Trends in Electrochemical Stability of Transition Metal Carbides and Their Potential Use As Supports for Low-Cost Electrocatalysts

Metal-Based Oxygen Reduction Electrocatalysts for Efficient Hydrogen Peroxide Production

Hydrogen peroxide (H2O2) is a high-value chemical widely used in electronics, textiles, paper bleaching, medical disinfection, and wastewater treatment. Traditional production methods, such as the anthraquinone oxidation process and direct synthesis, require high energy consumption, and involve risks from toxic substances and explosions. Researchers are now exploring photochemical, electrochemical, and photoelectrochemical synthesis methods to reduce energy use and pollution. This review focuses on the 2-electron oxygen reduction reaction (2e- ORR) for the electrochemical synthesis of H2O2, and discusses how catalyst active sites influence O2 adsorption. Strategies to enhance H2O2 selectivity by regulating these sites are presented. Catalysts require strong O2 adsorption to initiate reactions and weak *OOH adsorption to promote H2O2 formation. The review also covers advances in single-atom catalysts (SACs), multi-metal-based catalysts, and highlights non-noble metal oxides, especially perovskite oxides, for their versatile structures and potential in 2e- ORR. The potential of localized surface plasmon resonance (LSPR) effects to enhance catalyst performance is also discussed. In conclusion, emphasis is placed on optimizing catalyst structures through theoretical and experimental methods to achieve efficient and selective H2O2 production, aiming for sustainable and commercial applications.

First-Principles Calculations and Machine Learning of Hydrogen Evolution Reaction Activity of Nonmetallic Doped β-Mo2C Support Pt Single-Atom Catalysts

The most widely used catalyst for the hydrogen evolution reaction (HER) is Pt, but the high cost and low abundance of Pt need to be urgently addressed. Single-atom catalysts (SACs) have been an effective means of improving the utilization of Pt atoms. In this work, we used a nonmetal (NM = B, N, O, F, Si, P, S, Cl, As, Se, Br, Te, and I) doped β-Mo2C (100) C-termination surface as the support, with Pt atoms dispersed on the support surface to construct Pt@NM-Mo2C. Using density functional theory (DFT) calculations, we selected catalysts with excellent HER activity. Among 117 candidate catalysts, 49 catalysts exhibited ideal catalytic performance with Gibbs free energy of hydrogen intermediate (H*) adsorption (ΔGH*) values less than 0.2 eV. The ΔGH* values of 16 catalysts were even lower than that of Pt (ΔGH* ≈ 0.09 eV), with PtI@N2/4-a-Mo2C demonstrating the best performance (ΔGH* = -0.01 eV). Combined with electronic structure analysis, we could understand the impact of charge transfer between Pt and the underlying NM atoms on the strength of the Pt-H bond, thereby promoting HER activity. Using machine learning (ML), we identified that the primary influencing factors of the HER catalytic activity in the Pt@NM-Mo2C system were the Bader charge transfer of Pt (NePt), the d-band center of Pt (εdPt), and the atomic radius of NM (RNM), with NePt having the greatest impact on the HER catalytic activity.

Polyoxometalate-HKUST-1 composite derived nanostructured Na-Cu-Mo2C catalyst for efficient reverse water gas shift reaction

Transforming CO2 to CO via reverse water-gas shift (RWGS) reaction is widely regarded as a promising technique for improving the efficiency and economics of CO2 utilization processes. Moreover, it is also considered as a pathway towards e-fuels. Cu-oxide catalysts are widely explored for low-temperature RWGS reactions; nevertheless, they tend to deactivate significantly under applied reaction conditions due to the agglomeration of copper particles at elevated temperatures. Herein, we have synthesized homogeneously distributed Cu metallic nanoparticles supported on Mo2C for the RWGS reaction by a unique approach of in situ carburization of metal-organic frameworks (MOFs) using a Cu-based MOF i.e. HKUST-1 encapsulating molybdenum-based polyoxometalates. The newly derived Na-Cu-Mo2C nanocomposite catalyst system exhibits excellent catalytic performance with a CO production rate of 3230.0 mmol gcat-1 h-1 with 100% CO selectivity. Even after 250 h of a stability test, the catalyst remained active with more than 80% of its initial activity.

Highly Efficient Acidic Electrosynthesis of Hydrogen Peroxide at Industrial-Level Current Densities Promoted by Alkali Metal Cations

Acidic H2O2 synthesis through electrocatalytic 2e- oxygen reduction presents a sustainable alternative to the energy-intensive anthraquinone oxidation technology. Nevertheless, acidic H2O2 electrosynthesis suffers from low H2O2 Faradaic efficiencies primarily due to the competing reactions of 4e- oxygen reduction to H2O and hydrogen evolution in environments with high H+ concentrations. Here, we demonstrate the significant effect of alkali metal cations, acting as competing ions with H+, in promoting acidic H2O2 electrosynthesis at industrial-level currents, resulting in an effective current densities of 50-421 mA cm-2 with 84-100 % Faradaic efficiency and a production rate of 856-7842 μmol cm-2 h-1 that far exceeds the performance observed in pure acidic electrolytes or low-current electrolysis. Finite-element simulations indicate that high interfacial pH near the electrode surface formed at high currents is crucial for activating the promotional effect of K+. In situ attenuated total reflection Fourier transform infrared spectroscopy and ab initio molecular dynamics simulations reveal the central role of alkali metal cations in stabilizing the key *OOH intermediate to suppress 4e- oxygen reduction through interacting with coordinated H2O.

