Recent advances in morphology control of platinum catalysts toward oxygen reduction reaction

Abstract

Exploring advanced platinum (Pt)-based electrocatalysts is vital for the widespread implementation of proton exchange membrane fuel cells (PEMFCs). Morphology control represents an effective strategy to optimize the behavior of Pt catalysts. In this work, an attempt is made to comprehensively review the effect of morphology control on the catalytic behavior of catalysts in the oxygen reduction reaction (ORR). First, the fundamental physicochemical changes behind morphology control, including exposing more active sites, generating appropriate lattice strains, and forming different crystalline surfaces, are highlighted. Then, recently developed strategies for tuning the morphologies of electrocatalysts, including core-shell structures, hollow structures, nanocages, nanowires, and nanosheets, are comprehensively summarized. Finally, an outlook on the future development of morphology control of Pt catalysts is presented, including rational design strategies, advanced in situ characterization techniques, novel artificial intelligence, and mechanical learning. This work is intended to provide valuable insights into designing the morphology and technological innovation of efficient redox electrocatalysts in fuel cells.

Graphical abstract

Keywords

morphology / platinum catalysts / electrocatalysis / ORR / PEMFC

1 Introduction

Due to the ongoing development and use of conventional energy sources, the severity of the energy crisis and environmental issues is increasing. Consequently, there is a growing focus on exploring and researching renewable and sustainable energy sources such as solar, hydrogen, and wind power [1]. As a renewable energy source, hydrogen exhibits an excellent combustion performance, a high calorific value, and high utilization rates. Its applications extend beyond conventional energy fields, encompassing emergent spheres like novel energy cars and hydrogen power creation, positioning it as a promising source of green energy [2]. However, fuel cells only have an energy conversion efficiency of 40%–60% in practice, leaving ample room for improvement in their efficiency [3]. Fuel cells, particularly proton exchange membrane fuel cells (PEMFCs), have emerged as a solution to the problem of the low conversion efficiency of hydrogen energy because PEMFCs offer numerous advantages, including quick startup times, high specific power, and ease of operation [4,5]. With these advantages, PEMFCs will play a vital role in the evolution of new energy vehicles in the coming years. Energy conversion in fuel cells is accompanied by two primary processes, oxygen reduction reaction (ORR) at the cathode and hydrogen oxidation reaction (HOR) at the anode. The ORR course is particularly irreversible and yields an ultra-high potential, which is a crucial factor that affects the response kinetics of the fuel cell [6]. During the ORR reaction, catalysts can diminish the energy required to activate the reaction and thereby improve the efficiency of fuel cells, making them vital in the endeavor to develop sustainable energy technologies.

Efficient ORR catalysts have received significant research attention in recent decades. Some non-precious metal catalysts have demonstrated good activity upon heteroatom doping and structural engineering modifications [7–11]. For instance, Wang and colleagues [10] erected nonplanar embedded (Fe₂S₂) cluster sites on nitrogen (N)-doped carbon planes. The co-regulation of atomic composition and spatial configuration realized effective oxygen adsorption and activation, resulting in an excellent ORR performance. Unfortunately, these non-precious metal catalysts pose challenges in terms of stability, and some materials may become inactive over long durations. Compared to non-precious metal catalysts, noble metals exhibit durability and corrosion resistance in acidic environments [12]. These attributes render them the primary materials utilized in ORR electrode catalysts. However, the high costs associated with noble metal catalysts currently limit their practical applications. Therefore, a more efficient and cost-effective catalyst for fuel cells is urgently needed. To reduce the excess potential at the cathode and accelerate the reaction rate, researchers have devised several optimization schemes for ORR electrocatalysts, including doping [13,14], single-atom catalysts [15], and morphology control [16]. Morphology control has become an area of interest due to its flexible and diverse design strategies. The optimal performance and stability of ORR catalysts depend on the rational design of the morphology, size, and crystal structure of the catalyst material. Various factors, such as strain, ligand, and small size, have the potential to enhance the surface area and electron transfer efficiency of catalysts through morphology control [17,18]. This technique can regulate the distribution and quantity of active sites, thereby improving the catalytic performance. Furthermore, the morphology of the catalyst impacts the electron transfer efficiency and the overall number of active sites available, allowing tailoring of catalytic activity through morphology control [19,20]. As an important research area in energy and environmental protection, the morphology control of ORR catalysts offers a vast range of applications and development potential.

For a complete understanding of the morphological control of Pt-based catalysts, recent advances in the morphological engineering of catalysts are reviewed in detail. First, focus is laid on the fundamental physical chemistry changes behind the morphology control of Pt-based catalysts. Then, the latest strategies for morphology design is reviewed. Finally, perspectives on the future development of ORR catalyst morphology control are provided, including rational design strategies, advanced in situ characterization techniques, as well as novel artificial intelligence and mechanistic learning. This work aims to provide a thorough overview of the morphology control of ORR catalysts to promote exploration and practical application in this field.

2 Effects of morphology control on Pt-based ORR catalysts

The ORR is a complex process comprising numerous basic steps and several multi-electron transfer intermediates. Under acidic conditions, the ORR on the cathode experiences a reduced speed, with the reaction rate being five orders of magnitude lower than the anodic HOR [21]. Researchers have typically pursued the 4-electron reaction pathway, represented by the reaction pathway in Eq. (1).

(1){\mathrm{O}}_2+ 4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O}

First, oxygen molecules attach to the active sites on the catalyst surface (*) and decompose to form adsorbed active carboxyl groups (*OOH). The *OOH subsequently combines with a proton to produce reactive oxygen (*O) and a water molecule. The *O then acquires a proton, forming a reactive hydroxyl group (*OH). Lastly, *OH merges with another proton to generate a water molecule. The specific steps of electrocatalytic ORR under acidic conditions are

(2)\text{*}+{\mathrm{O}}_2\left(\mathrm{g}\right)+{\mathrm{H}}^{+}\to \text{*}\mathrm{O}\mathrm{O}\mathrm{H}

(3)\text{*}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \text{*}\mathrm{O} +{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)

(4)\text{*}\mathrm{O} +{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \text{*}\mathrm{O}\mathrm{H}

(5)\text{*}\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+} +{\mathrm{e}}^{-}\to \text{*}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)

However, the slow response of the multiple electron transfer pathway, accompanied by an ultra-high potential, is one of the most critical challenges in the study of PEMFCs [22]. ORR reaction mechanisms can be understood with the help of first principles [23,24]. Intermediates including *OOH, *O, and *OH are involved in the 4-electron reaction pathway in acidic environments. At the PEMFC operating potential of 0.9 V, O₂ is initially sorbed to the catalyst surface, and then the *O sorbed undergoes protonation to form *OH. This process determines the velocity of the overall ORR process [25]. According to the principle of Sabatier, an electron must remain at a particular location for a sufficiently long time before it can be received and continue to react. This process requires a certain amount of time to counteract the effects of inhomogeneous distribution and thermal motion to bring the reaction to equilibrium. However, electrons that remain at the site for too long can also prevent subsequent reactions from proceeding. In addition, from a chemical bonding perspective, the essence of catalyst adsorption is to establish chemical bonds. Numerous intermediates (*OOH, *O, and *OH) are formed during ORR. *OOH and OH are attached to the target metal surface via single bonds, while *O forms a double bond with the metal surface. Consequently, the binding energies of *OOH, *O, and *OH are linearly related (Fig.1(a)) [26]. Based on this linear relationship, the activity of the catalyst can be described by the binding energy of the intermediate. A strong binding energy may reduce the reactivity by transferring electrons to O* or OH*, while a weak binding energy reduces the efficiency of electron binding to the intermediate [21]. The adsorption strength between the catalyst and the reaction intermediates exhibits a volcano distribution about the catalyst activity (Fig.1(b)). This indicates that Pt and Pd have moderate adsorption strengths, leading to good catalytic activity.

