Pt-Based Electrocatalysts with Special Three-Dimensional Morphology or Nanostructure

Xiangchun Tang; Jiaxiang Chen; Lina Liu; Shijun Liao

doi:10.7536/PC210218

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (7) : 1238-1248 DOI: 10.7536/PC210218

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Pt-Based Electrocatalysts with Special Three-Dimensional Morphology or Nanostructure

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Abstract

Fuel cell technology and its industrialization have been developed rapidly in China in recent years. However, the high cost of the fuel cell caused mainly by the using of precious Pt catalysts is still one of the most important factors restricting the development of fuel cell commercialization. It is of great significance to develop low Pt catalysts with much higher catalytic efficiency and lower Pt loadings. In recent years, Pt-based catalysts with three-dimensional morphology or/nanostructures have been emerged as a type of ultra-important low Pt catalysts, due to their special morphology/structures, their catalytic activity are usually much higher than that of the widely used Pt/C catalysts. In this paper, the research progress of Pt-based catalysts with special three-dimensional morphology(such as nanoframe structure, flower-like structure, nanocage structure, sea urchin structure, etc.) and their applications in fuel cells are reviewed. Meanwhile, some weaknesses and challenges of these catalysts are concluded. Furthermore, the future development and application of these catalysts are prospected.

Contents

1 Introduction

2 Pt-based electrocatalysts with three-dimensional nanoframe structures

3 Pt-based electrocatalysts with three-dimensional nanoflower morphologies

4 Pt-based electrocatalysts with other three-dimensional structures

5 Conclusions and outlooks

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Keywords

fuel cell / Pt-based catalysts / three-dimensional morphology / catalytic activity / stability

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Xiangchun Tang, Jiaxiang Chen, Lina Liu, Shijun Liao. Pt-Based Electrocatalysts with Special Three-Dimensional Morphology or Nanostructure. Progress in Chemistry, 2021, 33(7): 1238-1248 DOI:10.7536/PC210218

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1 Introduction

Fuel cell technology is known as the most important new energy technology in the 21st century because of its high energy conversion efficiency and environmental friendliness. In recent years, fuel cell technology and fuel cell industry have developed rapidly in China and the world.Due to the high cost caused by using a large amount of noble metal Pt as a catalyst and the insufficient durability caused by the degradation of Pt catalyst itself, it is still one of the most important factors restricting the development of fuel cell technology and industry. Therefore, the research and development of new Pt-based fuel cell catalysts with higher activity and stability is of great significance for the development of fuel cell technology and industry.

The platinum-based catalysts studied in recent years include platinum catalysts, platinum-transition metal alloy catalysts and platinum-transition metal intermetallic compound catalysts. Improving the activity and stability of Pt-based catalysts has become an important research topic in the field of fuel cells.The main research ideas can be summarized as follows: the design and control of catalyst morphology, the control of catalyst components, and the introduction of appropriate strain effect, defect effect and crystal plane effect in the catalytic system to improve the catalytic activity and stability, or the selection of different catalyst supports to improve the catalytic performance^[1⇓⇓~4]^[5,6]^[7⇓~9]^{[10⇓⇓~13]}^{[14⇓⇓⇓~18]}^[19,20]. The results show that the construction of Pt-based catalysts with special morphology is a very effective control strategy. The main ideas are as follows: (1) The specific surface area and atom utilization of the catalyst are improved by the design of special morphology. The results show that the active sites of the catalyst are located on the surface of the catalyst, and the Pt atoms in the nanoparticles often do not participate in the reaction, resulting in low utilization of Pt. Therefore, in order to improve the utilization of Pt atoms, we can design some catalysts with special morphology to expose more active sites on the surface of the catalyst. Three-dimensional structured nanomaterials often have high surface area, which can provide abundant active sites for reactant molecules and effectively improve the mass transfer and gas diffusion on the alloy surface, thus enhancing the electrocatalytic performance. (2) The construction of nano-materials with three-dimensional structure can often form high-index crystal planes with special structures and other crystal planes with special structural characteristics on the surface of materials, thus causing a significant increase in catalyst activity.

