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Review

Structural Regulation of Advanced Platinum-Based Core-Shell Catalysts for Fuel Cell Electrocatalysis

by
Xiaqing Wang
1,2,
Panpan Du
1,2,
Kun Cheng
1,2,*,
Xing Hua
1,2,
Ming Xie
1,2,
Yuyu Li
1,2,
Yun Zheng
1,2,
Yingying Wang
1,2,
Chaoran Pi
1,2 and
Shiming Zhang
3,*
1
School of Optoelectronic Materials & Technology, Jianghan University, Wuhan 430056, China
2
Key Laboratory of Flexible Optoelectronic Materials and Technology, Ministry of Education, Jianghan University, Wuhan 430056, China
3
Institute for Sustainable Energy/College of Sciences, Shanghai University, Shanghai 200444, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(3), 235; https://doi.org/10.3390/min15030235
Submission received: 15 January 2025 / Revised: 19 February 2025 / Accepted: 24 February 2025 / Published: 26 February 2025
(This article belongs to the Special Issue From Mantle to Market: Platinum Group Elements and Minerals and Their Geological Significance)
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Versions Notes

Abstract

:
Platinum (Pt), a precious metal extracted from minerals, plays an important role as a catalyst in energy conversion and storage devices. However, Pt is expensive and a limited resource, so it is crucial to maximize its utilization. In the electrocatalytic process, the improvement of its utilization is contingent on enhancing its mass and specific activities, a goal that can be significantly realized through the deposition of a Pt-based shell layer on a nanosubstrate material, thereby producing a core-shell structure. This review gives an important overview on the characteristics of Pt-based core-shell catalysts, the structural regulation of the core-shell, and its effects on the electrocatalytic performance. The core-shell structure can significantly increase the ratio of surface Pt atoms per unit mass of Pt particles. Moreover, the lattice mismatch between the core material and the platinum shell can generate strain, which can modulate the magnitude of the adsorption-desorption force of the platinum-based shell layer on the active intermediates, and thus contribute to the modulation of the catalytic performance. In addition to the aforementioned characteristics, the electrocatalytic performance of Pt-based core-shell catalysts is significantly influenced by the core and shell structures. The core-shell structures have unique advantages over other types of catalysts, leading to the development of advanced Pt-based catalysts.

1. Introduction

Platinum (Pt) group elements (PGEs), i.e., Os, Ir, Ru, Pd, Rh, and Pt, are extremely rare in the Earth’s crust and do not occur as separate natural ores [1,2,3], but rather in combination with tellurides, arsenides, antimonides, sulfides, and other complex minerals. The single elements extracted from them have good electrical conductivity, corrosion resistance, a high melting point, good catalytic properties, etc., and thus are widely used in automotive catalysts, jewelry, medical, chemical, and other energy conversion systems [4,5,6,7,8,9]. Among these elements, Pt has received the most attention,; however, currently there is a gap between supply and demand. According to the survey conducted in 2024, the demand for Pt reached 246 tons, which was 21 tons more than the supply in the same year. It is predicted that in 2025, the gap between the supply and demand of Pt will still exist. Given the high cost of Pt, it is imperative to enhance its utilization in various applications. For instance, in the production of a fuel cell vehicle (FCV), even with more advanced technology, necessitates 20 g of Pt metal, a value significantly higher than that of conventional fuel vehicles (8 g/vehicle on average). This strong dependence of FCVs on Pt is due to the fact that Pt-based catalysts perform much better than other types of catalysts in the catalytic reaction of fuel cells.
In the field of fuel cell catalysts, the current mainstream catalysts are Pt/C catalysts (i.e., 3–5 nm Pt nanoparticles loaded on carbon black supports) [10,11,12]. Although non-Pt catalysts cost less, they still have many disadvantages, including the thick catalyst layer, water flooding, and insufficient electrochemical stability [13]; thus, it is difficult to achieve commercialization in a short time. Pt/C catalysts are predominantly employed in acidic conditions, while ORR catalysts in alkaline conditions have been the subject of extensive research [14,15]. However, alkaline electrolytes are vulnerable to the toxic effects of carbon dioxide, necessitating the use of pure oxygen as an oxidant, which increases the complexity of the system. There are still some technical challenges in large-scale production and application, such as improving system stability and safety [16]. It is believed that Pt/C catalysts will remain the mainstream catalysts in the commercial application of fuel cells for quite a long period of time. In order to improve the utilization of Pt to reduce the catalyst cost, various synthesis strategies have been proposed for the development of high-performance Pt-based catalysts [17,18,19,20,21,22,23]. Typically, anchoring single metal atoms on different supports (e.g., metal oxides, metal monoliths, graphene, etc.), as schematically illustrated in Figure 1a–c, can significantly reduce the loading amount of Pt and enhance the catalytic activity in some catalytic applications [24]. However, the synthesis of single-atom catalysts is usually complicated, and it is difficult to accomplish the transition from laboratory scale to industrialization production. As shown in Figure 1d, alloying Pt with transition metals is one of the current research hotspots in the quest to realize high-performance Pt-M alloy catalysts [25]. However, the transition metals in the alloys often present problems such as corrosion and leaching, which leads to the collapse of the particle structure and degradation of the catalyst performance. The above problems can be effectively avoided by designing a Pt-based core-shell structure (Figure 1e) [26].
Pt-based core-shell catalysts are catalysts in which Pt elements are mainly present in the surface layer of the catalyst particles and play a dominant catalytic role in the catalytic process, while the interior of the particles is mainly composed of non-Pt elements. The “surface layer” and “interior” referred to here correspond to the “shell layer” and “core”, respectively. The shell layer can exist in the form of a Pt elemental substance or its alloy, and the atomic proportion in the entire particle is usually very small. The inner core may be composed of monomers, alloys, compounds of non-Pt elements, and their derived doping structures. The structural characteristics of core-shell catalysts give them the advantages of high Pt utilization, excellent activity, and good stability. In this paper, we review the characteristics of Pt-based core-shell catalysts, the regulation of core and shell structures, and their effects on catalytic performance. Especially, modulation strategies for Pt-based core-shell structures are summarized, both those for the core structures (precious metal core, non-precious metal core, transition metal carbide/nitride core, alloy core, and heteroelement-doping core) and shell structures (thickness, alloying, doping, and multi-shell layer). Also, a systematic summary and outlook are presented for the future development of Pt-based core-shell catalysts.

