestry biomass including polar wood chip, bamboo, wheat straw, rice straw, corncob, and rice hull into H2 fuel is realized, and the highest H2 generation rate achieves 30 µmol g-1 h-1 in the wheat straw system. Outwork affords efficient noble-metal-free catalysts with adjustable active sites for photo-thermal catalytic conversion of lignocellulosic biomass into H2.

Boosting Alkaline Hydrogen Evolution Reaction by Modulating D-Band Center in Bimetallic Sulfide Ni3S2-FeS Heterointerfaces

Hydrogen evolution reaction (HER) in alkaline electrolytes is considered to be the most promising industry-scale hydrogen (H2) production method but is limited to the lack of low-cost, efficient, and stable HER catalysts. Here, a universal and scalable electrodeposition-sulfidization modulation strategy is developed to directly grow the Ni3S2-FeS heterojunction nanoarray on the commercial Ni foam (Ni3S2-FeS@NF). The as-prepared Ni3S2-FeS@NF catalyst only requires a low overpotential of 71 and 270 mV to reach the current density of 10 and 500 mA cm-2 with a long-lasting lifetime of over 200 h. Moreover, the Ni3S2-FeS@NF catalyst can operate at industrial conditions (500 mA cm-2 at 70 °C) for over 200 h stably at a low cell voltage of 1.71 V in an alkaline exchange membrane water electrolysis (AEMWE) device, which indicates a great prospect for practical application. In addition, in situ Raman experiments and density functional theory (DFT) calculations reveal that the downshift of the d-band center and interfacial synergistic actions due to the electron transfer between Ni3S2 and FeS reduce the water spitting energy barrier and optimize H/O-containing intermediates absorption, thereby improving the HER intrinsic catalytic activity. This work provides an atomic-level insight into designing efficient HER heterogeneous catalysts.

Atomically Dispersed Fe1Mo1 Dual Sites for Enhanced Electrocatalytic Nitrogen Reduction

The electrocatalytic nitrogen reduction reaction (eNRR) is an attractive strategy for the green and distributed production of ammonia (NH3); however, it suffers from weak N2 adsorption and a high energy barrier of hydrogenation. Atomically dispersed metal dual-site catalysts with an optimized electronic structure and exceptional catalytic activity are expected to be competent for knotty hydrogenation reactions including the eNRR. Inspired by the bimetallic FeMo cofactor in biological nitrogenase, herein, an atomically dispersed Fe1Mo1 dual site anchored in nitrogen-doped carbon is proposed to induce a favorable electronic structure and binding energy. The as-prepared electrocatalyst (FeMo-NC) presents a maximum NH3 yield rate of 1.07 mg h-1 mgmetal-1 together with a Faradaic efficiency of 21.7% at -0.25 V vs RHE, outperforming many reported atomically dispersed non-noble metal electrocatalysts. Further density functional theory (DFT) calculations reveal that the Fe1Mo1 dual site activates *N2 most strongly via a side-on adsorption configuration and optimizes the binding energy of eNRR intermediates, thus lowering the limiting barrier during the overall hydrogenation and promoting NH3 generation.

Confined Mo2C/MoC heterojunction nanocrystals-graphene superstructure anode for enhanced conversion kinetics in sodium-ion batteries

Heterostructure design and integration with conductive materials play a crucial role in enhancing the conversion kinetics of electrode materials for metal-ion batteries. However, integrating nanocrystal heterojunctions into a conductive layer to form a superstructure is a significant challenge, mainly due to the difficulty in maintaining the structural integrity. Here we report a unique glucose-induced heterogeneous nucleation method that enables the independent manipulation of nucleation and growth of Mo2C/MoC heterojunction nanocrystals within 2D layers. Our investigations reveal that the rGO-Mo2C/MoC-rGO superstructure is formed by a topological transformation induced by subsequent heat treatment of the initial hydrothermally prepared rGO-MoO2-rGO precursor. This novel structure embeds Mo2C/MoC heterojunction nanocrystals within a 2D graphene matrix, providing enhanced mechanical stability, accelerated Na+ transport, and improved electron conduction. Ex situ XRD and Raman spectroscopy analyses reveal that the rGO-Mo2C/MoC-rGO superstructure significantly enhances the stability and reversibility of anodes. Leveraging these unique characteristics, the newly developed superstructural anode exhibits remarkable long-term cycling stability and outstanding rate performance. As a result, superstructure anodes demonstrate superior electrochemical capabilities, delivering a specific capacity of 106 mAh/g after enduring 5000 cycles at 1 A/g. Our study underscores the critical importance of superstructure design in propelling the advancement of battery materials.

3D Printing of Graphene Aerogel Microspheres to Architect High-Performance Electrodes for Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) efficiency is highly dependent on the electrocatalysts microstructure and the macrostructure of the electrodes. Herein, the graphene aerogel microspheres loaded with well-dispersed ultrafine Ni/Co nanoparticles catalyst is prepared through electro-spraying, in-situ crosslinking, freeze-drying, and pyrolysis, and then is utilized to print the HER electrode via direct ink writing (DIW). The obtained graphene-based aerogel microspheres possess peculiar cabbage-like mesoporous structures which allow ready access of reaction species to active sites, optimal mass transfer, and proton diffusion within the microspheres. DIW 3D printing achieves the ordered control on the periodic lattice macro-geometry and thus facilitates the fast gas bubble evolution and release from the electrode surface. The as-fabricated 3D electrode possesses a low overpotential of 341 mV at 10 mA cm-2, a decrease by 31.5% compared to 3D printed electrode directly from 2D graphene, and a low Tafel slope of 119.1 mV dec-1, 40% lower than that of the electrodes fabricated via directly casting the aerogel microspheres. Furthermore, the 3D-printed electrode of aerogel microspheres displays good HER stability. This work provides a good approach for constructing high-performance HER electrodes through 3D printing of graphene aerogel microspheres.