The strength of the bond between catalysts and reactants strongly correlates with the surface electron structures of the catalysts [27]. For instance, Chang and colleagues [28] found a compelling association between catalytic reactions and binary compositions. It is observed that the Pt alloy (PtAu) nanowires (NWs) catalytic activity reaches its maximum value when the proportion of Pt to Au is approximately 3:1. Notably, Pt alloy NWs with such a ratio show a significant contraction in interatomic bonding distances (Fig.2(a) and Fig.2(b)). As atoms become more closely arranged, the bond lengths between neighboring atoms reduce. Consequently, the decrease in the distance between atoms increases the overlap between electron clouds, facilitating electron movement between atoms and thereby reducing the chance of electron polarization in the presence of an electric field. This results in a relatively low dielectric constant. Furthermore, the decreased atomic distances lead to amplified Fermi contacts among atoms, greater overlap of atomic orbitals, and the creation of broader energy bands. As the interatomic interaction force increases, the center of the d-band gradually moves to lower energies. According to the d-band center theory, the position of the d-band center of the atoms on the surface of the base metal affects the strength of the adsorption of *O. When the d-band center on a metal surface is low, electrons enter anti-bonding orbitals, lowering the anti-bonding orbitals of the metal surface atoms and weakening the interactions between the metal surface atoms and other molecules or atoms. A lower d-band center leads to a greater number of unoccupied d orbitals on the surface, which increases the susceptibility of the metal surface to oxidation. Consequently, the strength of its adsorption diminishes. Conversely, an upward displacement of the d-band center pushes up the anti-bonding orbitals. During this time, electrons reflux, and fewer electrons are in anti-bonding orbitals. This increases the interactions between atoms on the metal surface and other molecules or atoms, increasing the strength of adsorption at the surface site [29]. This correlates well with the above findings.

The ligand effect and strain effects have an impact on the overall configuration and electronic structure of atoms. Although various methods are available to regulate the configuration and electronic structure of catalyst atoms, enhancing catalytic performance remains a challenge due to the intricate design of the catalyst composition and surface structure. For instance, Luo and colleagues [31] synthesized a new Pt–Ni alloy catalyst in the shape of a truncated octahedron through thermal annealing. The mass activity (MA) is 3.4 times larger compared to the unannealed sample, while the alloys remain well stabilized even after the accelerated degradation test. Adding other metal or non-metal molecules, such as a specific amount of Co, Fe, and other elements, can alter the electronic structure of Pt atoms. Simultaneously, introducing various non-metallic molecules during the surface modification process can adjust the charge state and electron structure [32]. The modulation of its electronic structure can be achieved through size and morphology modulation. For instance, Pt NWs have a higher catalytic activity than Pt nanoparticles [33]. This work will focus on the effect of the morphology of ORR catalysts on catalyst performance.

In addition, the surface of the crystal plays an important role in influencing the ORR activity. For example, the ORR activity of thin films of Pt single crystals is strongly related to the exposed crystal surface, with the order of surface reactivity from high to low being: high index facets (HIFs) > (111) > (100) [34]. Some specifically shaped nanocrystals based on Pt exhibit high-index crystalline surfaces that lead to elevated catalytic activity. For instance, Kuroki and colleagues [30] published a study on a Pt–Fe nanoparticle catalyst that was devoid of carbon and features a bead-like network structure formed by the connection of Pt–Fe nanoparticles (Fig.2(c)). They revealed that the ORR activity was positively influenced by the (211) and (311) high-index crystalline surfaces on the connected Pt–Fe network (Fig.2(d)), and the specific activity (SA) was 9 times greater than that of commercial Pt/C catalysts. Nevertheless, the greater surface energy poses an obstacle to the formation of the HIF structure [35]. Moreover, it is a challenge to improve the structural stability of the HIF during the ORR process.

3 Morphology control of catalysts

The catalytic process of ORR primarily occurs at the interfacial area between the catalyst and reaction medium, making size and shape critical elements in achieving exceptional catalytic activity. At elevated potentials, metallic atoms on the catalyst surface may become detached, resulting in a reduced catalytic efficiency. For instance, oxidation of the surface Pt atoms occurs at potentials exceeding 0.8 V, resulting in the corrosion and dissolution of the catalysts. This ultimately harms the efficacy and durability of the Pt system [36]. Additionally, the surface poisoning of Pt caused by the radical action in the ORR system also leads to the degradation of Pt. One effective solution is to optimize the crystal structure and increase the particle size of Pt particles [37]. Unfortunately, a larger particle size exposes fewer Pt surface active sites, leading to a decline in electrochemically active surface area (ECSA). Therefore, intricate Pt nanostructures are often applied to augment the number of active sites while at the same time reducing the decomposition or aggregation of Pt [38]. To optimize the Pt-based catalysts while minimizing the amount of Pt required, nanocatalyst morphologies, including 1 dimentional (D) NWs, 2D nanosheets (NSs), and 3D cubes or octahedrons, have been explored [16,20]. In this context, optimization and preparation strategies are summarized herein for different morphologies.

3.1 Nanorods, nanowires, and nanotubes

1D nanomaterials like nanorods (NRs), NWs, and nanotubes (NTs) generally have an anisotropic quality and possess a unique linear structure, which grants them exceptional physicochemical properties. The 1D structure presents a morphology of small diameter and long length. Therefore, NRs, NWs, and NTs exhibit an increased specific surface area compared to other nanoparticle morphologies, which will expose more catalytically active sites. Besides, the morphology of 1D nanomaterials enhances the free flow of charge in the longitudinal direction, thus significantly increasing their charge-carrying capacity. The structure of the linear shape is more firmly adsorbed on the carrier, which guarantees the structural stability of 1D nanomaterials [39]. In addition, the versatility of NRs, NWs, and NTs in terms of synthesis methods means that parameters such as diameter, wall thickness, and length can be precisely controlled when the growth of metal particles is performed on an atomic scale. Therefore, 1D nanomaterials typically offer high specific surface areas, charge-carrying capacities, structural stabilities, and atomically precise growth [16]. These characteristics make NRs, NWs, and NTs highly promising for a broad spectrum of electrocatalytic applications. The growth directions of NRs, NWs, and NTs are affected by many factors, such as catalyst morphology, and catalyst surface composition, and their growth directions are not easy to be controlled accurately. Therefore, templates for the control of metal particle growth are usually required for the preparation of 1D nanocatalysts. Lv et al. [40] synthesized Pt–Co NWs supported by carbon (Pt–Co NWs/C) through the use of cetyltrimethylammonium bromide (CTAB) as a soft template. During the ORR, the Pt–Co NWs/C demonstrated an MA of 0.125 A/mg_Pt and a 14.7% ECSA loss after accelerated durability test (ADT). These findings indicate that Pt–Co NWs/C possess an exceptional performance. Furthermore, Yang et al. [41] reported a Pt–Co NW network (NWN) using a similar surfactant-assisted soft template method (Fig.3(a) and Fig.3(b)). Accordingly, the electrocatalytic effect of the Pt–Co NWN catalysts is significantly improved.