2. Characteristics of Pt-Based Core-Shell Structures

To design a high-performance Pt-based catalyst, there are usually the following requirements [27]: (1) as many active sites as possible to provide enough reactive sites and matching energy levels for reactants and intermediates; (2) good corrosion resistance to allow a longer service life; (3) high electrical conductivity to improve the electron transfer rate. For Pt-based core-shell structure catalysts, the internal component of the conventional Pt-based particles is replaced by other materials, which not only saves the unutilized Pt atoms inside, but also serves to modify the electronic structure of Pt atoms in the shell layer [28,29,30,31,32,33,34]. Further, adjusting the composition and structure of the inner core and outer shell and optimizing the parameters such as the size and shape of the overall catalyst particles can improve the performance of Pt-based catalysts.
It is believed that Pt-based core-shell structures have the following characteristics, which make them a class of cutting-edge catalysts. (1) Their core is composed of parts containing Pt or non-Pt materials, avoiding the waste of internal Pt atoms. Particularly, this can significantly reduce the amount of Pt elements in the whole catalyst particles. (2) The core can have a modifying effect on the electron cloud structure of the shell layer, and thus can regulate the strength of the adsorption-desorption force of the Pt-based shell layer on the reactive intermediate species, which can improve the catalytic performance of the Pt-based shell layer. Meanwhile, changes of the component and structure of the core can also affect the catalytic selectivity. (3) Lattice mismatches between the atoms of the shell layer and the core material cause lattice strain in the shell layer, and the strain effect may serve to optimize the catalytic performance, mainly due to its ability to modulate the electronic structure of the catalyst surface layer, and thus the interaction force between the catalyst surface layer and the adsorbed species [35,36,37,38,39,40,41]. (4) The thickness of the shell layer can be flexibly adjusted: a thicker shell layer tends to enhance the intrinsic catalytic performance of Pt, while as the shell becomes thinner, the surface catalytic performance will be increasingly affected by the modification of the inner core electron cloud. Consequently, varying the shell thickness can effectively modulate both the surface catalytic ability and selectivity. (5) The composition and morphology of the Pt-based shell layer can also be precisely tailored to regulate the catalytic performance. In brief, the catalytic activity and selectivity of the whole catalyst particle can be effectively tuned by adjusting the core configuration, as well as shell layer composition, strain, and thickness.

3. Regulation of Core Structure and Its Effect on Catalytic Performance

The catalytic activity and stability of Pt-based core-shell catalysts can be regulated by the interaction between the shell and different types of cores [42,43,44,45,46,47]. It is particularly important to select the appropriate core material when designing the core-shell structure. Some researchers have proposed the constraints of kernel selection, as shown in Table 1 [48]. However, there is currently no known core material that can satisfy all the conditions simultaneously. In this section, we will discuss the currently popular materials used as the core, and review their synthesis methods and properties.

3.1. Precious Metal Core

Precious metals (Ru, Rh, Pd, Ag, Os, Ir, Au) as core materials forming a core-shell structure with Pt have been widely noted for their favorable catalytic performance during fuel cell electrochemical reactions. In particular, Pd@Pt catalysts demonstrate a substantial enhancement in catalytic activity in comparison to pure Pt catalysts [49,50]. Tests using a rotating disk electrode (RDE), combined with density-functional theory (DFT) calculations, showed that the oxygen reduction reaction (ORR) activity of Pt on different substrates, including Ru, Ir, Rh, Au, and Pd monomers, has a strong volcano correlation with the d-band center [51]. Notably, the platinum monolayer supported on Pd(111) (denoted by PtML/Pd(111)) is at the top of the volcano curve and shows improved ORR activity over pure Pt(111). This is due to the fact that the Pd@Pt (111) catalyst is located near the intersection of O2 dissociation (i.e., O–O bond breaking) and oxygen hydrogenation capacity. This stems from the fact that the Pd core exerts a slight contraction effect on the Pt–Pt bond, thus weakening the binding strength of the adsorbed oxygen intermediate (O–O bond), which is beneficial to enhance the ORR activity.
Mahesh et al. [52] synthesized Pd@Pt catalysts using electroless under potential deposition (electroless-UPD) method, which differs from conventional technology where the potential is controlled by an external power supply. In their approach, Pd@Cu with a monoatomic Cu shell layer was first prepared by electroless-UPD, and then a Cu monoatomic shell layer was transformed into a Pt monatomic layer by the galvanic replacement method. The advantage of this synthesis method is that it does not require surfactants or reducing agents, and the thickness of the shell layer can be precisely controlled from mono-atomic to several atomic layers. Comparing the performance with that of commercial Pt/C (20 wt%), the ORR activity of this catalyst is significantly higher.
In addition, Ru@Pt catalysts have been the focus of current research, which have shown a significant improvement in catalytic performance and stability for the electro-oxidation of formic acid and ethanol [53,54]. Alayoglu et al. [55] synthesized Ru@Pt catalysts with an average particle size of 4 nm using a polyol process, which were shown to be more preferential than H2 oxidation for CO oxidation, and could occur at lower temperatures. This opens up new possibilities for the development of anode catalysts that are resistant to CO poisoning in low-temperature fuel cells. El Sawy et al. [56] showed through proton exchange membrane (PEM) cell data that both Ru@Pt and Rh@Pt exhibited enhanced catalytic oxidation of both methanol (Figure 2a) and ethanol (Figure 2b), as shown in Figure 2, which may be attributed to the aforementioned lattice compression effect with ligand action at play.
Overall, precious metal core-shell catalysts typically have higher catalytic activity than pure platinum particles in different reactions, while possessing good stability. However, the high cost and process complexity of precious metal core-shell catalysts make it imperative to reduce the amount of precious metals used in them in the long run.

