Strain engineering of Pt-based electrocatalysts for oxygen reaction reduction

Abstract

Proton exchange membrane fuel cells (PEMFCs) are playing irreplaceable roles in the construction of the future sustainable energy system. However, the insufficient performance of platinum (Pt)-based electrocatalysts for oxygen reduction reaction (ORR) hinders the overall efficiency of PEMFCs. Engineering the surface strain of catalysts is considered an effective way to tune their electronic structures and therefore optimize catalytic behavior. In this paper, insights into strain engineering for improving Pt-based catalysts toward ORR are elaborated in detail. First, recent advances in understanding the strain effects on ORR catalysts are comprehensively discussed. Then, strain engineering methodologies for adjusting Pt-based catalysts are comprehensively discussed. Finally, further information on the various challenges and potential prospects for strain modulation of Pt-based catalysts is provided.

Graphical abstract

Keywords

strain engineering / Pt-based catalysts / oxygen reduction reaction (ORR) / catalytic performance / proton exchange membrane fuel cells (PEMFCs)

1 Introduction

Renewable energy generation is rapidly developing in the context of exacerbated global climate change. Owing to zero carbon emissions and high energy conversion efficiency, proton exchange membrane fuel cells (PEMFCs) are expected to play an essential role in the development of renewable energy based power grids [1–3]. The cathodic oxygen reduction reaction (ORR) is a key step in PEMFC, and its efficiency and controllability directly impact the energy efficiency and stability of fuel cells. However, the ORR kinetics are quite slow, leading to significant voltage losses under conventional operating conditions, severely restricting the overall performance of fuel cells. Therefore, developing efficient and durable ORR electrocatalysts remains a major obstacle that needs to be overcome to advance PEMFCs [4–6]. Platinum (Pt)-based materials are currently the preferred choice for accelerating ORR in PEMFC [7]. Yet, the activity of Pt-based catalysts cannot meet the demands under high operating conditions. Moreover, they are prone to corrosion and aggregation under severe operational circumstances, resulting in a significant performance degradation [8]. To address these issues, elaborately design and synthesis strategies are required [9,10].

It has been shown through experiments that ligand, strain, and ensemble effects can be used to adjust the electronic structure of the catalyst and enhance the catalytic activity [11–13]. Consequently, these surface engineering strategies are regarded as practical methods to upgrade the function of Pt-based ORR electrocatalysts [14]. Strain engineering can alter atomic spacing and create lattice mismatches [15]. These aberrations change the distribution of the metal ion electron cloud and its interactions, making it possible to precisely control the position of the d-band level of the metal. This control directly influences the adsorption capacity and the management of the rate of substance transport [16,17].

Technical terms are explained upon first use. In contrast to the ligand and integration effects [18], the strain effect is controllable within six atomic layers near the surface of a nanoparticle, allowing for a greater flexibility than the ligand effect (which is limited by diatomic layer failure) [19]. Furthermore, strain engineering changes the shape of the catalyst by purely physical means, without the addition of additional materials. This approach avoids the problems of chemical contamination which can affect the stability and selectivity of the catalyst [20]. However, the challenge in strain modulation of Pt-based catalysts lies in precisely controlling the catalyst lattice parameters. When regulating the lattice structure of catalysts at the nanoscale, applying external stresses requires solving the issues of uniform stress application and strain accumulation prevention at the catalyst surface or interface [21]. Although some studies have shown the potential for strain engineering, practical implementation of strain control still presents a challenge [22]. It necessitates a thorough comprehension of the lattice organization, electronic structure and reaction kinetics of metallic Pt and Pt alloys to achieve a balance between theory and practice [23].

In this paper, the strain effects on improving Pt-based electrocatalysts for ORR and present recent advancements in this area were emphasized. First, the internal mechanism of strain intervention in ORR step was deeply elucidated to explain the change of chemisorption energy of catalyst induced by strain. Then, the design and synthesis of strain-rich electrocatalysts (SRE) with adjustable adsorption capacity through strain engineering were summarized, and some methods to accurately control the strain state during catalyst synthesis were introduced. Finally, from the perspective of foundation, the practical strain adjusting the Pt-based catalysts were summarized and prospected.

2 Fundamental explanation of strain effects on ORR

2.1 Electronic properties of Pt-based catalysts

ORR comprises oxygen molecule diffusion and adsorption, electron transfer, and dissociation of reaction products [24]. The Pt-based catalyst adsorbs oxygen molecules, and electron transfer and chemical reactions occur on its surface. Initially, oxygen molecules are absorbed onto the catalyst surface, forming adsorbed oxygen species. These species are then activated to form oxygen-containing intermediates that participate in subsequent reactions [25,26].

Under rich-oxygen conditions, the ideal four-electron reduction reaction stage on the Pt(111) crystal plane is outlined as (* signifying the active component of enzyme of the catalyst)

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

(2)\begin{array}{rl}& {\mathrm{O}}_2+4({\mathrm{H}}^{+}+{\mathrm{e}}^{-})\to \ast \mathrm{O}\mathrm{O}\mathrm{H}+3({\mathrm{H}}^{+}+{\mathrm{e}}^{-})\to \\ & \ast \mathrm{O}+2({\mathrm{H}}^{+}+{\mathrm{e}}^{-})\to \ast \mathrm{O}\mathrm{H}+({\mathrm{H}}^{+}+{\mathrm{e}}^{-})\to 2{\mathrm{H}}_2\mathrm{O}\end{array}

Promoting the dissociative O−O bond of the oxygen ions (OOH) to avoid the formation of peroxides is key to realizing the four-electron pathway. Due to the close bond between O₂ and Pt, Pt-based catalysts inherently show a significant preference for the four-electron pathway. For the same reason, adsorption of molecular O₂ is not the reason for slow ORR reaction [27], and the adsorption/desorption mechanism of specific oxygen-containing species determines the rate [28]. The changes in Gibbs free energy along the ORR pathway are shown in Fig.1(a) [29]. During the ORR process, each oxygen-containing intermediate species participates in the reaction sequentially in different steps, leading to some correlation between their binding energies [30], which can be linearly approximated as ΔG_O = 2ΔG_OH and ΔG_OOH = ΔG_OH + 3.2 (Fig.1(d)). Therefore, researchers are capable of evaluating ORR occupation by using ΔG_O as the only descriptor and introducing a metric for the maximum catalytic activity based on the Sabatier principle [31], resulting in the classical ORR volcano plot for metal catalysts (Fig.1(d)). For instance, Pt, being a metal with a strong oxygen binding (more negative ΔG_OH), has its rate-determining step as the dissociation of *OH to form H₂O [32].