Based on the above advantages of three-dimensional Pt-based catalysts, Pt-based catalysts with 3D morphology show great application prospects in electrocatalytic reactions. Therefore, designing Pt-based catalysts with different three-dimensional morphologies has become a hot research topic in the field of fuel cells and electrocatalysis.

2 Three-dimensional framework Pt-based electrocatalyst

In 2014, Stamenkovic et al. First reported the Pt-based catalyst with three-dimensional framework structure. Because the catalyst has a high specific surface area and is rich in high-index crystal planes, the Pt mass specific activity can reach 5.7 A/mg Pt, which is more than 20 times that of the currently widely used Pt/C catalyst, making the research on three-dimensional framework Pt-based catalyst rapidly become a hot research topic in the field of electrocatalysis^[16]. Conventionally, the utilization efficiency of Pt can be improved by making the Pt-based catalyst particles smaller. However, the catalyst may suffer from severe agglomeration during use. The three-dimensional nano-framework shows unique structural advantages. Firstly, it has a highly open three-dimensional structure, a higher specific surface area, a higher density of active sites, and a more sufficient contact with reactant molecules, which can accelerate the reaction. Secondly, its structure is more stable and can resist the harsh reaction environment. Therefore, the three-dimensional metal nanoframework can effectively improve the activity and stability of the catalyst and reduce the amount of noble metal Pt^{[21⇓⇓⇓~25]}.

The synthesis of noble metal Pt-based nanoframeworks is mainly based on the following two strategies (as shown in Figure 1): (1) selective deposition and etching of active sites^[21]; (2) Dealloying of hollow or bi (poly) metal nanocrystals. In general, the synthesis mechanism includes an electric displacement reaction, an oxidation etching, diffusion of internal atoms, or a combination thereof.

Xia Younan's team selectively deposited Pt at the edges and corners of the Pd cube, and then removed the internal Pd core by chemical etching to form a Pt cube nanoframework composed of ridges less than 2 nm thick (Fig. 2)^[26]. The Br^- plays an important role in the deposition of Pt, and it is mainly adsorbed on the side of the Pd cube. The initial mass specific activity of the Pt cubic framework catalyst for ORR (0.45 A/mg Pt) was 1.5 times that of the commercial Pt/C catalyst (0.3 A/mg Pt). After 20 000 cycles of accelerated aging test (Accelerate Decay Testing, ADT), the mass specific activity of the catalyst is still 6 times that of the commercial Pt/C catalyst. It is believed that the highly open framework structure improves the ORR activity and stability of the catalyst by improving the utilization of Pt atoms and preventing the exfoliation and agglomeration of Pt nano-framework on the carbon support.

By introducing another element M (M = Fe, Co, Ni, Cu) to form a Pt-M alloy with Pt, the electronic structure of the Pt atom can be adjusted, thereby improving the intrinsic activity of the catalyst^{[27⇓⇓~30]}. Therefore, Pt-M alloy nanoframe exhibits more excellent electrocatalytic properties than monometallic Pt nanoframe. Stamenkovic et al. reported a rhombic dodecahedral Pt₃Ni nanoframework (Pt₃Ni NFs/C) with a Pt (111) shell (fig. 3)^[16]. The structure has large specific surface area, and more active sites can be exposed on the inner surface and the outer surface, so that the structure can be fully contacted with oxygen molecules; At the same time, the surface of the catalyst has a Pt (111) layer with a thickness of two atomic layers, and the change of the distance between Pt atoms on the surface induces the generation of strain, thereby reducing the adsorption energy of Pt and oxygen-containing intermediates and showing extremely high ORR catalytic activity. The mass specific activity and area specific activity of Pt/C at 0.95 V are 36 times and 22 times higher than those of commercial Pt/C, respectively. The catalyst also has good stability, as shown in fig. 4. After 10,000 cycles, the surface area of the Pt/C catalyst decreases by 40%, while the structure of the Pt₃Ni nanoframework (Pt₃Ni NFs/C) with the surface Pt (111) layer does not change, and the loss of activity of the catalyst is negligible. The results show that the high stability is due to the fact that the surface Pt layer effectively prevents the dissolution of Ni in the interior.

