Recent Progress of 2D Pt-Group Metallic Electrocatalysts for Energy-Conversion Applications

Abstract

With the rapid growth of energy demand, the development of efficient energy-conversion technologies (e.g., water splitting, fuel cells, metal-air batteries, etc.) becomes an important way to circumvent the problems of fossil fuel depletion and environmental pollution, which motivates the pursuit of high-performance electrocatalysts with controllable compositions and morphologies. Among them, two-dimensional (2D) Pt-group metallic electrocatalysts show a series of distinctive architectural merits, including a high surface-to-volume ratio, numerous unsaturated metal atoms, an ameliorative electronic structure, and abundant electron/ion transfers channels, thus holding great potential in realizing good selectivity, rapid kinetics, and high efficiency for various energy-conversion devices. Considering that great progress on this topic has been made in recent years, here we present a panoramic review of recent advancements in 2D Pt-group metallic nanocrystals, which covers diverse synthetic methods, structural analysis, and their applications as electrode catalysts for various energy-conversion technologies. At the end, the paper also outlines the research challenges and future opportunities in this emerging area.

1. Introduction

With the advent of the era of digital civilization, the global demand for energy has increased rapidly to meet the convenience of modern daily life [1,2]. In order to solve the problem of the depletion of fossil fuel resources and the ensuing environmental pollution, advanced energy-conversion technologies (e.g., water splitting, fuel cells, metal–air batteries, etc.) have attracted extensive attention from both academic and industrial fields [3,4]. Compared with the traditional internal combustion engine, the electrochemical systems based on the above new technologies are able to convert or generate energy with extremely high efficiency and low greenhouse gas emissions [5,6]. In particular, electrocatalysts play a key role as the core component of these energy-conversion devices, as they improve the selectivity, reaction rate, and overall efficiency [7,8,9]. However, the limited activity and poor stability of conventional electrocatalysts have severely hindered their large-scale applications [10,11,12]. Therefore, the major challenge is to develop advanced electrocatalysts and improve their performance to enable the widespread application of clean energy technologies [13,14,15].

Pt-group metals (e.g., Pt, Pd, Rh, Ru, Ir, etc.) have long been regarded as the mainstream electrocatalysts of current energy-conversion systems because of their superior catalytic capacity arising from the unique d-band structure [16,17,18,19]. Nevertheless, given that the high costs and scarcity of Pt-group metals constrain their large-scale commercialization, great emphasis has been placed on developing optimization strategies to enhance both their catalytic activity and atomic utilization efficiency [20,21,22]. Therefore, great efforts have been devoted to the development of nanostructured Pt-group metallic nanocrystals [23,24,25]. Typically, most commercial electrocatalysts are made from small Pt-group metallic nanoparticles supported by carbon black matrices, which can increase the metal dispersibility and electroactive surface areas [26,27]. However, zero-dimensional (0D) Pt-group nanoparticles with large surface free energy easily suffer from agglomeration, dissolution, and Ostwald ripening phenomena during their practical use, resulting in an obvious deterioration in the electrocatalytic properties [28,29,30]. In the meantime, the limited contact area between particle-shaped Pt-group metals and carbon substrate through the “particle-to-face” contact model also renders a relatively weak synergistic coupling effect, which largely restrains the performance improvements [31,32,33]. Within this context, the rational design and precise synthesis of multi-dimensional Pt-group metallic nanocrystals with various controlled morphologies have become a hot topic in the field of electrocatalysis [34,35,36].

Since the successful exfoliation of graphene in 2004, two-dimensional (2D) materials have become a hot research topic over the last couple of decades due to their excellent electrical, magnetic, optical, mechanical, and thermal properties [37,38]. Researchers’ exploration of novel 2D nanomaterials has significantly enriched the library of such materials, including transition metal disulfides (TMDs), MXenes, hexagonal boron nitride (h-BN), metal–organic frameworks (MOFs), covalent organic frameworks (COFs), metals, metal oxides, metal hydroxides, and so on [39,40]. These materials find applications across electronics, petroleum, chemical industries, aerospace, military, pharmaceuticals, medicine, and pharmaceutics [41,42,43]. As an important subset of 2D materials, 2D Pt-group metallic nanomaterials have garnered widespread attention in recent years [44,45]. Specially, the ultrathin 2D Pt-group metallic nanocrystals with a high surface-to-volume ratio commonly provide maximal exposure of active sites as well as minimize the charge transfer and mass diffusion pathways, thereby enhancing the atom utilization efficiency and electrocatalytic kinetics [46,47]. Moreover, the unique 2D morphology typically features undercoordinated metal atoms, which confer high chemical reactivity [48,49]. It is also noted that the adsorption strength of reaction intermediates on the catalyst surface should fall within the optimal “volcano” range based on Sabatier’s principle [50,51]. Both overly strong and overly weak adsorption can impair overall catalytic performance [52,53], which puts requirements on the rational architectural design of 2D Pt-group metallic catalysts. On the other hand, the combination of Pt-group metallic nanosheets with nanocarbon supporting materials can form a 2D/2D heterostructure through a “face-to-face” contact model [54,55]. Such a sophisticated configuration has been demonstrated to effectively stabilize the Pt-group metal/carbon integral structure and generate extra concerted effects, thus affording a prolonged durability and boosted catalytic reactivity [56,57].

In the past few years, significant advances have been achieved in the design and fabrication of 2D Pt-group metallic nanocrystals, which have been extensively employed as electrocatalysts for diverse energy-conversion technologies [58,59]. Considering the fast-growing attention on this aspect, here, we intend to systematically summarize the latest advancements in this emerging field. As presented in Figure 1, this review first outlines diverse synthetic strategies of 2D Pt-group metallic electrocatalysts and discusses their respective characteristics and applicability. Afterwards, we highlight the electrocatalytic performances of various 2D Pt-group metallic nanocrystals towards different energy-conversion applications, including hydrogen evolution reactions (HERs), oxygen evolution reactions (OERs), oxygen reduction reactions (ORRs), formic acid oxidation reactions (FAORs), and alcohol oxidation reactions (AORs). To finalize, this review also proposes the current challenges and opportunities associated with 2D Pt-group metallic electrocatalysts and provides a perspective for future research.