The actual adsorption and desorption capacity of oxygenated substances usually depends on the electronic properties of the catalyst [33]. Using the density functional theory (DFT), Xu et al. [33] developed a method to calculate the relationship between the intermediate adsorption energy and the electrode potential (Fig.1(c)). During chemisorption, 2p orbitals of the atomic oxygen successively couple to metal sp and d-bands [34], resulting in a binding state with a lower energy than the renormalization band and an anti-binding state with a higher energy than the d-band (Fig.1(c)). The energy of the anti-bonding orbital is close to the Fermi level, so it is partially occupied [35]. For transition metals, the contribution of the sp-band is the same [36]. It is therefore the coupling with the d-electron that determines the main trend of the chemisorption energy. The position of the d-band, which is relevant to the Fermi level, is the critical surface feature that determines reactivity [37]. The adsorption energy varies depending on where the d-band center (the local average of the d-band electron energy) is relative to the Fermi level. As a good indicator of the adsorption energy of the material, the d-band center is often used to describe the adsorption characteristics of metals [19,22,38]. For oxygen adsorption, a higher d-band center results in a higher negative ΔG_O. For instance, hydrogen atoms can be adsorbed on (111) crystal planes of different metals. It is shown in Fig.1(b) that the d-band energy of Pt and Ni is higher than that of Cu and Au, and the d-band is closer to the Fermi level. In Ni(111) and Pt(111), anti-bond orbital is located above the Fermi level after bonding, indicating the lack of electrons, whereas in Au(111) and Cu(111), the anti-bond orbital is located below the Fermi level and occupied by electrons [39]. The adsorption of hydrogen atoms on Ni(111) and Pt(111) is characterized by increased stability and stronger chemical adhesion compared to Cu(111) and Au(111).

According to the d-band theory, the ORR activity can be maximized by adjusting the electronic characteristics of the catalyst to have the best d-band center to obtain the best oxygen binding energy [40]. The changes in Pt coordination number and Pt atomic spacing caused by strain can interact with the d-band center position. In particular, lattice strain effects change the interatomic coupling matrix, linked to the atomic d-band width and interatomic bonding. The change of atomic spacing will make the lattice structure relax or shrink. When the strain causes the d-band width to narrow or widen, in order to ensure that the d-band filling and the total number of d-band remain unchanged, the average d-band energy (i.e., the d-band center energy) must change (the rise and fall of the d-band center depends on the metal type), thus changing the electron state density on the metal surface of the interface [44]. The adsorption energy of the reactant on its surface is affected (Fig.2(e) and Fig.2(f)). For Pt-based ORR catalysts, it is generally understood that the optimal catalyst should have an oxygen binding energy of 0.2 eV lower than the Pt(111) [45].

Typically, strain induced changes in the electronic structure of Pt required techniques such as X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectra (XAS) [41]. For instance, replacing Pt with Cu in the synthetic Pt−Cu alloy causes varying degrees of Pt lattice contraction due to the smaller Cu atomic radius compared to Pt [42]. Pt lattice shrinkage was determined by direct XAS evidence and evaluating lattice constants by (111) peak positions measured in the X-ray diffraction (XRD) patterns (Fig.2(a)). Pt lattice shrinkage was calculated to be 1.15%, 2.68%, 3.83%, 4.60%, 5.36%, and 6.90%, respectively. The XPS analysis revealed that the Pt 4f_7/2 peak shifted toward a lower binding energy as the lattice shrinked, suggesting that the degree of 5d orbital electron filling increases with lattice shrinkage. Furthermore, reduction of empty 5d orbital led to decrease of d-band center, weakening adsorption strength of oxygen-containing materials [43], as shown in Fig.2(b) and 2(c). Similar changes in the electronic structure were also observed in the lattice strain caused by different support material (Fig.2(d)) [37].

Tensile strain and compression strain on strain affect the electronic structure and detailed insight into the electrochemical properties, i.e., the strain has been predictor parameters into the electric catalytic behavior.

2.2 Compression and tensile strain regulation

The reaction intermediates of ORR show a strong binding to the Pt-based catalyst, leading to problematic product removal. It is proposed in this paper that reducing the binding strength of ORR intermediates by compressive strain could improve the performance of Pt-based catalysts in ORR. According to the d-band theory, under compressive strain, Pt atomic spacing contracts and Pt atomic interactions increase, leading to increased energy level splitting and wave function overlap, which in turn widens the d-band [38]. Pt, as a late transition metal, has a more than half-filled d-band. d-band stretching reduces its occupancy. To conserve charge, the d-band center shifts down to maintain its d-band occupancy. Descent of the d-band leads to a reduced filling of the anti-bonding state from adsorbate interaction with the d-band of Pt, weakening the interaction strength and resulting in a weaker adsorption [46,47].

For example, adding M atoms with a smaller atomic radius than the Pt atoms will make the Pt lattice shrink and move the Pt d-band center away from the Fermi energy level [48]. Within a certain range, a higher compressive strain leads to a higher catalytic activity. By modifying the Pt/Nd precursor synthesis process [25], different atomic ratios of Pt−Nd/C alloys can be produced (Fig.3(a) and 3(b)). Due to lattice compression strain, the shorter Pt−Pt atomic spacing causes overlap of the electron bands, and the d-band center moves downward to weaken the adsorption (Fig.3(e)). In addition, the electronegativity differences ({\chi }_{\text{Nd}} = 1.14 and {\chi }_{\text{Pt}} = 2.28) of the alloy metal lead to the electron mobility between the Pt−Nd, which widens the Pt d-band. Eventually Pt_6.1Nd/C and Pt_7.8Nd/C, respectively have a compressive strain of 3% and 1.6%, and their mass activity (MA), respectively is 3.5 and 2.7 times that of the commercial carbon-supported platinum (Pt/C) (Fig.3(c) and Fig.3(d)).