Heterostructures coupling ultrathin metal carbides and chalcogenides

Non-layered transition metal carbides (TMCs) and layered transition metal dichalcogenides (TMDs) are two well-studied material families that have individually received considerable attention over the past century. In recent years, with the shift towards two-dimensional materials and heterostructures, a field has emerged that is focused on the structure and properties of TMC/TMD heterostructures, which through chemical conversion exhibit diverse types of heterostructure configuration that host coupled 2D-3D interfaces, giving rise to exotic properties. In this Review, we highlight experimental and computational efforts to understand the routes to fabricate TMC/TMD heterostructures. Furthermore, we showcase how controlling these heterostructures can lead to emergent electronic transport, optical properties and improved catalytic properties.

Solid-State Synthesis of Niobium Carbide Electrocatalyst in the Presence of Vanadium as Suppressor of Energy Input and Greenhouse Gas Emission

Solid-state reaction (SSR) is a widely adopted method for functional inorganic material syntheses. Unlike intricate systems emerging from chemically unstable precursor usage, the SSR can proceed from stable precursor couples using simple apparatuses. However, this reaction is associated with high temperatures that overcome solid-state diffusion. Moreover, solid-state syntheses of technologically crucial carbides lead to greenhouse gas emissions. Therefore, exploring an extrinsic component to suppress these challenges is vital to confronting global energy and environmental issues. This study reports that the presence of an ordinary element, vanadium (V), changes the routes of the SSR of niobium carbide (NbC), producing NbC efficiently and cleanly. 1000 °C is far below the temperature required to obtain NbC from a precursor couple of Nb2O5 and C, i.e., approximately 1500 °C is required. However, a carbon substitute, vanadium carbide, completely consumed Nb2O5 before reaching 1000 °C and consummated NbC crystallization for 10 h. Furthermore, NbC crystallites were observed using X-ray diffraction from 770 °C, and their formation was primarily accompanied by VNbO4, rather than being routed through NbO2 produced for the Nb2O5-C combination. The obtained NbC contained V as a dopant in the 15-50% range (NbC:V), and the relative abundance was correlated with the preparation temperature. Mass analyses revealed that the formation of NbC/V is barely associated with greenhouse gas emissions because of the sizable thermodynamic driving force for primarily forming vanadium oxide byproducts. Device performance using NbC/V was also assessed for a standard electrochemical hydrogen evolution reaction.

Niobium Carbide and Tantalum Carbide as Nitrogen Reduction Electrocatalysts: Catalytic Activity, Carbophilicity, and the Importance of Intermediate Oxidation States

Significant interest in the electrocatalytic reduction of molecular nitrogen to ammonia (the nitrogen reduction reaction: NRR) has focused attention on transition metal carbides as possible electrocatalysts. However, a fundamental understanding of carbide surface structure/NRR reactivity relationships is sparse. Herein, electrochemistry, DFT-based calculations, and in situ photoemission studies demonstrate that NbC, deposited by magnetron sputter deposition, is active for NRR at pH 3.2 but only after immersion of an ambient-induced Nb2O5 surface layer in 0.3 M NaOH, which leaves Nb suboxides with niobium in intermediate formal oxidation states. Photoemission data, however, show that polarization to -1.3 V vs Ag/AgCl restores the Nb2O5 overlayer, correlating with electrochemical measurements showing inhibition of NRR activity under these conditions. In contrast, a similar treatment of a sputter-deposited TaC sample in 0.3 M NaOH fails to reduce the ambient-induced Ta2O5 surface layer, and TaC is inactive for NRR at potentials more positive than -1.0 V even though a significant cathodic current is observed. A TaC sample with surface oxide partially reduced by Ar ion sputtering in UHV prior to in situ transfer to UHV exhibits a restored Ta2O5 surface layer after electrochemical polarization to -1.0 V vs Ag/AgCl. The electrochemical and photoemission results are in accord with DFT-based calculations indicating greater N≡N bond activation for N2 bound end-on to Nb(IV) and Nb(III) sites than for N2 bound end-on to Nb(V) sites. Thus, theory and experiment demonstrate that with respect to NbC, the formation and stabilization of intermediate (non-d0) oxidation states for surface transition metal ions is critical for N≡N bond activation and NRR activity. Additionally, the Nb suboxide surface, formed by immersion in 0.3 M NaOH of ambient-exposed NbC, is shown to undergo reoxidation to catalytically inactive Nb2O5 at -1.3 V vs Ag/AgCl, possibly due to hydrolysis or other, as yet not understood, phenomena.

