Hydrogen Embrittlement as a Conspicuous Material Challenge─Comprehensive Review and Future Directions

Hydrogen is considered a clean and efficient energy carrier crucial for shaping the net-zero future. Large-scale production, transportation, storage, and use of green hydrogen are expected to be undertaken in the coming decades. As the smallest element in the universe, however, hydrogen can adsorb on, diffuse into, and interact with many metallic materials, degrading their mechanical properties. This multifaceted phenomenon is generically categorized as hydrogen embrittlement (HE). HE is one of the most complex material problems that arises as an outcome of the intricate interplay across specific spatial and temporal scales between the mechanical driving force and the material resistance fingerprinted by the microstructures and subsequently weakened by the presence of hydrogen. Based on recent developments in the field as well as our collective understanding, this Review is devoted to treating HE as a whole and providing a constructive and systematic discussion on hydrogen entry, diffusion, trapping, hydrogen-microstructure interaction mechanisms, and consequences of HE in steels, nickel alloys, and aluminum alloys used for energy transport and storage. HE in emerging material systems, such as high entropy alloys and additively manufactured materials, is also discussed. Priority has been particularly given to these less understood aspects. Combining perspectives of materials chemistry, materials science, mechanics, and artificial intelligence, this Review aspires to present a comprehensive and impartial viewpoint on the existing knowledge and conclude with our forecasts of various paths forward meant to fuel the exploration of future research regarding hydrogen-induced material challenges.

The Isoelectronic Dopant in the Z-Scheme SnS2/β-As Heterostructure Enhancing Photocatalytic Overall Water Splitting

The catalytic field aims to decrease reaction barriers, accelerate reaction processes, and enhance the selectivity toward a target product. This study uses first-principles calculations to design a modified direct Z-scheme SnS2/β-As heterostructure as a potential photocatalyst for overall water splitting. Our previous investigations have demonstrated that the SnS2/β-As heterostructure can realize a hydrogen evolution reaction (HER) under light, while the oxygen evolution reaction (OER) follows a pathway involving the intermediate HOOH*. Interestingly, by substituting an S atom of SnS2 with a Se or Te atom, the rate-determining step of the OER is significantly reduced from 3.76 eV to 2.56 or 2.22 eV. Moreover, the OER can occur directly without the transition via HOOH*. Isoelectronic doping effectively trades off the adsorption strength of OER intermediates and promotes the OER process. This work highlights the dual benefits of isoelectronic doping, namely lowering the reaction barrier of the rate-determining step and promoting the selectivity of end products. These findings provide insights into the rational design of high-efficiency photocatalysts for water splitting.

Low-spin state of Fe in Fe-doped NiOOH electrocatalysts

Doping with Fe boosts the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). To understand this effect, we have employed state-of-the-art electronic structure calculations and thermodynamic modeling. Our study reveals that at low concentrations Fe exists in a low-spin state. Only this spin state explains the large solubility limit of Fe and similarity of Fe-O and Ni-O bond lengths measured in the Fe-doped NiOOH phase. The low-spin state renders the surface Fe sites highly active for the OER. The low-to-high spin transition at the Fe concentration of ~ 25% is consistent with the experimentally determined solubility limit of Fe in NiOOH. The thermodynamic overpotentials computed for doped and pure materials, η = 0.42 V and 0.77 V, agree well with the measured values. Our results indicate a key role of the low-spin state of Fe for the OER activity of Fe-doped NiOOH electrocatalysts.

Computational screening toward transition metal doped vanadium carbides in different crystal planes for efficient hydrogen evolution: a first-principles study

In the present work, we evaluated the hydrogen evolution reaction (HER) performance of transition metal (Co, Fe, Ni, Mn, and Mo) doped vanadium carbides (VC). In addition, the doping atoms were screened separately on the (100), (110) and (111) crystal planes to analyze the differences in HER activities. Among all the calculated models, Mn-VC(100) exhibited the best catalytic hydrogen evolution performance with a Gibbs free energy for hydrogen adsorption (ΔG_H*) of 0.0012 eV. Doping Mn greatly improved the HER performance of VC(100) by enhancing the adsorption of hydrogen on the catalyst surface. The analysis of the electronic density of states and charge transfer confirmed that doping transition metal atoms into the surfaces of the VC model successfully optimized the electronic structure and promoted catalytic reaction kinetics. Besides, the relationship between the catalytic activity and pH value of different models was considered, and doping Co atoms on the (100) crystal plane could effectively modify the pH value range applicable for the efficient HER. Interestingly, even if the same metal atoms were doped, various active sites of VC models exhibited different catalytic performances due to disparate exposed crystal planes and pH values. This indicates that the main exposed crystal surfaces and the pH range of application need to be considered when selecting the appropriate doping element for the catalyst.