Multifunctional Strategies of Advanced Electrocatalysts for Efficient Urea Synthesis

The electrochemical reduction of nitrogenous species (such as N2, NO, NO2 -, and NO3 -) for urea synthesis under ambient conditions has been extensively studied due to their potential to realize carbon/nitrogen neutrality and mitigate environmental pollution, as well as provide a means to store renewable electricity generated from intermittent sources such as wind and solar power. However, the sluggish reaction kinetics and the scarcity of active sites on electrocatalysts have significantly hindered the advancement of their practical applications. Multifunctional engineering of electrocatalysts has been rationally designed and investigated to adjust their electronic structures, increase the density of active sites, and optimize the binding energies to enhance electrocatalytic performance. Here, surface engineering, defect engineering, doping engineering, and heterostructure engineering strategies for efficient nitrogen electro-reduction are comprehensively summarized. The role of each element in engineered electrocatalysts is elucidated at the atomic level, revealing the intrinsic active site, and understanding the relationship between atomic structure and catalytic performance. This review highlights the state-of-the-art progress of electrocatalytic reactions of waste nitrogenous species into urea. Moreover, this review outlines the challenges and opportunities for urea synthesis and aims to facilitate further research into the development of advanced electrocatalysts for a sustainable future.

Material design of biodegradable primary batteries: boosting operating voltage by substituting the hydrogen evolution reaction at the cathode

Transient primary batteries (TPBs) degrade after use without leaving harmful toxic substances, providing power sources for developing low-invasive and environmentally benign sensing platforms. Magnesium and zinc, both abundant on Earth, possess low anodic potentials and good biodegradability, making them useful as anode materials. However, molybdenum, a biodegradable metal, causes the hydrogen evolution reaction (HER) at the cathode, reducing the operating voltage of cells because of its low cathodic potential. In this review, we examine recent material designs to increase the operating voltage by introducing alternative electrochemical reactions at the cathode, including the oxygen reduction reaction, metal-ion intercalation into transition metal oxides, and halogen ionization, all of which have higher cathodic potentials than the HER. After discussing the characteristics, constituents, and demonstration of TPBs, we conclude by exploring their potential as power sources for implants, wearables, and environmental sensing applications.

Hetero-structured Ru-Mo2C nanoparticles loaded on N,P co-doped carbon for a pH universal hydrogen evolution reaction

Considering the extensive research studies on Ru and Mo2C for the hydrogen evolution reaction (HER), the construction of heterostructure catalysts containing both Ru and Mo2C nanoparticles can further improve the catalytic activity by the synergistic effect of different species. In this work, we report the synthesis of N,P co-doped carbon coated Ru and Mo2C nanoparticles for the HER using a ZnMo metal-organic framework (MOF) as the precursor. The addition of phosphomolybdic acid during the synthesis of a Zn-MOF not only provides a Mo source for the formation of Mo2C, but also induces a nano-bowl structure. After anchoring RuCl3 in the ZnMo-MOF and thermal annealing, Ru and Mo2C nanoparticles encapsulated in N,P doped carbon (Ru-Mo2C@NPC) were synthesized and the nano-bowl morphology was well preserved. Due to the co-existence of the highly active catalytic species Ru and Mo2C, a large specific surface area owing to the evaporation of Zn during the calcination process, and the nano-bowl-like morphology that boosts mass transfer, Ru-Mo2C@NPC exhibits good catalytic activity for the HER over a wide pH range, with overpotentials of 62, 64 and 170 mV in 0.5 M H2SO4, 1 M KOH and 1 M PBS solution at 10 mA cm-2, surpassing the values for many Ru and Mo2C based catalysts. This work provides a feasible route for designing high performance HER catalysts.

Coupling High-Entropy Core with Rh Shell for Efficient pH-Universal Hydrogen Evolution

Constructing high-entropy alloys (HEAs) with core-shell (CS) nanostructure is efficient for enhancing catalytic activity. However, it is extremely challenging to incorporate the CS structure with HEAs. Herein, PtCoNiMoRh@Rh CS nanoparticles (PtCoNiMoRh@Rh) with ∼5.7 nm for pH-universal hydrogen evolution reaction (HER) are reported for the first time. The PtCoNiMoRh@Rh just require 9.1, 24.9, and 17.1 mV to achieve -10 mA cm-2 in acid, neutral, and alkaline electrolyte, and the corresponding mass activity are 5.8, 2.79, and 91.8 times higher than that of Rh/C. Comparing to PtCoNiMoRh nanoparticles, the PtCoNiMoRh@Rh exhibit excellent HER activity attributed to the decrease of Rh 4d especially 4d5/2 unoccupied state induced by the multi-active sites in HEA, as well as the synergistic effect in Rh shell and HEA core. Theorical calculation exhibits that Rh-dyz, dx2, and dxz orbitals experience a negative shift with shell thickness increasing. The HEAs with CS structure would facilitate the rational design of high-performance HEAs catalysts.