The tensile strain in Pt-based catalysts seems unlikely to catalyze ORR because the strain shifts the d-band center of the Pt metal upwards, enhancing the interaction with the reaction intermediate, leading to solid adsorption and active site occupation, hindering ORR [49]. However, this view is not always accurate, as has been demonstrated by experimental results and theoretical calculations [145]. It is typically achievable to introducing tensile strain on the Pt(110) lattice plane to improve the activity. When there exists compression strain along the in-plane direction, the Pt(110) plane aligns with the [110] and [001] directions of Pt atomic separation. Therefore, introducing tensile strain into the Pt(110) plane consisting of highly undercoordinated surface atoms counteracts the effect of “stretching” the Pt(110) plane by out-of-plane relaxation, resulting in an increased ORR activity [16]. Simultaneously, imposing tensile strain in the direction of [001] modifies the adsorption energy of oxygen adsorption sites on the Pt(110) facets, rendering them more conducive for ORR (Fig.4(e)–Fig.4(g)).

For example, rhombohedral dodecahedral Pt@Pd core-shell nanoparticles (NPs) composed entirely of Pt(110) surfaces were obtained by seed-mediated Pt epitaxy growth on Pd seeds (Fig.4(a)). The method examines the negative influence of compression strain on Pt(110) to explore the strain–activity relationship. Although the Pt shell is subjected to compressive strain, the degree of strain gradually decreases to zero as the shell thickness increases (Fig.4(b)). These results indicate that Pt@Pd particles have their lowest activity at a shell thickness of only 5 to 6 atoms (which is also their highest compression stress), and that this negative effect begins to decrease and eventually vanishes with increasing number of atoms in their shell (Fig.4(c) and Fig.4(d)). DFT calculations show that the face-centered cubic (FCC) hollow sites (“h”), which originally have no real contribution to the overall ORR, are activated under tensile strain in the [001] direction, while the bridge sites (“b2”) in the [110] direction are activated along the [001] direction and the strong Pt−O bonding energy is weakened [50]. Therefore, the exposed Pt(110) crystal surface shows a superior ORR electrocatalytic performance. Introducing tensile strain during synthesis can increase catalyst stability and activity to some extent. For example, doping Pt NPs with non-metallic N produced a tensile strain of about 0.6% within the Pt lattice [51]. Although N doping slightly improved ORR activity over regular Pt/C, it gave the catalyst an exceptional ORR durability. After 20000 accelerated durability tests (ADTs), the ORR activity of N-doped Pt NPs decreased to 3.7%, whereas commercial Pt/C catalysts decreased to 27.9% after the same number of ADTs.

In short, the main objective of inducing tensile or compression stress in a Pt catalyst is to reduce the robust Pt/O bonding on the Pt crystal surface. A few studies have indicated the impact of strain on the adsorption conduct of typical low-index crystal faces of Pt. On the Pt(111) and Pt(100) crystal facets, there is a notable linear correlation between the degree of strain exerted and the adsorption energies of oxygen-containing species. Compressive strains weaken adsorption, while tensile strains enhance it. For Pt(110), the adsorption energy remains fairly constant under compressive strain, but is significantly weakened by tensile strain. Therefore, it is necessary to modify the surface properties of the catalysts and to precisely tune their adsorption behavior according to various theoretical guidelines.

2.3 Strain test methods

The complete characterization of SRE is useful for analyzing its lattice strain and further evaluating its catalytic capabilities. HRTEM is the most popular and direct method for measuring strain. HRTEM can probe lattice strain at the nanometer scale and provide atomic-level images and lattice information. The analysis of the HRTEM images of the catalyst can help make direct judgments on atomic spacing, measuring the degree of the change in the lattice constant and lattice distortion [52]. For example, by comparing the measured sample atomic spacing and HRTEM images of standard material, the strain can be quantitatively calculated in detail (Fig.5(a) and Fig.5(b)).

Strain calculation methods based on HRTEM images often only reflect the strain information of local regions or specific particles [53]. In such a case, evaluating the strain situation frequently necessitates the use of additional methods such as XRD, pair distribution function (PDF) and extended X-ray absorption fine structure (EXAFS). The crystal structure information of the sample is obtained by XRD, and the position of the diffraction peak is inversely proportional to the plane spacing (constant incidence) [54]. Therefore, the lattice strain can be inferred by analyzing the peak displacement and peak width in the XRD pattern [55,56]. The reverse Fourier transform (Rietveld analysis) method can be used to further extract detailed information such as lattice parameters, lattice distortion, and crystal defects. For example, when the lattice parameters of Pt NPs are adjusted based on the shape memory alloy (SMA) [57], compared with the peak position of the original Pt, the peak of Pt(111) after tensile and compressive strain moves to a low angle and a high angle respectively, which means that the strain changes the lattice spacing of Pt (Fig.5(d) and Fig.5(e)).

Compared with XRD, PDF is more effective for the characterization of amorphous materials and catalysts containing amorphous structures (Fig.5(f)). By analyzing the X-ray or neutron diffraction data, the probability distribution function of the atomic spacing in the sample can be calculated, providing information on lattice strain, crystal defects, and impurities [58]. The PDF method is the average information of subtle changes in crystal structure and the resolving power of local distortion is relatively low [59,60]. In contrast, the EXAFS method can provide a higher resolution, enabling detailed studies of the atomic environment near specific locations. Compared to XRD, EXAFS has a higher resolution in detecting catalyst lattice strain and the ability to directly observe local structures [61]. Compared with the distribution function method, EXAFS has a higher resolution, sensitivity and the ability to directly observe local structures in the detection of catalyst lattice strain [62]. It can realize element selectivity study and is suitable for various catalyst forms (Fig.5(c)) [63].

Due to the fact that the strain can be formed by different mechanisms, a single strain characterization technique has some difficulties in accurately chara-cterizing the strain state. Therefore, it is significantly important to synthesize multiple representations to accurately analyze changes. For example, HRTEM is used to qualitatively analyze the lattice defects and crystal plane spacing of Pt-based catalysts [64], and XRD measurements quantitatively analyze the degree of lattice strain [65]. The lattice strain distribution information at atomic level can be obtained by PDF analysis [66]. Finally, through EXAFS analysis, the local structure information around the Pt atom can be obtained to further understand the effect of lattice strain [67].

3 Strain engineering strategies to improve ORR performance

To achieve the strain regulation technique, it is critical to select an appropriate strain induction method. Typically, the following methods are advised: incorporating appropriate additives during the preparation process that distort the lattice as Pt atoms form particles or films, for example doping the metal; controling of catalyst morphology by the preparation process, compressing some Pt atoms by surrounding regions, creating strain; loading catalyst onto a support with controlled lattice parameters to indirectly change nanocatalyst lattice parameters by transforming the lattice parameters of the upport; and generation of strain by applying force or pressure, for instance, compressing or twisting the catalyst with mechanical tools, leading to the straining of some Pt atoms by dislocation and other effects. These methods include core-shell, size and shape changes, loading deformations, doping defects and alloying. Tab.1 shows the strain-induced mechanism, i.e., the properties, of some of the strain-rich Pt-based catalysts involved in this paper.

