Theoretical study of the mechanism of the hydrogen evolution reaction on the V2C MXene: Thermodynamic and kinetic aspects

Hydrogen Binding Energy Is Insufficient for Describing Hydrogen Evolution on Single-Atom Catalysts

The design principles for metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) in the hydrogen evolution reaction (HER) have been extensively studied. Yet, consensus remains elusive, hindering advancements in hydrogen energy technologies. Although the hydrogen binding energy (ΔGH*) has long been used as a key HER descriptor during the past two decades, originating from the activity volcano of metallic surfaces, its applicability to HER SACs has been met with significant controversy. Herein, we investigate the effects of HO*/O* poisoning and H* coverage on SACs with varied metal centers and coordination environments using pH-dependent surface Pourbaix diagrams at the reversible hydrogen electrode (RHE) scale and microkinetic modeling. Our findings reveal that HO* poisoning, realistic H* adsorption strengths at active metal sites, and the potential HER activity at the coordinating N-sites are crucial factors that should be considered for accurate descriptor development. Experimental validation using a series of M-phthalocyanine/CNT catalysts (M = Co, Ni, Cu) confirms the theoretical predictions, with excellent agreement in exchange current densities and the role of N-sites in Ni/Cu-phthalocyanine/CNT catalysts. This work provides answers to a long-lasting debate on HER descriptors by establishing ΔGH* and ΔGHO* as a combined HER descriptor for SACs, offering new guidelines for catalyst design.

High-Yield Delamination of Hydrothermally-Etched V2CTx

The delamination of hydrothermally etched V2CTx has presented challenges, with limited reports of an effective delamination process. X-ray diffraction data indicate that excess lithium and lithium salts in the reaction mixture interact with the multilayered MXene surfaces in the interlayer space, impeding intercalants that would separate the nanosheets. The removal of this salt with a dilute acid solution is the key step to enable the synthesis of a delaminated MXene with a markedly higher yield in comparison to that of traditional HF-etched (and delaminated) V2CTx. Because this yield is substantial, the sample can be centrifuged to produce 20 mL of a concentrated (25 mg mL-1) sample. Due to the removal of excess water and dissolved O2, this concentrated sample shows improved stability toward oxidation and can withstand ambient conditions over the course of a year.

Corrosion-resistant titanium-based electrodes synergistically stabilized with polymer for hydrogen evolution reaction

The economic and reasonable design of highly stable and corrosion-resistant electrodes is fundamental to achieving the industrial-scale hydrogen productions via water electrolysis, but electrodes' premature failures are often caused by corrosion and stress damage. Therefore, these challenges are successfully solved by utilizing conductive and crack-resistant polyaniline "stabilizer" with a mild chemical plating process to construct the catalytic electrode on a titanium substrate (15 %PANI-NiB@Ti) in the present work. The 15 %PANI-NiB@Ti catalytic electrodes have been in continuous operation for 350 h at the current density of 200 mA cm-2 with the high efficiency of 98.4 % in a 323.15 K environment. With the high economy and universality, the catalytic electrodes have good catalytic performance and reliability in the extreme industrial environments, such as high temperature, air, and high current density. Except for the above advantages, the 15 %PANI-NiB@Ti catalytic electrodes also have good cracking resistance, which provides a novel and feasible approach to the industrial application of transition metal catalytic electrodes.

Untangling the role of single-atom substitution on the improvement of the hydrogen evolution reaction of Y2NS2 MXene in acidic media

The production of hydrogen (H2) fuel through electrocatalysis is emerging as a sustainable alternative to conventional and environmentally harmful energy sources. However, the discovery of cost-effective and efficient materials for this purpose remains a significant challenge. In this study, we explore the potential of the transition-metal-substituted Y2NS2 MXene as a promising candidate for hydrogen production through the hydrogen evolution reaction (HER). Using density functional theory (DFT) calculations, we first analyzed the Pourbaix diagram, and dissolution potential which showed the stability and resistance to corrosion of the sulfur termination. Later, we address the kinetic limitations of HER on bare Y2NS2 by introducing single-atom substitutions of Y atoms with 3d transition metals. Nine distinct structures were evaluated, revealing that Fe-substituted Y2NS2 exhibits the highest HER activity under acidic conditions, as indicated by volcano plot analyses. Further investigation of the bonding characteristics and electronic density of states highlights the crucial role of Fe d-orbitals and the weak interactions at the sulfur-terminated surface in enhancing the HER efficiency. These findings provide insights into the design of advanced, cost-effective materials for HER catalysis, paving the way for their application as efficient electrochemical catalysts across a wide pH range.

Molten Salt-Assisted Synthesis of Catalysts for Energy Conversion

A breakthrough in manufacturing procedures often enables people to obtain the desired functional materials. For the field of energy conversion, designing and constructing catalysts with high cost-effectiveness is urgently needed for commercial requirements. Herein, the molten salt-assisted synthesis (MSAS) strategy is emphasized, which combines the advantages of traditional solid and liquid phase synthesis of catalysts. It not only provides sufficient kinetic accessibility, but effectively controls the size, morphology, and crystal plane features of the product, thus possessing promising application prospects. Specifically, the selection and role of the molten salt system, as well as the mechanism of molten salt assistance are analyzed in depth. Then, the creation of the catalyst by the MSAS and the electrochemical energy conversion related application are introduced in detail. Finally, the key problems and countermeasures faced in breakthroughs are discussed and look forward to the future. Undoubtedly, this systematical review and insights here will promote the comprehensive understanding of the MSAS and further stimulate the generation of new and high efficiency catalysts.