Engineering heterostructured Mo2C/MoS2 catalyst with hydrophilicity/aerophobicity via carbothermal shock for efficient alkaline hydrogen evolution

The exploration of high-performance hydrogen evolution reaction (HER) catalysts is conducive to the development of clean hydrogen energy, yet still remains a challenge. Herein, we rapidly synthesize the Mo2C/MoS2 heterostructure on carbon paper (Mo2C/MoS2-CP) via carbothermal shock in only two seconds. The construction of the Mo2C/MoS2 heterostructure regulates the electronic structure of the Mo site and facilitates charge transfer during the HER process. Moreover, the catalyst exhibits enhanced hydrophilicity and aerophobicity, facilitating optimal electrolyte-catalyst interaction and efficient hydrogen bubble detachment for accelerated mass transfer. Consequently, Mo2C/MoS2-CP exhibits superior intrinsic alkaline HER activity, and excellent stability for 100 h. This finding provides a novel insight into the development of outstanding HER catalysts.

In Situ Engineering Multifunctional Active Sites of Ruthenium-Nickel Alloys for pH-Universal Ampere-Level Current-Density Hydrogen Evolution

Developing robust non-platinum electrocatalysts with multifunctional active sites for pH-universal hydrogen evolution reaction (HER) is crucial for scalable hydrogen production through electrochemical water splitting. Here ultra-small ruthenium-nickel alloy nanoparticles steadily anchored on reduced graphene oxide papers (Ru-Ni/rGOPs) as versatile electrocatalytic materials for acidic and alkaline HER are reported. These Ru-Ni alloy nanoparticles serve as pH self-adaptive electroactive species by making use of in situ surface reconstruction, where surface Ni atoms are hydroxylated to produce bifunctional active sites of Ru-Ni(OH)2 for alkaline HER, and selectively etched to form monometallic Ru active sites for acidic HER, respectively. Owing to the presence of Ru-Ni(OH)2 multi-site surface, which not only accelerates water dissociation to generate reactive hydrogen intermediates but also facilitates their recombination into hydrogen molecules, the self-supported Ru90Ni10/rGOP hybrid electrode only takes overpotential of as low as ≈106 mV to deliver current density of 1000 mA cm-2, and maintains exceptional stability for over 1000 h in 1 m KOH. While in 0.5 m H2SO4, the Ru90Ni10/rGOP hybrid electrode exhibits acidic HER catalytic behavior comparable to commercially available Pt/C catalyst due to the formation of monometallic Ru shell. These electrochemical behaviors outperform some of the best Ru-based catalysts and make it attractive alternative to Pt-based catalysts toward highly efficient HER.

Ultrafine Co-MoC particles in porous carbon derived from polyoxometalate-based metal organic framework for efficient hydrogen evolution reaction

Accurately regulating ultrafine molybdenum carbide (MoC)-based catalysts is a significant challenge in the rational design of hydrogen evolution reaction (HER) electrocatalysts. Herein, under the guidance of the first principle calculations, we proposed an in-situ polyoxometalate-confined strategy for creating uniformly distributed ultrafine Co-MoC bimetallic nanoparticles in porous carbon nanostars, with the assistance of precisely designed metal-organic framework (MOF). The Co-MoC@C electrocatalyst has a high specific surface area of 969 m2·g-1 because of the conductive carbon substrate with abundant mesopores, which makes for exposing more active sites of Co-MoC nanocrystals (∼1.5 nm) and facilitating electron/ion transport. Thus, Co-MoC@C electrocatalyst shows the excellent electrochemical activity with overpotentials of 88.4 mV and 66.6 mV at a current density of 10 mA·cm-2 under acidic and alkaline conditions, respectively. The in-situ polyoxometalate-confined strategy will provide a new guideline for the design and preparation of efficient HER electrocatalysts.

Alkali cation stabilization of defects in 2D MXenes at ambient and elevated temperatures

Transition metal carbides have been adopted in energy storage, conversion, and extreme environment applications. Advancements in their 2D counterparts, known as MXenes, enable the design of unique structures at the ~1 nm thickness scale. Alkali cations have been essential in MXenes manufacturing processing, storage, and applications, however, exact interactions of these cations with MXenes are not fully understood. In this study, using Ti3C2Tx, Mo2TiC2Tx, and Mo2Ti2C3Tx MXenes, we present how transition metal vacancy sites are occupied by alkali cations, and their effect on MXene structure stabilization to control MXene's phase transition. We examine this behavior using in situ high-temperature x-ray diffraction and scanning transmission electron microscopy, ex situ techniques such as atomic-layer resolution secondary ion mass spectrometry, and density functional theory simulations. In MXenes, this represents an advance in fundamentals of cation interactions on their 2D basal planes for MXenes stabilization and applications. Broadly, this study demonstrates a potential new tool for ideal phase-property relationships of ceramics at the atomic scale.