Synergistically Tuning Surface States of Hierarchical MoC by Pt-N Dual-Doping Engineering for Optimizing Hydrogen Evolution Activity

Catalytic performance can be greatly enhanced by rational modulation of the surface state. In this study, reasonable adjustment of the surface states around the Fermi level (EF ) of molybdenum carbide (MoC) (α phase) via a Pt-N dual-doping process to fabricate an electrocatalyst named as Pt-N-MoC is performed to promote hydrogen evolution reaction (HER) performance over the MoC surface. Systematically experimental and theoretical analyses demonstrate that the synergistic tuning of Pt and N can cause the delocalization of surface states, with an increase in the density of surface states near the EF . This is beneficial for accumulating and transferring electrons between the catalyst surface and adsorbent, resulting in a positively linear correlation between the density of surface states near the EF and the HER activity. Moreover, the catalytic performance is further enhanced by artificially fabricating a Pt-N-MoC catalyst that has a unique hierarchical structure composed of MoC nanoparticles (0D), nanosheets (2D), and microrods (3D). As expected, the obtained Pt-N-MoC electrocatalyst exhibits superb HER activity with an extremely low overpotential of 39 mV@10 mA cm-2 as well as superb stability (over 24 d) in an alkaline solution. This work highlights a novel strategy to develop efficient electrocatalysts via adjusting their surface states.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Mo2C@C nanofibers film as durable self-supported electrode for efficient electrocatalytic hydrogen evolution

Self-supported electrocatalytic thin films consist 3D conducting network and well-embedded electrocatalysts, which endows the advantage in mass flow kinetics and durability for large-scale water splitting. Synthesis of such self-supported electrode still remains a big challenge due to the difficulty in the control over the 3D conducting network and the simultaneous growth of catalyst with well attachment on the conducting fibers. Herein, a self-supported Mo₂C@carbon nanofibers (Mo₂C@C NF) film has been successfully fabricated with outstanding electrocatalytic performance under optimized pyrolysis temperature and precursors mass ratio conditions. During the carbonation process, the Mo₂C nanoparticles (∼16 nm) were simultaneously grown and well dispersed on the inter-connected carbon nanofibers, which formed a 3D conducting network. The as-formed 3D carbon network was strong enough to support direct electrocatalytic application without additional ink or supporting substrates. This particular electrode structure facilitated easy access to the active catalytic sites, electron transfer, and hydrogen diffusion, resulting in the high hydrogen evolution reaction activity. A low overpotential of 86 mV was needed to achieve 10 mA cm^-2current density with outstanding kinetics metric (Tafel 43 mV dec^-1) in 1 M KOH. Additionally, the self-supported Mo₂C@C NF film, a binder-free electrode, exhibited extraordinary stability of more than 340 h.

Density functional theory studies of transition metal carbides and nitrides as electrocatalysts

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N₂RR), CO₂ reduction reaction (CO₂RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts.

Metal-Nitrogen-Carbon Cluster-Decorated Titanium Carbide is a Durable and Inexpensive Oxygen Reduction Reaction Electrocatalyst

Clusters of nitrogen- and carbon-coordinated transition metals dispersed in a carbon matrix (e. g., Fe-N-C) have emerged as an inexpensive class of electrocatalysts for the oxygen reduction reaction (ORR). Here, it was shown that optimizing the interaction between the nitrogen-coordinated transition metal clusters embedded in a more stable and corrosion-resistant carbide matrix yielded an ORR electrocatalyst with enhanced activity and stability compared to Fe-N-C catalysts. Utilizing first-principles calculations, an electrostatics-based descriptor of catalytic activity was identified, and nitrogen-coordinated iron (FeN₄ ) clusters embedded in a TiC matrix were predicted to be an efficient platinum-group metal (PGM)-free ORR electrocatalyst. Guided by theory, selected catalyst formulations were synthesized, and it was demonstrated that the experimentally observed trends in activity fell exactly in line with the descriptor-derived theoretical predictions. The Fe-N-TiC catalyst exhibited enhanced activity (20 %) and durability (3.5-fold improvement) compared to a traditional Fe-N-C catalyst. It was posited that the electrostatics-based descriptor provides a powerful platform for the design of active and stable PGM-free electrocatalysts and heterogenous single-atom catalysts for other electrochemical reactions.