Unveiling the Synergy between Surface Terminations and Boron Configuration in Boron-Based Ti3C2 MXenes Electrocatalysts for Nitrogen Reduction Reaction

The performance of B-containing Ti3C2 MXenes as catalysts for the nitrogen reduction reaction (NRR) is scrutinized using density functional theory methods on realistic models and accounting for working conditions. The present models include substituted and adsorbed boron along with various mixed surface terminations, primarily comprising -O and -OH groups, while considering the competitive hydrogen evolution reaction (HER) as well. The results highlight that substituted and low-coordinate adsorbed boron atoms exhibit a very high N2 adsorption capability. For NRR, adsorbed B atoms yield lower limiting potentials, especially for surfaces with mixed -O/-OH surface groups, where the latter participate in the reaction lowering the hydrogenation reaction energy costs. The NRR does also benefit of having B adsorbed on the surface which on moderate -OH terminated model displays the lowest limiting potential of -0.83 V, competitive to reference Ru and to HER. The insights derived from this comprehensive study provide guidance in formulating effective MXene-based electrocatalysts for NRR.

Recent progression in MXene-based catalysts for emerging photocatalytic applications of CO2 reduction and H2 production: A review

The development of advanced materials for efficient photocatalytic H2 production and CO2 reduction is highly recommended for addressing environmental issues and producing clean energy sources. Specifically, MXenes have emerged as two-dimensional (2D) materials extensively used as high-performance cocatalysts in photocatalyst systems owing to their outstanding features of structure and properties such as high conductivity, large specific surface area, and abundant active sites. Nevertheless, there is a lack of deep and systematic studies concerning the application of these emerging materials for CO2 reduction reaction (CRR) and H2 production (HER). This review first outlines the essential features of MXenes, encompassing the synthesis methods, composition, surface terminations, and electronic properties, which make them highly active as cocatalysts. It then examines the recent progress in MXene-based photocatalysts, emphasizing the synergy achieved by coupling MXenes as co-catalysts with semiconductors, utilizing MXenes as a support for the consistent growth of photocatalysts, leading to finely dispersed nanoparticles, and exploiting MXene as exceptional precursors for creating MXene/metal oxide photocomposite. The roles of engineering surface terminations of MXene cocatalysts, MXene quantum dots (QDs), and distinctive morphologies in MXenes-based photocatalyst systems to enhance photocatalytic activity for both HER and CRR have been explored both experimentally and theoretically using DFT calculations. Challenges and prospects for MXene-based photocatalysts are also addressed. Finally, suggestions for further research and development of effective and economical MXenes/semiconductors strategies are proposed. This comprehensive review article serves as a valuable reference for researchers for applying MXenes in photocatalysis.

Pd:In-Doped TiO2 as a Bifunctional Catalyst for the Photoelectrochemical Oxidation of Paracetamol and Simultaneous Green Hydrogen Production

The integration of clean energy generation with wastewater treatment holds promise for addressing both environmental and energy concerns. Focusing on photocatalytic hydrogen production and wastewater treatment, this study introduces PdIn/TiO2 catalysts for the simultaneous removal of the pharmaceutical contaminant paracetamol (PTM) and hydrogen production. Physicochemical characterization showed a high distribution of Pd and In on the support as well as a high interaction with it. The Pd and In deposition enhance the light absorption capability and significantly improve the hydrogen evolution reaction (HER) in the absence and presence of paracetamol compared to TiO2. On the other hand, the photoelectroxidation of PTM at TiO2 and PdIn/TiO2 follows the full mineralization path and, accordingly, is limited by the adsorption of intermediate species on the electrode surface. Thus, PdIn-doped TiO2 stands out as a promising photoelectrocatalyst, showcasing enhanced physicochemical properties and superior photoelectrocatalytic performance. This underscores its potential for both environmental remediation and sustainable hydrogen production.

Why efficient bifunctional hydrogen electrocatalysis requires a change in the reaction mechanism

Hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) are both two-electron processes that culminate in the formation or consumption of gaseous hydrogen in an electrolyzer or a fuel cell, respectively. Unitized regenerative proton exchange membrane fuel cells merge these two functionalities into one device, allowing to switch between the two modes of operation. This prompts the quest for efficient bifunctional electrode materials catalyzing the HER and HOR with reasonable reaction rates at low overpotentials. In the present study using a data-driven framework, we identify a general criterion for efficient bifunctional performance in the hydrogen electrocatalysis, which refers to a change in the reaction mechanism when switching from cathodic to anodic working conditions. The obtained insight can be used in future studies based on density functional theory to pave the design of efficient HER and HOR catalysts by a dedicated consideration of the kinetics in the analysis of reaction mechanisms.