Lee et al. Reported a PtNi composite framework catalyst with a dendritic structure embedded in the PtNi nanoframework^[24]. After 10 000 cycles of ADT test, the mass specific activity of PtNi composite framework/C lost only 15. 3%, while that of Pt/C lost 50%. In addition, it is found that the multi-component alloys with more component effects have more advantages in improving the catalytic activity and stability of the electrocatalyst. Wang Erkang's team successfully synthesized PtRuNi nanoframeworks (PtRuNi FDs) surrounded by ultra-fine Ru nanobranches by a two-step method^[31]. With the increase of Ni content, the PtRuNi nanotree framework gradually evolved into PtRuNi nanotrees (PtRuNi NDs). In acidic environment, PtRuNi FDs exhibited the best MOR mass specific activity (1.49 A/mg Pt), which was 2.9 times that of commercial Pt/C (0.52 A/mg Pt) and superior to many reported catalysts^[32].

PtCu nanoframe is another common Pt-based nanoframe electrocatalyst. The reported PtCu octagonal and PtCu rhombic dodecahedron have excellent catalytic performance^[25,33]^[34]. By changing the ratio of Pt and Cu precursors, Shen Peikang's team prepared two different octagonal nanoframeworks (PtCu₂CONFs and PtCu UONFs) with high-index crystal planes by one-pot method^[25]. Interestingly, the PtCu₂CONFs is composed of eight symmetric legs with sharp angles, which are surrounded by n (111)- (111) high-index planes, such as (553), (331), and (221). Compared with the low-index crystal planes, the high-index crystal planes contain a high density of step atoms and kink atoms, the coordination number of these atoms (CN = 6,7) is less, and the chemical activity of these atoms is high, so they are easy to interact with reactant molecules, break chemical bonds and become catalytic active centers, so the high-index crystal surfaces generally have higher activity^[15]. Benefiting from the accessible surface with 3D structure and multiple high-index crystal planes, the PtCu₂ catalyst exhibits excellent electrocatalytic performance and resistance to CO poisoning. In the MOR reaction, the catalyst showed a higher peak current density, up to 155.9 mA/cm². Compared with Pt/C, the MOR catalytic activity of Pt/C was increased by 7 times, and the onset potential was decreased by 205 mV. More importantly, the PtCu₂ catalyst showed better stability in the face of harsh electrochemical reaction conditions, and its ECSA decreased by 8. 5% after 2000 cycles of voltage scanning, while the ECSA of PtCu and Pt/C decreased by 20. 5% and 80%, respectively. In addition, the subject combined a catalyst (PtCu OFAs) having a novel octagonal nanoframework structure with high-index crystal planes, which has abundant step atoms and a plurality of high-index crystal planes ( (553), (332), (331), (311), and (221) and (211) crystal planes) (fig. 5)^[33].

The catalyst exhibits an ultra-high mass specific activity of 3.26 A/mg Pt, which is 20 times the mass specific activity of the commercial Pt/C catalyst (0.16 A/mg Pt). In addition, the catalyst also showed high stability, and its structure remained intact after 10 000 cycles of ADT test, while its composition and catalytic performance did not change significantly, and its mass specific activity still reached 2. 95 A/mg Pt, which was 47 times that of Pt/C. In addition to the synthesis of PtCu catalysts with octagonal nanoframework structure with high-index crystal planes, Shen Peikang's team also synthesized Ru-doped PtCu rhombic dodecahedral PtRuCu nanoframework composed of (553), (332), (331) and (221) high-index crystal planes.

Huang Xiaoqing's team synthesized PtCu rhombic dodecahedral nanoframework through a simple one-pot method (Figure 6)^[34]. The catalyst showed excellent catalytic activity and stability for methanol oxidation reaction (MOR). Compared with the Pt/C catalyst, the mass specific activity (0.98 a/mg Pt) of the catalyst is improved by 0.72 times, and the area specific activity (2.35 mA/cm²Pt) is improved by 1.77 times. After 200 cycles of CV scanning, the PtCu framework catalyst still has a mass specific activity of 0.39 A/mg Pt, which is 3.9 times that of Pt/C catalyst (0.1 A/mg Pt). They believe that the high activity is attributed to the highly open three-dimensional nanoframework structure with high Pt atom utilization. Lee et al. Reported a PtCu @ PtCuNi core-shell nanoframework consisting of a PtCu dendritic structure and a PtCuNi alloy framework (fig. 7)^[35]. The inner PtCu dendritic structure is used to support the outer PtCuNi alloy framework to increase the structural stability of the catalyst. At the same time, PtCu @ PtCuNi catalyst has excellent ORR activity, and its mass specific activity and area specific activity are respectively 11 times and 16 times that of commercial Pt/C (Fig. 8). In addition, the team enhanced the stability of PtCu nanoframework by doping Co to improve the stability of the catalyst^[36]. Compared with Pt/C and PtCu RNF catalysts, Co-PtCu RNF alloy nanoframework showed higher catalytic activity and stability for MOR and ORR.