The NR, NW, and NT can be synthesized using micellar or microemulsion techniques to achieve mesoporous structures through electrolysis. Alternatively, compact structures can be obtained using template-free, soft, and hard template methods via electrodeposition [42]. For example, using Te NWs and Pluronic F-127 templates, Fu et al. [43] created 1D mesoporous hierarchical nanostructures of PtCu (ODHMNs, Fig.3(c)) featuring large mesopores. The utilization of Te NWs as a hard template ensured the formation of 1D nanostructures of PtCu alloys, while F-127, as a soft template facilitated the creation of PtCu hierarchical porous nanostructures (Fig.3(d)). The mesopores on the surface, the hollow internal structure, and the 1D morphology enable the PtCu ODHMNs to exhibit an enhanced electrocatalytic performance, with an MA of 0.31 A/mg_Pt at 0.85 V. While previous research has demonstrated that grain boundary (GB) sites located on the catalyst surface can notably alleviate the kinetic barrier to ORR, their introduction may also result in adverse effects on the morphology and composition of the catalyst surface [44]. To investigate the role of GB on the defective sites on the catalyst surface, Kabiraz et al. [45] synthesized GB-rich PtCo NWs (Pt–Co GB-NWs, Fig.3(e)) under mechanical stirring and obtained single-crystalline PtCo NWs (Pt–Co SC-NWs, Fig.3(f)) via a magnetic field reactor. Due to the presence of surface GB, the Pt–Co GB-NWs leached more elemental Co during the ORR process, which might lead to strong *OH binding, thus reducing the ORR activity [46]. The presence of GB disrupts the surface morphology of the catalyst, implying that improving the orderliness of the crystal structure is a viable means of optimizing the activity and durability of the catalyst.

Recently, significant research has been conducted on the preparation processes and properties of various NRs, NWs, and NTs. For example, Fidiani and coworkers [47] reported a process for fabricating gas diffusion electrodes (GDEs) with arrays of Pt-based alloy NRs. They obtained Au-doped Pt–Ag NRs in a single step by wet chemical reduction (Fig.4(a)–4(c)). The results of the density functional theory (DFT) calculations indicate that the surface O binding energy on Au-doped PtAg (−5.46 eV) is lower compared to PtAg (−5.63 eV) and pure Pt (−5.53 eV). Thus, it can be inferred that Au plays a crucial role in effectively reducing O binding energy to the Pt (111) surface for better ORR kinetics. Meanwhile, Au doping can reduce the MA loss of the PtAg NR array GDE to some extent, ensuring the durability of the electrode (Fig.4(d) and 4(e)). In addition, this in situ-grown NR array GDE on carbon is directly applicable as fuel cell cathodes, simplifying the preparation. Additionally, Yao et al. [48] reported an unusual impact of Pt NW size on ORR. They found that as the diameter decreased from 2.4 to 1.1 nm, the ORR activity and durability grew steadily. This was attributed to the fact that as the diameter decreased, the compressive strain and the proportion of low coordination sites simultaneously increased.

It is noteworthy that electron transfer has the potential to occur between two catalyst particles while undertaking ORR, ascribed to the unrestricted movement of Pt²⁺ in the electrolyte and the capacity of the conductive C support to transport electrons. Unfortunately, this transfer of electrons disturbs the quasi-equilibrium state of the polarized electric field that arises between small negatively charged particles and large positively charged particles [49]. As a result, particles of different sizes aggregate, i.e., “electrochemical Ostwald ripening” [50]. To avoid this phenomenon, Cao et al. [51] reported a TiO₂/CNT composite support and then used the glycol reduction method to prepare PtCo/TiO₂/CNT catalysts. Owing to the porous architecture provided by the TiO₂ film, the Pt–Co nanoparticles can be stably adsorbed on the support while the TiO₂ film is encapsulated on the CNTs (Fig.4(f) and Fig.4(g)). The TiO₂ film not only reduces the shedding of the PtCo nanoparticles during the catalytic process but also plays a protective role for the CNTs. As a result, the stability of the PtCo/TiO₂/CNT nanocatalysts is significantly improved. To reduce the dissolution of non-precious metal elements in perchloric acid, Gao et al. [52] reported PtGa alloy NWs that exhibited strong p-d hybridization interactions. The durability of the catalysts improved (only a 15.8% reduction in MA after 30000 durability tests) due to the strong p-d hybridization interactions between Ga and Pt. This led to an improvement in the electronic structure of the catalyst surface, increased the oxidation resistance of Pt, and reduced the leaching of Ga. This suggests that the hybridization of Pt with p-block metal promotes the endurance of ORR catalysts.

Furthermore, generating HIFs of Pt on Pt-based alloy NWs is an efficient approach to boost ORR performance. For example, Cao et al. [53] prepared an Au-doped PtCo NWs with a rugged surface by combining electrochemical substitution with the solvothermal method. Due to the large number of HIFs formed on the surface of Au-doped PtCo NWs, such as (211) and (311), in combination with the NW structure and surface doping with inert Au atoms, the ORR activity and durability of PtCo NWs were significantly enhanced (the MA of 1.94 A/mg_Pt, only 21% loss of MA after 20000 potential cycles). Tetteh and colleagues [54] reported a PtCo@Pt core-shell NW with ordered intermetallic PtCo cobalt cores and compression-strained HIFs (221) in Pt shells. This structure demonstrated an exceptional ORR activity with an MA of up to 1.3 A/mg_Pt. Based on DFT calculations, the face-centered cubic (fcc) PtCo crystals exhibited a −4.2% strain compression in comparison to the fcc Pt crystals, resulting in a decrease in adsorption energy. Moreover, the binding on the (111) surface is significantly weakened due to the presence of Co, which is detrimental to the ORR process. However, the stronger adsorption on the fcc vacancy on the (221) surface positively affects the weak binding of *O and thus shows an excellent ORR activity.

3.2 Nanosheets

NSs possess a distinctive lamellar structure, resulting in exceptional physicochemical characteristics, including increased anisotropy, larger specific surface area, and superior mechanical properties [55]. The benefits of NS morphology for electrochemical catalysis encompass three primary aspects, a specific surface area, a distinctive structure, and robust electrical conductivity [56]. The electrocatalytic reaction only occurs at the surface. Therefore, NS-structured materials with an elevated surface area/volume ratio expose additional active sites, thereby increasing the rate of electron transfer to improve the catalytic efficiency of ORR. Secondly, NS structures typically possess a substantial specific surface area and widely dispersed active sites, which results in challenges for hazardous substances to impact every active site across the entire surface area. This even dissemination of active sites facilitates the active retention of a part of the catalyst surface, thus improving the durability of the catalyst.

Additionally, NSs exhibit an adjustable surface electronic structure, which aids in elevating the electrical conductivity and catalytic activity [57]. Chen and colleagues [58] synthesized Pd@PtNi nanostructures via a hydrothermal approach, employing Pd nanostructures for core and Pt–Ni atoms for surface deposition, resulting in NSs with a Pd/PtNi core-shell structure. The higher ORR catalytic activity of Pd@PtNi NSs with an MA of 1.038 A/mg_Pt+Pd can be attributed to the fact that more active sites of surface atoms are exposed in the NS structure. The distinctive configuration of the sheet regulates the lattice compressive/tensile strains of the surface atoms, thereby enhancing the inherent activity of the corresponding active sites [59]. After the inclusion of Pd, the strain and electronic ligand effects facilitate the interaction between the in situ intermetal complex, serving as the core and the Pt shell in the fcc phase, thereby enhancing the ORR activity and prolonging the lifetime of the catalyst [60].