3.2. Non-Precious Metal Core

It has been shown that the generation of a thin shell layer of Pt on a suitable non-precious metal (e.g., Fe, Co, Cu, etc.) core not only reduces the amount of precious metal Pt used, but also improves the catalytic activity of the catalyst [57,58,59,60,61,62].
Yang et al. [63] systematically investigated M@Pt12 (M = Fe, Co, Tc, Ru, Os; 12: 12 Pt atoms on the shell) core-shell nanoclusters by using ab initio DFT calculations with the generalized gradient approximation and found that they have strong resistance to structural deformation and oxidation (oxidized by adsorbed oxygen). Among them, the stability and catalytic activity of the Fe@Pt12 structure are greatly improved. However, it is difficult to control the easy oxidation and corrosion of iron elements during the synthesis reaction process, often resulting in the difficulty of obtaining a pure iron core [57,64]. As a result, there are fewer studies that utilize monolithic iron as the core material.
The Co@Pt structure has also been studied; for example, Wang et al. [65] successfully synthesized it in an organic solution using seed-mediated growth. Figure 3a shows the synthesis steps of the seed-mediated method [66], where the precursor solution is injected into a vessel containing a mixture of seeds, reducing agents, capping agents, and colloidal stabilizer. The metal precursor is reduced (or decomposed) to form zero-valent metal atoms and heterogeneously nucleates on the surface of the seed, and the nuclei eventually form nanocrystals by continuous growth. The Co@Pt catalyst showed a 10-fold increase in ORR activity over the commercial Pt/C catalyst and a loss of no more than 13% after 30,000 electrochemical cycles, as shown in Figure 3b. In addition, the Co leaching rate of this catalyst was only 5% after 30,000 cycles, compared to the commercial Pt3Co/C catalyst, which exhibited a Co leaching rate of 8% after just 5000 cycles. This shows that the structure of the catalyst can effectively inhibit the leaching of core elements.
Foucher et al. [67] successfully synthesized monodisperse Cu@Pt nanoparticles using a solvothermal method, which controls the thickness of the shells, thereby increasing the exposed surface area per unit mass of Pt and maintaining a narrow size distribution. This results in an electrochemically active surface area (ECSA) of Pt in excess of 200 m2/g. This is much higher than that of commercial Pt/C catalysts and other Pt-based catalysts currently on the market. In addition, the ECSA is above 120 m2/g even after 15,000 cycles. This approach can be extended to the synthesis of non-Pt-shell materials such as Cu@Rh, Cu@Ir, etc., with satisfactory results in terms of stability and activity.
Although introducing non-precious metals as the inner core can improve the activity and stability of the catalysts, these non-precious metals generally have weak acid and alkali resistance, causing elemental leaching problems [58,68,69,70]. Additionally, the lattice mismatch between the Pt shell layer and these metals (e.g., Fe, Co, and Cu) is relatively large, which complicates the formation of a homogeneous Pt-based shell layer. This results in the aggregation of some Pt atoms into small clusters which, on the one hand, makes some Pt atoms exhibit catalytic properties similar to those of bulk Pt, thereby preventing the modification and improvement of the catalytic performance through the inner core. On the other hand, the discontinuous Pt shells formed by these small Pt clusters weaken the protection of the inner core, potentially exacerbating the oxidative dissolution of the inner-core elements [58,71].

3.3. Transition Metal Carbide/Nitride Core

Transition metal carbides (TMCs) are a class of substances with good properties such as acid and alkali resistance, good electrical conductivity, and a high melting point. Some studies have reported that tungsten (W) and molybdenum (Mo) carbides have Pt-like catalytic properties [72,73], which has led to increasing interest in exploring TMCs as core materials for catalysts. This interest is reflected in several key advantages. (1) TMCs have sp electronic properties similar to those of Pt, which promotes a good adhesion between Pt and TMCs. This interaction helps to inhibit undesirable phenomena such as agglomeration and migration of Pt atoms [74,75,76]. (2) Carbon atoms in TMCs tend to form stable carbides with transition metals, which prevents dissolution and segregation of transition metals [77]. (3) Compared to the corresponding parent metal, the presence of carbon can reduce the lattice mismatch between the parent metal and the Pt element. This reduction minimizes the lattice strain effect induced by the parent metal below the Pt shell layer, optimizing the binding energy between the Pt shell layer and surface adsorbates during the catalytic reaction process. As a result, it is conducive to the improvement of the catalytic activity [78,79,80].
Hunt et al. [78] in 2016 successfully synthesized WC@Pt using a high-temperature self-assembly method. In this synthesis strategy, the Pt shell layer is generated without the use of a reducing agent, but by carburization, which leads to the spontaneous polarization of Pt atoms onto the WC metal surface to form the core-shell structure. It is worth mentioning that the layer of atoms in direct contact with the Pt atoms in this structure can only be tungsten atoms, not carbon atoms. DFT calculations show that the bond energy of Pt–WC is 90 kJ/mol higher than that of Pt–Pt, which can effectively prevent the Pt atoms on the surface of Pt–WC particles from escaping the particle surface during electrochemical operation. The ability of the material to resist methanol oxidation reaction (MOR) after 10,000 cycles is improved by an order of magnitude over commercial Pt/C. This provides a new direction for the synthesis method of TMC-centered core-shell catalysts. Furthermore, Wang et al. [79] used the same method to synthesize TaC@Pt in 2024, as shown in Figure 4. This novel approach leverages the protective effect of a hard SiO2 shell to inhibit the Ostwald ripening growth process of the internal particles during high-temperature heat treatment. As a result, a core-shell structure is successfully achieved, with a single or a few layers of Pt atoms encapsulating on the surface of the TaC inner core, showing high durability in ORR and MOR.
Transition metal nitride (TMN), which has similar properties to TMC, was synthesized by Garg et al. [80] using high-temperature nitridation of TiWC@Pt to produce the TiWN@Pt catalyst. In this structure, the electronic states of the d orbitals in the valence shell layer of the Pt atoms are more stable, which attenuates the binding energy with CO and exhibits higher CO tolerance. The TiWN@Pt catalyst demonstrates superior catalytic performance in certain catalytic scenarios where the adsorbate binds too strongly to Pt or where CO poisoning problem is a concern on the Pt surface.
TMN and TMC usually have good electrical conductivity and high stability, which allows for smoother electron transport and less leaching of the core elements. However, TMN and TMC are often prone to oxidization during the preparation process, and such cores may have interfacial compatibility problems with the shell material under certain conditions, which may result in a decrease in material stability. In addition, their fabrication process usually requires harsh and complex conditions, which is detrimental to the development of industrialization [81].