Synergistic engineering of shell thickness and core ordering to boost the oxygen reduction performance

When benchmarked against the extended Pt(111), slightly weaker adsorption and stronger cohesion properties of surface Pt are required to improve activity and durability for the oxygen reduction reaction, respectively, making it challenging to meet both requirements on one surface. Here, using Pt(111) over-layers stressed and modified by Pt-TM (TM = Fe, Co, Ni, V, Cu, Ag, and Pd) intermetallics as examples, we theoretically identified ten promising catalysts by synergistically tailoring the skin thickness and substrate chemical ordering to simultaneously achieve weak adsorption and strong cohesion. More specifically, compared with Pt(111), all candidates exhibit 10-fold enhanced activity, half of which show improved durability, such as mono-layer skin on L1₂-Pt₃Co or Pt₃Fe, double-layer Pt on L1₃-Pt₃Ni or Pt₃Cu, and triple-layer skin on L1₁-PtCu, while double- or triple-layer skin on L1₀-PtCo or PtNi and double-layer skin on L1₂-PtFe₃ show slightly poor durability. Although L1₀ and L1₂ based nanocrystals have been demonstrated extensively as outstanding catalysts, L1₁ and L1₃ ones hold great application potential. The coexistence of high activity and durability on the same surface is because of the different responses of surface adsorption and cohesion properties to the strain effects and ligand effects. When intermetallic-core@Pt-shell nanocrystals are constructed using this slab model, the necessity of protecting or eliminating low-coordinated Pt and the possibility of maximizing Pt(111) facets and core ordering by morphology engineering were highlighted. The current discovery provides a new paradigm toward the rational design of promising cathodic catalysts.

Significance of density functional theory (DFT) calculations for electrocatalysis of N2 and CO2 reduction reactions

Density functional theory (DFT) based computational methods have shown great significance in developing high-performance electrocatalysts. In this perspective, we briefly summarized the state-of-the-art research progress of electrocatalysts for the nitrogen reduction reaction (NRR) and CO₂ reduction reaction (CO₂RR), which are important processes for the conversion of common molecules into value-added products. With the help of DFT calculations, various modulation strategies are employed to improve the catalytic activity and performance of NRR and CO₂RR electrocatalysts. DFT calculations are performed to confirm the surface catalytic sites, evaluate the catalytic activity, reveal the possible reaction mechanisms, and design novel structures with high catalytic performance. By discussing the currently applied computational methods and conditions during the calculations, we outlined our concerns on the prospects and future challenges of DFT calculations in electrocatalysis studies.

Micro-kinetic Description of Electrocatalytic Reactions: The Role of Self-organized Phenomena

In this perspective we proposed a workflow for the construction of micro-kinetic models that consists of at least four stages, starting with information gathering that allows proposing possible reaction mechanisms....

Using the Isalos platform to develop a (Q)SAR model that predicts metal oxide toxicity utilizing facet-based electronic, image analysis-based, and periodic table derived properties as descriptors

Engineered nanoparticles (NPs) are being studied for their potential to harm humans and the environment. Biological activity, toxicity, physicochemical properties, fate, and transport of NPs must all be evaluated and/or predicted. In this work, we explored the influence of metal oxide nanoparticle facets on their toxicity towards bronchial epithelial (BEAS-2B), Murine myeloid (RAW 264.7), and E. coli cell lines. To estimate the toxicity of metal oxide nanoparticles grown to a low facet index, a quantitative structure–activity relationship ((Q)SAR) approach was used. The novel model employs theoretical (density functional theory calculations) and experimental studies (transmission electron microscopy images from which several particle descriptors are extracted and toxicity data extracted from the literature) to investigate the properties of faceted metal oxides, which are then utilized to construct a toxicity model. The classification mode of the k-nearest neighbour algorithm (EnaloskNN, Enalos Chem/Nanoinformatics) was used to create the presented model for metal oxide cytotoxicity. Four descriptors were identified as significant: core size, chemical potential, enthalpy of formation, and electronegativity count of metal oxides. The relationship between these descriptors and metal oxide facets is discussed to provide insights into the relative toxicities of the nanoparticle. The model and the underpinning dataset are freely available on the NanoSolveIT project cloud platform and the NanoPharos database, respectively.

