Progress in electrocatalytic hydrogen evolution of transition metal alloys: synthesis, structure, and mechanism analysis

Hierarchically Structured Catalysts Toward Sustainable Hydrogen Economy: Electro- and Thermo-Chemical Pathways

Hydrogen, as an important clean energy source, plays a more and more crucial role in decarbonizing the planet and meeting the global climate challenge due to its high energy density and zero-emission. The demand for sustainable hydrogen is increasing drastically worldwide as driven by the global shift towards low-carbon energy solutions. Thermochemical catalysis process dominates hydrogen production at scale given its relatively mature technology and commercialization status, as well as the established manufacturing infrastructure. While due to its environmentally friendly nature and growing abundant sources of renewable electricity, the electrochemical path for hydrogen production is rising as a major alternative to the thermochemical means. Nevertheless, hierarchically structured catalysts and devices have gradually taken the center stage toward replacing the traditional counterparts, especially with the rapid advancement of the design and manufacture of such ordered nanostructure assemblies toward high activity, efficient mass transport, and superb stability. In this review, the latest progress of the hierarchically structured catalysts for hydrogen production have been surveyed on electro- and thermo- chemical pathways comparatively. It covers the structure designs of atomic dispersion, nanoscale surfaces and interfaces for achieving highly active and durable catalysts, components, and devices. Both electrochemical and thermochemical approaches are reviewed in terms of the vast design details, engineered benefits, and understandings of various Pt-group metal (PGM) and non-PGM based transition metal catalysts for hydrogen production. As the growing trend, brief discussions are also presented toward the high-level assembly and manufacture of complexly structured components and devices at scale in the electrochemical and thermochemical energy systems.

Alloy Reconstruction in Pyrolytic Bowknot-like Heteronuclear CoFe Clusters for Electrocatalytic Application

The construction of doped molecular clusters is an intriguing way to perform bimetallic doping for electrocatalysts. However, efficiently harnessing the benefits of a doping strategy and alloy engineering to create a nanostructure for electrocatalytic application at the molecular level has consistently posed a challenge. Here we propose an in situ reconstruction strategy aimed at producing an alloy nanostructure through a pyrolysis process, originating from bowknot-like heterometallic clusters. The Schiff base, denoted as ligand L1 (o-vanillin ethylenediamine), was introduced as a precursor to coordinate Fe and Co metals, thereby yielding a heteronuclear metal cluster [(FeCo)(L1)2O]CH3CN. Subsequently, a comprehensive investigation of the in situ reconstruction process [(FeCo)(L1)2O](CH3CN) → [(FeCo)(L1)2O] → [M-O-M/M-O] [CH3+/CH3O+/H2C═N/C2H5+/C4H4+] → [FeCo/Fe3O4/Fe2O3/Co3O4][carbon layer] led to the formation of MOx/CoFe@NC-700 during the pyrolysis. This process reveals that the metals Fe and Co in the clusters undergo partly in situ evolution into FeCo alloys, resulting in the successful preparation of MOx/CoFe@NC (M = Fe, Co) nanomaterials that leverage the advantages of both doping strategies and alloy engineering. The synergistic interaction between alloy particles and metal oxides establishes active sites that contribute to the excellent oxygen evolution (OER) and hydrogen evolution (HER) catalytic behaviors. Notably, these materials exhibit outstanding OER and HER properties under alkaline conditions, with overpotentials of 191 and 88 mV for OER and HER, respectively, at 10 mA cm-2. Investigation of the in situ conversion of Schiff base bimetal clusters into alloy materials through pyrolysis offers a novel strategy for advancing electrocatalytic applications.

Architectural Decoration of Bioleached NiFeP Surfaces by Co3O4 Flowers for Efficient Electrocatalytic Hydrogen Generation

In the quest for sustainable hydrogen production via water electrolysis, the development of high-performance, noble-metal-free catalytic systems is highly desired. Herein, we proposed an innovative strategy for the development of an electrocatalyst by refining the surface characteristics of a NiFeP alloy through microbiological techniques and subsequent enrichment of active sites by tailoring 3D hierarchical flower-like structures with intact and interconnected two-dimensional (2D) Co3O4. The resultant 3D Co3O4@NiFeP-5/24h has a porous structure comprised of intercrossed nanoparticles covering the entirety of the catalytic surface. This design ensures comprehensive electrolyte ion penetration and facilitates the release of gas bubbles while reducing bubble adhesion rates. Remarkably, the Co3O4@NiFeP-5/24h electrode demonstrates superior hydrogen evolution (HER) performance in an alkaline medium, characterized by its high stability, low overpotential (106 mV at a current density of 10 mA cm-2), and reduced Tafel slope (98 mV dec-1). Besides, the minimized interfacial contact resistance among the phases of electrode and electrolyte emphasizes the high HER performance of the 3D Co3O4@NiFeP-5/24h electrode. The innovative design and fabrication strategy employed herein holds significant potential for advancing the field of water-splitting electrocatalysis, offering a promising path toward the rational design and development of noble-metal-free electrocatalysts.