3.4. Alloy Core

As mentioned earlier, both precious and non-precious metals have their limitations when used individually as the cores. So, combining precious and non-precious metals into a composite core holds the potential to achieve a synergistic effect. In fact, PtM (M = Ni, Fe, Co, etc.) alloys have been extensively studied as core materials, and this type of structure is considered a promising class of catalysts. These PtM alloys offer several advantages, particularly in terms of reducing Pt loading while enhancing electrocatalytic activity [33,82,83,84]. Compared to pure Pt nanoparticles, PtM alloys have less Pt content and their area-specific activity is significantly improved due to strain effects and electronic state modification effects. During the alloying process, transition metal atoms with relatively small radii enter into the Pt lattice structure, leading to a decrease in the Pt lattice size and hence the formation of compressive strains [25]. In addition, due to the electronegativity difference between Pt and the transition metal, charge transfer will occur between the two, which will change the electron filling state of the Pt-d orbitals and can modulate the catalytic reaction performance of the Pt shell layer to some extent.
For example, PtNi@Pt catalysts have been widely studied due to their high ORR activity. Cui et al. [84] prepared Pt1Ni1@Pt/C core-shell structured catalysts with a PtNi alloy core and Pt as a shell layer on in situ converted defective carbon-containing supports with an average size of 2.6 nm by a controlled carbon defect anchoring strategy. The strong interaction between Pt and carbon defect sites effectively inhibited the migration or agglomeration of Pt-based nanoparticles. As shown in Figure 5, even after more than 70,000 cycles of electrochemical testing, only 1.6% activity decay was observed. Moreover, the Pt1Ni1@Pt/C catalyst possessed high mass and specific activities for the oxygen reduction reaction, reaching 1.424 ± 0.019 A/mgPt and 1.554 ± 0.027 mA/cm2Pt, respectively, which were higher than those of the Pt/C catalysts prepared under similar synthesis conditions (0.157 ± 0.042 A/mgPt and 0.172 ± 0.022 mA/cm2Pt) by about 8.1 and 8.0 times.
For PtFe@Pt catalysts, a large number of theoretical and experimental results have shown that they are excellent catalysts for ORR and MOR [85,86,87]. Liu et al. [85] synthesized PtFe@Pt core-shell catalyst by using a simple chemical reduction method followed by heat treatment in an inert atmosphere without H2. Compared to commercial Pt/C and disordered PtFe alloy catalysts, the PtFe@Pt core-shell catalyst showed superior catalytic activity and durability. During potentiostatic cycling, the ECSA of the commercial Pt/C and PtFe/CNT-disordered electrocatalysts reached a maximum value after about 1000 cycles, followed by a rapid decline. The maximum ECSA loss of was observed to be 19.4% for commercial Pt/C and 40.8% for PtFe/CNT-disordered, respectively. In contrast, the ECSA of the PtFe@Pt/CNT core-shell electrocatalyst gradually increased and reached a stable value at 8000 cycles, which had a significantly enhanced electrochemical stability compared with the other two electrocatalysts. The mechanism can be explained by the fact that in the early stage of electrochemical cycling, the Fe atoms in the near-surface layer gradually leached out to form a Pt-rich surface layer, and this Pt-rich surface layer structure has a strong resistance to electrochemical oxidation. As a result, the ECSA tends to increase at the beginning of the cycle and is maintained for a long time at the end of the cycle.
In addition to PtNi and PtFe, PtCo has also emerged as a well-studied core material for electrocatalysis [88]. Pan and coworkers [89] prepared a high-performance and durable cathode catalyst for polymer electrolyte fuel cells by a scalable leaching method. The catalyst, consisting of an ordered face-centered tetragonal structure (fct) CoPt (L10-CoPt) core and a thin Pt shell layer, possessed an initial mass activity of 0.6 A/mgPt and achieved the target of less than 40% mass activity decay after 30,000 accelerated durability tests (ADT). Compared with conventional CoPt alloys, this work employs an atomically ordered structure to stabilize the Co atoms within the CoPt lattice. This characteristic structure effectively suppresses the elemental leaching problem and thus improves the durability of the catalyst.
Overall, the incorporation of non-precious metals into the catalyst system serves two primary functions. On the one hand, the cost of precious metal catalysts can be effectively reduced. On the other hand, the alloying material can regulate the electronic structure of Pt, which can enhance its resistance to “poisoning” and inhibit the leaching of non-precious metals from the core. However, challenges persist in screening the optimal alloying core material, and the fine regulation of the shell’s electronic state by the alloying core remains insufficiently explored.