2. Synthesis of 2D Pt-Group Metallic Electrocatalysts

To date, great efforts have been made to explore efficient synthesis strategies for the construction of 2D Pt-group metallic electrocatalysts [60,61]. These methods can be broadly classified into five categories: carbon monoxide (CO)-confined growth methods, self-assembly methods, template methods, in situ growth methods, and other synthesis methods [62,63]. In this section, we focus on the synthetic processes and growth mechanisms of various 2D Pt-group metallic electrocatalysts based on the above five strategies.

2.1. CO-Confined Growth Methods

The use of CO as a surface ligand to achieve the confined growth of 2D Pt-group metallic nanocrystals has emerged as an efficient synthetic method rooted in solution chemistry [64,65,66]. Typically, Pt-group metal atoms tend to form densely packed three-dimensional (3D) structures, which necessitate strong binding forces to inhibit their growth and stacking [67,68]. In this respect, CO functions as an effective capping agent to facilitate the attainment of 2D Pt-group metallic nanostructures by preferentially binding to the (111) facets of metal atoms [69,70]. As a typical example, Huang et al. conducted a pioneering study on the fabrication of free-standing hexagonal Pd nanosheets by utilizing CO as a surface capping agent [71]. Building upon this foundational work, Zhao’s group successfully prepared ultrathin hexagonal Pd nanosheets with three different sizes by utilizing CO as a surface capping agent [72]. It was observed that a surface remodeling phenomenon occurred during the electrocatalytic reduction of CO₂ by defect-rich ultrathin Pd nanosheets. As illustrated in Figure 2a–c, the morphology of the ultrathin-sheet layer remained almost unchanged, while some nanopores penetrated the basal surface. Interestingly, the pristine Pd nanosheets with major Pd(111) crystalline sites were then transformed into wrinkled lamellar structures prevalent in electrocatalytically active Pd(100) sites. In another work, Zheng’s group prepared freestanding Rh nanosheets with long edge lengths from 500 to 1300 nm by adjusting the CO pressure in the range of 0.5–2.0 atm [73].

In addition, a series of carbonyl-containing compounds, such as formaldehyde, Mo(CO)₆, W(CO)₆, and so on, were also employed to release CO molecules for the preparation of 2D Pt-group metallic nanoarchitectures [76,77]. For instance, Yang et al. successfully synthesized ultrathin Rh nanosheets with abundant grain boundaries with the help of formaldehyde by a kinetic control synthesis method [74]. As depicted in Figure 2d, the reduction of polyvinylpyrrolidone (PVP)-coordinated Rh³⁺ ions by formaldehyde initially yielded Rh nanoparticles, which could be further converted to grain-boundary-rich Rh nanosheets by the controlled release of CO through the in situ decomposition of formaldehyde. In another work, Wang and his collaborators successfully constructed an ultrathin curved Pd-Cu bimetallene with a thickness of 1.82 nm, during which the in situ-generated CO from the decomposition of N,N-dimethylformamide could facilitate the growth of the 2D structure [78]. The ultrathin curved geometry and the bimetallic composition effectively modulate the local chemical environment, thereby exposing sufficient active sites and optimizing the oxygen adsorption energy. Similarly, the controllable construction of diverse 2D Pt-group metal-based alloy nanostructures, including PdCu, PdCo, PdPtNi, and PdPtMoCrCoNi nanosheets, was also realized using various carbonyl-containing compounds [79,80,81,82].

Although the use of CO gas and metal carbonyl compounds exhibits considerable potential for the confined growth of 2D Pt-group metallic nanocrystals, their implementation typically necessitates specialized laboratory equipment and stringent reaction conditions or presents an inherent risk of metal impurity contamination [83,84]. To address these issues, Ando et al. developed a green synthesis strategy employing 2,4,6-trichlorophenyl formate (TCPF) as an organic CO precursor [75]. In this synthetic process, the urea-derived ammonia would facilitate the in situ CO gas generation within the TCPF framework, which could serve both as a reducing agent and a surface-capping agent to effectively suppress oxide formation and impurity contamination, thereby enabling the high-yield synthesis of “clean” Pd nanosheets with a narrow size distribution (Figure 2e–g).

2.2. Self-Assembly Methods

Self-assembly is a highly versatile method for constructing nanostructured functional systems from a diverse array of small structural units [85,86]. The self-assembly process enables precise control over the composition, coordination environment, and nanoscale structure of catalysts, facilitating the optimization of their catalytic activity [87,88]. In recent years, self-assembly methods have been widely used to create various 2D Pt-group metallic nanoarchitectures with desired morphologies and compositions [89,90].

For example, Zu et al. synthesized a series of 2D iridium-based nanosheets through a nanoconfined self-assembly strategy utilizing block copolymers with stabilized ends embedded in lamellar micelles [91]. As shown in Figure 3a, the self-assembly process was initially performed to form lamellar organic–inorganic composite micelles, and then the residual tetrahydrofuran and water were evaporated at 100 °C to obtain organic–inorganic PS₁₉₇-b-PEO₁₁₄/Ir composites. Different lamellar micellar composites were prepared by adjusting the mass ratios of Ir³⁺/PS₁₉₇-b-PEO₁₁₄. Finally, the ordered mesoporous Ir-IrO_x/C catalysts were in situ generated after calcination in an argon atmosphere. Remarkably, these nanosheets featured highly ordered interlayer channels (~20 nm) and uniformly distributed Ir-IrO_x nanoparticles (~2 nm) (Figure 3b–e), affording numerous catalytically active centers. In another study, Xie’s group demonstrated the efficient synthesis of lamellar-assembled PdNi super-nanosheets with optimized Pd/Ni molar ratios by a controllable self-assembly method [92]. It was found that the resulting product showed a bunched-nanosheet structure comprising spontaneously lamellar stacking of several thin nanosheets (Figure 3f,g), which not only possessed enhanced stability and improved electron and mass transfer rates but also ensured a large surface area due to the layer-by-layer hierarchical nanostructure. Furthermore, Tang’s group proposed a facile one-pot simultaneous step-by-step self-assembly synthesis strategy to prepare self-supported porous Pd nanosheets composed of crosslinked ultrathin nanowires assisted by poly(diallyldimethylammonium chloride) [93]. As shown in Figure 3h,i, the lateral size of the obtained Pd nanosheets was up to about 2.5 μm, and the corresponding thickness was around only 10 nm. Additionally, it was discovered that different dimensions of Pd nanostructures (including 3D nanoflowers, 2D nanosheets, and 1D nanochains) could be prepared by precisely controlling the pH values of the precursor solution.