How the in situ monitoring of bulk crystalline phases during catalyst activation results in a better understanding of heterogeneous catalysis

The present Highlight article shows the importance of the in situ monitoring of bulk crystalline compounds for a more thorough understanding of heterogeneous catalysts at the intersection of catalysis, materials science, crystallography and inorganic chemistry. Although catalytic action is widely regarded as a purely surface-bound phenomenon, there is increasing evidence that bulk processes can detrimentally or beneficially influence the catalytic properties of various material classes. Such bulk processes include polymorphic transformations, formation of oxygen-deficient structures, transient phases and the formation of a metal-oxide composite. The monitoring of these processes and the subsequent establishment of structure-property relationships are most effective if carried out in situ under real operation conditions. By focusing on synchrotron-based in situ X-ray diffraction as the perfect tool to follow the evolution of crystalline species, we exemplify the strength of the concept with five examples from various areas of catalytic research. As catalyst activation studies are increasingly becoming a hot topic in heterogeneous catalysis, the (self-)activation of oxide- and intermetallic compound-based materials during methanol steam and methane dry reforming is highlighted. The perovskite LaNiO3 is selected as an example to show the complex structural dynamics before and during methane dry reforming, which is only revealed upon monitoring all intermediate crystalline species in the transformation from LaNiO3 into Ni/La2O3/La2O2CO3. ZrO2-based materials form the second group, indicating the in situ decomposition of the intermetallic compound Cu51Zr14 into an epitaxially stabilized Cu/tetragonal ZrO2 composite during methanol steam reforming, the stability of a ZrO0.31C0.69 oxycarbide and the gas-phase dependence of the tetragonal-to-monoclinic ZrO2 polymorphic transformation. The latter is the key parameter to the catalytic understanding of ZrO2 and is only appreciated in full detail once it is possible to follow the individual steps of the transformation between the crystalline polymorphic structures. A selected example is devoted to how the monitoring of crystalline reactive carbon during methane dry reforming operation aids in the mechanistic understanding of a Ni/MnO catalyst. The most important aspect is the strict use of in situ monitoring of the structural changes occurring during (self-)activation to establish meaningful structure-property relationships allowing conclusions beyond isolated surface chemical aspects.

Enhanced Electrochemical H2O2 Production via Two-Electron Oxygen Reduction Enabled by Surface-Derived Amorphous Oxygen-Deficient TiO2-x

The electrochemical oxygen reduction reaction (ORR) is regarded as an attractive alternative to the anthraquinone process for sustainable and on-site hydrogen peroxide (H₂O₂) production. It is however hindered by low selectivity due to strong competition from the four-electron ORR and needs efficient catalysts to drive the 2e^- ORR. Here, an acid oxidation strategy is proposed as an effective strategy to boost the 2e^- ORR activity of metallic TiC via in-site generation of a surface amorphous oxygen-deficient TiO_2-x layer. The resulting a-TiO_2-x/TiC exhibits a low overpotential and high H₂O₂ selectivity (94.1% at 0.5 V vs reversible hydrogen electrode (RHE)), and it also demonstrates robust stability with a remarkable productivity of 7.19 mol g_cat.^-1 h^-1 at 0.30 V vs RHE. The electrocatalytic mechanism of a-TiO_2-x/TiC is further revealed by density functional theory calculations.

Nominal Effect of Mg Intercalation on the Superconducting Properties of 2H-NbSe2

2H-NbSe₂ is a phonon-mediated, Fermi-surface topology-dependent multiband superconductor with an incommensurate charge-density wave (CDW) that coexists at a local level with superconductivity. Usually, the intercalation in 2H-NbSe₂ enriches the CDW, enhances the c-axis lattice parameter, and distorts the Fermi surface, which result in a decrease in the superconducting transition temperature (T_c). The rate of decrease of T_c depends on the electronic structure, size, valence, magnetic nature, and electronegativity of the intercalating species. Herein, we report an unusual effect of Mg intercalation on the superconductivity of 2H-Mg_xNbSe₂ (x = 0.0, 0.02, 0.06, 0.08, 0.10, and 0.12) synthesized by a high-temperature solid-state reaction method. Unlike other s- and p-block elements/species as intercalants (Rb, Sn, Ga, and Al) that have a sharp detrimental effect on the T_c of 2H-NbSe₂ within 1-5% of intercalation, Mg is found to be an exception. Upon Mg intercalation up to x = 0.06, no remarkable changes in T_c as compared to the parent 2H-NbSe₂ (T_c ∼ 6.7 K) are observed, and further intercalation results in a small decrease in T_c (for x = 0.12, T_c = 6.2 K). From heat-capacity measurements, it is inferred that superconducting Mg-intercalated 2H-NbSe₂ exhibits strong electron-phonon coupling. Electronic structure calculations on two s-block element intercalated compounds of formula M_0.125NbSe₂ (M = Mg, Rb) show that Rb s-, p-, and d-states completely overlap with the Nb d states, while the Mg s states lie in a low-energy region as compared to Nb d states, indicating a weak interaction between the intercalant and the Nb sublattice in Mg_0.125NbSe₂ as compared to Rb_0.125NbSe₂. These results suggest that the electronic states of the Nb network in 2H-NbSe₂ are least altered with Mg intercalation, which could be one of the reasons for the minimal effect on the T_c with intercalation.

PMO12@ZIF-8/ZnO-derived hierarchical porous molybdenum carbide as efficient electrocatalysts for hydrogen evolution

MoC@NC is an N-doped hierarchical porous graphite carbon-coated MoC nanoparticles with outstanding HER activity.