3.5. Heteroelement-Doping Core

Pt-M (Fe, Co, Ni, etc.) alloy cores have been widely studied, but most of the current alloys have a disordered face-centered cubic structure, in which the Pt and M atoms are randomly arranged. This disordered arrangement can easily lead to the oxidation and leaching of the M atoms, thus reducing the catalyst’s lifetime. The ordered face-centered tetragonal structure has better stability than the disordered Pt-M nanoparticles. However, the main difference between a “doped structure” and an “alloy structure” is that doping is usually the introduction of a small amount of additives into the crystal structure of the original material as dopants, which usually does not change the basic structure of the original material, and mainly changes some of the material’s specific physical or chemical properties of the material.
Liang et al. [90] reported that tungsten (W)-doped L10-PtCo was used as a core-shell catalyst with a tungsten (W)-doped core (Figure 6a). The catalyst was firstly synthesized by wet synthesis of ordered Pt-Co nanoparticles, followed by high-temperature annealing and reduction. The nanoparticles were then acid-washed to remove the cobalt atoms from the surface, resulting in ordered L10-W-PtCo nanoparticles (Figure 6a). Also, this core-shell catalyst was employed to fabricate the membrane electrode assemblies (MEAs) and tested for full-cell performance (Figure 6b–e). The results showed an initial mass activity of 0.57 A/mgPt, along with a significant improvement in stability. Notably, the catalyst exhibited only 17.5% loss in MA after 30,000 ADT cycles, and less than 30% loss after 50,000 cycles. In contrast, the commercial Pt/C catalyst showed considerable performance loss after 30,000 cycles. Analysis by DFT and extended X-ray absorption fine structure (EXAFS) tests showed that the compressive strain and low surface energy of the Pt shell layer were attributed to the ~6 at% W doping, which also stabilized the structure of the PtCo ordered intermetallic compounds, and adjusted the atomic spacing between Pt–Pt to regulate the catalytic performance.
In addition, nitrogen (N)-doped core-shell catalyst with ordered intermetallic Int-PtNiN as the core also showed good performance in MEA tests [91]. The researchers prepared the nanocatalysts by primary annealing synthesis, and DFT calculations confirmed that the N doping caused moderate tensile strain (0.2%–1.5%) in the catalyst particles. This strain was favorable for catalyzing the transition from free hydroxyl groups to water molecules, thus increasing the ORR rate. After 30,000 ADT test cycles, the mass activity of the catalyst particles was still as high as 1.33 A/mgPt (with only 27.3% depletion), which was 7.4 times higher than that of the initial commercial Pt/C counterpart. In addition, the doped N element can serve to inhibit the leaching of non-precious metals and thus enhance the stability of the catalyst particles.
In conclusion, the catalyst material activity and stability can be enhanced by doping the alloy core, which is attributed to the doping-induced lattice shrinkage of the core, which induces compressive strain in the shell layer. This not only protects the kernel and improves the stability of the catalyst, but also promotes the transformation of free hydroxyl groups to water molecules, thus enhancing the catalytic activity of oxygen reduction. However, there are many problems with the doping of the inner core, such as the difficulty in achieving precise structural doping and the difficulty in the synthesis process.

4. Regulation of Shell Structure and Its Effect on Catalytic Performance

4.1. Pt Shell Thickness

It is a common perception that thicker Pt shells may lead to higher stability. However, while thicker Pt shells increase the Pt loading, they may also inhibit the electronic state modification effect of the core on the surface layer [92,93,94,95,96]. So, the optimal shell thickness must be determined according to different application requirements.
Taking the core-shell TaC/Pt catalyst as an example, Wang et al. [79] synthesized Pt shell layers with different thicknesses corresponding to 0.2, 0.6, 0.9, and 1.2 ML (monolayer), as shown in Figure 7. TaC/Pt with relatively thin shells (0.2 and 0.6 ML) exhibited higher MOR mass and specific activities compared to commercial Pt/C (Ptcomm). For TaC@Pt with shell thicknesses below 1 ML, at the interface between Pt and surface-exposed TaC, CO-reactive intermediate species adsorbed on Pt atoms and OH-reactive intermediate species adsorbed on the TaC surface may react to produce CO2. This reaction can reduce the coverage of CO intermediate species on Pt atoms, and thus, more Pt active sites can be released.
Of course, the shell thickness should be appropriately adjusted for different cores. For example, Gan et al. [97] showed that Pt shells with 3 to 5 atomic layers could give the best catalytic activity when PtNi3 was used as the core. In this study, the authors analyzed the thickness of the Pt shell layer of alloyed PtxNi1-x nanoparticles using aberration-corrected scanning transmission electron microscopy (STEM) coupled with the electron energy loss spectroscopy (EELS) line technique. Typically, a self-assembled core-shell fine structure was observed, and a spherical Ni-rich second shell layer underneath the Pt shell layer seems to play a major role in the catalytic activity of PtxNi1−x. The Ni-rich inner shell leads to a greater degree of compressive strain in the Pt shell, which leads to the enhancement of the catalytic activity of the PtNi3 catalyst for ORR, as shown in Figure 8.

4.2. Pt Shell Alloying

In addition to modulating the thickness of the Pt shell layer, alloying the Pt shell layer with other elements has also been investigated. Ruthenium (Ru) has been proposed for alloying Pt shells due to its well-established bifunctional mechanism that enhances the resistance of the shell layer to CO “poisoning”. One of the distinguishing features of Ru compared to other metals is its ability to provide hydroxyl adsorption sites (Ru-OHads) on the catalyst surface. These sites can help the neighboring reactive Pt atoms, which adsorb CO intermediate species (Pt-COads) to catalyze the CO oxidation [98]. For this reason, the Ru element has been widely incorporated into Pt shells to form PtRu alloy shell structures [99,100,101,102,103,104,105].
As early as 2004, Zhang et al. [106] reported a class of ORR electrocatalysts consisting of Pt and another late transition metal (Ir, Ru, Rh, Re, or Os) as a monolayer alloy deposited on the crystalline surface of Pd(108) single crystals or on the surface of plain Pd nanoparticles (loaded on carbon supports), which could greatly enhance the performance of the catalysts. Li et al. [107] successfully synthesized ultrathin hollow Pt3Co alloy spheres, based on Co cores using wet chemistry. The ECSA of this Pt3Co/C catalyst is about 8 times greater than that of commercial Pt/C, and mass activity is about 4 times higher. Also, this type of hollow shell-layer structure tends to increase the electrochemical stability dramatically (Figure 9a).
In general, for multi-metallic Pt-based shell-layer structure catalysts, the following factors can affect the mass and specific activities of Pt: (1) the thickness of the Pt shell layer [44,108], (2) the roughness of the Pt shell layer surface (i.e., the average coordination number of Pt atoms on the surface [108]), and (3) the type of other elements alloyed with Pt. It is worth mentioning that the roughness of the Pt shell layer often comes from the rough surface structure formed after the dissolution of other elements. This increased roughness will indirectly decrease the average coordination number of Pt. It has been reported in the literature that such a decrease leads to excessively strong adsorption of reactive intermediate species, which can hinder the migration of reactive intermediate species at the catalytic interface and complicate the desorption of products, ultimately leading to a decrease in catalytic activity [44].