The carrier of the nano-framework is generally a carbon carrier, and the strength of the interaction between the carrier and the catalyst affects the stability and charge transfer of the catalyst. Therefore, it is necessary to study the nano-framework supported by different carbon support materials. Tian Xinlong et al. Reported the Pt₃Cu nano-framework supported by TiN carrier, namely the Pt₃Cu/TiN nano-framework^[37]. The catalyst showed excellent ORR activity and stability, and its mass specific activity reached 2.43 A/mg Pt, which was 11.6 times that of commercial Pt/C. The area specific activity is 5.32 mA/cm²Pt, which is 14.4 times that of Pt/C. Under the same ADT conditions, the Pt₃Cu/TiN exhibits higher stability and less performance degradation than the Pt₃Cu/C. The high activity and stability of the Pt₃Cu/TiN nanoframe are attributed to more charge transport, the unique nanoframe structure and the synergistic effect of anchoring Pt and TiN carriers. In addition to the significant influence of the support on the activity and stability of Pt-based electrocatalysts, the ordering degree of the alloy also has a significant effect on the catalytic performance. Ordered alloy catalysts generally exhibit higher activity and stability than disordered alloy catalysts^[38]. Joo et al. Successfully synthesized an atomically ordered intermetallic PtCu nanoframe (O-PtCu NFs/C) (fig. 9), which showed high ORR activity, durability, and chemical stability (fig. 10)^[39].

PtCo nanoframe also shows excellent electrocatalytic activity and stability. Yang Peidong et al synthesized a highly open three-dimensional PtCo nanoframe catalyst by HNO₃ etching, which shows excellent ORR activity in acid and good stability^[40]. In addition, the mass specific activity of PtCo catalyst for MOR reached 4. 28 A/mg Pt in alkaline medium, which was 4 times higher than that of commercial Pt/C. The experimental results show that the high activity of PtCo catalyst is attributed to the weakening of the binding between the carbonaceous poison intermediates and Pt. Zhu et al. Designed and prepared a PtCo ND-NFs catalyst, which consists of an internal Pt-Co nano-dendrite (ND) and an external Pt layer framework (NFs)^[41]. The catalyst has a high mass specific activity of 0.939 a/mg Pt, which is 5 times that of commercial Pt/C (0.187 a/mg Pt). After 50 000 cycles of ADT test, the mass specific activity of the catalyst can still reach 0.566 A/mg Pt, which is 3 times of the initial activity of commercial Pt/C.

In addition to the common PtNi, PtCu and PtCo nanoframeworks, other Pt-based nanoframeworks such as PtAu, PtPd, PtRuNi, PtCuMn, PtNiSn and PtPdRhAg also show excellent electrocatalytic properties^[42]^[43]^[31]^[44]^[45]^[46].

3 Three-dimensional flower-like Pt-based electrocatalyst

Pt-based nanoflowers with three-dimensional (3D) interconnected porous structures have attracted considerable attention. Such structures have high porosity, highly rough surfaces, a high density of corner and edge sites, and sufficient adsorption sites for all relevant reaction molecules in a narrow space to facilitate relevant reaction kinetics^[47]. In addition, the flower-like structure can effectively prevent the catalyst from agglomerating^[48].