2.3. Template Methods

Template methods have long been regarded as an easy-to-use strategy that employs a template as the primary configuration to control, influence, and alter the shapes and sizes of Pt-group metallic nanocrystals [94,95,96]. The metal component is commonly deposited into the pores or onto the surface of the template by physical or chemical means, after which the template is removed to obtain the desired metallic forms [97,98]. One important contribution was that Li et al. demonstrated the template synthesis of self-standing mesoporous Pt nanosheets [99]. As illustrated in Figure 4a, solvent evaporation was used to induce the orderly self-assembly of spherical micelles composed of poly (styrene-b-2-vinylpyridine-b-ethylene oxide), which were confined within an ultrathin layer across the entire substrate surface. Afterwards, metal Pt deposition around the micelles was accomplished by a two-step reduction process utilizing two different reducing agents. After the removal of the micelles and the substrate, continuous self-standing ultrathin mesoporous Pt nanosheets (~15 nm thick) were obtained (Figure 4b,c).

Xu et al. demonstrated the facile powerful bottom-up synthesis of ultrathin Pd nanosheets (PdNS) with efficiently controlled surface facets through a template-assisted solution-phase growth [100]. By modulating the head group functionality of surfactants, PdNSs with exposed low-index {100}, {110}, and {111} facets were selectively produced. Consequently, PdNS{100} demonstrated superior electrocatalytic activity and stability relative to PdNS{110} and PdNS{111}. In addition, Yamamoto and co-workers pioneered a new template method for synthesizing molecularly thin Pt metal nanosheets using solid surfactant crystals as precursors [101]. They first prepared the surfactant crystals with planarly arranged Pt complexes and subsequently controlled the morphology and thickness of 2D surfactant crystals by recrystallization followed by a UV-ozone treatment and reduction process under H₂/Ar flow. As a result, the derived Pt metal nanosheets exhibited varying thicknesses ranging from 1.5 to 3.0 nm while retaining the structural features of the 2D surfactant crystals.

Figure 4. (a) Illustration describing the synthesis of the meso-Pt nanosheets; (b) cross-section SEM and (c) top-view SEM of the meso-Pt nanosheets (from ref. [99] with permission); (d,e) HAADF-STEM images of PdAg nanosheets at different resolutions (from ref. [102] with permission); (f) HAADF-STEM, (g,h) TEM and (i) AFM images of the PdRhBP nanosheets (from ref. [103] with permission).

Figure 4. (a) Illustration describing the synthesis of the meso-Pt nanosheets; (b) cross-section SEM and (c) top-view SEM of the meso-Pt nanosheets (from ref. [99] with permission); (d,e) HAADF-STEM images of PdAg nanosheets at different resolutions (from ref. [102] with permission); (f) HAADF-STEM, (g,h) TEM and (i) AFM images of the PdRhBP nanosheets (from ref. [103] with permission).

In order to further improve the catalytic ability of 2D Pt-group metallic electrocatalysts, multi-element synergistic template strategies have also been developed. As an example, Teng et al. utilized lamellar micelles as soft templates to chemically constrain the growth of the 2D Pd structure and simultaneously employed silver as a co-metal to modulate the reduction kinetics of Pd nanocrystals [102]. The prepared PdAg nanosheets displayed a homogeneous and monodisperse sheet-like nanostructure with an average lateral diameter of 150 nm (Figure 4d). Unlike the traditional nanosheets with smooth and intact surfaces, PdAg nanosheets showed a snowflake-like morphology with abundant active dendrites (Figure 4e). Li and co-workers successfully prepared B and P co-doped PdRh (PdRhBP) nanosheets by using pre-synthesized PdRh nanosheets as the compositional templates, as well as NaBH₄ and NaH₂PO₂ as the B and P sources, respectively [103]. The resultant quaternary PdRhBP material could not only completely preserve the 2D morphology of the pristine PdRh nanosheets to form an ultrathin curled structure (Figure 4f–i) but could also optimize the electronic structure of the catalytically active sites through the synergistic effects of multiple elements. To address the difficulties of the traditional template method such as the residual surfactant blocking the active site, Lee’s group employed Ni-Fe layered double hydroxides (LDHs) as a 2D substrate to prepare 2D Pt nanostructures with ligand-free clean catalytic surfaces [104]. This design strategy endowed the resulting hybrid catalyst with surfactant-free and atomically ultra-exposed heterointerfaces, which were beneficial in increasing the Pt-atom utilization efficiency.

2.4. In Situ Growth Methods

The in situ growth of 2D Pt-group metallic nanocrystals onto advanced supporting materials with large surface area and high electrical conductivity is also an efficient strategy to reduce the precious metal loading as well as stabilize their thin-sheet nature [105,106,107]. Among the various carbonaceous matrixes, graphene or reduced graphene oxide (RGO) has been recognized as a superior substrate for the growth and dispersion of diverse 2D Pt-group metallic nanostructures [108]. For instance, Yang et al. achieved the preparation of ultrathin PdMo bimetallene in situ immobilized on the RGO carrier through a robust stereo-assembly procedure [109]. As seen from Figure 5a–d, the highly-curved PdMo bimetallic nanolayers were almost transparent, which were uniformly distributed on the RGO nanosheets through direct “face-to-face” contact. Notably, the presence of PdMo bimetallene on the RGO substrate effectively prevented the nanosheets from restacking, and this unique 2D/2D heterostructure provided multiple catalytically active sites. In another work, He et al. proposed a solid-phase synthesis method to produce 2D ultrathin Pd and its alloy nanosheets (PdFe, PdCo and PdNi) on the RGO carrier [110]. TEM observation revealed that thin Pd nanosheets with rich jagged edges were flatly laid on the RGO support with an average size of about 20 nm. In comparison with the conventional wet-chemical synthetic route, the solid synthesis of these Pd-based nanosheets did not require the use of any templates or surfactants, and in the meantime, the catalysts could be produced in larger quantities.