Implicit Solvation Methods for Catalysis at Electrified Interfaces

Implicit solvation is an effective, highly coarse-grained approach in atomic-scale simulations to account for a surrounding liquid electrolyte on the level of a continuous polarizable medium. Originating in molecular chemistry with finite solutes, implicit solvation techniques are now increasingly used in the context of first-principles modeling of electrochemistry and electrocatalysis at extended (often metallic) electrodes. The prevalent ansatz to model the latter electrodes and the reactive surface chemistry at them through slabs in periodic boundary condition supercells brings its specific challenges. Foremost this concerns the difficulty of describing the entire double layer forming at the electrified solid–liquid interface (SLI) within supercell sizes tractable by commonly employed density functional theory (DFT). We review liquid solvation methodology from this specific application angle, highlighting in particular its use in the widespread ab initio thermodynamics approach to surface catalysis. Notably, implicit solvation can be employed to mimic a polarization of the electrode’s electronic density under the applied potential and the concomitant capacitive charging of the entire double layer beyond the limitations of the employed DFT supercell. Most critical for continuing advances of this effective methodology for the SLI context is the lack of pertinent (experimental or high-level theoretical) reference data needed for parametrization.

First-Principles Study of Pt-Based Bifunctional Oxygen Evolution & Reduction Electrocatalyst: Interplay of Strain and Ligand Effects

We examined the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) of Pt-based Pt3M/Pt nanoalloy catalysts (where M represents a 3d transition metal) for bifunctional electrocatalysts using spin-polarized density functional theory calculations. First, the stability of the Pt3M/Pt catalyst was investigated by calculating the bulk formation energy and surface separation energy. Using the calculated adsorption energies for the OER/ORR intermediates in the modeled catalysts, we predicted the OER/ORR overpotentials and potential limiting steps for each catalyst. The origins of the enhanced catalytic reactivity in Pt3M/Pt catalysts caused by strain and ligand effects are explained separately. In addition, compared to Pt(111), the OER and ORR activities in a Pt3Ni/Ptskin catalyst with a Pt skin layer were increased by 13.7% and 18.4%, respectively, due to the strain and ligand effects. It was confirmed that compressive strain and ligand effects are key factors in improving the catalytic performance of OER/ORR bifunctional catalysts.

Machine Learning in Screening High Performance Electrocatalysts for CO2 Reduction

Converting CO₂ into carbon-based fuels is promising for relieving the greenhouse gas effect and the energy crisis. However, the selectivity and efficiency of current electrocatalysts for CO₂ reductions are still not satisfactory. In this paper, the development of machine learning methods in screening CO₂ reduction electrocatalysts over the recent years is reviewed. Through high-throughput calculation of some key descriptors such as adsorption energies, d-band center, and coordination number by well-constructed machine learning models, the catalytic activity, optimal composition, active sites, and CO₂ reduction reaction pathway over various possible materials can be predicted and understood. Machine learning is now realized as a fast and low-cost method to effectively explore high performance electrocatalysts for CO₂ reduction.

Two-Dimensional Graphdiyne-Confined Platinum Catalyst for Hydrogen Evolution and Oxygen Reduction Reactions

Pt-based materials are the state-of-the-art catalysts for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR); however, there is still much room to improve the catalytic activity and enhance the stability of Pt-based catalysts. In this work, two-dimensional (2D) graphdiyne (GDY) with uniform distributed pores was applied to cover the Pt surface for catalyzing HER and ORR through density functional theory (DFT) calculations. The 2D confinement induced by GDY was found to improve the catalytic performance of the Pt catalyst from three aspects: (1) the 2D covering layer increases the stability of the Pt catalyst through forming the heterogeneous interface of GDY/Pt(111); (2) GDY/Pt(111) shows better catalytic activities of HER and ORR, with the smaller average overpotential values of 0.26 and 0.51 V, respectively, compared with those (0.29 V for HER, 0.62 V for ORR) on the Pt catalyst; (3) the confinement effect of GDY weakens the adsorption energy of CO to -1.81 eV (average value) from -2.14 eV on Pt(111), inhibiting CO poisoning. This work sheds new light on 2D confinement effects for HER and ORR, which opens up a new strategy for improving the catalytic performance of Pt-based catalysts.

Intermetallic FePt@PtBi Core-Shell Nanoparticles for Oxygen Reduction Electrocatalysis

The development of active and stable platinum (Pt)-based oxygen reduction reaction (ORR) electrocatalysts with good resistance to poisoning is a prerequisite for widespread practical application of fuel cells. An effective strategy for enhancing the electrocatalytic performance is to tune or control the physicochemical state of the Pt surface. Herein, we show a general surface-engineering approach to prepare a range of nanostructured Pt alloys by coating with alloy PtBi shells. FePt@PtBi core-shell nanoparticles showed the best ORR performance with a mass activity of 0.96 A mg_Pt ^-1 and a specific activity of 2.06 mA cm^-2 , respectively 7 times and 11 times those of the corresponding values for benchmark Pt/C. Moreover, FePt@PtBi shows much better tolerance to methanol and carbon monoxide than conventional Pt-based electrocatalysts. The observed comprehensive enhancement in ORR performance of FePt@PtBi can be attributed to the increased compressive strain of the Pt surface due to in-plane shearing resulting from the presence of the large Bi atoms in the surface-structured PtBi overlayers, as well as charge displacement via Pt-Bi bonding which mitigates crossover issues.