The most common way to synthesize three-dimensional nanoflowers is the one-pot wet chemical synthesis method, which can synthesize nanocrystals with controllable size and shape on a large scale. In addition, Wang Aijun's team developed a simple one-pot method to prepare Pt₃Co nanoflowers by co-reducing acetylacetone platinum (II) and acetylacetone cobalt (III) in oleylamine^[49]. No seed or template was used in this synthesis process, and HDPC was both a stabilizer and a structure-directing agent, which played an important role in the formation of well-dispersed Pt₃Co nanoflowers. Compared with solid Pt₃Co nanoparticles and commercial Pt/C, the prepared Pt₃Co nanoflowers showed higher catalytic performance for oxygen reduction reaction (ORR). At the same time, the Pt₃Co nanoflowers showed more excellent catalytic activity and stability. The improved catalytic performance of Pt₃Co nanoflowers is mainly attributed to the porous three-dimensional interconnected structure, the increased specific surface area, the ligand effect and the bifunctional interaction between Pt and Co. In 2018, the team also synthesized a uniform Pt₆₆Ni₃₄ nanoflower (Fig. 11), which clearly shows that the outer surface of the Pt₆₆Ni₃₄NFs has abundant steps and ladder atoms from high-resolution transmission electron microscopy^[50]. In addition, there were some defect sites (fig. 12). Its high density of atomic step sites and defects provide abundant active sites for catalytic reactions. In addition, the selective area electron diffraction (SAED) pattern shows that the Pt₆₆Ni₃₄NFs has four bright concentric rings, which correspond to (111), (200), (220) and (311) planes from inside to outside, respectively, indicating the polycrystalline nature of the prepared Pt₆₆Ni₃₄NFs. The onset potential of the Pt₆₆Ni₃₄NFs was -28 mV, close to that of Pt/C (-27 mV), and lower than that of the Pt₂₅Ni₇₅NCs(-53 mV) and the Pt₇₇Ni₂₃NCs(-61 mV), showing significantly improved HER activity. After the durability test, the ECSA values of the Pt₆₆Ni₃₄NFs and Pt/C are about 95% and 68% of their original values, respectively, indicating that the surface structure of the Pt₆₆Ni₃₄NFs has not changed significantly. At the same time, the polarization curves of the electrode in acidic medium did not change significantly after 1000 cycles. The catalytic activity of Pt₆₆Ni₃₄NFs in alkaline medium showed a low decay within 1000 cycles and a relatively flat decline after 2000 cycles, indicating that the catalytic performance of Pt₆₆Ni₃₄NFs in alkaline medium was stable. Liu et al. Developed a simple one-pot wet chemical method to synthesize Pd @ PtNi core-shell nanoflowers (CSNFs) supported on multi-walled carbon nanotubes (MWNCTs)^[51]. Brij 58 was shown to be a structure-directing agent for the formation of nanoflowers, while ascorbic acid acted as a reducing agent for the formation of core-shell structures. By adjusting the molar ratio of Pd and Pt, Pd @ PtNi/MWCNTs CSNFs showed higher ORR activity and longer durability in alkaline electrolyte than PtNi/MWCNTs nanoflowers and commercial Pt/C. The results showed that both the core-shell structure and the porous characteristics of the nanoflowers were beneficial to enhance the catalytic performance. Similarly, Pt-Pd/C flower-like nanostructures, PtNi, PtAg flower-like nanocrystals, PtCoRu nanoflowers, etc. Synthesized by a one-pot wet chemical method also showed excellent catalytic performance^[52]^[50,53]^[54]^[55].

Electrodeposition techniques are also relatively simple and can selectively deposit metals or alloys on active substrates with both proton and electron conduction at room temperature. Its advantage is that it can control the nucleation, deposition amount and particle size with specific morphology by changing the deposition parameters (including current density, time, medium, etc.). Kalpana et al. Deposited three-dimensional ultrafine Pt nanoflowers directly on a carbon-coated gas diffusion layer electrode (C-GDL) by a single-step electrodeposition method^[56]. Sha and Li et al. Deposited flower-like Pt catalysts on TiO₂ nanotube arrays by pulse electrodeposition at room temperature^[57]. The morphology and the exposed crystal planes of the Pt catalyst can be adjusted by changing the concentration of the H₂SO₄ in the electrolyte precursor or by replacing the H₂SO₄ with HCl. The results showed that the Pt nanoflowers with (111) crystal face exhibited the best catalytic performance for MOR when the concentration of H₂SO₄ was 0.25 M.