Besides, the orientation of surface atoms also has a considerable effect on the ORR performance of the catalysts owing to the strong anisotropy of the NSs. This property renders the charge transfer rate and the electrocatalytic efficiency mainly subject to the crystallographic orientation [57]. For example, Song et al. [61] reported a facile method for preparing Pt monolayer (Pt_ML) catalysts on PdNS/tungsten (W)–Ni/C 2D layer substrates (Fig.5(a)). It was shown that the –OH detachment peak of Pt_ML/PdNS/WNi/C was shifted to a higher potential by about 75 mV compared to that of commercial Pt/C, suggesting that oxygen-containing substances were more easily detached on Pt_ML/PdNS/WNi/C and had a higher ORR catalytic activity. The research showed that the (111) orientation on the surface of PdNSs led to heightened electrocatalytic properties [60,62]. Since the (111) crystal plane has the highest density of Pt atoms, the orbitals of the atoms are close together, leading to mutual influence and an overlap of energy levels. This overlap increases the coupling between the respective energy levels. Thus, the splitting of the electronic energy levels is increased, broadening the energy bands. As the energy levels of orbitals with different angular momenta shift, the d-band center decreases, regulating the adsorption strength of Pd and leading to an improved catalytic activity. Furthermore, the concave edges of PdNS lead to additional shape contraction, which boosts the stability of the Pt_ML/PdNS/WNi/C structure [60].

Ultrathin NS frequently substitutes noble metals with transition metals that have excellent coordination with impurities and exhibit robust interaction with them. Consequently, this substitution leads to high activity rates combined with a low ΔE [65]. For this purpose, Chen and colleagues [63] produced an ultrathin Pt–Fe nanostructure containing 6.7 wt.% of closely dispersed Pt atoms via a surfactant-free solvothermal method (Fig.5(b) and Fig.5(c)). Due to the benefits of both single-atom catalysts and ultra-thin 2D nanostructures, which reveal additional active atomic sites and facilitate optimization of the electron structure, highly dispersed single-atom catalysts are fabricated over high specific surface area substrates. This allows for adjacent atoms to be contacted at the same time, improving the performance of catalysts (MA of FePt NSs is 0.566 A/mg_Pt at 0.9 V and remains at 95.38% after CO tolerance experiments, Fig.5(d)).

In oxygen reduction catalysts, the incorporation of heteroatoms can significantly impact the electronic structure, surface active sites, and catalytic activity of the catalyst. Heteroatoms typically possess distinct electronegativities and incorporating them as dopants can modify the electronic structure of catalysts, resulting in a superior ORR catalytic activity [66]. For instance, doping with N atoms can lead to local electron enrichment or sparing, affecting the adsorption and dissociation of reaction intermediates, which in turn optimizes the kinetics of ORR and improves the efficiency of the reaction. Tran and colleagues [64] developed a Pt-based catalyst (CoN–Pt/CoNC-2D), as shown in Fig.5(e)–Fig.5(g) which was loaded onto porous carbon doped with a Co–N NS structure via a solvothermal method. This was achieved by using nanosheeted Co-based metal-organic frameworks as templates for Pt deposition [64]. The ORR catalytic efficiency of CoN–Pt/CoNC-2D catalysts is enhanced significantly due to the synergistic interaction between CoN–Pt and CoN-2D, and the improved reactant transport efficiency resulting from the structured CoN-2D carriers in the nanospheres (MA of the CoN–Pt/CoNC-2D is 2.14 A/mg_Pt at 0.8 V). This research provides new avenues for the exploration of novel Pt-based catalysts and non-platinum group metal (non-PGM) support complexes.

The coordination number of the catalyst is a potential characterization of the electronic state of the environment and the structure energy relationship [67]. The unsaturated coordination of the active site will promote the adsorption of oxygen species on the catalyst, thus affecting the catalytic performance. The construction of twin boundaries and high index surfaces can effectively reduce the coordination number and regulate the adsorption strength, which has a favorable effect on the ORR catalytic activity [63,67]. Wei et al. [68] used a one-pot solvothermal technique to fabricate a flower-shaped nanocatalyst composed of Co and Pt, with a high concentration of Pt. The researchers optimized the performance of the NSs by tuning the properties of the high-index facets and defects present (Fig.6(a)–Fig.6(c)). In this case, the lower coordination number due to defective sites, high index surfaces, and twin boundaries in the catalyst reduces the adsorption of oxygen species. Consequently, Pt-rich PtCo nanoflowers (NFs) showed an outstanding mass-specific ORR activity, with values of 2.63 A/mg_Pt and 11.23 mA/cm² (Fig.6(d)). Furthermore, the structure rich in Pt enhances the resistance of Co to corrosion, which improves the durability of Pt-rich PtCo NFs (Fig.6(e)). DFT calculations showed that for active sites with low coordination numbers, the d-band centers are located higher, which is favorable for the adsorption efficiency of the intermediates. It is concluded that the catalytic behavior can be predicted based on the coordination number, which further provides an efficient design scheme for atomically controllable and defect-enriched ORR electrocatalysts. Additionally, Liu and colleagues [69] fabricated Pd@PtNi core-shell NFs (CSNFs) which were supported by multi-walled carbon nanotubes (MWCNTs) using a one-pot wet chemical method (Fig.6(f)–Fig.6(i)). The catalyst combines both the core-shell and porous morphology of the NF, which exhibits a reduced density and a permeable structure that facilitates the transport of reactive oxygen species and thus exhibits an outstanding ORR catalytic activity. The mass-specified kinetic current densities (j_mass) of Pd₂@PtNi/MWCNTs CSNFs is 0.0733 A/mg_PGM and the E_1/2 of the catalyst migrates 44 mV after the ADT experiment.

3.3 Nanopolyhedra

Nanopolyhedra are nanomaterials with non-spherical geometries composed of different crystalline surfaces, such as cubes, octahedral, and dodecahedra, whose crystal structures are usually symmetric and regular, yet the size and shape of each crystalline surface can be different [70–72]. There exist numerous methods for the preparation of Pt-based polyhedral nanoparticles. One approach involves utilizing a reducing agent to reduce metal ions to their corresponding metal nanocrystals, with adjustments to reaction conditions and surfactant additions resulting in different nanopolyhedra morphologies. Second, metal ions can also be reductively deposited on the surface of the template, which can be silica gel or polystyrene microspheres, etc., by a reduction deposition process, where changes in the shape and size of the template result in different nanopolyhedral morphologies. Alternatively, the morphology of the nanopolyhedra can be controlled by depositing the metal precursor on the substrate and controlling the direction of crystal growth using ligands and the like. It should be noted that the arrangement of atoms on the crystal surface can impact the adsorption of oxygen molecules or other reactive substances on the surface. Furthermore, the electronic structure of a crystalline surface can affect the mechanisms of electron transport and reactions, potentially resulting in a more efficient electron transport due to a more favorable electronic energy level structure. Some crystalline surfaces can unveil additional active sites, creating more chances for reactions. For example, Pt(111) and PtNi(111) crystal faces are notable instances of this [73,74]. In the following sections, focus will be laid on some common polyhedral structures of nanocatalysts, such as nanocubes, nanoctahedrons, nanodecahedrons, and icosahedrons.