4.3. Pt Shell Doping

In addition to alloying the Pt-based shell layer, a further effective method of achieving lower cost and higher activity of Pt-based core-shell catalysts is to replace some Pt atoms in the shell with a second metal. This can be achieved by doping the shell layer with other atoms, such as Rh [109], Mo [110], Ga [111], Co [112], and so on.
Feizabadi et al. [112] successfully doped the Pt shell layer of a Pd@Pt core-shell structure with the Co element to obtain a Pd@Pt-Co catalyst (Figure 9b), which exhibited an octahedral morphology. It was shown that the activity and stability were significantly enhanced compared to the undoped structure, with only a 2% loss in initial activity after 20k cycles. The results suggest that this improvement may be due to the contraction of the Pt surface lattice induced by Co doping, which can moderately enhance the oxygen adsorption on the Pt surface.

4.4. Multi-Shell Layer

In recent years, multi-shell layer structures have also been developed which, unlike the aforementioned shell-layer alloying structure, improve the stability of the overall catalyst particles by introducing a second shell layer to protect the inner core elements. Chen et al. [113] proposed a double-shell confinement strategy in which a titanium (Ti)-rich layer is introduced underneath the Pt shell layer (Figure 9c). This additional layer can help prevent the leaching of Fe atoms from the inner core, thus protecting the internal PtFeTi intermetallic structure. The DS-PtFe0.6Ti0.4 catalyst exhibited an ORR mass activity of 1.04 A/mgPt, with only a 13.5% drop after 30,000 ADT cycles.
Minerals 15 00235 g009
Figure 9. (a) Line sweep curves of hollow Pt3Co/C before and after 5000 cycles, and the EDS line sweep of Pt3Co in the inset (reproduced with permission from [107], published Elsevier, 2016). (b) Synthesis process of Pd@Pt-Co nanoparticles (reproduced with permission from [112], published by American Chemical Society, 2023). (c) Schematic of the multi-shell layer structure, with the inset showing the polarization curves of double-shell (DS)-PtFe0.6Ti0.4 and commercial Pt/C, before and after 30,000 potential cycles in the range 0.60~0.95 V (reproduced with permission from [113], published by American Chemical Society, 2024).
Figure 9. (a) Line sweep curves of hollow Pt3Co/C before and after 5000 cycles, and the EDS line sweep of Pt3Co in the inset (reproduced with permission from [107], published Elsevier, 2016). (b) Synthesis process of Pd@Pt-Co nanoparticles (reproduced with permission from [112], published by American Chemical Society, 2023). (c) Schematic of the multi-shell layer structure, with the inset showing the polarization curves of double-shell (DS)-PtFe0.6Ti0.4 and commercial Pt/C, before and after 30,000 potential cycles in the range 0.60~0.95 V (reproduced with permission from [113], published by American Chemical Society, 2024).
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5. Summary and Outlook

In the field of fuel cell electrocatalysts, the consumption of Pt is huge. It is worth mentioning that non-precious metal catalysts are emerging as alternative materials to Pt-based catalysts [114,115,116,117]; for example, Fe-N-C, chalcocite, spinel, and other metal oxides (e.g., MnO2 and TiO2). However, due to the limitations of the production technology and catalytic effect, non-precious metal catalysts will not replace Pt-based catalysts for a long time, so it is exceptionally important to improve the utilization of Pt. The essence of high Pt utilization is to improve its mass and specific activities. Regulating the core-shell structure can have a significant effect in improving the utilization of Pt and the intrinsic activities of Pt-based electrocatalysts.
So far, most of the various Pt-based catalysts with a core-shell structure are stuck at the laboratory development stage. Moreover, there is often a certain discrepancy in the performance data between that of a half-cell based on a rotating disk electrode (RDE) and a full-cell based on membrane electrode assembly (MEA). For example, the limiting current during fuel cell operation is at least 3 orders of magnitude higher than that under RDE testing. Thus, it is a crucial task to study core-shell structure Pt-based catalysts from the RDE-level test to the MEA-level evaluation.
In addition to the Pt-based particles themselves, the catalyst support is also a very important topic [118,119]. At present, there is a lack of research on the structural design of combining the core-shell metal-based particles with various supports [120]. Importantly, the identification of a structural relationship between catalyst particles and the supports can produce practical catalysts and will promote the commercial development of the related industry chain. Usually, the catalyst particles often need to be loaded onto the surface of the carbon support. Carbon black has good electric conductivity, and an appropriate amount of functional groups on its surface can allow a strong metal-support interaction. In addition, non-carbon materials can also be used as catalyst supports, and the key disadvantage is poor electronic conductivity. To solve this issue, heteroelement doping has been found to be an effective strategy for improving the conductivity of non-carbon materials to enhance the activity [121,122,123]. Overall, carbon black is still the most popular catalyst support, and more research should be focused on the metal-support interaction and the innovative interface structure.
In this paper, the currently popular and representative Pt-based core-shell structure electrocatalysts are reviewed from the aspects of synthesis strategy, structural regulation of core-shell, and electrocatalytic mechanism. For different catalytic reactions, the efficient electrocatalysts can be obtained only after an in-depth understanding of the material properties and modulation mechanisms, which is not only applicable to fuel cell electrocatalysis, but also to industrial catalysis, photocatalysis, etc. We believe that in the future, by deepening the research on the mechanisms of various catalytic reactions, it is highly likely to obtain Pt-based core-shell catalysts with ultra-low Pt usage, high performance, and long life for the electrochemical energy storage and conversion technologies.