Engineering interfacial sulfur migration in transition-metal sulfide enables low overpotential for durable hydrogen evolution in seawater

Hydrogen production from seawater remains challenging due to the deactivation of the hydrogen evolution reaction (HER) electrode under high current density. To overcome the activity-stability trade-offs in transition-metal sulfides, we propose a strategy to engineer sulfur migration by constructing a nickel-cobalt sulfides heterostructure with nitrogen-doped carbon shell encapsulation (CN@NiCoS) electrocatalyst. State-of-the-art ex situ/in situ characterizations and density functional theory calculations reveal the restructuring of the CN@NiCoS interface, clearly identifying dynamic sulfur migration. The NiCoS heterostructure stimulates sulfur migration by creating sulfur vacancies at the Ni3S2-Co9S8 heterointerface, while the migrated sulfur atoms are subsequently captured by the CN shell via strong C-S bond, preventing sulfide dissolution into alkaline electrolyte. Remarkably, the dynamically formed sulfur-doped CN shell and sulfur vacancies pairing sites significantly enhances HER activity by altering the d-band center near Fermi level, resulting in a low overpotential of 4.6 and 8 mV at 10 mA cm-2 in alkaline freshwater and seawater media, and long-term stability up to 1000 h. This work thus provides a guidance for the design of high-performance HER electrocatalyst by engineering interfacial atomic migration.

Recent advances of doping strategy for boosting the electrocatalytic performance of two-dimensional noble metal nanosheets

Benefiting from the ultrahigh specific surface areas, massive exposed surface atoms, and highly tunable microstructures, the two-dimensional (2D) noble metal nanosheets (NSs) have presented promising performance for various electrocatalytic reactions. Nevertheless, the heteroatom doping strategy, and in particular, the electronic structure tuning mechanisms of the 2D noble metal catalysts (NMCs) yet remain ambiguous. Herein, we first review several effective strategies for modulating the electrocatalytic performance of 2D NMCs. Then, the electronic tuning effect of hetero-dopants for boosting the electrocatalytic properties of 2D NMCs is systematically discussed. Finally, we put forward current challenges in the field of 2D NMCs, and propose possible solutions, particularly from the perspective of the evolution of electron microscopy. This review attempts to establish an intrinsic correlation between the electronic structures and the catalytic properties, so as to provide a guideline for designing high-performance electrocatalysts.

Rechargeable Zn-H2O hydrolysis battery for hydrogen storage and production

Reactive metals hydrolysis offers significant advantages for hydrogen storage and production. However, the regeneration of common reactive metals (e.g., Mg, Al, etc.) is energy-intensive and produces unwanted byproducts such as CO2 and Cl2. Herein, we employ Zn as a reactive mediator that can be easily regenerated by electrolysis of ZnO in an alkaline solution with a Faradaic efficiency of >99.9 %. H2 is produced in the same electrolyte by constructing a Zn-H2O hydrolysis battery consisting of a Zn anode and a Raney-Ni cathode to unlock the Zn-H2O reaction. The entire two-step water splitting reaction with a net energy efficiency of 70.4 % at 80 °C and 50 mA cm-2. Additionally, the Zn-H2O system can be charged using renewable energy to produce H2 on demand and runs for 600 cycles only sacrificing 3.76 % energy efficiency. DFT calculations reveal that the desorption of H* on Raney-Ni (-0.30 eV) is closer to zero compared with that on Zn (-0.87 eV), indicating a faster desorption of H* at low overpotential. Further, a 24 Ah electrolyzer is demonstrated to produce H2 with a net energy efficiency of 65.5 %, which holds promise for its real application.

Platinum-Grafted Twenty-Five Atom Gold Nanoclusters for Robust Hydrogen Evolution

A robust hydrogen evolution is demonstrated from Au25(PET)18]- nanoclusters (PET = 2-phenylethanethiol) grafted with minimal platinum atoms. The fabrication involves an electrochemical activation of nanoclusters by partial removal of thiols, without affecting the metallic core, which exposes Au-sites adsorbed with hydrogen and enables an electroless grafting of platinum. The exposed Au-sites feature the (111)-facet of the fcc-Au25 nanoclusters as assessed through lead underpotential deposition. The electrochemically activated nanoclusters (without Pt loading) show better electrocatalytic reactivity toward hydrogen evolution reaction than the pristine nanoclusters in an acidic medium. The platinum-grafted nanocluster outperformed with a lower overpotential of 0.117 V vs RHE (RHE = Reversible Hydrogen Electrode) compared to electrochemically activated nanoclusters (0.353 V vs RHE ) at 10 mA cm-2 and is comparable with commercial Pt/C. The electrochemically activated nanoclusters show better reactivity at higher current density owing to the ease of hydrogen release from the active sites. The modified nanoclusters show unique supramolecular self-assembly characteristics as observed in electron microscopy and tomography due to the possible metallophilic interactions. These results suggest that the post-surface modification of nanoclusters will be an ideal tool to address the sustainable production of green hydrogen.