3.3.1 Nanocubes

Catalysts with cubic structures tend to possess a high surface area, and their surfaces can be rich in lattice defects, twin interfaces, etc., offering multiple active sites and thus increasing their catalytic efficiency. The cubic structure has a large equilibrium surface/volume ratio, which reduces the influence of surface energy, providing a good stability. Furthermore, the generation of local strains in the lattice could impact the electronic structure of the catalyst surface. This fine-tuning of the electronic structure might improve the electronic state of the active site, resulting in the formation of stable intermediates and ultimately leading to a favorable 4-electron reaction pathway.

Practically, the application of catalysts for cubic structures tends to couple with other structures, which in turn leads to a better electrocatalytic performance. Wang et al. [75] deposited high-density Pt octahedra on cubic Pd crystals using glucose as a reducing agent (Fig.7(a)–Fig.7(h)). As the glucose concentration increases, the deposition increases, but the size of the loaded Pt octahedral islands decreases (Fig.7(i)). The Pd NC catalysts loaded with Pt octahedral shells possessed an enhanced catalytic performance due to the excellent behavior of the (111) crystalline face of Pt for ORR [76]. The catalysts yielded an MA of 0.164 A/mg_Pt, quadrupling the value of the Pd@Pt NC catalysts with the exposed (100) faces. In addition, to further increase the exposure of Pt atoms, a thin layer of Pt is often deposited on nanoscale substrates [77,78], although the Pt layer is subject to dissolution during the catalytic process. To improve the activity and lifetime of Pt-based core-shell catalysts, He et al. [79] used amorphous Pd phosphide (a-Pd-P) as a substrate and deposited a sub-monolayer of Pt on Pd NCs to prepare Pd@a-Pd-P@Pt core-shell catalysts (Fig.7(j)–Fig.7(m)). High-angle annular dark-field STEM (HAADF-STEM) images reveal the amorphous crystal structure and confirm that the obtained Pt(100) shells have a thin atomic layer thickness. DFT calculations were used to predict the diffusion energies of Pd and Pt atoms in catalysts, indicating their resistance to solvation with elements. As the diffusion energy increases, the catalyst stability increases. DFT analyses indicate that the Pd atoms in a-Pd-P possess an equivalent diffusion resistance (~7.62 eV), which is considerably greater than that in the crystalline Pd nucleus (5.23 eV). Furthermore, the diffusion resistance of Pt on Pd@a-Pd-P@Pt (~7.47 eV) is substantially higher than that on Pd@Pt (~5.52 eV). It is suggested that the robust bond between Pt and Pd hinders the movement of Pt atoms on the a-Pd-P facade, leading to the exceptional durability of Pd@a-Pd-P@Pt (Fig.7(n)). This research extends the utilization of amorphous materials in electrocatalyst design and contributes to understanding the morphology of catalysts.

3.3.2 Nanoctahedrons

It can be directly deduced from the nature of nanocubes that the surfaces of octahedral catalysts have similarly high specific surface areas and are enriched with features such as lattice defects and twinned interfaces, resulting in a greater number of active sites. In addition, octahedra possess more crystalline surfaces, which may have structurally different crystal faces, lattice orientations, and atomic arrangements. These different surfaces may have different surface energies, active sites, and surface reaction activities, and thus may enhance the selectivity of the catalysts for ORR. Uniform particle size, consistent morphology, exemplary catalytic activity, and selectivity make the octahedral catalyst a morphologically controllable and highly efficient catalyst. Stamenkovic and colleagues [80] reported on the high activity of a single-crystal surface of Pt₃Ni(111), which showed an ORR activity 10 times higher than that of the Pt(111) surface. Since octahedral PtNi nanoparticles show only the active (111) surface, this prompted the researchers to explore Pt-alloyed nanoparticles with a selective octahedral morphology. For example, Cui et al. [81] reported PtNi nan-octahedrons obtained by solvothermal synthesis, and they found that the shape of the catalyst could be controlled by the choice of precursor ligands. Owing to the exposure of the PtNi nanoparticles (111) facets and the octahedral morphology, the PtNi octahedron exhibits an excellent ORR activity with an MA of up to 1.45 A/mg_Pt.

In addition, to achieve control of the atomic arrangement in both the bulk and on the surface, Xie et al. [82] loaded octahedral PtCo nanoparticles onto carbon supports, which were then treated by annealing and hydrothermal methods to obtain ultrathin Pt-skin-covered octahedral PtCo nanoparticles (fct-Pt–Co@Pt/C, Fig.8(a) and Fig.8(b)). The intermetallic Pt–Co core and thin Pt shell of fct-Pt-Co@Pt/C, with the (111) plane dominating on the surface (Fig.8(c)–Fig.8(f)), results in superior electrocatalytic activity (MA of 2.82 A/mg_Pt). However, the yield of catalysts by solvothermal methods is insufficient to satisfy commercial and industrial demand. To address the issue of poor catalyst yield, Niu et al. [83] reported a simple droplet reactor (Fig.8(g)). Pt(acac)₂, Ni(acac)₂, and W(CO)₆ were mixed and reduced in a droplet at 230 °C to yield PtNi octahedral alloy nanoparticles that are highly active (Fig.8(h)) and have an 11-fold higher MA than the Pt/C catalysts (2.67 A/mg_Pt). The W(CO)₆ undergoes decomposition at 170–230 °C to produce CO gas, which separates the precursor solution into separate droplets. Additionally, CO acts as a reducing agent and promotes the development of (111) facets on the nanocrystals [84]. Moreover, the (111) facet of fcc Pt–Co alloy exhibits a considerable ORR efficacy [85]. However, the dissolution of Co at the operating voltage and acidic conditions considerably shrinks the lifespan of the catalyst. To enhance the stability of the alloy, Zhu and colleagues [86] described a method for in situ synthesis of Pt₃Co octahedral nanoparticles doped with Ru on carbon. This method avoids surfactants and enables the synthesis of large quantities, up to 0.68 g per single batch. In addition, according to the DFT calculations, both Pt₃Co/C and Ru–Pt₃Co/C catalysts exhibited lower onset potentials in the binding step of the ORR process, indicating a preference for the dissociation step. Furthermore, Pt₃Co/C exhibited a significantly lower onset potential (0.51 V) compared to Ru–Pt₃Co/C (0.66 V). This suggests that the inclusion of Ru atoms can effectively regulate the desorption properties of oxygen intermediates, thus improving the ORR characteristics up to MA of 1.05 A/mg_Pt.

Furthermore, a simpler strategy is directly reducing precursor powders in a reducing gas atmosphere. This solid-phase reduction method not only allows for higher catalyst yields but also eliminates the potential hazard of organic solvent contamination of the catalyst. Zhang et al. [84] impregnated Pt(acac)₂ and Ni(acac)₂ onto carbon support and then reduced them directly in a CO/H₂ atmosphere to obtain octahedral Pt–Ni/C in a single step. As CO molecules are primarily adsorbed on the (100) plane of Pt and the (111) plane of Ni, the growth of the (111) plane of PtNi becomes severely restricted and becomes the main exposed crystalline plane, which facilitates the growth of PtNi nanoparticles into an octahedral morphology. H₂ can function as both a reducing agent for the metal precursors and a means of transport. The octahedral Pt_1.5Ni/C is surrounded by (111) planes and thus exhibits an excellent ORR activity, with a j_mass of up to 1.96 A/mg_Pt at 0.9 V. The present work employs the solid-phase synthesis method, not only simplifying the synthesis route of the catalyst but also realizing the mass production of Pt–Ni/C catalyst, which is a reliable reference for exploring the process of large-scale production of Pt-based nanocatalysts.