Sequeira CA, Figueiredo JL, Šljukić B. Carbon-Supported Mo2C for Oxygen Reduction Reaction Electrocatalysis

Molybdenum carbide (Mo2C)-based electrocatalysts were prepared using two different carbon supports, commercial carbon nanotubes (CNTs) and synthesised carbon xerogel (CXG), to be studied from the point of view of both capacitive and electrocatalytic properties. Cation type (K+ or Na+) in the alkaline electrolyte solution did not affect the rate of formation of the electrical double layer at a low scan rate of 10 mV s-1. Conversely, the different mobility of these cations through the electrolyte was found to be crucial for the rate of double-layer formation at higher scan rates. Molybdenum carbide supported on carbon xerogel (Mo2C/CXG) showed ca. 3 times higher double-layer capacity amounting to 75 mF cm-2 compared to molybdenum carbide supported on carbon nanotubes (Mo2C/CNT) with a value of 23 mF cm-2 due to having more than double the surface area size. The electrocatalytic properties of carbon-supported molybdenum carbides for the oxygen reduction reaction in alkaline media were evaluated using linear scan voltammetry with a rotating disk electrode. The studied materials demonstrated good electrocatalytic performance with Mo2C/CXG delivering higher current densities at more positive onset and half-wave potential. The number of electrons exchanged during oxygen reduction reaction (ORR) was calculated to be 3, suggesting a combination of four- and two-electron mechanism.

The rapid production of multiple transition metal carbides via microwave combustion under ambient conditions

Transition metal carbides (TMCs) have attracted great interest owing to their potential applications for energy storage and electrocatalysis. But the synthesis of high-quality TMCs usually needs high energy consumption, long reaction times, or dangerous chemical reagents, limiting their practical application. Here, a microwave combustion method is developed to rapidly (∼2 min) produce transition metal carbides under ambient conditions. The as-synthesized TMCs (W₂C, VC, Fe₃C, NbC, TaC, and Mo₂C) and rGO composites exhibit outstanding catalytic performance for the hydrogen evolution reaction (HER) over a broad range of pH values. This work highlights a novel strategy for the design and synthesis of transition metal carbides.

Non-Precious Metal Graphene-Based Catalysts for Hydrogen Evolution Reaction

Developing sustainable and renewable energy sources is critical as higher and higher global energy and environmental challenges arise. Hydrogen has the highest mass/energy density of any fuel and is considered one of the best sources of clean energy. Water splitting is regarded as one of the most promising solutions for hydrogen production on a large scale. Highly efficient, durable, and cost-effective catalysts for hydrogen evolution reaction (HER) are critical in the realization of this goal. Among the many materials proposed, graphene-based materials offer some unique properties for HER catalysis. In this review, we present recent progress on development of graphene-based electrocatalysts toward HER throughout the past few years.

Spontaneous Synthesis of Silver-Nanoparticle-Decorated Transition-Metal Hydroxides for Enhanced Oxygen Evolution Reaction

The fabrication of metal-supported hybrid structures with enhanced properties typically requires external energy input, such as pyrolysis, photolysis, and electrodeposition. In this study, silver-nanoparticle-decorated transition-metal hydroxide (TMH) composites were synthesized by an approach based on a spontaneous redox reaction (SRR) at room temperature. The SRR between silver ions and TMH provides a simple and facile route to establish effective and stable heterostructures that can enhance the oxygen evolution reaction (OER) activity. Ag@Co(OH)_x grown on carbon cloth exhibits outstanding OER activity and durability, even superior to IrO₂ and many previously reported OER electrocatalysts. Experimental and theoretical analysis demonstrates that the strong electronic interaction between Ag and Co(OH)₂ activates the silver clusters as catalytically OER active sites, effectively optimizing the binding energies with reacted intermediates and facilitating the OER kinetics.

Three-Dimensional N-Doped Carbon Nanotube Frameworks on Ni Foam Derived from a Metal-Organic Framework as a Bifunctional Electrocatalyst for Overall Water Splitting

Rational design of bifunctional, high-performance, and stable non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great importance and challenging for the realization of overall water splitting. Metal-organic frameworks (MOFs) have been intensively studied as pyrolyzing precursors to prepare electrocatalysts. However, the aggregation of powder and the low conductivity of polymer binders have limited the applications of powder electrocatalysts. Therefore, the direct growth of MOFs on conductive and porous substrates will be a favorable way to prepare efficient electrocatalysts for electrocatalytic water splitting. Herein, we report a facile strategy for constructing three-dimensional N-doped carbon nanotube frameworks derived from metal-organic framework on Ni foam as a bifunctional electrocatalyst for overall water splitting. The resulting electrocatalyst exhibits excellent stability and high OER and HER activity with rather low overpotentials of 230 and 141 mV at 10 mA/cm² in 1.0 M KOH, respectively. Specifically, the as-synthesized electrodes were used as both the cathode and anode for overall water splitting with 10 mA/cm² at a cell voltage of only 1.62 V. The outstanding electrocatalytic performance is mainly attributed to a large number of accessible active sites of Co nanoparticles dispersed by the N-doped carbon nanotubes (CNTs) and the ultra-high surface area of CNT frameworks. The presented strategy offers a novel approach for developing MOF-derived nanocarbon materials on Ni foam for electrocatalysis and electrochemical energy devices.