Design Engineering, Synthesis Protocols, and Energy Applications of MOF-Derived Electrocatalysts

The core reactions for fuel cells, rechargeable metal-air batteries, and hydrogen fuel production are the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), which are heavily dependent on the efficiency of electrocatalysts. Enormous attempts have previously been devoted in non-noble electrocatalysts born out of metal-organic frameworks (MOFs) for ORR, OER, and HER applications, due to the following advantageous reasons: (i) The significant porosity eases the electrolyte diffusion; (ii) the supreme catalyst-electrolyte contact area enhances the diffusion efficiency; and (iii) the electronic conductivity can be extensively increased owing to the unique construction block subunits for MOFs-derived electrocatalysis. Herein, the recent progress of MOFs-derived electrocatalysts including synthesis protocols, design engineering, DFT calculations roles, and energy applications is discussed and reviewed. It can be concluded that the elevated ORR, OER, and HER performances are attributed to an advantageously well-designed high-porosity structure, significant surface area, and plentiful active centers. Furthermore, the perspectives of MOF-derived electrocatalysts for the ORR, OER, and HER are presented.

High-entropy materials for catalysis: A new frontier

Entropy plays a pivotal role in catalysis, and extensive research efforts have been directed to understanding the enthalpy-entropy relationship that defines the reaction pathways of molecular species. On the other side, surface of the catalysts, entropic effects have been rarely investigated because of the difficulty in deciphering the increased complexities in multicomponent systems. Recent advances in high-entropy materials (HEMs) have triggered broad interests in exploring entropy-stabilized systems for catalysis, where the enhanced configurational entropy affords a virtually unlimited scope for tailoring the structures and properties of HEMs. In this review, we summarize recent progress in the discovery and design of HEMs for catalysis. The correlation between compositional and structural engineering and optimization of the catalytic behaviors is highlighted for high-entropy alloys, oxides, and beyond. Tuning composition and configuration of HEMs introduces untapped opportunities for accessing better catalysts and resolving issues that are considered challenging in conventional, simple systems.

Selective Halogenation of Pyridines Using Designed Phosphine Reagents

Halopyridines are key building blocks for synthesizing pharmaceuticals, agrochemicals, and ligands for metal complexes, but strategies to selectively halogenate pyridine C-H precursors are lacking. We designed a set of heterocyclic phosphines that are installed at the 4-position of pyridines as phosphonium salts and then displaced with halide nucleophiles. A broad range of unactivated pyridines can be halogenated, and the method is viable for late-stage halogenation of complex pharmaceuticals. Computational studies indicate that C-halogen bond formation occurs via an S_NAr pathway, and phosphine elimination is the rate-determining step. Steric interactions during C-P bond cleavage account for differences in reactivity between 2- and 3-substituted pyridines.

Perspective on theoretical methods and modeling relating to electro-catalysis processes

Theoretical methods and models for the description of thermodynamics and kinetics in electro-catalysis, including solvent effects, externally applied potentials, and many-body interactions, are discussed.

Metallic nanostructures with low dimensionality for electrochemical water splitting

Metallic nanostructures with low dimensionality (one-dimension and two-dimension) possess unique structural characteristics and distinctive electronic and physicochemical properties including high aspect ratio, high specific surface area, high density of surface unsaturated atoms and high electron mobility. These distinctive features have rendered them remarkable advantages over their bulk counterparts for surface-related applications, for example, electrochemical water splitting. In this review article, we highlight the recent research progress in low-dimensional metallic nanostructures for electrochemical water splitting including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fundamental understanding of the electrochemistry of water splitting including HER and OER is firstly provided from the aspects of catalytic mechanisms, activity descriptors and property evaluation metrics. Generally, it is challenging to obtain low-dimensional metallic nanostructures with desirable characteristics for HER and OER. We hereby introduce several typical methods for synthesizing one-dimensional and two-dimensional metallic nanostructures including organic ligand-assisted synthesis, hydrothermal/solvothermal synthesis, carbon monoxide confined growth, topotactic reduction, and templated growth. We then put emphasis on the strategies adopted for the design and fabrication of high-performance low-dimensional metallic nanostructures for electrochemical water splitting such as alloying, structure design, surface engineering, interface engineering and strain engineering. The underlying structure-property correlation for each strategy is elucidated aiming to facilitate the design of more advanced electrocatalysts for water splitting. The challenges and perspectives for the development of electrochemical water splitting and low-dimensional metallic nanostructures are also proposed.