Author Contributions

Conceptualization, K.C.; methodology, software, validation, X.W. and S.Z.; formal analysis, Y.W.; investigation, X.W., P.D., K.C., Y.Z. and X.H.; resources, P.D., Y.L. and C.P.; data curation, X.W., Y.Z. and Y.W.; writing—original draft preparation, X.W. and P.D.; writing—review and editing, K.C. and S.Z.; visualization, C.P.; supervision, M.X. and X.H.; project administration, M.X., Y.L. and X.H.; funding acquisition, K.C. and S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Research Fund of Jianghan University (Grant No. 2023KJZX09) and the National Natural Science Foundation of China (Grant No. 22272105) and the Natural Science Foundation of Shanghai (Grant No. 23ZR1423900).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Different types of single atom catalysts with single metal atoms anchored on (a) metal oxides, (b) metal surfaces, and (c) graphene (reproduced with permission from [24], published by American Chemical Society, 2013). (d) Transition metals regulate ligand and strain effects to promote the oxygen reduction reaction of Pt alloy catalysts (reprinted with permission from [25], published by American Chemical Society, 2023). (e) Schematic diagram of Pt-based core-shell structure (reproduced with permission from [26], published by American Chemical Society, 2022).
Figure 1. Different types of single atom catalysts with single metal atoms anchored on (a) metal oxides, (b) metal surfaces, and (c) graphene (reproduced with permission from [24], published by American Chemical Society, 2013). (d) Transition metals regulate ligand and strain effects to promote the oxygen reduction reaction of Pt alloy catalysts (reprinted with permission from [25], published by American Chemical Society, 2023). (e) Schematic diagram of Pt-based core-shell structure (reproduced with permission from [26], published by American Chemical Society, 2022).
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Figure 2. Polarization curves of oxidation on the anode of samples with different compositions of (a) 0.1 M methanol and (b) 0.1 M ethanol at 80 °C (reprinted with permission from [56], published by IOP, 2020).
Figure 2. Polarization curves of oxidation on the anode of samples with different compositions of (a) 0.1 M methanol and (b) 0.1 M ethanol at 80 °C (reprinted with permission from [56], published by IOP, 2020).
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Figure 3. (a) General strategy of the seed-mediated method (reproduced with permission from [66], published by John Wiley and Sons, 2017). (b) STEM dark field map of Co@Pt core-shell particles and their mass-activity comparative data with Co-Pt alloys and pure Pt particles (loaded on carbon supports) after a certain number of electrochemical cycling numbers (reproduced with permission from [65], published by American Chemical Society, 2018).
Figure 3. (a) General strategy of the seed-mediated method (reproduced with permission from [66], published by John Wiley and Sons, 2017). (b) STEM dark field map of Co@Pt core-shell particles and their mass-activity comparative data with Co-Pt alloys and pure Pt particles (loaded on carbon supports) after a certain number of electrochemical cycling numbers (reproduced with permission from [65], published by American Chemical Society, 2018).
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Figure 4. Schematic of high-temperature self-assembly synthesized TaC@Pt particles (reproduced with permission from [79], published by John Wiley and Sons, 2024).
Figure 4. Schematic of high-temperature self-assembly synthesized TaC@Pt particles (reproduced with permission from [79], published by John Wiley and Sons, 2024).
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Figure 5. ORR durability of Pt1Ni1@Pt/C and com-Pt/C. (a) CV curves and (b) iR-corrected ORR polarization curves of Pt1Ni1@Pt/C after every 10 k potential cycles in 0.1 M HClO4. (c) CV curves and (d) iR-corrected ORR polarization curves of com-Pt/C after every 10 k potential cycles in 0.1 M HClO4. (e) MA and SA comparison betweenPt1Ni1@Pt/C and com-Pt/C during ADT (reprinted with permission from [84], published by Springer Nature, 2024).
Figure 5. ORR durability of Pt1Ni1@Pt/C and com-Pt/C. (a) CV curves and (b) iR-corrected ORR polarization curves of Pt1Ni1@Pt/C after every 10 k potential cycles in 0.1 M HClO4. (c) CV curves and (d) iR-corrected ORR polarization curves of com-Pt/C after every 10 k potential cycles in 0.1 M HClO4. (e) MA and SA comparison betweenPt1Ni1@Pt/C and com-Pt/C during ADT (reprinted with permission from [84], published by Springer Nature, 2024).
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Figure 6. (a) Illustration of synthetic route to L10-W-PtCo NPs. (b) H2-O2 fuel-cell polarization curves of commercial Pt/C and L10-W-PtCo/C at 80 °C with a backpressure of 150 kPaabs. (c) H2-O2 fuel cell polarization curves of L10-W-PtCo/C after 50,000 voltage cycles at 80 °C. (d) Pt/C after 30,000 voltage cycles at 80 °C. (e) MA of L10-W-PtCo/C and Pt/C at 0.9 ViR-free before and after voltage cycles at 80 °C (black and red dashed lines indicate the DOE beginning-of-life (BOL) and end-of-life (EOL) targets, respectively) (reproduced with permission from [90], published by John Wiley and Sons, 2019).