Over the past few years, 2D transition-metal carbides/nitrides (MXene) have shown great potential to serve as a novel matrix for the confined growth of various Pt-group metallic nanocrystals because of their large accessible surface areas, tunable termination groups, good hydrophilic properties, and high metallic conductivity [113,114,115]. Typically, our group presents a robust and convenient wet-chemical approach for the construction of a 2D/2D heterojunction consisting of ultrathin Rh nanosheets in situ grown on Ti₃C₂T_x MXene [111]. As shown in Figure 5e, 2D Ti₃C₂T_x MXene nanolayers were first produced from bulk Ti₃AlC₂ MAX powder using a LiF and HCl etching route, followed by the growth of Rh seeds on the MXene surface. Afterward, formaldehyde was decomposed to generate plentiful CO molecules with the assistance of pyridine, resulting in the gradual formation of 2D Rh nanosheets on MXene nanoflakes. The generated Rh nanosheets were rhombic in shape with sizes between 10 and 15 nm, which could provide abundant unsaturated Rh atoms with optimized electronic structure (Figure 5f,g). Furthermore, we recently reported the successful fabrication of Ti₃C₂T_x MXene-supported Pd nanosheets via a heterointerface engineering strategy that involves the decomposition of Mo(CO)₆ to produce CO molecules. As seen from Figure 5h–j, most Pd nanosheets appeared as a regular hexagonal shape with sizes between 10 and 35 nm, which were intimately attached to the MXene matrix through a strong atomic interaction at the heterointerfaces [112].

2.5. Other Synthesis Methods

Besides the aforementioned four principal strategies, other synthesis methods have also been developed for the construction of 2D Pt-group metallic electrocatalysts. For example, the epitaxial growth method involves the deposition of atoms from a thin crystalline layer onto a single-crystal substrate, thereby replicating the substrate’s atomic arrangement and yielding high-quality interfaces [116]. In a typical study, Du’s group reported an epitaxial seed growth method to build a 2D heterostructure consisting of Pt nanoparticles grown on Pd nanosheets [117]. Notably, a rapid reduction kinetic could make the Pt nanoparticles evenly deposited on the Pd nanosheets, while a slow reduction kinetic led to the preferential growth of Pt nanoparticles on the edges of Pd nanosheets (Figure 6a–c). Density functional theory (DFT) calculations showed that the Pd(111)–Pt interfaces induced a more pronounced electron-deficient state of Pd carriers, affording a lower-band center and stronger intermetallic interaction.

Another interesting advance is that Ding et al. designed a new 2D heterostructure with Au nanoparticles intercalated in Pd nanosheets via a facile wet chemical method combining seed growth, plating substitution and assembly in a single step (Figure 6d) [118]. It was found that most Au particles were not coated on the Pd nanosheets but buried within the body of Pd nanosheets. As shown in Figure 6e, a large number of spherical Au nanoparticles with a diameter of ~15 nm were intercalated within ultrathin Pd nanosheets to form an interesting egg-waffle-like morphology, which contributed to efficient hot-carrier generation that could mediate a direct transfer of plasmonic energy to the metal-adsorbate complex.

3. Applications of 2D Pt-Group Metallic Electrocatalysts for Energy Conversion

Two-dimensional Pt-group metallic nanocrystals have become a research hotspot in the field of electrocatalysis due to their attractive architectural features, including atomic-level thin-layer structure, large specific surface area, and plenty of unsaturated ligand atoms. As core components of novel energy-conversion devices, electrode catalysts play crucial roles in boosting the reaction efficiency and improving the overall output power [119,120,121,122,123]. In this section, the recent advances on the development of 2D Pt-group metallic nanocrystals as electrocatalysts are systematically summarized and discussed based on five typical electrocatalytic reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), formic acid oxidation reaction (FAOR), and alcohol oxidation reaction (AOR).

3.1. Hydrogen Evolution Reaction

Electrochemical water splitting has been widely recognized as a highly promising hydrogen production technology due to its advantages of high efficiency, environmental friendliness, and sustainability [124,125,126]. A typical electrolyzer system comprises three essential components: an electrolyte, an anode, and a cathode [127,128]. When an appropriate external voltage is applied to the electrodes, water molecules dissociate at the cathode to generate hydrogen gas, while oxygen evolves at the anode [129,130]. The cathodic half-reaction, known as the HER, constitutes the fundamental electrochemical process in electrocatalysis [131,132]. Similar to most electrochemical reactions, HER requires efficient catalysts to optimize the reaction kinetics and minimize the overpotential [133,134,135]. Currently, commercial HER electrocatalysts predominantly employ Pt-based materials, which dominate the field owing to their exceptional catalytic activity and ultralow overpotential, while their high costs severely limit large-scale commercial applications [136,137,138].

To address this issue, great efforts have been devoted to the exploration and utilization of 2D Pt-group metallic electrocatalysts, which show huge potential for cost reduction and HER performance improvement [139,140]. For example, Zhang and co-workers successfully prepared a series of ultrathin RuPdM (M = Ni, Co, Fe) nanosheets for the first time and investigated their HER activity [141]. As displayed in Figure 7a,b, optimized trimetallic RuPdNi nanosheets exhibited a unique 2D lamellar structure with an atomic-level thickness of about six atomic layers (≈1.6 nm) as well as uniform dispersions of Ru, Pd, and Ni throughout the whole nanosheets. Electrochemical tests revealed that the Ru₃₈Pd₃₄Ni₂₈ electrocatalyst required a low overpotential of only 20 mV at a current density of 10 mA cm⁻², superior to the widely used Pt/C and Pd/C catalysts. DFT calculations elucidated that the introduction of 3D transition metals (Fe, Co, and Ni) was able to induce the modulation of RuPd electronic structures to enhance the intrinsic catalytic activity.