Engineering Non-precious Trifunctional Cobalt-Based Electrocatalysts for Industrial Water Splitting and Ultra-High-Temperature Flexible Zinc-Air Battery

Developing efficient, robust, and cost-effective trifunctional catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) at high current density and high temperature is crucial for water splitting at industry-level conditions and ultra-high-temperature Zinc-air battery (ZAB). Herein, cobalt nanoparticles well-integrated with nitrogen-doped porous carbon leaves (Co@NPCL) by direct annealing of core-shell bimetallic zeolite imidazolate frameworks is synthesized. Benefiting from the homogeneous distribution of metallic Co nanoparticles, the conductive porous carbon, and the doped N species, the as-fabricated Co@NPCL catalysts exhibit outstanding trifunctional performances with low overpotentials at 10 mA cm-2 for HER (87 mV) and OER (276 mV), long-lasting lifetime of over 2000 h, and a high half-wave potential of 0.86 V versus RHE for ORR. Meanwhile, the Co@NPCL catalyst can serve as both cathode and anode for water splitting at industrial conduction, and exhibit a stable cell voltage of 1.87 V to deliver a constant catalytic current of 500 mA cm-2 over 60 h. Moreover, the excellent trifunctional activity of Co@NPCL enables the flexible ZAB to operate efficiently at ultra-high temperature of 70 °C, delivering 162 mW cm-2 peaks power density and an impressive stability for 4500 min at 2 mA cm-2.

Atomically dispersed Iridium on Mo2C as an efficient and stable alkaline hydrogen oxidation reaction catalyst

Hydroxide exchange membrane fuel cells (HEMFCs) have the advantages of using cost-effective materials, but hindered by the sluggish anodic hydrogen oxidation reaction (HOR) kinetics. Here, we report an atomically dispersed Ir on Mo2C nanoparticles supported on carbon (IrSA-Mo2C/C) as highly active and stable HOR catalysts. The specific exchange current density of IrSA-Mo2C/C is 4.1 mA cm-2ECSA, which is 10 times that of Ir/C. Negligible decay is observed after 30,000-cycle accelerated stability test. Theoretical calculations suggest the high HOR activity is attributed to the unique Mo2C substrate, which makes the Ir sites with optimized H binding and also provides enhanced OH binding sites. By using a low loading (0.05 mgIr cm-2) of IrSA-Mo2C/C as anode, the fabricated HEMFC can deliver a high peak power density of 1.64 W cm-2. This work illustrates that atomically dispersed precious metal on carbides may be a promising strategy for high performance HEMFCs.

Efficient and durable seawater electrolysis with a V2O3-protected catalyst

The ocean, a vast hydrogen reservoir, holds potential for sustainable energy and water development. Developing high-performance electrocatalysts for hydrogen production under harsh seawater conditions is challenging. Here, we propose incorporating a protective V2O3 layer to modulate the microcatalytic environment and create in situ dual-active sites consisting of low-loaded Pt and Ni3N. This catalyst demonstrates an ultralow overpotential of 80 mV at 500 mA cm-2, a mass activity 30.86 times higher than Pt-C and maintains at least 500 hours in seawater. Moreover, the assembled anion exchange membrane water electrolyzers (AEMWE) demonstrate superior activity and durability even under demanding industrial conditions. In situ localized pH analysis elucidates the microcatalytic environmental regulation mechanism of the V2O3 layer. Its role as a Lewis acid layer enables the sequestration of excess OH- ions, mitigate Cl- corrosion, and alkaline earth salt precipitation. Our catalyst protection strategy by using V2O3 presents a promising and cost-effective approach for large-scale sustainable green hydrogen production.

Synergistic effect of dual phase cocatalysts: MoC-Mo2C quantum dots anchored on g-C3N4 for high-stability photocatalytic hydrogen evolution

The extensive examination of hexagonal molybdenum carbide (β-Mo2C) as a non-noble cocatalyst in the realm of photocatalytic H2 evolution is predominantly motivated by its exceptional capacity to adsorb H+ ions akin to Pt and its advantageous conductivity characteristics. However, the H2 evolution rate of photocatalysts modified with β-Mo2C is limited as a result of their comparatively low ability to release H through desorption. Therefore, a facile method was employed to synthesize carbon intercalated dual phase molybdenum carbide (MC@C) quantum dots (ca. 3.13 nm) containing both α-MoC and β-Mo2C decorated on g-C3N4 (gCN). The synthesis process involved a simple and efficient combination of sonication-assisted self-assembly and calcination techniques. 3-MC@C/gCN exhibited the highest efficiency in generating H2, with a rate of 4078 µmol g-1h-1 under 4 h simulated sunlight irradiation, which is 13 times higher than pristine gCN. Furthermore, from the cycle test, 3-MC@C/gCN showcased exceptional photochemical stability of 65 h, as it maintained a H2 evolution rate of 40 mmol g-1h-1. The heightened level of activity observed in the 3-MC@C/gCN system can be ascribed to the synergistic effects of MoC-Mo2C that arise due to the existence of a carbon layer. The presence of a carbon layer enhanced the transmission of photoinduced electrons, while the MoC-Mo2C@C composite served as active sites, thereby facilitating the H2 production reaction of gCN. The present study introduces a potentially paradigm-shifting concept pertaining to the exploration of novel Mo-based cocatalysts with the aim of augmenting the efficacy of photocatalytic H2 production.

Synergetic Catalytic Effect between Ni and Co in Bimetallic Phosphide Boosting Hydrogen Evolution Reaction

The application of electrochemical hydrogen evolution reaction (HER) for renewable energy conversion contributes to the ultimate goal of a zero-carbon emission society. Metal phosphides have been considered as promising HER catalysts in the alkaline environment, which, unfortunately, is still limited owing to the weak adsorption of H* and easy dissolution during operation. Herein, a bimetallic NiCoP-2/NF phosphide is constructed on nickel foam (NF), requiring rather low overpotentials of 150 mV and 169 mV to meet the current densities of 500 and 1000 mA cm-2, respectively, and able to operate stably for 100 h without detectable activity decay. The excellent HER performance is obtained thanks to the synergetic catalytic effect between Ni and Co, among which Ni is introduced to enhance the intrinsic activity and Co increases the electrochemically active area. Meanwhile, the protection of the externally generated amorphous phosphorus oxide layer improves the stability of NiCoP/NF. An electrolyser using NiCoP-2/NF as both cathode and anode catalysts in an alkaline solution can produce hydrogen with low electric consumption (overpotential of 270 mV at 500 mA cm-2).