Figure 6. (a) Illustration of synthetic route to L10-W-PtCo NPs. (b) H2-O2 fuel-cell polarization curves of commercial Pt/C and L10-W-PtCo/C at 80 °C with a backpressure of 150 kPaabs. (c) H2-O2 fuel cell polarization curves of L10-W-PtCo/C after 50,000 voltage cycles at 80 °C. (d) Pt/C after 30,000 voltage cycles at 80 °C. (e) MA of L10-W-PtCo/C and Pt/C at 0.9 ViR-free before and after voltage cycles at 80 °C (black and red dashed lines indicate the DOE beginning-of-life (BOL) and end-of-life (EOL) targets, respectively) (reproduced with permission from [90], published by John Wiley and Sons, 2019).
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Figure 7. (a,b) Comparison of mass activity and specific activity and CV plots of methanol oxidation for different thicknesses of shell layers versus commercial Pt/C. (c) Mass activities of Ptcomm and Pt/TaC 0.6 ML (the symbols TaC@Pt 0.6 ML are used in this paper) measured at various potentials at steady-state. (d) CV plots of Ptcomm and Pt/TaC 0.6 ML for the oxidation of methanol in the presence of CO saturated 0.1 mol/L HClO4 and the inset showing the CV maps obtained in the absence of CO. (e) STEM images of Pt/TaC_0.6 ML before and after 10,800 ADT cycles and the corresponding elemental EDX maps and (f) ECSA retention. (g) ORR curves of Pt/TaC with different shell thicknesses and Pt/Ccomm. (h) Specific activity and mass activity of Ptcomm and Pt/TaC core-shell nanoparticles at 0.9 V (reproduced with permission from [79], published by John Wiley and Sons, 2024).
Figure 7. (a,b) Comparison of mass activity and specific activity and CV plots of methanol oxidation for different thicknesses of shell layers versus commercial Pt/C. (c) Mass activities of Ptcomm and Pt/TaC 0.6 ML (the symbols TaC@Pt 0.6 ML are used in this paper) measured at various potentials at steady-state. (d) CV plots of Ptcomm and Pt/TaC 0.6 ML for the oxidation of methanol in the presence of CO saturated 0.1 mol/L HClO4 and the inset showing the CV maps obtained in the absence of CO. (e) STEM images of Pt/TaC_0.6 ML before and after 10,800 ADT cycles and the corresponding elemental EDX maps and (f) ECSA retention. (g) ORR curves of Pt/TaC with different shell thicknesses and Pt/Ccomm. (h) Specific activity and mass activity of Ptcomm and Pt/TaC core-shell nanoparticles at 0.9 V (reproduced with permission from [79], published by John Wiley and Sons, 2024).
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Figure 8. (a) Structural model of distinctly different compositional core-shell fine structures of dealloyed PtxNi1−x catalysts. (b) Initial CV curves in N2-saturated 0.1 M HClO4 aqueous solution. (c) CV curves in N2-saturated 0.1 M HClO4 aqueous solution after electrochemical dealloying. (d) polarization curves of ORR by linear scanning voltammetry from 0.06 to 1.0 V at 5 mV/s in O2-saturated 0.1 M HClO4 aqueous solution. (e) Comparison of mass activities and specific activities at 0.9 V (reproduced with permission from [97], published by American Chemical Society, 2012).
Figure 8. (a) Structural model of distinctly different compositional core-shell fine structures of dealloyed PtxNi1−x catalysts. (b) Initial CV curves in N2-saturated 0.1 M HClO4 aqueous solution. (c) CV curves in N2-saturated 0.1 M HClO4 aqueous solution after electrochemical dealloying. (d) polarization curves of ORR by linear scanning voltammetry from 0.06 to 1.0 V at 5 mV/s in O2-saturated 0.1 M HClO4 aqueous solution. (e) Comparison of mass activities and specific activities at 0.9 V (reproduced with permission from [97], published by American Chemical Society, 2012).
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Table 1. Constraints proposed by some researchers in the selection of ideal core material (reprinted with permission from [48], published by MIT libraries, 2016).
Table 1. Constraints proposed by some researchers in the selection of ideal core material (reprinted with permission from [48], published by MIT libraries, 2016).
1Earth-abundant and inexpensive.
2Metallic electrical conductivity and acidic/alkaline corrosion resistance.
3Melting point >2000 °C.
4Shell material is insoluble in the core lattice.
5Shell monolayer formation on the core is favorable, and interfacial bonding is strong.
6Core has minimal or favorable modifications to shell work function and d-band
7Readily manufactured and independently tunable core-shell architectures
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Wang, X.; Du, P.; Cheng, K.; Hua, X.; Xie, M.; Li, Y.; Zheng, Y.; Wang, Y.; Pi, C.; Zhang, S. Structural Regulation of Advanced Platinum-Based Core-Shell Catalysts for Fuel Cell Electrocatalysis. Minerals 2025, 15, 235. https://doi.org/10.3390/min15030235

AMA Style

Wang X, Du P, Cheng K, Hua X, Xie M, Li Y, Zheng Y, Wang Y, Pi C, Zhang S. Structural Regulation of Advanced Platinum-Based Core-Shell Catalysts for Fuel Cell Electrocatalysis. Minerals. 2025; 15(3):235. https://doi.org/10.3390/min15030235

Chicago/Turabian Style

Wang, Xiaqing, Panpan Du, Kun Cheng, Xing Hua, Ming Xie, Yuyu Li, Yun Zheng, Yingying Wang, Chaoran Pi, and Shiming Zhang. 2025. "Structural Regulation of Advanced Platinum-Based Core-Shell Catalysts for Fuel Cell Electrocatalysis" Minerals 15, no. 3: 235. https://doi.org/10.3390/min15030235

APA Style

Wang, X., Du, P., Cheng, K., Hua, X., Xie, M., Li, Y., Zheng, Y., Wang, Y., Pi, C., & Zhang, S. (2025). Structural Regulation of Advanced Platinum-Based Core-Shell Catalysts for Fuel Cell Electrocatalysis. Minerals, 15(3), 235. https://doi.org/10.3390/min15030235

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