Composites of graphene-Mo2C rods: highly active and stable electrocatalyst for hydrogen evolution reaction

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.

Recent Advancements in Two-Dimensional Layered Molybdenum and Tungsten Carbide-Based Materials for Efficient Hydrogen Evolution Reactions

Green and renewable energy is the key to overcoming energy-related challenges such as fossil-fuel depletion and the worsening of environmental habituation. Among the different clean energy sources, hydrogen is considered the most impactful energy carrier and is touted as an alternate fuel for clean energy needs. Even though noble metal catalysts such as Pt, Pd, and Au exhibit excellent hydrogen evolution reaction (HER) activity in acid media, their earth abundance and capital costs are highly debatable. Hence, developing cost-effective, earth-abundant, and conductive electrocatalysts is crucial. In particular, various two-dimensional (2D) transition metal carbides and their compounds are gradually emerging as potential alternatives to noble metal-based catalysts. Owing to their improved hydrophilicity, good conductivity, and large surface areas, these 2D materials show superior stability and excellent catalytic performances during the HER process. This review article is a compilation of the different synthetic protocols, their impact, effects of doping on molybdenum and tungsten carbides and their derivatives, and their application in the HER process. The paper is more focused on the detailed strategies for improving the HER activity, highlights the limits of molybdenum and tungsten carbide-based electrocatalysts in electro-catalytic process, and elaborates on the future advancements expected in this field.

Supported Nanostructured MoxC Materials for the Catalytic Reduction of CO2 through the Reverse Water Gas Shift Reaction

Mo_xC-based catalysts supported on γ-Al₂O₃, SiO₂ and TiO₂ were prepared, characterized and studied in the reverse water gas shift (RWGS) at 548-673 K and atmospheric pressure, using CO₂:H₂ = 1:1 and CO₂:H₂ = 1:3 mol/mol reactant mixtures. The support used determined the crystalline Mo_xC phases obtained and the behavior of the supported nanostructured Mo_xC catalysts in the RWGS. All catalysts were active in the RWGS reaction under the experimental conditions used; CO productivity per mol of Mo was always higher than that of unsupported Mo₂C prepared using a similar method in the absence of support. The CO selectivity at 673 K was above 94% for all the supported catalysts, and near 99% for the SiO₂-supported. The Mo_xC/SiO₂ catalyst, which contains a mixture of hexagonal Mo₂C and cubic MoC phases, exhibited the best performance for CO production.

Boosted Photoelectrochemical N2 Reduction over Mo2C In Situ Coated with Graphitized Carbon

Photoelectrochemical N₂ reduction reaction (PEC NRR) is a promising method to solve the problems of environmental protection and energy sustainability. However, the strong chemical stability of the N≡N bond and competitive hydrogen evolution reaction (HER) cause the nonideal efficiency of N₂ → NH₃ conversion in actual operation. For the first time, a Mo₂C/C heterostructure was fabricated as a PEC cathode for N₂ reduction under environmental conditions. The Mo₂C/C heterostructure could effectively decrease the coverage of hydrogen spillover and inhibit the competitive HER, resulting in a desirable selectivity for N₂ activation. Meanwhile, the decoration of the C shell further promoted the stability and conductivity of Mo₂C. Mo sites of Mo₂C were considered as activation centers, which played a dominant role in the final PEC performance. An optimal NH₃ yield rate of up to 6.6 μg h^-1 mg^-1 was achieved with the Mo₂C/C heterostructure, which was almost 3 times that with pristine C. The faradic efficiency (FE) of the Mo₂C/C heterostructure was 37.2% at 0.2 V (vs Ag/AgCl). This work not only provides an insight into the interplay between the Mo₂C/C heterostructure and N₂ activation, but also reveals its great potential in NH₃ synthesis by a green route.