Moreover, Cai et al. demonstrated through DFT simulations that the essence of the Ru-H binding energy depended on the strong interaction between the 4dz² orbital of Ru and the 1s orbital of H [142]. The charge transfer between the Ru and CoNi substrate caused the band center of Ru to shift downwards, which resulted in a suitable Gibbs free energy of 0.022 eV for H^* in the RuCo structure. To verify this prediction, RuCo alloy nanosheets with different proportions were prepared by a combined precipitation and electrochemical reduction process. It can be seen from Figure 7c–f that Ru atoms were embedded into the Co matrix to form an optimized coordination structure, which exhibited an ultra-low overpotential of 10 mV at a current density of 10 mA cm⁻² and a small Tafel slope of only 20.6 mV/dec in the alkaline medium, more competitive than the commercial Pt/C catalyst. Jin et al. reported the fabrication of holey large-area 2D Ru nanosheets by the reductive phase transition of monolayer RuO₂ nanosheets [144]. Their study revealed that the metal coordination number was a critical parameter governing the catalytic HER performance of the resultant Ru nanosheets. With an optimal coordination number of Ru atoms of ~10.2, the holey metallic Ru nanosheets achieved a remarkably low overpotential of 38 mV at 10 mA cm⁻² in the presence of 1 M KOH, outperforming that of bulk Ru metal and commercial 20 wt% Pt/C electrocatalysts.

Recent theoretical and experimental research has also demonstrated that the intrinsic strains in atomically thin 2D alloys are associated with their specific thickness or curvature, which is able to greatly modulate the surface electronic structure and energy of the reaction intermediates, thus promoting catalytic activity [145,146]. Specifically, tensile strains induce lattice expansion and thus weaken the interatomic interactions, which could lead to an upward shift of the center of the d-band and increase the adsorption energy between the absorbed molecules and the catalyst [147,148]. In contrast, compressive strains have the opposite effects, shifting the d-band center downward and decreasing the adsorption energy between the catalysts and adsorbates [149,150]. According to the Sabatier principle, the optimal catalyst should have a reactant and intermediate adsorption energy that is neither too high nor too low [151,152], which implies that a suitable intrinsic strain is very beneficial to boost the catalytic efficiency of 2D Pt-group metallic electrocatalysts. Based on this, Guo’s group proposed a novel strategy to synthesize ultrathin curved PdIr bimetallene with a thickness of about five atomic layers (Figure 7g) [143]. Theoretical simulations proved that the surface strain effects endowed the PdIr bimetallene with plenty of concave–convex featured electro-active regions. Accordingly, the PdIr bimetallene/C catalyst with a Pd/Ir atomic ratio of 4: 1 manifested the best HER performance with an overpotential of 34 mV at 10 mA cm⁻² and a Tafel slope of 58.3 mV dec⁻¹, superior to the PdIr nanoparticle/C and Pt/C catalysts (Figure 7h,i). In addition, Yin et al. prepared curled and perforated PdIn bimetallene with abundant defective sites through a one-step solvothermal method, which exhibited excellent activity and stability for both HER and methanol oxidation reaction (MOR) [153]. In a typical MOR-HER co-electrolysis system assembled with the bifunctional PdIn bimetallene as both the anode and cathode, a current density of 100 mA cm⁻² could be driven by a tank voltage of only 1.26 V, which was 0.93 V lower than that of a conventional water electrolysis system without methanol.

3.2. Oxygen Evolution Reaction

As another half reaction of electrochemical water splitting at the anode, the oxygen evolution reaction has also emerged as a promising energy-conversion technology for renewable hydrogen production [154,155]. Noteworthily, in comparison with the HER, the OER process exhibits inherently slower kinetics, primarily due to its complex four-electron transfer mechanism involving proton-coupled steps [156,157]. This kinetic limitation manifests as substantial overpotential requirements, which critically decreases the overall energy-conversion efficiency of the electrolysis system [158,159]. Consequently, the development of high-performance OER catalysts remains a key research focus for optimizing the performance of the electrochemical water splitting device [160,161].

Ir-based materials have been considered as the current state-of-the-art acidic OER catalysts due to their good stability under harsh acidic conditions [162,163]. However, the intrinsic activity of Ir-based catalysts is insufficient, which severely limits the OER efficiency [164,165]. To address this problem, researchers have focused on the morphological regulation of Ir-based catalysts with various 2D configurations, which have achieved a significant increase in catalytic activity [166,167]. As a typical example, Yamauchi’s group developed a soft polymeric micelle template-assisted construction of 2D mesoporous Ir nanosheets and investigated their OER performance [168]. As presented in Figure 8a,b, the as-derived metallic Ir sample appeared to be an interconnected thin-sheet network, where a large number of mesopores with a diameter of around 15 nm were homogeneously distributed over whole nanosheets. By virtue of their unique architectural features, the mesoporous Ir nanosheets exhibited an unusual OER performance with a low overpotential of 240 mV at 10 mA cm⁻² and a Tafel slope of 49 mV dec⁻¹ in 0.5 M H₂SO₄ electrolyte, much better than that of commercial Ir black and IrO₂ catalysts (Figure 8c,d). In addition, Chatterjee and co-workers proposed the top-down synthesis of nanoporous Ir nanosheets for catalyzing the OER process [169]. As shown in Figure 8e,f, these Ir nanosheets have a lateral size of several microns, a thickness of about 100 nm, and rich porosity. Impressively, the nanoporous Ir nanosheets exhibited stable electrocatalytic properties over 50,000 accelerated stress test cycles in the polymer electrolyte membrane water electrolyzer (PEMWE) (Figure 8g). Yang’s group prepared a hexagonal nanosheet structure with lattice distortion and edge-connected [IrO₆] octahedra to effectively improve the OER performance [170]. In 0.1 M HClO₄ electrolyte, the mass activity of this newly designed catalyst was up to 217 mA mg_Ir⁻¹ at 1.55 V, which was 13 times higher than that of rutile iridium dioxide.