3.1 Strain in core-shell structured catalysts

The strain of core-shell structural catalysts arises from a discrepancy in the lattice parameters of the core and shell metals. Furthermore, lattice mismatch at the core-shell interface results in lattice shape alterations. Pt-based core-shell NPs consist of Pt as the outer material and non-Pt as the inner material. The crystal structure of a thin shell typically aligns with that of the core, causing compressive strain in the Pt shell layer when the core metal lattice constant is small. Conversely, the Pt shell layer experiences tensile strain. For example, the lattice constant of Pt (3.920 Å) is greater than that of Pd (3.890 Å) and less than the lattice constant of Au (4.080 Å) [83]. The Pt as shell deposition on the Pd NPs, the mismatch between Pt and Pd at the interface generates about 0.8% of the lateral compression strain [84,85]. When the Pt shell is deposited on the Au NPs, it has the tensile strain [86]. The Pd−Au alloy with Pd:Au = 9:1 (molar ratio) was used as the core material to synthesize Pd₉Au₁@Pt core-shell catalyst. Applying Vegard’s law, the core alloy lattice is 3.909 Å, resulting in the stress compression within the Pt shell, which is beneficial to ORR [87]. In addition, as a form of alloying, the intermetallic compound as the core metal, or the development of a certain shape of particles will boost catalytic action and stability even further. For instance, Pt shell layer was placed onto a Pt−Co intermetallic core [68], resulting in octahedral nanocrystals of Pt−Co@Pt that exposes the Pt{111} facets (Fig.6(a) and Fig.6(b)). Co atoms are incorporated into the Pt lattice in an orderly way, and the alloying of Pt and Co produces the best binding energy of oxygen-containing species [51,88], resulting in a better ORR activity (Fig.6(c) and Fig.6(d)). The core-shell lattice mismatch not only induces compressive strain in the shell layer, but also significantly reduces Pt and Co atom movement.

The regulation of the strain effect can be realized in the six monolayers near the surface of the NPs, and compared with the ligand effect, the strain effect can achieve a larger range of performance regulation [89]. Key factors such as the number of shell layers, lattice mismatch, growth mode and core shape regulate the strain state of core-shell catalysis. As an instance, the strain state of a core-shell catalyst decreases as the number of shells increases. Core-shell Pd@Pt electrocatalysts is synthesized by a solvothermal method. The Pt atomic layer thickness increases from 3.0 to 14.0 monolayers when the reaction temperature is increased from 140 to 180 °C, with an increase of about 3 ML/10 °C in reaction temperature. It can be seen from the XPS test that the lattice constant changes as the number of shell layers of Pt increases (Fig.6(e) and Fig.6(f)), and the surface strain of Pt ranges from −1.85% to −0.18% [70]. Among these, the 3 to 4 layered Pd@Pt catalyst has the best mass activity (MA = 0.95 \text{A/m}{\text{g}}_{\text{Pt}}), which is about 5.3 times higher than the equivalent of a normal Pt/C. The same conclusion is supported in Ir@Pt core-shell catalysts which were calculated and evaluated using DFT. It was found that there exist different degrees of lattice compression in the shells of Pt with different shell numbers, making the surface overpotential of Ir@Pt lower than that of pure Pt. Monolayer or bilayer Ir@Pt has the lowest ORR overpotential and is considered a potential ORR catalyst [90]. The core-shell structure allows full exposure of the precious metal atoms that act as catalysts to the surface, enhancing its exploitation. The use of non-precious metals in the core can also reduce costs [71]. For example, monodisperse Pt/PtP₂ core-shell NPs were designed and prepared on N-and phosphorus-doped carbon (defined as Pt/PtP₂@NPC) (Fig.6(g)). The strain of the Pt shell increases catalyst ORR activity, and the ultrathin Pt shell reduces Pt loading while exposing the catalyst site to molecular oxygen. The SA and MA of Pt/PtP₂@NPC in 0.1 mol/L HClO₄ solution were 0.508 mA·cm^-2 and 0.724 \text{A/m}{\text{g}}_{\text{Pt}}, respectively (Fig.6(h)). It is significantly superior to commercial Pt catalysts (MA = 0.098 \text{A/m}{\text{g}}_{\text{Pt}} and SA = 0.142 mA·cm^-2) [69,91].

Common techniques for synthesizing polymetallic core-shell nanostructures include electrochemical, colloidal and chemical reduction approaches [5]. Electrochemical methods include underpotential deposition (UPD) and dealloying. Underpotential electrodeposition can achieve more precise control and easily induce the formation of smaller NPs [94]. It is often used as sacrificial metal, replacing the copper atoms covering the core metal to deposit the Pt shell evenly [95]. The dealloying method is appropriate for polymetallic systems and facilitates the production of heterogeneous nanostructures. The surface of core-shell NPs is enriched through the electrochemical elimination of more non-Pt gold from Pt alloys. For instance, the surface of Ag−Pd−Pt nano frames underwent successful dealloying into Pd−Pt shells via cyclic voltammetry [96]. Colloidal synthesis uses the gelling properties of colloidal particles to create metal particles on their surface, while chemical reduction uses reducing agents to reduce metal ions. These metal NPs are then coated with shell material using techniques like co-precipitation or sol-gel, creating a core-shell structure. Taking seed growth as an example, metal grains (the core of NPs) in solution encourage ions to crystallize and grow. This technique resolves the challenge of metal ions requiring traversal of high-energy barriers during nuclear [97]. In addition, the shaping of the core can induce and regulate the lattice structure and the exposed crystalline surface of the metal shell layer. For example, seed-mediated growth of Pd@Pt core-shell tetrahedra under five atomic layers allows the shell metal to replicate the lattice structure of the underlying seeds and grow around the Pd(111) surface (Fig.6(i)). The Pd@Pt tetrahedron will be transformed into a truncated octahedral shape with the Pt(111) crystal face exposed [93]. In addition, this technique can successfully synthesize various core-shell structures with even single-layer shells [79,98,99]. The one-pot reduction method can also effectively prepare Pt NPs. To illustrate, PtNi/C catalysts with Ni-rich cores and Pt-rich shells can be prepared by a simple one-pot process using acetaldehyde/tetraethylene glycol reagent and polyethylpyrrolidone (PVP) as structural guides [92] (Fig.6(j)). HAADF-STEM observed a typical lattice fringe of 1.111 Å along the FCC plane of the Pt shell, i.e., there is compressive strain in the Pt shell.