Green tea extract assisted green synthesis of reduced graphene oxide: Application for highly sensitive electrochemical detection of sunset yellow in food products

The search to find simple, cost-effective, environmentally friendly method for synthesising of reduced graphene oxide (rGO) has motivated the use of various natural materials. Also, monitoring of sunset yellow (SY) level in foods due to the potential negative side effects is imperative. In this study, tea extract was explored as reducing and stabilizing agent for synthesising of rGO. The rGO modified carbon paste electrode (rGO/CPE) was used as a highly sensitive electrochemical sensor for the detection of SY. The rGO/CPE, due to the large surface area, showed strong enhancement effect on electrochemical oxidation of SY. Under optimized conditions, linear range between 0.05 and 10 µM with a detection limit of 27 nM could be achieved. The proposed sensor was used to determine the amount of SY in food products with satisfactory results, and the results were in good agreement with the results obtained by UV-Vis spectroscopy.

RuNi Nanoparticles Embedded in N-Doped Carbon Nanofibers as a Robust Bifunctional Catalyst for Efficient Overall Water Splitting

Developing high-performance, low-cost, and robust bifunctional electrocatalysts for overall water splitting is extremely indispensable and challenging. It is a promising strategy to couple highly active precious metals with transition metals as efficient electrocatalysts, which can not only effectively reduce the cost of the preparation procedure, but also greatly improve the performance of catalysts through a synergistic effect. Herein, Ru and Ni nanoparticles embedded within nitrogen-doped carbon nanofibers (RuNi-NCNFs) are synthesized via a simple electrospinning technology with a subsequent carbonization process. The as-formed RuNi-NCNFs represent excellent Pt-like electrocatalytic activity for the hydrogen evolution reaction (HER) in both alkaline and acidic conditions. Furthermore, the RuNi-NCNFs also exhibit an outstanding oxygen evolution reaction (OER) activity with an overpotential of 290 mV to achieve a current density of 10 mA cm-2 in alkaline electrolyte. Strikingly, owing to both the HER and OER performance, an electrolyzer with RuNi-NCNFs as both the anode and cathode catalysts requires only a cell voltage of 1.564 V to drive a current density of 10 mA cm-2 in an alkaline medium, which is lower than the benchmark of Pt/C||RuO2 electrodes. This study opens a novel avenue toward the exploration of high efficient but low-cost electrocatalysts for overall water splitting.

Enhanced Overall Water-Splitting Performance: Oleylamine-Functionalized GO/Cu2ZnSnS4 Composite as a Nobel Metal-Free and NonPrecious Electrocatalyst

Using emergent highly proficient and inexpensive non-noble metal-based bifunctional electrocatalysts to overall water splitting reaction is a pleasingly optional approach to resolve greenhouse gases and energy anxiety. In this work, oleylamine-functionalized graphene oxide/Cu2ZnSnS4 composite (OAm-GO/CZTS) is prepared and investigated as a higher bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The OAm-GO/CZTS shows brilliant electrocatalytic performance and durability toward H2 and O2 in both acidic and basic media, with overpotentials of 47 mV for HER and 1.36 V for OER at a current density of 10 mA cm-2 and Tafel slopes of 64 and 91 mV dec-1, respectively, which are extremely higher to those of transition metal chalcogenide and as good as of commercial precious-metal catalysts.

Graphene Nanoarchitectonics: Recent Advances in Graphene-Based Electrocatalysts for Hydrogen Evolution Reaction

Under the double pressures of both the energy crisis and environmental pollution, the exploitation and utilization of hydrogen, a clean and renewable power resource, has become an important trend in the development of sustainable energy-production and energy-consumption systems. In this regard, the electrocatalytic hydrogen evolution reaction (HER) provides an efficient and clean pathway for the mass production of hydrogen fuel and has motivated the design and construction of highly active HER electrocatalysts of an acceptable cost. In particular, graphene-based electrocatalysts commonly exhibit an enhanced HER performance owing to their distinctive structural merits, including a large surface area, high electrical conductivity, and good chemical stability. Considering the rapidly growing research enthusiasm for this topic over the last several years, herein, a panoramic review of recent advances in graphene-based electrocatalysts is presented, covering various advanced synthetic strategies, microstructural characterizations, and the applications of such materials in HER electrocatalysis. Lastly, future perspectives on the challenges and opportunities awaiting this emerging field are proposed and discussed.