Porous Molybdenum Carbide Nanostructures Synthesized on Carbon Cloth by CVD for Efficient Hydrogen Production

Molybdenum carbide (Mo₂ C) is a promising noble-metal-free electrocatalyst for the hydrogen evolution reaction (HER), due to its structural and electronic merits, such as high conductivity, metallic band states and wide pH applicability. Here, a simple CVD process was developed for synthesis of a Mo₂ C on carbon cloth (Mo₂ C@CC) electrode with carbon cloth as carbon source and MoO₃ as the Mo precursor. XRD, Raman, XPS and SEM results of Mo₂ C@CC with different amounts of MoO₃ and growth temperatures suggested a two-step synthetic mechanism, and porous Mo₂ C nanostructures were obtained on carbon cloth with 50 mg MoO₃ at 850 °C (Mo₂ C-850(50)). With the merits of unique porous nanostructures, a low overpotential of 72 mV at current density of 10 mA cm^-2 and a small Tafel slope of 52.8 mV dec^-1 was achieved for Mo₂ C-850(50) in 1.0 m KOH. The dual role of carbon cloth as electrode and carbon source resulted into intimate adhesion of Mo₂ C on carbon cloth, offering fast electron transfer at the interface. Cyclic voltammetry measurements for 5000 cycles revealed that Mo₂ C@CC had excellent electrochemical stability. This work provides a novel strategy for synthesizing Mo₂ C and other efficient carbide electrocatalysts for HER and other applications, such as supercapacitors and lithium-ion batteries.

Zirconium Oxycarbide: A Highly Stable Catalyst Material for Electrochemical Energy Conversion

Metal carbides and oxycarbides have recently gained considerable interest due to their (electro)catalytic properties that differ from those of transition metals and that have potential to outperform them as well. The stability of zirconium oxycarbide nanopowders (ZrO_0.31C_0.69), synthesized via a hybrid solid‐liquid route, is investigated in different gas atmospheres from room temperature to 800 °C by using in‐situ X‐ray diffraction and in‐situ electrical impedance spectroscopy. To feature the properties of a structurally stable Zr oxycarbide with high oxygen content, a stoichiometry of ZrO_0.31C_0.69 has been selected. ZrO_0.31C_0.69 is stable in reducing gases with only minor amounts of tetragonal ZrO₂ being formed at high temperatures, whereas it decomposes in CO₂ and O₂ gas atmosphere. From online differential electrochemical mass spectrometry measurements, the hydrogen evolution reaction (HER) onset potential is determined at −0.4 V_RHE. CO₂ formation is detected at potentials as positive as 1.9 V_RHE as ZrO_0.31C_0.69 decomposition product, and oxygen is anodically formed at 2.5 V_RHE, which shows the high electrochemical stability of this material in acidic electrolyte. This peopwery makes the material suited for electrocatalytic reactions at anodic potentials, such as CO and alcohol oxidation reactions, in general.

First-principles study of vanadium carbides as electrocatalysts for hydrogen and oxygen evolution reactions

Vanadium carbides have attracted much attention as highly active catalysts in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), while a satisfactory understanding of the underlying mechanisms still remains a challenge. Herein we apply first-principles calculations to systematically analyze the crystal structures, electronic properties, free energies during the HER and OER processes, surface energies and crystal formation energies of the three types of vanadium carbides, i.e., V4C3, V8C7 and VC. We show that all these vanadium carbides are metallic, which enables efficient electron transport from the bulk to the surface of the catalysts. All these vanadium carbides exhibit excellent HER performance but show poor OER catalytic activity. In particular, the V8C7 (110) surface shows the best catalytic performance for its relatively small |ΔG(H*)| value (-0.114 eV) for HER. Emergence of natural carbon vacancies gives rise to large surface energy, proper hydrogen adsorption energy, low crystal formation energy and weak bond strength in V8V7, which guarantees its leading position among the three vanadium carbides. In addition, a remarkable resemblance between VC/V8C7 and Pt in their electronic structures on (110) and (111) surfaces are found, which indicates a Pt-like HER mechanism in these vanadium carbides. Our results thus bring new insights to the theoretical understanding of the excellent HER performance of vanadium carbides.