3.2 Strain in alloy

Mixed with Pt atoms doped with one or two different element atoms, the synthesis of new NPs induces local lattice strain by changing the metallic bond length, as a result of the differences in size between atoms in the Pt lattice [72]. Typically, alloying with another metal of smaller atomic size than Pt induces compressive strain [100]. For example, due to smaller atomic radius (0.124 nm) of Ni compared to that of Pt (0.137 nm), there is a size mismatch when Ni atoms replace and occupy some Pt lattice positions [101]. Additionally, Ni atoms occupy a lattice position further from the surrounding Pt atoms. This size mismatch causes the surrounding Pt atoms to converge toward the nickel atoms, resulting in a slight lattice shrinkage and compressive strain in the crystal structure [105]. PtNi metallic NPs with varying atomic percentages loaded with carbon (Ketjen black, KB) were synthesized by applying multiple current pulses to the precursor by means of high-temperature shock [102]. The end outcome is represented as PtNi (a:b)/KB, where a:b indicates the Pt:Ni input percentage. Based on TEM observations, the lattice spacing of PtNi/KB with various compositions falls within the range of 0.219 to 0.223 nm. It is clear that PtNi alloy is present by comparing the lattice spacing of pure Pt(111) (0.227 nm) and pure Ni(111) (0.203 nm). The strain is caused by the nanoparticle composition of the PtNi alloy as well as dislocation structure induced by ultrafast quenching during synthesis. Other methods of producing PtNi NPs have yielded similar results. For example, supported PtNi NPs are produced by high-temperature reduction, which promotes atomic diffusion and ordering [72]. The small-sized PtNi catalyst (Pt_2.3Ni@OMC) was obtained by incorporating PtNi₃ NPs (diameter ≈ 4.3 nm) into ordered mesoporous carbon (OMC) matrix material, exploiting the binding influence of OMC to prevent sintering. The OMC has a networked pore structure, narrow pore size distribution and limited particle growth during reduction, resulting in finely distributed metal nanocrystals (Fig.7(c)). The study showed that the d-band center of Pt_2.3Ni is lower than pure Pt, and there exists 2.5% compressive strain in the lattice, and the downward shift of the d-band center reduces the O binding energy and accelerates dissociation of surface oxygenates(Fig.7(d)). It was also observed that smaller particles resulted in a larger specific surface area, leading to a better ORR catalytic activity [103,104].

Particular processes are used to manufacture alloyed catalyst, such as the dealloying strategy, where leaching of alloy atoms causes vacancy shrinkage in the surface layer. It creates additional compressive stress and exposes more active noble metal atoms, which naturally causes the lattice constant to shrink [106,107]. The Pt−Cu alloy catalyst with a low Pt was synthesized by solvothermal synthesis and electrochemical dealloying, and the compressive strain was introduced on the Pt through lattice dislocation between the Pt-rich surface and the alloy core [75]. Electron microscopic analysis showed significant changes in shape and size of NPs due to dealloying, which resulted in the lattice strain and porosity of Pt, leading to an optimal d-band center position and ECSA of catalyst. In the electrochemical dealloying catalysts (Cu₁₁Pt₈₉, Cu₁₇Pt₈₃, and Cu₂₃Pt₇₇), Cu₁₇Pt₈₃ showed an excellent ORR activity in acidic media, with a limiting current density, an SA of 5.4 mA/cm², and an MA of 0.157 A/mg_Pt, respectively.

Traditional Pt-based alloys are usually disordered solid solutions composed of a mixture of two or more metals (or a few non-metals), in which the atoms are randomly distributed on a lattice. In the process of alloy formation, lattice parameters can also be modulated by special means to form ordered intermetallic compounds [108]. In contrast to disordered alloys, the atoms of each component element of ordered intermetallic compounds have a specific location, allowing for predictable regulation of the structure and electronic effects [109]. The reason for their enhanced catalytic activity is the same, i.e., the strain effect caused by the change in atomic spacing and d-band coupling between different metals [110]. The difference is that the ordered structure can induce a greater lattice shrinkage. This helps to regulate the d-band center position [111]. Due to low enthalpy of formation and strong interatomic interaction [112], the relatively close atomic arrangement of intermetallic compounds ensures its acid corrosion resistance to a certain extent, significantly improving its oxidation resistance and corrosion resistance, thus achieving an excellent cycle durability [113,114]. PtFe intermetallic compounds with different order degrees [76] were compared and analyzed, and the PtFe/Pt core-shell structure was obtained by dealloying to analyze the relationship between activity and order degree. It is verified by XRD that PtFe with different order degrees has different lattice constants (Fig.7(a)). The order degree of PtFe with a higher order degree (expressed as PtFe-H) and PtFe with a medium order degree (expressed as PtFe-M) was calculated as 74.3% and 34.7%, respectively. The planar spacing of the two (111) faces is 2.211 and 2.227 Å, respectively. Fig.7(b) visually shows the highly ordered and small lattice spacing of PtFe-H. It is proved that the lattice shrinkage of PtFe-H with a higher order degree is larger, and the SA and MA of PtFe-H/Pt reach 3.03 mA/cm and 2.12 A/mg_Pt, which are far greater than those of PtFe-M/Pt (0.88 mA/cm, 0.67 A/mg_Pt) (Fig.7(e)–Fig.7(h)). This suggests that there is a substantial positive relationship among the level of organization and its ORR characteristics of an alloy or intermetallic compound.