In addition to one-pot method and electrodeposition, Guo et al. Successfully synthesized PtNi alloy nanoflowers (PtNi-H/MWCNTs) supported on multi-walled carbon nanotubes (MWCNTs) by in-situ sacrificial template method^[58]. The reduction peak potential of the Pt oxide of the PtNi-H/MWCNTs nanoflowers has a positive shift of 26 mV compared to the commercial Pt/C-JM, indicating a weaker binding of oxidized species on the surface of the PtNi-H/MWCNTs nanoflowers. The weak oxygen affinity of ORR is helpful to enhance the electrocatalytic activity of ORR. After 800 cycles, the half-wave potential of commercial Pt/C-JM had a negative shift of 10 mV, while the half-wave potential of PtNi-H/MWCNTs nanoflowers had almost no negative shift. This directly demonstrates that PtNi-H/MWCNTs nanoflowers are more stable to ORR than commercial Pt/C-JM.

4 Pt-based electrocatalysts with other three-dimensional morphologies/structures

In addition to nano-framework and nano-flower-like structure, the more common three-dimensional Pt-based catalytic materials include nano-cage, nano-hollow structure, nano-sponge, nano-sea urchin and so on. In addition, PtCu-O octahedral nanostructures constructed by nano-dendrites, cushion-like Pt-Ag octahedral alloy nanostructures, concave PtCo nanostructures, aerogel networked Pt-Sn nanowires, PtCu (Au) nanowire networks and so on also show excellent electrocatalytic properties due to their special 3D morphology characteristics^[59]^[60]^[61]^[62]^[63~64].

Similar to the nano-framework structure, the nano-cage structure also eliminates the influence of inert atoms inside the structure, thus maximizing the atomic utilization of the catalyst. Recently, Sun Xueliang's team designed a novel trimetallic Pt-Pd-Ni octahedral nanocage structure^[65]. Compared with commercial Pt/C, the ORR mass specific activity (1. 17A/mg Pt) of the octahedral Pt-Pd-Ni nanocages is improved by about 10 times, and the area specific activity (3.8 mA/cm²Pt) is improved by about 25 times. The enhanced activity is attributed to the synergistic effect of the exposed Pt (111) crystal face and the hollow structure, and the influence of the doping of Ni atoms on the geometric structure of the catalyst. In addition, the nanocage material has excellent stability^[66,67]. For example, Li et al. Deposited Pt shells several atomic layers thick on Pd to form icosahedral core-shell nanoparticles (fig. 13)^[68]. Ultrathin Pt-rich nanocages with icosahedral symmetry and (111) crystal planes were obtained by selective etching of Pd. The ORR activity of the Pt-rich icosahedral nanocages decreased only slightly after stability testing^[68]. In addition to the common single-shell nanocages, the porous double-shell nanocages can also effectively improve the utilization efficiency of Pt atoms and accelerate the mass transfer in the reaction process^[69].

There are many methods to synthesize 3D hollow structures, and etching is the most commonly used method^[70]. Fu et al. Prepared 3D PtNi hollow nanochains using nickel as a sacrificial template^[71]. Liu et al. Synthesized hollow PtAg alloy nanotube aerogels by etching Ag^[72]. However, the etching process involves many complex steps, which will inevitably lead to the waste of metal precursors. The one-pot method is considered to be a simple method to synthesize 3D hollow structures, which can maximize the use of metal precursors, and the capping agent plays an important role in the synthesis process. The mechanism is that the capping molecules always interact strongly with the metal precursors and/or the surface atoms of the nanocrystals, which can affect the reduction potential of the precursors to adjust the reduction kinetics, thus changing the growth habits of different crystal faces and ultimately affecting the shape and structure of the products. Feng's group has developed a simple one-pot co-reduction method, which can control the synthesis of 3D porous PtAg hollow nanochain networks (HNCNs). Benefiting from the unique 3D interconnected porous hollow structure and the synergistic effect between bimetals, Pt₇₅Ag₂₅HNCNs catalysts have higher catalytic performance for ORR than Pt₃₇Ag₆₃HNCNs, Pt₄₅Ag₅₅HNCNs and Pt/C catalysts^[73]. Specifically, the mass specific activity of the Pt₇₅Ag₂₅HNCNs is 0.37 A/mg Pt at 0.81 V (vs RHE), which is 2.5 times higher than that of Pt/C.