Considering that Ir is one of the rarest elements, with an annual production of only 5~8 tons, its limited supply can hardly meet the demand for large-scale development of PEMWE [172,173]. In this context, metallic Ru has recently been regarded as a promising Ir-alternative electrode catalyst for the OER in acidic conditions owing to its more abundant reserve and relatively lower cost [174,175]. For instance, Kong et al. reported the rational design and solvothermal production of 2D free-standing ultrathin Ru nanosheets with clean surfaces [171]. In particular, these Ru nanosheets were mildly heat-treated in the atmospheric environment to allow the conversion of metallic Ru surface to oxide form while maintaining their 2D thin-sheet structure for the OER application (Figure 8h,i). As shown in Figure 8j,k, the onset overpotential of the ruthenium oxide nanosheets was determined to be 220 mV, significantly better than that of commercial Pt/C (430 mV). In the meantime, an overpotential of only 260 mV was required for the Ru-based nanosheets to derive a current density of 10 mA mg⁻¹, which was lower than that required for the Ru powder catalyst. Furthermore, the current density increased exponentially with applied potential, and the corresponding Tafel plot analysis indicated that the Ru nanosheets exhibited the lowest Tafel slope of only 54 mV dec⁻¹. In addition, Yuan et al. proposed that a stable Pt₅Se₄ monolayer could be obtained from bulk Pt₅Se₄ crystals by mechanical exfoliation [176]. Surface Pt atoms in this monolayer exhibited quasi-neutral charge states analogous to Pt(111) surfaces. This electronic configuration may confer exceptional electrocatalytic properties. Regarding OER, the overpotential of monolayer Pt₅Se₄ was calculated to be 0.35 V, comparable to those of Ru and Ir oxide catalysts (0.37 and 0.56 V).

3.3. Oxygen Reduction Reaction

Proton exchange membrane fuel cells (PEMFCs) have been recognized as one of the core technologies for building a sustainable hydrogen energy system due to their high energy-conversion efficiency, zero pollution, low working temperature, and facile cell configuration, which have received extensive attention from both academia and industry in recent years [177,178]. In particular, the cathodic ORR is a crucial electrochemical process that directly influences the energy-conversion efficiency of PEMFCs [179,180]. Currently, the overall performance of PEMFCs has been largely limited by the slow kinetics of the ORR process [181,182]. As is known, Pt and Pt-derived electrocatalysts can effectively catalyze the ORR, but their high cost and insufficient stability have severely hindered the scalability of the PEMFC technology. In addition, low-buffering-capacity solutions can significantly change the local pH at the electrode/solution interface, which in turn affects the reaction path and kinetic rate, which can be visualized through the fluorescence imaging of the interface [183]. Therefore, the development of highly active and long-lasting ORR electrocatalysts has become a key challenge to promote the hydrogen economy and popularize clean energy technology [184,185]. Typically, the performance evaluation for Pt-based catalysts relies on a variety of electrochemical techniques: (1) hydrogen adsorption/desorption analysis via cyclic voltammetry (CV) using stationary catalyst-loaded electrodes to calculate the electrochemical surface area (ECSA); (2) CO stripping tests (following the DOE protocol) for ECSA calculation and anti-toxicity analysis; and (3) CV and linear sweep voltammetry (LSV) techniques for the assessment of mass activity and cycling stability [186,187]. In order to deeply analyze the electrocatalytic ORR mechanism, the rotating ring disk electrode (RRDE) coupled with online differential electrochemical mass spectrometry (DEMS) provides a powerful tool to reveal the reaction pathway by real-time monitoring of the reaction intermediates and gas products [188,189].

To enhance the catalytic ORR performance, researchers have developed a variety of Pt-group metallic nanosheet systems through various structural design concepts [190,191]. For example, Zhang and co-workers utilized microwave-assisted heating to deposit a significant number of Pt atoms into PdCu nanosheets to form a series of ternary PtPdCu nanosheets [192]. As presented in Figure 9a–d, the as-obtained ternary nanosheets possessed a 2D wavy nanostructure, where Pt, Pd, and Cu elements were uniformly distributed over the nanosheets. It was found that the PtPdCu alloy effects could induce the lattice distortion of Pt atoms, and the resulting compressive strain was essential to improving the ORR ability. As a result, the optimized Pt₃₈Pd₅₀Cu₁₂ nanosheet catalyst expressed the best catalytic optimized activity with an ECSA of 118.7 m²/g_Pt+Pd and a mass activity of 0.93 A/mg_Pt+Pd at 0.9 V. After 10,000 cycles of an accelerated durability testing (ADT) cycling test, the Pt₃₈Pd₅₀Cu₁₂ catalyst exhibited an attenuation of only 9.2% in terms of ECSA and 11.4% in terms of mass activity. Additionally, the Pt₃₈Pd₅₀Cu₁₂ nanosheets also realized a maximum power density of 796 mW cm⁻² in the single-cell test, far surpassing that of traditional Pt/C catalyst (606 mW cm⁻²). Guo’s group reported the construction of Pt-on-Pd dendritic nanosheets made from Pt branches grown in situ on the surface of ultrathin Pd nanosheets (Figure 9e,f) [193]. Strikingly, the separate dendritic nanostructure could inhibit the clustering of the active sites and create electronic coupling effects between Pd-Pt bimetallic crystals, which effectively improved the catalytic ORR capacity. As depicted in Figure 9g, the specific activity and mass activity of Pt-on-Pd dendritic nanosheets were 3.0 and 2.2 times higher than those of a commercial Pt/C catalyst, respectively, accompanied by superior cycling performance.

Two-dimensional Pt-group metallic nanosheets with rich wrinkles have been proved to not only significantly improve the electrolyte penetration and transport but also inhibit the aggregation of metal nanolayers, commonly leading to enhanced electrocatalytic ORR efficiency. In this aspect, Zhang et al. synthesized highly wrinkled Pd nanosheets with a bent cross-linked structure to increase the compressive strain for rapid electrocatalytic kinetics [194]. Consequently, the prepared highly wrinkled Pd nanosheets depicted excellent ORR properties in acidic media with 26 mV more positive half-wave potential and much better stability when compared with commercially available Pd/C and common Pd nanosheet catalysts. Theoretical simulations disclosed that the compressive strain arising from the wrinkles was able to downshift the d-band center of the Pd component, which could reduce the reaction barriers of the rate-determining step for the ORR process. As another example, Chen et al. successfully prepared ultrathin wrinkled FePt nanosheets decorated with high-density neighboring dispersed Pt atoms and investigated their ORR performance [195]. As shown in Figure 9h,i, the synthesized FePt nanosheets were only 1.2 nm thick and consisted of about 5–6 atomic nanolayers, along with numerous Pt atoms positioned on the wrinkled surface. Such a neighboring-atom electrocatalyst retained the merits of conventional single-atom catalysts and meanwhile was able to synergistically catalyze the reactions that require two or more neighboring sites. As a result, the wrinkled FePt nanosheets manifested a four-electron reduction pathway, a large electrochemical active surface area of 545.5 m² g⁻¹, stable mass activity, as well as good CO tolerance towards the ORR (Figure 9j).