Intermetallic compounds are usually prepared by thermal annealing and direct liquid phase synthesis. The thermal annealing method usually requires a disordered alloy on the basis of a secondary annealing treatment in a suitable atmosphere to promote the diffusion of metal atoms in the lattice to form an ordered structure. For example, a disordered Pd−Pt−Fe alloy catalyst named D-Pd_1−xPt_xFe/C was synthesized at an annealing temperature of 500 °C after the precursor was obtained by spray dehydration (SPD). The well-structured Pd−Pt−Fe intermetallic compounds were prepared at a 600 °C annealing temperature. The characterization test found that the lattice constant of Pd_0.75Pt_0.25Fe and D-Pd_0.75Pt_0.25Fe increased significantly, and the lattice shrinkage was 0.06% and 3.12% respectively, and the XRD pattern also showed a slight negative shift [73]. Due to the strong d-orbital interaction between the two metals, the atoms in intermetallic compounds are highly ordered [116,117]. However, it is generally necessary to control annealing temperature and time and select the appropriate carrier and coating layer to prevent the agglomeration of nanocrystals at high temperatures. Intermetallic arranged fct-PtFe NPs were created, for example, by impregnating dopamine hydrochloride solution and annealing at high temperatures [115]. N-doped carbon shells were formed by in situ pyrolysis of dopamine during thermal annealing. This carbon cladding effectively prevents NPs from coalescing (Fig.7(i)). The size of ordered fct-PtFe NPs obtained by this method is only 6.5 nm, while the diameter of uncoated PtFe NPs is 10 nm. The inert coating method can also be used for one-step pyrolysis. The N-doped carbon encapsulation approach, for example, is used in the manufacture of Pt−Fe intermetallic mixtures (Fig.7(j)). Coated carbon shells may successfully avoid particle aggregation and Oswald maturation during processing with heat or curiosity cycling. Carbon shells may potentially improve the anti-toxicity of Pt−Fe NPs [74]. Direct liquid-phase synthesis allows ordered structures to be obtained at a lower temperature and avoids sintered NPs at a higher temperature. Solvent boiling point limits reaction to temperature phases below that of solvent, thus limiting the transformation of semi-ordered phases into ordered phases.

3.3 Strain caused by the support

NPs are deposited onto a flat carrier made of other materials to deform by physically transforming the carrier, such as by applying an external force or changing temperature. Due to the interaction between the nanoparticle and the carrier, the nanoparticle can also undergo a small amount of deformation driven by the force [57]. On the other hand, the local lattice mismatch between NPs and deposited support is also one of the causes of high lattice strain. In this respect, the formation mechanism of nanoparticle strain on deformable carriers is similar to that of shell strain in core-shell structures. The study of strain effects using this method focuses on finding suitable tunable carriers to flexibly and efficiently control the tensile and compressive lattice strains of NPs while avoiding the influence of additional effects. For example, Pt NPs were deposited on L_0.5CO (Li_0.5CoO₂) and LCO (LiCoO₂) carriers by taking advantage of the adjustable volume and lattice spacing of lithium-ion battery electrode materials during charge and discharge [77]. During the charging process, the LCO is converted to L_0.5CO because half of the Li is extracted to form Li⁺, and its Co-O layer spacing expands from 4.69 to 4.82 Å. The lattice of the loaded Pt NPs increases due to the Co-O layer spacing, resulting in tensile strain. In contrast, during discharge, Li⁺ is inserted into L_0.5CO, and its Co-O layer spacing is reduced from 4.82 to 4.69 Å, forming a compressive strain in the loaded Pt NPs. Experiments have shown that the carrier can flexibly control the lattice strain in the tensile or compressive direction up to approximately 10%.

In another example, the bidirectional shape memory effect of NiTi SMA was utilized for strain engineering of 5 and 10 nm thick Pt nanofilms deposited on SMA substrates [57]. The thermo-mechanical SMA produces stress during the reversible phase transformation process and thus exerts strain on Pt nanofilms. The phase transition is only related to temperature (Fig.8(a)), i.e., a functional relationship is established between strain and temperature. The XRD test shows that under different strain states, Pt(111) peak position displaces obviously (Fig.8(b) and 8(c)). Finally, a compression strain of 1.93% and a tensile strain of 0.76% were achieved on the 5 nm Pt film. Compared with the original Pt, the half-wave potential of 5 nm Pt under compression strain is positively shifted by 27 mV, and the kinetic rate constant is increased by 52% (Fig.8(d)–8(f)). It should be noted that this method of providing strain through the substrate is often limited by the properties of the substrate material, the load transfer efficiency between the weak substrate and the nanofilm, and the intrinsic elastic strain limit of the Pt film, and cannot achieve the expected degree of strain. For example, for a 10 nm Pt film, the NiTi substrate induced only about 1.10% elastic compression strain, followed by a large plastic deformation of Pt. In another case, Pt metal clusters were synthesized in solution by self-assembly of CO molecular metal clusters (MMCs) (Fig.8(g)) and attached to carbon carriers [118]. Due to the existence of a substantial number of Pt-C bonds, XAFS and PDF analyses revealed a strong contact across Pt nanoclusters and the Vulcan framework (Fig.8(h)–8(i)). On the one hand, the interaction made the nanoclusters stable on the Vulcan framework. On the other hand, the very low particle size and high strain of Pt nanoclusters increased the electrochemical activity of the samples relative to Pt/C by 5.24 times. It is also proven that the interaction between clusters and support can significantly affect the catalytic performance of the catalyst.

The strain distribution and size of the supported catalyst are affected not only by the support [119], but also by the thickness of the Pt shell deposited on the support and the particle size of Pt NPs. The Cu-UPD-Pt displacement technique was used to deposit Cu monolayer on clean Pt₃Ni/C to reduce K₂PtCl₄, thereby controlling the number of Pt layers, which was used effectively for laying down a Pt monolayer on a variety of platforms [120]. Because the lattice parameter of Ni in the base Pt₃Ni alloy is smaller than Pt, compressive strain is generated. The Pt layer on the substrate also has a compressive strain under the interaction of the substrate. The single-layer Pt lattice shrinks by about 2%, although the stress energy steadily decreases as the total amount of levels increases, with only approximately 1% strain in the 2 and 3-layer Pt shells. When more than 4 atomic layers of Pt are deposited, the compression strain caused by the Pt₃Ni alloy is already negligible. According to TEM and HAADF-STEM measurements, the particle size increased to 5.5 nm after the deposition of the second Pt layer on the Pt₃Ni surface and further increased to 7.3 nm after the deposition of the sixth Pt layer. As the Pt coating and particle size grew, the SA of catalyst rapidly decreased from (0.9 ± 0.04) mA/cm² to (0.44 ± 0.04) mA/cm². No significant activity decrease was observed over Pt shells, demonstrating that the compressive strain in the Pt layer significantly affected the catalytic performance of the catalysts.