Synthesis of Mo2C and W2C Nanoparticle Electrocatalysts for the Efficient Hydrogen Evolution Reaction in Alkali and Acid Electrolytes

The synthesis of low cost, high efficacy, and durable hydrogen evolution electrocatalysts from the non-noble metal group is a major challenge. Herein, we establish a simple and inexpensive chemical reduction method for producing molybdenum carbide (Mo2C) and tungsten carbide (W2C) nanoparticles that are efficient electrocatalysts in alkali and acid electrolytes for hydrogen evolution reactions (HER). Mo2C exhibits outstanding electrocatalytic behavior with an overpotential of -134 mV in acid medium and of -116 mV in alkaline medium, while W2C nanoparticles require an overpotential of -173 mV in acidic medium and -130 mV in alkaline medium to attain a current density of 10 mA cm-2. The observed results prove the capability of high- and low-pH active electrocatalysts of Mo2C and W2C nanoparticles to be efficient systems for hydrogen production through HER water electrolysis.

Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C/MoS2 Hybrid Structure via Chemical Conversion of Molybdenum Disulfide

The epitaxial synthesis of molybdenum carbide (Mo₂C, a 2D MXene material) via chemical conversion of molybdenum disulfide (MoS₂) with thermal annealing under CH₄ and H₂ is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo₂C from MoS₂ and an atomically sharp metallic/semiconducting hybrid structure via partial conversion of the semiconducting 2D material. Mo₂C/MoS₂ hybrid junctions display a low contact resistance (1.2 kΩ·μm) and low Schottky barrier height (26 meV), indicating the material's potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo₂C grows and forms a Mo₂C/MoS₂ hybrid structure. The results show that Mo₂C conversion is initiated at the MoS₂ edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS₂ forms through epitaxial growth of Mo₂C. This work provides the area-controllable synthesis of a manufacturable MXene from a transition metal dichalcogenide material and the formation of a metal/semiconductor junction structure. The present results will be of critical importance for future 2D heterojunction structures and functional device applications.

In situ synthesis of molybdenum carbide/N-doped carbon hybrids as an efficient hydrogen-evolution electrocatalyst

The development of non-precious metal based electrocatalysts for the hydrogen evolution reaction (HER) has received more and more attention over recent years owing to energy and environmental issues, and Mo based materials have been explored as a promising candidate. In this work, molybdenum carbide/N-doped carbon hybrids (Mo₂C@NC) were synthesized facilely via one-step high-temperature pyrolysis by adjusting the mass ratio of urea and ammonium molybdate. The Mo₂C@NC consisted of ultrasmall nanoparticles encapsulated by N-doped carbon, which had high specific surface area. They all exhibited efficient HER activity, and the Mo₂C@NC with a mass ratio of 160 (Mo₂C@NC-160) showed the best HER activity, with a low overpotential of 90 mV to reach 10 mA cm⁻² and a small Tafel slope of 50 mV dec⁻¹, which was one of the most active reported Mo₂C-based electrocatalysts. The excellent HER activity of Mo₂C@NC-160 was attributed to the following features: (1) the highly dispersed ultrasmall Mo₂C nanoparticles, which exhibited high electrochemically active surface areas; (2) the synergistic effect of the N-doped carbon shell/matrix, which facilitated the electron transport.

The molybdenum carbide/N-doped carbon hybrids (Mo₂C@NC) were synthesized facilely via one-step high-temperature pyrolysis, which exhibited efficient electrocatalytic hydrogen evolution performance.

Three-dimensional supramolecular phosphomolybdate architecture-derived Mo-based electrocatalytic system for overall water splitting

With phosphomolybdate supramolecular architecture-derived Mo₂C@NC and MoO₂@NC as cathode and anode, an efficient electrolyzer was constructed, which possessed excellent overall water splitting activity.

Nanocatalysts for hydrogen evolution reactions

Hydrogen fuel is among the cleanest renewable resources and is the best alternative to fossil fuels for the future.

Reduced graphene oxide and MoP composite as highly efficient and durable electrocatalyst for hydrogen evolution in both acidic and alkaline media

Materials based on earth-abundant elements can be developed for hydrogen evolution reactions to meet the future demand for eco-friendly and renewable energy sources based on hydrogen.