Figure 9. (a) HAADF and (b–d) the corresponding STEM-EDS elemental mapping images of Pt₃₈Pd₅₀Cu₁₂ NSs (from ref. [192] with permission); (e) TEM image of Pd NSs; (f) TEM image of Pd₁Pt₄ DNSs; (g) histogram of SAs and MA at 0.9 V vs. RHE (from ref. [193] with permission); (h) HAADF-STEM and (i) HRTEM images of FePt NS; (j) CV curves of FePt NSs, FePt NPs, and Pt/C (from ref. [195] with permission).

Figure 9. (a) HAADF and (b–d) the corresponding STEM-EDS elemental mapping images of Pt₃₈Pd₅₀Cu₁₂ NSs (from ref. [192] with permission); (e) TEM image of Pd NSs; (f) TEM image of Pd₁Pt₄ DNSs; (g) histogram of SAs and MA at 0.9 V vs. RHE (from ref. [193] with permission); (h) HAADF-STEM and (i) HRTEM images of FePt NS; (j) CV curves of FePt NSs, FePt NPs, and Pt/C (from ref. [195] with permission).

3.4. Formic Acid Oxidation Reaction

Direct formic acid fuel cells (DFAFCs), as a new type of clean energy device, have demonstrated significant potential for applications in portable electronic devices, new energy vehicles, and the aerospace industry due to their comprehensive advantages, including high safety, excellent energy-conversion efficiency, outstanding power density, low hazards emission, and simple structure [196,197,198]. The core of this fuel cell system lies in the catalytic performance of FAOR at the anode, which operates through a dual pathway competition mechanism: the direct pathway generates carbon dioxide under ideal conditions, while the indirect pathway generates toxic intermediates such as CO [199,200]. Currently, most conventional Pt-group metallic catalysts generate toxic CO intermediates via the indirect pathway, which negatively impacts the catalytic performance [201,202]. To address this challenge, the rapid surface desorption of CO and retention of active sites during the FAOR process have become a key point [203,204]. This has motivated the rational design and controlled preparation of 2D Pt-group metallic electrocatalysts with high FAOR activity and superior selectivity capable of facilitating the direct dehydrogenation pathway [205,206]. For instance, Zhou and co-workers proposed a novel composite catalyst comprising 2D Pd nanosheets decorated with SnO₂ nanoflakes for the FAOR (Figure 10a–c) [207]. Benefiting from the distinct physicochemical properties as well as the strong interactions between the heterointerfaces, the FAOR mass and specific activities of the resultant hybrid catalyst reached 4.96 A mg⁻¹_Pd and 216.95 A m⁻²_ECSA, which were 3.76 and 4.26 times higher than those of Pd nanosheets/C, respectively. In situ attenuated total reflection infrared analysis evidenced a promoted formate pathway for the SnO₂-decorated Pd nanosheets, thus affording a suppressed accumulation of CO molecules.

The precise synthesis of 2D Pd-based bimetallic alloy nanosheets is a feasible strategy to enhance the catalytic efficiency of the FAOR. One important contribution was that Zhang’s group innovatively synthesized ultrathin PdCu bimetallic alloy nanosheets with varying Cu/Pd ratios, which were further treated with ethylenediamine (EN) for the FAOR application [208]. As confirmed by the TEM and AFM characterization (Figure 10d,e), the resulting PdCu nanosheets presented an ultrathin nature with a thickness of only about 2.8 nm, which were composed of uniformly distributed Pd and Cu elements. In a 0.5 M H₂SO₄ and 0.25 M HCOOH electrolyte, the EN post-treated PdCu nanosheets exhibited excellent FAOR performance with a maximum ECSA value of 139.8 m² g_Pd⁻¹ and an ultrahigh mass activity of 1655.7 mA mg_Pd⁻¹ (Figure 10f). The dramatically enhanced activity of the EN-treated PdCu alloy nanosheets can be attributed to their ultrathin morphology, ameliorative electronic structure, bimetallic synergistic effects, and removal of inert oleic acid by EN treatment. In another work, Wei et al. successfully synthesized 2D PdSb-based nanosheets by doping the Sb element with a predominant metallic state into the Pd crystal structure (Figure 10g,h) [209]. Notably, metallic Sb operated through the synergistic effects of electronic modification and oxophilic properties, leading to the effective electrooxidation removal of CO species during the FAOR. As shown in Figure 10i,j, the specific and mass activities of the PdSb-based nanosheets with 69% Sb metallic state were significantly higher than those of Pd-based nanosheets and Pd/C catalysts.

3.5. Alcohol Oxidation Reactions

Direct alcohol fuel cells (DAFCs) achieve efficient energy conversion through the electrocatalytic oxidation of alcohol fuels (e.g., methanol, ethanol, ethylene glycol, etc.) [210,211,212]. Such electrocatalytic processes are environmentally friendly and sustainable, making DAFCs ideal alternatives to traditional heat engines [213,214,215,216]. In addition, DAFCs can perform well at various operating temperatures, thus holding huge potential for a wide range of applications [217,218]. Nevertheless, the intrinsic sluggish dynamics for the electrooxidation of alcohol molecules poses a great challenge to the large-scale commercialization of DAFCs [219,220,221]. For example, there are two competing pathways for the ethanol oxidation reaction (EOR), named “C1” and “C2”, which refer to the complete oxidation of ethanol to CO₂ (12-electron transfer, C1 pathway) and partial oxidation to CH₃COOH (4-electron transfer, C2 pathway), respectively [222,223]. Conventional catalyst systems generally exhibit low selectivity of the C1 pathway and high selectivity of the C2 pathway, which largely hinders the efficiency improvement [224,225]. Therefore, the development of efficient, highly-selective anode electrocatalysts that enhance the cleavage of C-C and C-H bonds for the complete oxidation of alcohol molecules is conducive to promoting the widespread application of DAFCs [226,227,228,229].