Transition-Metal Oxides/Carbides@Carbon Nanotube Composites as Multifunctional Electrocatalysts for Challenging Oxidations and Reductions

The rapid development of renewable-energy technologies such as water splitting, rechargeable metal-air batteries, and fuel cells requires highly efficient electrocatalysts capable of the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER). Herein, we report a facile sonication-driven synthesis to deposit the molecular manganese vanadium oxide precursor [Mn₄ V₄ O₁₇ (OAc)₃ ]^3- on multiwalled carbon nanotubes (MWCNTs). Thermal conversion of this composite at 900 °C gives nanostructured manganese vanadium oxides/carbides, which are stably linked to the MWCNTs. The resulting composites show excellent electrochemical reactivity for ORR and OER, and significant reactivity enhancements compared with the precursors and a Pt/C reference are reported. Notably, even under harsh acidic conditions, long-term OER activity at low overpotential is reported. In addition, we report exceptional activity of the composites for the industrially important Cl₂ evolution from an aqueous HCl electrolyte. The new composite material shows how molecular deposition routes leading to highly active and stable multifunctional electrocatalysts can be developed. The facile design could in principle be extended to multiple catalyst classes by tuning of the molecular metal oxide precursor employed.

Co3O4-nanoparticle-entrapped nitrogen and boron codoped mesoporous carbon as an efficient electrocatalyst for hydrogen evolution

Co₃O₄-nanoparticle-entrapped nitrogen and boron codoped mesoporous carbon was synthesized via the molten salt method. Melamine formaldehyde resin (MF resin) was used as the nitrogen and carbon precursor, and boric acid was utilized as the boron precursor. Furthermore, cobalt chloride was used as the cobalt precursor and the template for the formation of mesopores, which could also be removed and partly recovered by acid washing. The characterization results revealed that the as-obtained samples possessed mesoporous structures, with high cobalt, boron, and nitrogen content values. For the sample of Co_0.65B_0.3NC₈₀₀, the atomic content values of Co, N, and B are 2.3%, 8.87%, and 8.67%, respectively. Moreover, the carbonation temperature and the amount of salt template could both affect the mesoporous structures of the final samples and then affect the electrocatalytic activities for the hydrogen evolution reaction (HER). When the carbonation temperature was 800 °C, the sample of Co_0.65B_0.3NC₈₀₀ showed superior performance for the HER under basic conditions, with high current density, low overpotential, and good stability.

Rational Design of Atomic Layers of Pt Anchored on Mo2 C Nanorods for Efficient Hydrogen Evolution over a Wide pH Range

Transition metal carbide compound has been extensively investigated as a catalyst for hydrogenation, for example, due to its noble metal-like properties. Herein a facile synthetic strategy is applied to control the thickness of atomic-layer Pt clusters strongly anchored on N-doped Mo₂ C nanorods (Pt/N-Mo₂ C) and it is found that the Pt atomic layers modify Mo₂ C function as a high-performance and robust catalyst for hydrogen evolution. The optimized 1.08 wt% Pt/N-Mo₂ C exhibits 25-fold, 10-fold, and 15-fold better mass activity than the benchmark 20 wt% Pt/C in neutral, acidic, and alkaline media, respectively. This catalyst also represents an extremely low overpotential of -8.3 mV at current density of 10 mA cm^-2 , much better than the majority of reported electrocatalysts and even the commercial reference catalyst (20 wt%) Pt/C. Furthermore, it exhibits an outstanding long-term operational durability of 120 h. Theoretical calculation predicts that the ultrathin layer of Pt clusters on Mo-Mo₂ C yields the lowest absolute value of ΔG_H* . Experimental results demonstrate that the atomic layer of Pt clusters anchored on Mo₂ C substrate greatly enhances electron and mass transportation efficiency and structural stability. These findings could provide the foundation for developing highly effective and scalable hydrogen evolution catalysts.

Structural Design and Electronic Modulation of Transition-Metal-Carbide Electrocatalysts toward Efficient Hydrogen Evolution

As the key of hydrogen economy, electrocatalytic hydrogen evolution reactions (HERs) depend on the availability of cost-efficient electrocatalysts. Over the past years, there is a rapid rise in noble-metal-free electrocatalysts. Among them, transition metal carbides (TMCs) are highlighted due to their structural and electronic merits, e.g., high conductivity, metallic band states, tunable surface/bulk architectures, etc. Herein, representative efforts and progress made on TMCs are comprehensively reviewed, focusing on the noble-metal-like electronic configuration and the relevant structural/electronic modulation. Briefly, specific nanostructures and carbon-based hybrids are introduced to increase active-site abundance and to promote mass transportation, and heteroatom doping and heterointerface engineering are encouraged to optimize the chemical configurations of active sites toward intrinsically boosted HER kinetics. Finally, a perspective on the future development of TMC electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials in energy chemistry.

Recent progress in Pt and Pd-based hybrid nanocatalysts for methanol electrooxidation

Hybrid nanomaterials can combine merits of different components and modulate electronic states of Pt and Pd based nanocrystals simultaneously.