Ultra-Stable Ti Vacancies-Pt Atomic Clusters Structure on Titanium Oxycarbide Supports for High Current Density Hydrogen Evolution Reaction

Electrocatalysts with low Pt loading mass to achieve high current density (≥1 A cm-2) for hydrogen evolution reaction (HER) are still extremely challenging due to the limited intrinsic activity and weak stability of catalytic sites. The modulation of the electronic microenvironment of the support-Pt structure is crucial to enhance the intrinsic activity and stability of catalytic sites. Herein, an innovative titanium oxycarbide (TiVCO) solid solution with Ti vacancies (TiV) is proposed as support to anchor sub-nanoscale Pt atomic clusters (Pt ACs) and a stable "TiV-Pt ACs" structure is carefully designed. The electronic microenvironment of "TiV-Pt ACs" is indirectly optimized by an unsaturated C/O site near TiV. Thanks to this, novel "TiV-Pt ACs" structure (Pt@TiVCO) with low Pt loading mass (2.44 wt.%) exhibits excellent HER activity in acidic solution and the mass activity is more than ten times that of commercial 20% Pt/C at the overpotentials of 50 and 100 mV. Particularly, Pt@TiVCO shows amazing stability at high and fluctuating current density of 1-2 A cm-2 for 120 h. This work provides a novel and promising method to develop stable and low-loading Pt-based catalysts adapting to high current density.

Mo2C-Based Ceramic Electrode with High Stability and Catalytic Activity for Hydrogen Evolution Reaction at High Current Density

Developing robust electrodes with high catalytic performance is a key step for expanding practical HER (hydrogen evolution reaction) applications. This paper reports on novel porous Mo2C-based ceramics with oriented finger-like holes directly used as self-supported HER electrodes. Due to the suitable MoO3 sintering additive, high-strength (55 ± 6 MPa) ceramic substrates and a highly active catalytic layer are produced in one step. The in situ reaction between MoO3 and Mo2C enabled the introduction of O in the Mo2C crystal lattice and the formation of Mo2C(O)/MoO2 heterostructures. The optimal Mo2C-based electrode displayed an overpotential of 333 and 212 mV at 70 °C under a high current intensity of 1500 mA cm-2 in 0.5 m H2SO4 and 1.0 m KOH, respectively, which are markedly better than the performance of Pt wire electrode; furthermore, its price is three orders of magnitude lower than Pt. The chronopotentiometric curves recorded in the 50 - 1500 mA cm-2 range, confirmed its excellent long-term stability in acidic and alkaline media for more than 260 h. Density functional theory (DFT) calculations showed that the Mo2C(O)/MoO2 heterostructures has an optimum electronic structure with appropriate *H adsorption-free energy in an acidic medium and minimum water dissociation energy barrier in an alkaline medium.

Superstructure-Assisted Single-Atom Catalysis on Tungsten Carbides for Bifunctional Oxygen Reactions

Single-atom catalysis (SAC) attracts wide interest for zinc-air batteries that require high-performance bifunctional electrocatalysts for oxygen reactions. However, catalyst design is still highly challenging because of the insufficient driving force for promoting multiple-electron transfer kinetics. Herein, we report a superstructure-assisted SAC on tungsten carbides for oxygen evolution and reduction reactions. In addition to the usual single atomic sites, strikingly, we reveal the presence of highly ordered Co superstructures in the interfacial region with tungsten carbides that induce internal strain and promote bifunctional catalysis. Theoretical calculations show that the combined effects from superstructures and single atoms strongly reduce the adsorption energy of intermediates and overpotential of both oxygen reactions. The catalyst therefore presented impressive bifunctional activity with an ultralow potential gap of 0.623 V and delivered a high power density of 188.5 mW cm-2 for assembled zinc-air batteries. This work opens up new opportunities for atomic catalysis.

Biodegradable Mg-Mo2C MXene Air Batteries for Transient Energy Storage

Primary batteries are the fundamental power sources in small electronic gadgets and bio/ecoresorbable batteries. They are fabricated from benign and biodegradable materials and are of interest in environmental sensing and implants because of their low toxicity toward the environment and human body during decomposition. However, current bio/ecoresorbable batteries suffer from low operating voltages and output powers because of the occurrence of undesired hydrogen evolution reactions (HERs) at cathodes. Herein, Mo2C MXene was used as a cathode to achieve high operating voltage and areal power. Mo2C provides energy barriers for HERs in alkaline solutions, and such barriers suppress HERs and allow the oxygen reduction reaction to dominate at the cathode. The fabricated battery exhibits an operating voltage and areal power of 1.4 V and 0.92 mW cm-2, respectively. Degradation tests show that the full cell completely degrades within 123 days, leaving only Mo fragments from the electrode and biodegradable encapsulation. This study provides insights into bio/ecoresorbable batteries with high power and operating voltage, which can be used for environmental sensing.