Ultralow-content Pt nanodots/Ni3Fe nanoparticles: interlayer nanoconfinement synthesis and overall water splitting

Minimizing precious metal loading into electrocatalysts for water splitting is vital to promoting hydrogen energy technology toward practical applications. Low-content loading of precious-metal electrocatalysts is achieved by decorating precious metal nanostructures on co-electrocatalysts typically via surface confinement. Here, an electrocatalyst of ultralow-content Pt nanodots (0.71 wt%)/Ni3Fe nanoparticles on reduced oxidation graphene (Pt/Ni3Fe/rGO) is constructed for overall water splitting by pyrolyzing a single-source precursor PtCl63- guest-intercalated MgNiFe-layered double hydroxide (MgNiFe-LDH) host via a distinctive interlayer confinement. Consequently, Pt/Ni3Fe/rGO demonstrates attractive overpotentials of 240 and 76 mV at 10 mA cm-2 for the oxygen and hydrogen evolution reactions (OER and HER), respectively, outperforming those of its /Ni3Fe/rGO counterpart. Moreover, the Pt/Ni3Fe/rGO∥Pt/Ni3Fe/rGO electrolyzer generates a current density of 10 mA cm-2 at 1.55 V, with a retention of 92.4% after 50 h. Furthermore, the measured specific activity and low transfer resistance, as well as the density functional theory (DFT) calculations, indicate that the active Pt/Ni3Fe in Pt/Ni3Fe/rGO can optimize the adsorption/desorption of reaction intermediates and thus boost OER/HER kinetics, all of which lead to enhanced performance. The results demonstrate that such an interlayer confinement-based synthesis strategy can allow for the design of cost-effective precious nanodots as potential electrocatalysts.

Design and multilevel regulation of transition metal phosphides for efficient and industrial water electrolysis

Renewable energy electrolysis of water to produce hydrogen is an effective measure to break the energy dilemma. However, achieving activity and stability at a high current density is still a key problem in water electrolyzers. Transition metal phosphides (TMPs), with high activity and relative inexpensiveness, have become excellent candidates for the production of highly pure green hydrogen for industrial applications. In this mini-review, multilevel regulation strategies including nanoscale control, surface composition and interface structure design of high-performance TMPs for hydrogen evolution are systematically summarized. On this basis, in order to achieve large-scale hydrogen production in industry, the hydrogen evolution performance and stability of TMPs at a high current density are also discussed. Peculiarly, the practical application and requirements in proton exchange membrane (PEM) or anion exchange membrane (AEM) electrolyzers can guide the advanced design of regulatory strategies of TMPs for green hydrogen production from renewable energy. Finally, the challenges and prospects in the future development trend of TMPs for efficient and industrial water electrolysis are given.

Optimization strategies of high-entropy alloys for electrocatalytic applications

High-entropy alloys (HEAs) are expected to become one of the most promising functional materials in the field of electrocatalysis due to their site-occupancy disorder and lattice order. The chemical complexity and component tunability make it possible for them to obtain a nearly continuous distribution of adsorption energy curve, which means that the optimal adsorption strength and maximum activity can be obtained by a multi-alloying strategy. In the last decade, a great deal of research has been performed on the synthesis, element selection and catalytic applications of HEAs. In this review, we focus on the analysis and summary of the advantages, design ideas and optimization strategies of HEAs in electrocatalysis. Combined with experiments and theories, the advantages of high activity and high stability of HEAs are explored in depth. According to the classification of catalytic reactions, how to design high-performance HEA catalysts is proposed. More importantly, efficient strategies for optimizing HEA catalysts are provided, including element regulation, defect regulation and strain engineering. Finally, we point out the challenges that HEAs will face in the future, and put forward some personal proposals. This work provides a deep understanding and important reference for electrocatalytic applications of HEAs.