Principles and Methods for the Rational Design of Core-Shell Nanoparticle Catalysts with Ultralow Noble Metal Loadings

Conspecuts Commercial and emerging renewable energy technologies are underpinned by precious metal catalysts, which enable the transformation of reactants into useful products. However, the noble metals (NMs) comprise the least abundant elements in the lithosphere, making them prohibitively scarce and expensive for future global-scale technologies. As such, intense research efforts have been devoted to eliminating or substantially reducing the loadings of NMs in various catalytic applications. These efforts have resulted in a plethora of heterogeneous NM catalyst morphologies beyond the traditional supported spherical nanoparticle. In many of these new architectures, such as shaped, high index, and bimetallic particles, less than 20% of the loaded NMs are available to perform catalytic turnovers. The majority of NM atoms are subsurface, providing only a secondary catalytic role through geometric and ligand effects with the active surface NM atoms. A handful of architectures can approach 100% NM utilization, but severe drawbacks limit general applicability. For example, in addition to problems with stability and leaching, single atom and ultrasmall cluster catalysts have extreme metal-support interactions, discretized d-bands, and a lack of adjacent NM surface sites. While monolayer thin films do not possess these features, they exhibit such low surface areas that they are not commercially relevant, serving predominantly as model catalysts. This Account champions core-shell nanoparticles (CS NPs) as a vehicle to design highly active, stable, and low-cost materials with high NM utilization for both thermo- and electrocatalysis. The unique benefits of the many emerging NM architectures could be preserved while their fundamental limitations could be overcome through reformulation via a core-shell morphology. However, the commercial realization of CS NPs remains challenging, requiring concerted advances in theory and manufacturing. We begin by formulating seven constraints governing proper core material design, which naturally point to early transition metal ceramics as suitable core candidates. Two constraints prove extremely challenging. The first relates to the core modifying the shell work function and d-band. To properly investigate materials that could satisfy this constraint, we discuss our development of a new heat, quench, and exfoliation (HQE) density functional theory (DFT) technique to model heterometallic interfaces. This technique is used to predict how transition metal carbides can favorably tune the catalytic properties of various NM monolayer shell configurations. The second challenging constraint relates to the scalable manufacturing of CS NP architectures with independent synthetic control of the thickness and composition of the shell and the size and composition of the core. We discuss our development of a synthetic method that enables high temperature self-assembly of tunable CS NP configurations. Finally, we discuss how these principles and methods were used to design catalysts for a variety of applications. These include the design of a thermally stable sub-monolayer CS catalyst, a highly active methanol electrooxidation catalyst, CO-tolerant Pt catalysts, and a hydrogen evolution catalyst that is less expensive than state-of-the-art NM-free catalysts. Such core-shell architectures offer the promise of ultralow precious metal loadings while ceramic cores hold the promise of thermodynamic stability and access to unique catalytic activity/tunability.

Porous-carbon-based Mo2C nanocomposites as excellent microwave absorber: a new exploration

In this study, we report a porous-carbon-based Mo2C nanocomposite (NCs) as the microwave absorber via typical carbothermal reduction using metal-organic frameworks (MOFs) and polyoxometalates (POMs) as the precursors, which have been rarely applied in electromagnetic (EM) wave absorption areas. The elaborately designed NCs not only bring about good impedance matching, but also possess strong dissipation ability due to the large surface areas and porous features. Thanks to the material characteristics as well as structural advantages, the as-prepared Mo2C/C NCs with 20 wt% sample loading exhibit remarkable microwave absorbing performance. The minimum RL value reaches -49.19 dB at matching thickness of 2.6 mm, and the best effective bandwidth (RL < -10 dB) of 4.56 GHz at 1.70 mm was also achieved. Moreover, the NCs overcome the intrinsic drawback of traditional carbon materials, that is, centralized effective absorption always occurred at high frequency (>10 GHz) and the minimum RL value of the NCs shifted to 9.04 GHz. Clearly, in this study, we not only developed the Mo2C NC as the new light absorber, but also paved the way to synthesize other available transition metal carbides using MOFs and POMs.

Reactions of water and C1 molecules on carbide and metal-modified carbide surfaces

The formation of carbides can significantly modify the physical and chemical properties of the parent metals. In the current review, we summarize the general trends in the reactions of water and C1 molecules over transition metal carbide (TMC) and metal-modified TMC surfaces and thin films. Although the primary focus of the current review is on the theoretical and experimental studies of reactions of C1 molecules (CO, CO₂, CH₃OH, etc.), the reactions of water will also be reviewed because water plays an important role in many of the C1 transformation reactions. This review is organized by discussing separately thermal reactions and electrochemical reactions, which provides insights into the application of TMCs in heterogeneous catalysis and electrocatalysis, respectively. In thermal reactions, we discuss the thermal decomposition of water and methanol, as well as the reactions of CO and CO₂ over TMC surfaces. In electrochemical reactions, we summarize recent studies in the hydrogen evolution reaction, electrooxidation of methanol and CO, and electroreduction of CO₂. Finally, future research opportunities and challenges associated with using TMCs as catalysts and electrocatalysts are also discussed.

Tuning core–shell interactions in tungsten carbide–Pt nanoparticles for the hydrogen evolution reaction

Titanium doping enhances the stability and activity of tungsten carbide core–platinum shell nanoparticles for hydrogen evolution

Density functional study on the high catalytic performance of single metal atoms on the NbC(001) surface

We have studied the adsorption and electronic properties as well as the catalytic activities of group 9–11 single metal atoms deposited on NbC(001).