In Situ Growth of Interfacially Nanoengineered 2D-2D WS2/Ti3C2Tx MXene for the Enhanced Performance of Hydrogen Evolution Reactions

In line with current research goals involving water splitting for hydrogen production, this work aims to develop a noble-metal-free electrocatalyst for a superior hydrogen evolution reaction (HER). A single-step interfacial activation of Ti3C2Tx MXene layers was employed by uniformly growing embedded WS2 two-dimensional (2D) nanopetal-like sheets through a facile solvothermal method. We exploited the interactions between WS2 nanopetals and Ti3C2Tx nanolayers to enhance HER performance. A much safer method was adopted to synthesize the base material, Ti3C2Tx MXene, by etching its MAX phase through mild in situ HF formation. Consequently, WS2 nanopetals were grown between the MXene layers and on edges in a one-step solvothermal method, resulting in a 2D-2D nanocomposite with enhanced interactions between WS2 and Ti3C2Tx MXene. The resulting 2D-2D nanocomposite was thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses before being utilized as working electrodes for HER application. Among various loadings of WS2 into MXene, the 5% WS2-Ti3C2Tx MXene sample exhibited the best activity toward HER, with a low overpotential value of 66.0 mV at a current density of -10 mA cm-2 in a 1 M KOH electrolyte and a remarkable Tafel slope of 46.7 mV·dec-1. The intercalation of 2D WS2 nanopetals enhances active sites for hydrogen adsorption, promotes charge transfer, and helps attain an electrochemical stability of 50 h, boosting HER reduction potential. Furthermore, theoretical calculations confirmed that 2D-2D interactions between 1T/2H-WS2 and Ti3C2Tx MXene realign the active centers for HER, thereby reducing the overpotential barrier.

Interfacial Synergy in Mo2C/MoC Heterostructure Promoting Sequential Polysulfide Conversion in High-Performance Lithium-Sulfur Battery

A rational design of sulfur host is the key to conquering the"polysulfide shuttle effects" by accelerating the polysulfide conversion. Since the process involves solid-liquid-solid multistep phase transitions, purposely-engineered heterostructure catalysts with various active regions for catalyzing conversion steps correspondingly are beneficial to promote the overall conversion process. However, the functionalities of the materials surface and interface in heterostructure catalysts remain unclear. In this work, an Mo2C/MoC catalyst with abundant Mo2C surface-interface-MoC surface tri-active-region is developed by in situ converting the MoZn-metal organic framework. The experimental and simulation studies demonstrate the interface can catch long-chain polysulfides and promote their conversion. Instead, the Mo2C and MoC tend to accommodate the short-chain polysulfide and accelerate their conversion and the Li2S dissociation. Benefitting from the high catalytic ability, the Li-S battery assembled with the Mo2C/MoC-S cathode shows more discrete redox reactions and delivers a high initial capacity of 1603.6 mAh g-1 at 1 C charging-discharging rate, which is over twofolds of the one assembled using individual hosts, and 80.4% capacity can be maintained after 1000 cycles at 3 C rate. This work has demonstrated a novel synergy between the interface and material surface, which will help the future design of high-performance Li-S batteries.

Metal Electrocatalysts for Hydrogen Production in Water Splitting

The rising demand for fossil fuels and the resulting pollution have raised environmental concerns about energy production. Undoubtedly, hydrogen is the best candidate for producing clean and sustainable energy now and in the future. Water splitting is a promising and efficient process for hydrogen production, where catalysts play a key role in the hydrogen evolution reaction (HER). HER electrocatalysis can be well performed by Pt with a low overpotential close to zero and a Tafel slope of about 30 mV dec-1. However, the main challenge in expanding the hydrogen production process is using efficient and inexpensive catalysts. Due to electrocatalytic activity and electrochemical stability, transition metal compounds are the best options for HER electrocatalysts. This study will focus on analyzing the current situation and recent advances in the design and development of nanostructured electrocatalysts for noble and non-noble metals in HER electrocatalysis. In general, strategies including doping, crystallization control, structural engineering, carbon nanomaterials, and increasing active sites by changing morphology are helpful to improve HER performance. Finally, the challenges and future perspectives in designing functional and stable electrocatalysts for HER in efficient hydrogen production from water-splitting electrolysis will be described.

Electronic configuration regulation of single-atomic Mn sites mediated by Mo/Mn clusters for an efficient hydrogen evolution reaction

Tuning the electron distribution of metal single-atom active sites via bimetallic clusters is an effective way to enhance their hydrogen evolution reaction (HER) activity, but remains a great challenge. A biochar-based electrocatalyst (BCMoMn800-2) with both MnN4 active sites and Mo2C/Mn7C3 clusters was synthesized using in situ enriched Mo/Mn biomass as a precursor to trigger the HER. Various characterization and density functional theory (DFT) calculation results indicated that the presence of Mo2C/Mn7C3 clusters in BCMoMn800-2 effectively induced the redistribution of charges at MnN4 sites, reducing the energy of H* activation during the HER. In 0.5 M H2SO4, the overpotential was 27.4 mV at a current density of 10 mA cm-2 and the Tafel slope was 31 mV dec-1, and its electrocatalytic performance was close to that of Pt/C. The electrocatalyst also exhibited excellent electrocatalytic stability and durability. This work might provide a new strategy for solid waste recycling and constructing efficient HER electrocatalysts.