One representative example was published by Chen’s group, where hierarchical porous Rh nanosheets were prepared with the help of an emulsion-induced soft template method [230]. Structural characterization revealed that the sample presented a typical lamellar structure, which was constructed from the self-assembly of secondary Rh crystals with rough and porous surfaces (Figure 11a–c). Because of their abundant edge/grain boundary atoms and ultrathin porous structure, the hierarchical porous Rh nanosheets demonstrated enhanced MOR kinetics with a mass activity of 333 A g⁻¹ with a relatively low peak potential of 0.63 V, which was 5.8 and 3.3 times higher than those of commercial Pt black and Rh black catalysts, respectively (Figure 11d). In addition, Zhang’s group introduced a second metal Ag into Pd crystal lattices and successfully prepared hierarchically interconnected PdAg alloy nanosheets with a thickness of only seven atoms (Figure 11e,f), which were very beneficial to promote both charge transfer and mass transport during the EOR process [231]. As shown in Figure 10g, with a moderate molar ratio of Pd/Ag precursors, the optimized Pd₇Ag₃ nanosheets exhibited a remarkable EOR peak current density of 9365.9 mA mg⁻¹_Pd, which was 8.8 times higher than that of reference Pd nanosheets catalyst (1053.7 mA mg⁻¹_Pd).

Precise regulation of the active site distribution through the construction of multicomponent configurations has become an important research direction to enhance the catalytic performance of 2D Pt-group metallic materials [233,234]. Dong et al. reported the construction of free-standing ultrathin and highly flexible ternary alloy PtTeCu nanosheets and their use as MOR electrocatalysts (Figure 11h–j) [232]. The nanoporous ultrathin-sheet structure largely facilitated the exposure of Pt active sites, while the introduction of Te and Cu atoms induced an obvious downward shift of the d-band center of Pt and weakened the adsorption energy of the products on the ternary nanosheets. As a result, the PtTeCu nanosheets exhibited a high catalytic performance for the ethylene glycol oxidation reaction (EGOR) and MOR with mass activities of 7.1 and 4.9 A mg⁻¹, which were 6.5 and 6.1 times higher than those of commercial Pt/C, respectively (Figure 11k). Huang’s group developed a unique class of Rh/Rh-M nanosheets (M = Co, Mn, Fe, and Ni) by depositing Rh-M nanoparticles on the surface of a 2D Rh nanosheet substrate [235]. Noteworthily, CO could spill over from the Rh sites of ultrathin Rh nanosheets to the M sites of the Rh-M nanoparticles, thus leading to the weakening of CO adsorption on the Rh sites. In regard to the EOR catalysis, it was found that the optimal Rh₇₉Co₂₁ nanosheets with the CO spillover effect had a large current density of 485.14 mA mg_Rh⁻¹ and a CO₂ Faraday efficiency of 75.5%, significantly superior to those of bare Rh nanosheets.

4. Conclusions, Challenges, and Perspectives

Over the past few years, 2D Pt-group metallic nanocrystals have attracted considerable and widespread attention because of their significant application value in the field of clean energy-conversion devices due to their large specific surface area, plentiful unsaturated metal atoms, optimized electronic structure, and unimpeded electron/ion transfer pathways. In this review paper, we have outlined the effective methods for the controllable construction of 2D Pt-group metallic nanocrystals, comprising CO-confined growth methods, self-assembly methods, template methods, in situ growth methods, and so on. Moreover, this review has summarized the latest research progress on the applications of 2D Pt-group metallic nanocrystals as electrocatalysts for a variety of energy-conversion technologies (including HER, OER, ORR, FAOR, and AOR) and highlighted the core relationship between their nanoarchitectural features and electrocatalytic properties.

Although 2D Pt-group metallic electrocatalysts present promising opportunities for addressing critical energy and environmental challenges, their large-scale commercialization remains constrained by several fundamental limitations. Firstly, the thin-sheet Pt-group metallic nanocrystals with high surface free energies easily suffer from longitudinal re-stacking or re-aggregation during their practical usage, resulting in an obvious decrease in the number of catalytically active sites. Given this, it is recommended to combine 2D Pt-group metallic nanocrystals with high-quality supporting materials (e.g., graphene, MXene, etc.) to construct a 2D/2D heterostructure through a “face-to-face” interfacial interaction, which could largely maintain the catalytic advantages of the separated Pt-group metallic nanosheets. Meanwhile, identifying optimal material combinations from numerous possibilities presents a significant challenge; leveraging artificial intelligence (AI) methods synergistically with high-throughput computing techniques offers an accelerated pathway for discovering and evaluating efficient electrocatalysts based on 2D/2D heterostructures. Secondly, current synthesis processes for the 2D Pt-group metallic electrocatalysts commonly refer to the utilization of various capping agents, including surfactants, templates, and organic ligands, which cannot be effectively removed through conventional purification methods without compromising the structural integrity of the nanosheets. Therefore, it is highly desirable to develop novel surfactant- and template-free synthetic strategies to avoid the excessive use of capping agents and keep the catalytic crystal faces clean. Thirdly, considering the high costs and scarcity of most Pt-group metals, it is suggested to introduce appropriate inexpensive transition metal elements to construct multicomponent metallic systems, especially high-entropy alloys and intermetallic compounds, which could not only reduce the usage amount of Pt-group metals to lower the manufacturing costs but also boost the intrinsic catalytic activity through alloy, strain, lattice distortion, and cocktail effects. Finally, the dynamic surface reconstruction of 2D Pt-group metallic electrocatalysts during diverse energy-conversion reactions obscures the precise identification of actual catalytical active centers, thereby hindering the elucidation of underlying catalytic mechanisms, which requires the exploration and utilization of various in situ characterization techniques combined with advanced theoretical analyses to unveil the structural evolution of active centers and establish robust guidelines for rational catalyst design.

Encouragingly, despite the above research challenges, researchers are still enthusiastic about 2D Pt-group metallic electrocatalysts, with more and more young scholars devoted to this field. With the continuous innovation of nanomaterials as well as breakthroughs in surface/interface regulation technology, the development of this field is expected to catalyze the technological innovation of core devices, such as next-generation PEMFC and metal–air batteries. Such breakthroughs will not only reshape the energy-production and -consumption systems but may also trigger revolutionary changes in green transportation, distributed energy supply, and other application scenarios, thus providing key technological supports for the realization of carbon neutrality.