3.4 Strain caused by defects

The strain caused by crystal defects is different from that caused by lattice mismatches. Structural defects are the direct cause of crystal lattice distortion and atomic stacking density change, and the bond length and electronic structure of nearby atoms are changed under the influence of defects, i.e., lattice distortion in these defects leads to the strain in NPs, and micro-strain provides a compressed lattice constant for the catalytic site [78,121,122]. For example, hollow PtNi/C NPs with a Ni content of approximately 10% is thermally annealed to obtain microcrystalline PtNi/ CNPs (spongy PtNi particles) with lattice strain and microstrain [123]. Fig.9(a) compares the defect density and ORR performance of several different PtNi catalysts. The lattice strain is induced by the mixing of Pt with Ni, which results in a lattice misfit. However, structural defects (atomic vacancies occurring in PtNi microcrystals and grain boundaries connecting PtNi microcrystals) cause atoms to deviate from their ideal positions, and the resulting compressive microstrain (local lattice strain) results in more contracted PtNi lattice parameters [124]. The XRD peak broadening is shown in XRD tests (Fig.9(b)) [125,126]. Defects are common in solidified substances and are classified as point flaws, line flaws, surface flaws, and volume defects, which directly or indirectly affect the performance of solid materials [80]. In hollow PtNi/C NPs, the lattice distortion caused by defects induces strain to change the Pt−Pt bond distance, changes the electronic structure of Pt, and optimizes the binding energy of Pt and ORR intermediates [127]. On the other hand, grain boundaries increase corresponding to the mean collaboration variety of atoms on the exterior and increase their ORR function [118,128]. For example, a spongy Pt–tellurium NRs (PtTe₂ NRs) is synthesized into highly distorted weakly distorted Pt NR (W-Pt NRs) and highly distorted Pt NR (H-Pt NR) by a continuous chemical and electrochemical coupling process [129](Fig.9(g)). The HRTEM clearly shows the presence of discontinuities and a high number of inconsistencies (Fig.9(e) and Fig.9(f)), and the discontinuities cause irregular faceted boundaries that significantly improve the overall strain effect, leading to an improved ORR performance (Fig.9(h) and Fig.9(i)).

In addition, after synthesizing the catalyst with the original structural morphology, part of the metal atoms are etched off by acid leaching or electrochemical corrosion to rearrange the surface atoms of the catalyst, and a large number of highly distorted and misplaced high-activity reaction sites are added on the basis of the original complex morphology of the catalyst, thus improving the overall activity of the catalyst [78]. For example, PtTe alloy NPs with layered SSs were first synthesized by using the solvothermal method, and then some Te atoms were leached by electrochemical method to obtain distorted Pt SSs/C [80]. On the one hand, Te leaching works with Pt enrichment to expose more active sites, and on the other hand, the compressive strain caused by surface defects and nanopores contribute to enhancing the ORR activity due to distorted structures resulting from electrochemical Te dissolution.

Defects in catalysts are usually introduced during the synthesis process. For example, 2D porous Pt nanosheets composed of interwoven ultra-thin nanowires were successfully prepared by a simple NaCl templating process [130] (Fig.9(c)). During the preparation process, NaCl microcrystals that had undergone recrystallization had a homogeneous coating of chloroplatinic acid, and in the subsequent gas–solid reaction, H₂ induced anisotropic diffusion of Pt atoms, resulting in the formation of 2D perforated Pt nanosheets on the edge of the NaCl template through the particle adhesion process (Fig.9(d)). Porosity-rich nanosheets are actually composed of staggered and zigzag nanowires with rough surfaces and uneven diameters, and grain boundaries are often observed between adjacent grains in the nanowires. In addition, defects can be used to assist in the synthesis of catalysts. For example, by introducing defects to promote the diffusion of metal atoms within NP, a more efficient structural transformation is achieved. In the dumbbell-like structure Fe₃O₄−FePt NPs, Fe₃O₄ in the NPs is decreased through the heat treatment to cause structural changes, and vacancy arises to encourage metal atom movement and the production of a perfectly arranged framework. In the other case, the NPs structure was changed by removing the defect site. For example, ultra-thin PtAgPb core-shell nanosheets were synthesized by oxidation etching of surface-constrained stacking faults [81]. The newly formed atoms were continuously oxidized back to ions to obtain PtAgPb nanosheets containing a large number of stacked faults. By etching these defect regions by oxidation, single crystal core-shell PtAgPb nanosheets with compressive strain are formed, and the crystallizm (stacking faults to single crystals) and structure (solid to core-shell) of the nanosheets are changed.

3.5 Strain induced by size and shape variation

The strain phenomenon of NPs caused by particle size is related to lattice structure and surface energy [131]. When the nanoparticle is smaller than a certain size (generally thought to be less than 50 nm), the lattice structure is affected and defects such as grain boundaries and dislocations are prone to appear [132]. Defects introduce strain energy into the crystal and generate internal strain, similar to the defect-induced strain mechanism. When the material is subjected to external forces, it is more likely to slip, further damaging the crystal structure [17,133]. On the other hand, smaller particles have a higher surface area, and the chemical potential is proportional to the surface area. Since NPs spontaneously tend to be more stable, in order to reduce this chemical potential, the surface atoms need to be compressed so that the surface can be as small as possible [134,135]. That is, smaller NPs usually have a larger lattice compression strain. For example, a PtCoCu alloy system was prepared to study the internal relationship between particle size and strain [136], and the nanocrystallite size was controlled by adjusting the alloy ratio. Six alloy catalysts with identical structures (Pt₅₀Co₃₃Cu₁₇, Pt₅₁Co₂₇Cu₂₂, Pt₅₀Co₂₆Cu₂₄, Pt₄₇Co₂₆Cu₂₇, Pt₄₂Co₂₄Cu₃₂, Pt₄₂Co₂₁Cu₃₇) were prepared in total (Fig.10(a1)–Fig.10(a4)). The grain size decreases from 10.9 nm (Pt₅₀Co₃₃Cu₁₇) to 2.98 nm (Pt₄₇Co₂₆Cu₂₇) and then to 4.07 nm (Pt₄₂Co₂₁Cu₃₇) by increasing the Cu content. The TEM and XRD results show that the compressive strain gradually increases from 2.38% to 2.84% (Fig.10(b)–Fig.10(d)).