3.3.3 Nanododecahedron and nanoicosahedron

Nanododecahedron and nanoicosahedron catalysts share similar polyhedral structures and physicochemical properties. In fact, for icosahedral and dodecahedral nanocatalysts, strategies to further optimize the catalyst structure, including hollow and core-shell structures, are usually employed to improve catalytic activity. Recently, Liao and colleagues [87] synthesized dodecahedral nanoparticles composed of Fe N-coordinated hollow nanoparticles (Fe-NC) incorporated into carbon via an Fe-doped zeolite-type imidazoline framework (ZIF-8) precursor. Then Pt deposition was performed by impregnation reduction temperature to obtain Pt@Fe-NC catalysts. Exposure of Pt to the (111) crystalline surface in Pt@Fe-NC, along with multiple active sites on the catalyst surface, enhances its activity. Additionally, the hollow porous structure allows for effective mass exchange, resulting in a 1.34 A/mg_Pt increase in the activity of Pt@Fe-NC. Zhu et al. [88] prepared Pd@Pt–Ir core-shell nanocatalysts via atom-by-atom deposition with Pt atoms and Ir atoms on various shapes of Pd seeds (e.g., cubic, octahedral, or icosahedral). Twinned Pt–Ir(111) faces enclose the icosahedral Pd@Pt–Ir structure, and the surface tensile strain that would result from the twinned boundaries favors the addition of Pt atoms to the outer shells of the Pd@Pt icosahedron, resulting in the Pt shells being subjected to compressive strain [89]. This strain allows the Pd@Pt–Ir icosahedron to demonstrate an excellent activity, exhibiting an MA of 1.88 A/mg_Pt. In addition, Zhu et al. [90] synthesized icosahedral Pt₃M NCs fully bonded by (111) faces by a one-pot hydrothermal technique using glucosamine as a reducing agent and structure-directing agent (Fig.9(a)). Furthermore, the redox potential of the Pt₃Ni/C catalyst was positively shifted to a significant degree as a result of highly active Pt(111) crystalline facets exposure. This suggests a weakening of reactive oxygen species adsorption, which lead to an improvement in the ORR catalytic activity, with an MA of 1.761 A/mg_Pt (Fig.9(b)). Notably, Fourier transform infrared (FTIR) spectroscopic measurements show a more pronounced redshift of the N–H peak and a stronger coordination effect of Pt⁴⁺ with glucosamine relative to Ni²⁺ (Fig.9(c) and Fig.9(d)). Therefore, it can be inferred that glucosamine has an inhibitory effect on the reduction of Pt⁴⁺, which in turn favors the formation of the PtNi core/Pt shell structure.

Considering the correlation between the surface strain of Pt and catalytic activity, Ahn and colleagues [91] devised a method to create rhombic dodecahedron Pd@Pt core-shell crystals bonded by (110) facets by adjusting the number of Pt atomic layers (Fig.9(e)–9(g)). Among the low-index facets of fcc metals, the (110) facet consists of surface atoms and possesses a more open structure than the (111) and (100) facets. Therefore, the rhombic dodecahedral Pd@Pt core-shell catalysts attached to the (110) facets exhibit an enhanced ORR performance. Notably, increasing the number of available Pt atomic layers increases both the SA and MA. However, when the Pt content exceeds a certain level, a decrease in the MA occurs (Fig.9(h)). Furthermore, the DFT results reveal that the relationship between strain and activity of the Pt(110) surface results from the in-plane strain response to the applied out-of-plane strain. Hence, to realize a promising Pt-based catalyst, the effects of both in- and out-of-plane surface strain exerted upon the d-band centers should be considered.

The selection of nanoparticle morphologies for electrochemical catalysts is dependent on both the application requirements and preparation capabilities. Consequently, the choice of nanoparticle morphology for a specific electrochemical reaction requires meticulous consideration. The distinct surface areas, crystal structures, and electrochemical activities of nanoparticles having different morphologies may result in differing catalytic properties during electrochemical reactions. 1D nanocatalysts like NWs and NTs have high aspect ratios that increase electron and proton transportation rates. However, their preparation involves complicated procedures and requires precise control of the orientation of square catalysts. On the other hand, NSs offer a high specific surface area that improves the conductivity of catalysts and exposes more active sites, but their mechanical properties are inferior to nanopolyhedra. By comparison, cubic crystals demonstrate a consistent structural pattern and are easily produced, making them beneficial for applications involving catalytic materials. However, their small specific surface area poses a limitation for certain high-surface-area catalytic reactions. Polyhedral nanocatalysts such as octahedra, dodecahedra, and icosahedra demonstrate a high symmetry and uniform crystal structures, aiding the exposure of highly active crystalline surfaces to Pt-based catalysts. However, the elaborate synthesis conditions required for these unique structures can result in higher costs and complexity in the preparation of catalysts. Therefore, various factors must be taken into account including the complexity of preparation, performance requirements, and costs when choosing the most appropriate type of catalyst for practical uses.

3.4 Core-shell structure

The integration of non-noble metals into noble metal catalysts is a prevalent approach to regulating the electronic structure of such catalysts [92]. Unfortunately, non-noble metals are unstable and tend to leach during ORR. One approach is to apply a coating of noble metal to the non-noble metal to form a core-shell structure, which regulates not only the electron structure of the thin shell of noble metal on the surface but also protects the core of the non-noble metal inside [93]. The core-shell structure possesses unique physicochemical properties as a layered configuration with sturdy internal chemical bonds and feeble interlayer coupling owing to electron confinement effects [94]. From a strain effect perspective, the growth of Pt shells on the surface resulted in a reduction of the Pt–Pt interatomic distance, thereby controlling the chemisorption strength of ORR intermediates. Considering the ligand effect, the donor acceptance behaviors of neighboring atoms have an immediate bearing on the electron structure of the active site, leading to the displacement of the Pt d-band center and thus a tuning of the adsorption strength.

Furthermore, the core-shell structure houses different active sites. Liu et al. [95] synthesized a catalyst that had a Co₂P/Pt core-shell structure demonstrating exceptional ORR activity and stability (Fig.10(a)). Co₂P/Pt employs Co₂P as its core and Pt as its shell layer. Co₂P is a phosphide metal featuring a lattice constant similar to that of Pt, allowing for the uniform loading of the Pt shell on the surface. This arrangement reveals additional active sites, enhancing the performance of the catalysts (960 mA/mg_Pt for MA and 2.3 mA/cm² for SA). The impact of strain effects on the Pt(111) surface was investigated through DFT simulations that explored the variation of ΔE_O values under alternating transverse strains of compression and tension. The results indicate that the Co₂P(100)/Pt(111) interface is primarily impacted by strain. In contrast, the ligand effect is more prominently observed at the interfaces Co₂P(010)/Pt(111) and Co₂P(001)/Pt(111) (Fig.10(b)). The ligand effect impacts the binding energy of surface Pt-O. However, excessively strong binding energy hinders the desorption of reactive oxygen species, thereby affecting ORR detrimentally. The Co₂P(001)/Pt(111) interface theoretically presents the optimal configuration for Co₂P/Pt owing to the appropriate strain effect and ligand effect. Among these, the compressive strain of Pt results in a negative shift of the ΔE_O value, which comes closer to the optimal ΔE_O value (0 eV), thereby demonstrating a superb catalytic activity. Recently, Weber et al. [93] explored the influence of the chemical state of Co species within the cores of Pt-rich-shelled core-shell nanoparticles on ORR durability. The chemical state of Co atoms inside the Pt_xCo_1−x and their stability during the ORR process and after various accelerated stress tests (ASTs) were analyzed (Fig.10(c)). Notably, the precursor catalyst produces cobalt oxides during electrochemical dealloying activation. The stability of particle cores composed of metal and oxidized Co species is considerably higher than that of other pure metal Co cores documented in the literature [96]. It is evident from the X-ray photoelectron spectroscopy (XPS) data that the cobalt oxide compound is considerably stable (Fig.10(d)). This suggests that cobalt oxide species positively impacted the ORR process, thereby improving the durability of the Pt_xCo_1−x core-shell catalyst.