A Highly Active Porous Mo2C-Mo2N Heterostructure on Carbon Nanowalls/Diamond for a High-Current Hydrogen Evolution Reaction

Developing non-precious metal-based electrocatalysts operating in high-current densities is highly demanded for the industry-level electrochemical hydrogen evolution reaction (HER). Here, we report the facile preparation of binder-free Mo2C-Mo2N heterostructures on carbon nanowalls/diamond (CNWs/D) via ultrasonic soaking followed by an annealing treatment. The experimental investigations and density functional theory calculations reveal the downshift of the d-band center caused by the heterojunction between Mo2C/Mo2N triggering highly active interfacial sites with a nearly zero ∆GH* value. Furthermore, the 3D-networked CNWs/D, as the current collector, features high electrical conductivity and large surface area, greatly boosting the electron transfer rate of HER occurring on the interfacial sites of Mo2C-Mo2N. Consequently, the self-supporting Mo2C-Mo2N@CNWs/D exhibits significantly low overpotentials of 137.8 and 194.4 mV at high current densities of 500 and 1000 mA/cm2, respectively, in an alkaline solution, which far surpass the benchmark Pt/C (228.5 and 359.3 mV) and are superior to most transition-metal-based materials. This work presents a cost-effective and high-efficiency non-precious metal-based electrocatalyst candidate for the electrochemical hydrogen production industry.

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.

Application of Oxygen-Group-Based Amorphous Nanomaterials in Electrocatalytic Water Splitting

Environmentally friendly energy sources (e.g., hydrogen) require an urgent development targeting to address the problem of energy scarcity. Electrocatalytic water splitting is being explored as a convenient catalytic reaction in this context, and promising amorphous nanomaterials (ANMs) are receiving increasing attention due to their excellent catalytic properties.Oxygen group-based amorphous nanomaterials (O-ANMs) are an important component of the broad family of ANMs due to their unique amorphous structure, large number of defects, and abundant randomly oriented bonds, O-ANMs induce the generation of a larger number of active sites, which favors a better catalytic activity. Meanwhile, amorphous materials can disrupt the inherent features of conventional crystalline materials regarding electron transfer paths, resulting in higher flexibility. O-ANMs mainly include VIA elements such as oxygen, sulfur, selenium, tellurium, and other transition metals, most of which are reported to be free of noble metals and have comparable performance to commercial catalysts Pt/C or IrO2 and RuO2 in electrocatalysis. This review covers the features and reaction mechanism of O-ANMs, the synthesis strategies to prepare O-ANMs, as well as the application of O-ANMs in electrocatalytic water splitting. Last, the challenges and prospective remarks for future development in O-ANMs for electrocatalytic water splitting are concluded.

Recent progress of molybdenum carbide based electrocatalysts for electrocatalytic hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) is one of the most promising and clean strategies to prepare hydrogen on a large scale. Nevertheless, the efficiency of HER is greatly restricted by the large overpotential at the anode, and it is necessary to develop low cost electrocatalysts with excellent performance and stability. Molybdenum carbide has shown great potential in the field of HER due to its unique electronic structure and physical and chemical properties. In this paper, the current progress of molybdenum carbide-based catalysts for HER is summarized. The influence of phase structure, nanostructure, heterostructure and heteroatoms doping on its catalytic performance is discussed in detail. Especially, the catalytic mechanisms are analyzed according to structural characterization and theoretical calculation results. Finally, the challenges and prospects for the further development of molybdenum carbide-based catalysts for HER are put forward to guide the progress of this field.

Electrocatalytic hydrogen evolution with a copper porphyrin bearing meso-(o-carborane) substituents

A newly designed copper complex of 5,15-bis(pentafluorophenyl)-10,20-bis(o-carborane)porphyrin (1) was synthesized and tested for the electrocatalytic hydrogen evolution reaction (HER). In acetonitrile, 1 was much more efficient than Cu 5,15-bis(pentafluorophenyl)-10,20-diphenylporphyrin (2) for electrocatalytic HER by shifting the catalytic wave to the anodic direction by 190 mV. In aqueous media, 1 also outperformed 2 by achieving higher current densities under smaller overpotentials. This enhancement was attributed to the aromatic and the strong electron-withdrawing properties of o-carborane groups. This work is significant to address the crucial effects of meso-(o-carborane) substituents of metal porphyrins on boosting the electrocatalytic HER.

Controlled synthesis of Mo2C micron flowers via vapor-liquid-solid method as enhanced electrocatalyst for hydrogen evolution reaction

Mo2C demonstrates excellent performance in catalysis, and it has been found to possess excellent hydrogen evolution reaction (HER) catalytic activity and highly efficient nitrogen fixation. The catalytic activity of Mo2C is greatly influenced and restricted by the preparation method. Sintering and carbon deposition, which affect the catalytic activity of Mo2C, are inevitable in the traditional vapor-solid-solid (VSS) process. In this study, we report the controllable synthesis of α-Mo2C micron flowers by adjusting the growth temperature via a vapor-liquid-solid (VLS) process. The density of the Mo2C micron flowers is closely related to the concentration of Na2MoO4 aqueous solution. The as-grown Mo2C micron flowers dispersed with Pt are validated to be an enhanced collaborative electrocatalyst for HER against Pt/VSS-Mo2C.