In addition to being affected by strain, small-particle Pt NPs provides a higher specific surface area for intermediates in the electrocatalytic process, but small-particle Pt crystals inevitably have structures such as steps, kinks, and edges, and these sites show a very strong oxygen binding energy [138,139]. The strain effect and the crystal structure together influence the ORR activity. This means that it is not possible to increase the ORR activity by reducing the particle dimensions. As an example, the average particle dimensions grows from 1.84 to 4.65 nm as the Pt layer raises (2–10 layers), when Cu-UPD-Pt layer by layer growth is used to regulate the Pt particle size [137] (Fig.10(e1) and Fig.10(e2)). After combining the influence of strain effect and structure, it was found that the MA increased by 2 times in the range of 1.3–2.2 nm and decreased with the increase of particle size, However, across all particle size Pt NPs, the Pt/C catalyst with a common particulate size of 2.5 nm shows the highest MA (Fig.10(f) andFig.10(g)).

The shape or exposure surface of the catalyst can indeed affect its catalytic performance [140]. The strain in the catalyst is also affected by its shape or exposed surface, as certain shapes and exposed surfaces can introduce local stresses on the catalyst surface and cause strain to appear [141]. For example, compared with Pt−Cu−Mn pentagonal NFs (PNFs), Pt−Cu−Mn UNFs have an ultra-thin structure (Fig.10(h1) and 10(h2)), and the ultra-thin structure in nanocrystals is favorable for the generation of compressive strain [82]. According to aberration-corrected STEM test results, the planar spacing of the Pt−Cu−Mn UNFs ridges showed significant compression compared to Pt−Cu−Mn PNF, resulting in a compressive strain of about 1.5% (Fig.10(i)). In addition, the coordination number of surface atoms is related to the shape of the catalyst. By controlling the crystal surface and morphology of metal NPs, the adsorption energy of the intermediate can be regulated without changing the chemical composition, thus affecting the reaction rate. The surface oxygen bonding energy of the catalyst was used as the descriptor of catalytic activity by modeling the tetrahedron Pd@Pt NP (TH NPs) and the spheroidal truncated octahedron Pd@Pt NP (SP NPs). From 1 to 2.6 nm, the surface shrinkage of TH NPs decreased, and the corresponding O binding energy increased rapidly by 0.88 eV, and the O binding energy was positively correlated with the particle size. In contrast, when SP NPs is less than 1 nm, the larger surface shrinkage (4.7%) does not weaken O binding as TH NPs does. With the particle size increasing from 1 to 2.2 nm, the O binding energy of SP NPs decreases by 0.59 eV despite of the 0.7% decrease in surface shrinkage. Later, with the increase of particle size, the O binding energy increases rapidly, showing a volcanic-like change [142].

4 Summary and prospect

Pt and Pt-based alloys are frequently used as catalysts for ORR because of their remarkable catalytic properties. Lattice strain can alter the electronic structure of the metal, and controlling lattice parameters on the nanoparticle surface and ligand environment affects adsorption capacity, intermediates and essential steps such as electron transfer capability, thus affecting the catalytic performance. The regulation of lattice strain to optimize catalytic properties holds significant theoretical implications and practical applications. Noteworthy progress has been made in researching the strain regulation of ORR catalysts. Core-shell synthesis, alloying doping, defect introduction, deformable coating, and size and shape tuning techniques have been described to alter lattice configuration and strain state.

1) Synthetic core-shell structure: By adjusting the lattice mismatch between the core and shell layers, strain modulation of Pt particles is possible. This technique improves the atomic utilization and stability of the Pt particles, while regulating their catalytic activity and selectivity.

2) Doping to form alloy: Incorporation of additional metal elements into the Pt lattice leads to the creation of Pt alloys. Alloying can induce lattice strain, alter Pt electronic structure and surface adsorption properties, and modulate catalytic efficiency and stability.

3) Introduction of defects: Dislocations and grain boundaries can change lattice configuration and stress, and ultimately affect stability and catalytic activity. This methodology allows for strain modulation in Pt catalysts by controlling the density and type of defects.

4) Coating on a deformable substrate: Pt films or NPs are applied to a deformable substrate, and substrate deformation is transferred to the Pt particles to induce strain. An external stress or temperature change on the flexible substrate can be used to modulate the strain of the Pt catalyst.

5) Adjust size and shape: controlling the size and shape of Pt particles can induce lattice distortion and strain effects. Reduction in Pt particle size leads to increased surface area and surface strain, which can improve catalytic activity and stability.

To summarize, different methods of induction have different advantages and no blanket statement can be made about the effectiveness of any single strategy. Identifying the most effective technique requires evaluating the specific catalytic reaction and its proposed application. In addition, the combination of several approaches can create synergies which can further enhance catalyst performance.

Moreover, research into strain regulation of ORR catalysts is still in its early stages, with the introduction and regulation of strain encountering multiple challenges. Presently, existing synthetic methods can only roughly control the size of strain and the direction, rendering precise regulation of a formidable task. In most cases, the lattice parameters of doped materials or alloys do not match completely with the substrate during the synthesis process. Simultaneously, the presence of imperfections hinders the creation and regulation of strain. Defects in the lattice, dislocations or irregular distribution of doping atoms additionally impact the transfer and distribution of strain. This leads to uneven strain and arduous management scenarios. Furthermore, although certain experimental and theoretical analyses link strain to catalytic performance, the specific mechanisms involved remain unclear. Theoretical simulation of Pt-based catalysts is used to describe their crystal structure, surface morphology and adsorption properties, which require accurate calculation methods and mode.

For the further development of Pt-based catalyst for ORR strain engineering, thorough research should be conducted in the following areas:

1) Optimization of catalyst synthesis method: Advanced synthesis technology is used to achieve greater crystallinity and state control, resulting in an improved catalyst stability and activity. High strain catalysts with excellent electrocatalytic properties can be produced by atomic layer deposition, evaporation or high temperature quenching, or develop SRE synthesis measures suitable for mass production.

2) Single crystal plane strain regulation: Studies show that compared to the commonly observed low exponential crystal faces on Pt, high exponential crystal faces exhibit a superior catalytic activity due to their higher degree of order. Catalyst crystal surface orientation and structure can be systematically modified to activate catalytic sites, enable enhanced proton transport efficiency, and subsequently improve ORR reaction rate and stability.

In conclusion, ORR catalyst with strain regulation is a very promising research topic that will play a crucial part in energy conversion and storage in the not-too-distant future. With the continuous maturity of related technologies, the ORR catalyst strain regulation technology will also provide new ideas and new methods for other catalyst research and application, and promote the sustainable development of the whole catalyst field.

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