The core-shell structure frequently synergizes with other structures to advance the performance of the catalysts. For instance, according to a study by Bharadwaj et al. [97], incorporating the core-shell effect into Cu@Pt NR resulted in an increased electrocatalytic activity. They aimed to investigate the source of ORR activity by modeling 1D Pt NRs (Fig.10(e)) and concluded that Cu@Pt core-shell catalysts displayed the most consistent performance. According to the effects of strain and charge transport, the incorporation of a core-shell structure into Cu@Pt NR can regulate the position of the d-band center of the Pt shell, resulting in an improved ORR activity. Chen and colleagues [58] fabricated Pd@PtNi NSs core-shell catalysts utilizing Pd as the core and PtNi NSs as the shell with varied thicknesses via hydrothermal synthesis. By utilizing ultra-thin layers and a core-shell structure, the Pd cores can be fully covered, and surface alloying can be achieved through surface atomic diffusion. This approach results in outstanding catalytic performance for Pd@PtNi NSs, with appropriate Pt shell thicknesses exhibiting an MA of 1.038 A/mg_Pt+Pd. To maximize the efficiency of Pt utilization, methods for the preparation of core-shell nanoparticles were extensively investigated, such as underpotential deposition [98], and chemical reduction [99]. However, the mass production of core-shell nanoparticles was severely limited by the reliance on substrates during their synthesis. The continuously prepared process offers a viable approach to increasing the yield of core-shell nanoparticles [100]. For instance, Hashiguchi et al. [101] developed structurally controllable Pd@Pt core-shell nanoparticles with a high ORR activity using a flow process. It is noteworthy that they use diethylene glycol as a capping agent, which not only achieves a uniform distribution of Pd@Pt core-shell nanoparticles over the activated carbon but also ensures the activity of the catalysts to a certain extent. This continuous flow synthesis method, using a straightforward and inexpensive process, holds enormous importance for the manufacturing of Pt-based core-shell catalysts in industries.

3.5 Hollow and porous structure

The surface adsorption strength of nanocatalysts can be further regulated by controlling the tiny crystalline surface of nanoparticles [102]. Unlike lattice mismatches in core-shell structures, which create tensile or compressive stresses on surface atoms, the modulation of the tiny crystalline surfaces of nanoparticles by hollow and porous structures results in different catalytic activities and selectivities due to the different microfacets with different surface energies [103,104]. Furthermore, it should be noted that the catalytic process is both random and inhomogeneous on the catalyst surface. However, the hollow and porous structure of this catalyst greatly increases its total surface area, providing exposure to additional active sites and causing a significant increase in the activity of catalysts [105]. Hollow and porous nanostructures, such as nanoframes and nanocages were demonstrated to significantly boost the electrocatalytic performance of Pt-based catalysts. “Frames” refers to nanostructures consisting solely of borders without particle surfaces, while “cages” have small planar surfaces and multiple cavities on these surfaces compared to “frames.” Unlike “frames,” “cages” have nanoparticles with small planar surfaces and numerous cavities on these surfaces. This porous structure facilitates the entry of reactants into the interior of the nanoparticles, confining them to the nanoscale, increasing their collision frequency, and subsequently exposing additional active sites to accelerate the catalytic process [106]. In this regard, a CuPt nanocage (CuPt-NC, Fig.11(a)–Fig.11(c)) intermetallic structure was reported by Dhavale & Kurungot [107]. This unique nanocage structure has a large specific surface area and structural stability, thus improving the MA of CuPt-NC (0.32 A/mg_Pt). Specifically, the changes in the electronic structure and the geometric nature of the CuPt structure shorten the interatomic distance of the Pt atoms, thus causing a decrease in the adsorption energy. Moreover, the hollow structure exposes more active sites and facilitates the transfer of molecules within the structure further promoting the catalytic efficiency. Additionally, Eid et al. [108] reported the synthesis of a hollow PtPdRu dendritic nanocage with cavities and porous dendritic shells (Fig.11(d) and Fig.11(e)). This nanocage not only facilitated more active sites, but its consequent confinement effect favored the mass transfer of active intermediates. Synergistically, the electronic and strain impacts originated by the trimetallic components prompted superior catalytic activity [109–111].

Unfortunately, the porous structure of nanoparticles with numerous cavities increases susceptibility to agglomeration or detachment from their carrier, reducing the stability of the nanoparticle structure [103]. Ultimately, this leads to a reduction in the lifetime of the catalyst. In addressing the aforementioned issues, it has been determined that 1D nanostructures featuring numerous attachment points exhibit substantially reduced chances of agglomeration or detachment from the carrier in comparison to nanoparticles with a sole point of contact on the carrier. Additionally, their inherent anisotropy, heightened flexibility, and superior electrical conductivity enhance the stability of the catalysts [112]. For instance, Tian et al. [33]. synthesized 1D Pt–Ni beam nanospheres (BNSs) through a one-pot solvothermal method. Afterward, the Ni material was etched in acidic conditions to yield 1D beam Pt–Ni alloy nanocages (BNCs, Fig.11(f) and Fig.11(g)). DFT calculations show that the ΔE_O* values of the Pt₄Ni and Pt₃Ni skin are weaker than on Pt(111) by 0.17 and 0.11 eV, respectively, and thus exhibit admirable properties (MA of 3.52 A/mg_Pt). The Pt-O bond strength is strongly correlated with the 2.5% compressive strain in the Pt₄Ni and Pt₃Ni surface layers, which shifts the center of the d-band of the surface Pt atoms downwards, resulting in a weakening of ΔE_O*. Moreover, the PtNi-BNCs/C show a negligible loss in performance after 50000 cycles of long-term durability testing. This is attributed to the bundled structure that ensures the stability of the catalyst structure and to the Pt skin that prevents further leaching of Ni (Fig.11(h) and Fig.11(i)). van der Vliet et al. [105] obtained nanostructured thin films (NSTFs) of Pt alloy through physical vapor deposition. They then utilized magnetron sputtering to deposit Pt alloy NSTF catalysts on molecular solid whisker arrays, which resulted in the formation of mesoporous-structured thin film electrocatalysts. The sidewalls of the catalyst in the NSTF, along the whiskers, consist mainly of tightly bound whisker tips, creating densely stacked, rough, corn cob-like features on the surface. This maximizes the catalyst surface area for effective usage and prevents corrosive degradation between the carbon carriers and metal, resulting in improved catalyst stability.

The crystal structure, pore size, and specific surface area are crucial factors that favorably influence the catalytic performance and selectivity of octahedral catalysts. Chen and colleagues [113] synthesized compositionally tunable PtCu hollow octahedral nanoparticles (HONs) possessing a controllable near-surface structure. This was achieved by utilizing octahedral Cu-rich PtCu alloys in a hydrothermal approach. Due to the effect of near-surface alloying and the unique hollow structure, the PtCu/C HONs demonstrated a substantially higher MA (2.60 A/mg_Pt) and stability. In addition, Jiang et al. [114] reported the synthesis of porous PtNi octahedral nanoparticles using PtNi octahedral nanoparticles as precursors. The octahedral PtNi nanoparticles were subjected to chemical etching, leading to the development of a porous open structure, as illustrated in Fig.12(a) and Fig.12(b). The SA and MA of porous octahedral PtNi nanoparticles in the ORR witnessed a significant increase, respectively, with an SA at 2.45 mA/cm² and an MA at 914 mA/mg_Pt. Apparently, due to the larger specific surface area of the open-structured nanocatalysts, more active sites and alloying phases were exposed, resulting in an improved electrochemical activity. Not coincidentally, hollow porous dodecahedral frameworks exhibit a similar selectivity. Chen et al. [115] prepared Pt–Co rhombic dodecahedra and then etched the inner Co-rich phases with nitric acid, leaving the Pt-rich phases at the edges, which in turn resulted in the Pt–Co nanoframe (Fig.12(c)–Fig.12(f)). The hollow internal structure of the PtCo nanoframe causes a decrease in the binding energy of oxygenated species by further shifting the d-band center downward. Under 0.95 V_RHE conditions in an acidic electrolyte, its initial SA and MA are up to 0.80 mA/cm² and 0.40 A/mg_Pt. After 10000 ADT cycles (Fig.12 (g)), these values decreased to 0.75 mA/cm² and 0.34 A/mg_Pt, respectively. In addition, the Pt–Co nanoframe and commercial Pt/C catalysts show a significant E_1/2 difference of 26 mV in the ORR polarization curves, indicating an improved ORR kinetics of the former (Fig.12(h)).

The structural and chemical instability of nanoframe continues to be a significant challenge. Enhancing the chemical stability of catalysts, in addition to designing stable framework structures, can be achieved by implementing ordered atomic arrangements within intermetallic structures [116]. Nevertheless, obtaining an ordered atomic arrangement of intermetallic structures through irregular alloys usually requires high-temperature annealing. To circumvent agglomeration and deformation of nanoframe under these conditions, Kim and colleagues [117] utilized a silica-coating-mediated approach to preparing Pt–Cu nanoframe (O–PtCu NF/C) with ordered intermetallic L1₁ structures. This intermetallic structure is characterized by atomic ordering and coupling of strain and ligand effects. According to DFT calculations, the ΔE_O value of the intermetallic PtCu L1₁ structure (327 meV) is lower than that of the L1₀ structure (356 meV), and the activity enhancement coefficient of the L1₁ structure (3.6) is higher than that of the L1₀ structure (2.1). This indicates that the intermetallic PtCu L1₁ structure favorably affects its MA (2.47 A/mg_Pt) and SA (4.69 mA/cm_Pt). The studies above suggest that optimizing the structure of the nanoframe and regulating the ordering of intermetallic atoms can enhance the performance of the nanoframe.

4 Summary and prospect

The control of morphology in ORR catalysts is an essential area of research aimed at enhancing the activity and durability of catalysts through adjustments to parameters such as crystal structure, morphology structure, and active surface sites. Currently, nanoscale catalysts demonstrate a great potential for ORR applications, particularly with precious metal catalysts being expensive. Therefore, the need for low-cost and high-efficiency catalysts is increasingly pressing. Over the past few decades, scientists have devoted considerable effort to controlling the morphology of ORR catalysts to find more effective strategies to overcome this problem. This paper provides a detailed discussion of the encouraging morphology modulation schemes in acidic ORR, as well as the source of activity and stability. For instance, the surface area can be effectively increased by adjusting the morphology of catalysts, such as NWs, NSs, and nanopolyhedra, which can enhance the interaction between the reactant molecules and the catalyst and enhance its activity. In addition, methods such as surface modification, hybridization structure, controlling the nucleus, and selecting the appropriate crystal surface can optimize the structure and improve the catalytic activity and stability of the catalyst. With the continuous innovation and development of the nanomaterial synthesis technology, the field of morphology modulation for ORR catalysts has undergone a rapid and significant progress. In addition, other materials, such as MXene and 2D transition metal discs, have been extensively utilized for the modulation of ORR catalyst morphology. These innovative materials possess abundant surface-active sites and adjustable structures and thus have a wide range of potential applications in ORR. For the development of ORR catalyst morphology control, the following aspects are suggested as a reference:

1) Innovations in morphologic modulation strategies. New synthesis methods and techniques should be developed, including electrochemical deposition, template method, etc., to prepare more complex and diverse catalyst morphologies. For example, during the catalyst design process, the structural morphology can be combined with the surface morphology design to trade off the activity and stability. Of course, it is necessary not to be limited to the existing synthesis technology and instead to expand the design ideas and combine various advanced technologies, for example, nanotechnology, photovoltaic technology, and biotechnology to provide new ways to explore catalyst morphology control.

2) Advanced in situ characterization techniques. Advanced in situ monitoring and characterization techniques can help analyze the dynamic process of ORR and deeply investigate the mechanism of catalyst morphology on the catalytic performance of ORR. With the advancement of materials science, researchers frequently utilize various in situ characterization methods to analyze catalyst structure, thus establishing a constitutive relationship between catalyst morphology and performance. For instance, techniques such as in situ XRD, in situ Raman, in situ infrared (IR), in situ X-ray absorption spectroscopy (XAS), and in situ atomic force microscopy (AFM) are commonly used in this type of research. These in situ characterizations provide real-time observation of chemical reactions, substance structure, and morphology, offering information on reaction intermediates and analytical insights into catalytic reaction changes. These observations aid in analyzing reaction mechanisms, leading to further advancements in catalytic materials.

3) Artificial intelligence and machine learning. With the continuous progress of science and technology, the experimental design of human–computer interaction is becoming increasingly normalized. Mechanical learning, as an efficient data processing method, is widely used in the field of material design, such as experimental data integration and analysis, material performance prediction, data-driven material design, active site identification, and automated experimental design. It is foreseeable that mechanical learning will bring more intelligent and efficient methods for catalyst morphology regulation, which will not only improve the efficiency of catalyst design and reduce the cost of trial and error, but also help discover new catalysts that are difficult to discover by traditional experimental methods. This is of great practical significance for the promotion of green chemistry and energy transition.

4) Advanced strategies for morphology control in practical fuel cells. Considerable work has been done to modify the morphology and improve the performance of ORR electrocatalysts at the laboratory level. However, studying the performance of catalysts using laboratory RDE electrochemical tests is not enough. Successful real-life implementation of the research results requires consideration of the practical effects demonstrated by the catalysts in fuel cells as well as the large-scale production of low-cost catalysts.

In summary, ORR catalyst morphology control has a broad application prospect and research space. Undoubtedly, ORR catalysts that are more relevant to practical applications can only be designed by considering reasonable catalyst design strategies, advanced in situ characterization techniques, novel artificial intelligence and mechanical learning screening, as well as the cost and mass preparation of catalysts. Through continuous exploration, innovation, and breakthroughs, further achievements in ORR catalyst morphology control will significantly contribute to realizing sustainable energy development.

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