Three-dimensional (3D) porous nanosponges have special multifunctional catalytic nanostructures, and have good prospects for electrocatalytic applications due to their interconnected nanoscale skeletons and voids. Porous nanosponge structures of various Pt-based alloys, including PtCu, PtAu, PtMo, and PtRu, have been previously reported^[74]^[75]^[76]^[77]. Tran et al. Synthesized graphene-encapsulated 3D-PtNi nanosponge (G-PtNi NS) at room temperature using formic acid as a reducing agent and carbon quantum dots as a structure-directing agent (fig. 14)^[78]. The mass specific activity and area specific activity of G-PtNi NS were 1.15 a/mg Pt and 1.95 mA/cm²Pt, which were 6.1 and 3.9 times higher than those of Pt/C, respectively. DFT calculations demonstrate the excellent ORR performance of 3D G-PtNi NS, which is attributed to the graphene shell, which alters the oxygen affinity of the PtNi nanosponge. Yang Xiurong's team expanded the kinetic-controlled sodium borohydride reduction method to synthesize bimetallic PtRh alloy nanosponges (NSs) at room temperature without adding capping agents^[79]. Their composition can be adjusted by changing the ratio of Pt and Rh precursors. When used as unsupported catalysts for MOR, the prepared PtRh NSs exhibited composition-dependent electrochemical activity and durability, and in all cases were superior to pure Pt NSs and commercial Pt/C catalysts.

In nature, sea urchins rely on their prickly skin to protect themselves, which has inspired chemists to develop urchin-like or dendritic nanocrystals for high-performance catalysis. On the one hand, it increases the surface area and therefore provides more reaction sites; On the other hand, it can reduce agglomeration. This design is particularly attractive for precious metals and their alloys because it reduces the amount of precious metal used, thereby reducing costs. Nanoscale echino-like Pt, PtNi, PtAu, PtPd, etc. Have been reported, and there are also many related reports on ternary Pt-based dendritic nanocatalysts. For example, Wang et al. Developed ternary PtPdRu dendritic nanocages with 3D electrocatalytic surfaces by using polyvinylpyrrolidone (PVP) as a stabilizer and ascorbic acid (AA) as a reducing agent in a selective chemical etching strategy^[80]^[81]^[82]^[83]^[84]. Zhou's group prepared dendritic porous three-dimensional PtPdRu (PtPdRu-3D) nanocatalysts by a simple one-pot hydrothermal method using P123 as reducing agent, stabilizer and structure-directing agent^[85]. Compared to zero-dimensional PtPdRu (PtPdRu-0D), two-dimensional PtPdRu (PtPdRu-2D), and commercial Pt/C, PtPd-Ru-3D exhibits an ultra-high electrochemical surface area (ECSA) and shows excellent electrocatalytic activity for MOR, which is 2.9 times higher than that of PtPd-Ru-0D, 1.3 times higher than that of PtPd Ru-2D, and 1.7 times higher than commercial Pt/C.

5 Conclusion and prospect

Pt-based catalysts with three-dimensional special morphology are a kind of highly efficient electrocatalysts with great development in recent years (as shown in Table 1), in terms of material preparation technology, activity and structure characterization, and mechanism exploration.A lot of research work has been carried out and many important results have been achieved. Several important preparation methods have been formed, and some basic understanding of its activity and mechanism has been obtained. However, this type of catalyst still has the following problems:

(1) The preparation process is complex, which leads to the fact that the current research mainly stays at the stage of micro-preparation in the laboratory, and it is necessary to further explore a simple, repeatable and easy-to-scale preparation technology;

(2) At present, the research of this kind of catalyst is still at the level of rotating disk electrode, and there is almost no report on the application of this kind of catalyst in single fuel cell;

(3) The current 3D Pt-based catalyst model can not well reflect the real reaction conditions of the catalyst, so it is necessary to establish a theoretical calculation model closer to the real environment as soon as possible, and develop a new and efficient theoretical calculation method.

(4) The understanding of the structure formation mechanism and catalytic mechanism of these catalysts is still insufficient.

Future research should focus on: exploring and developing new preparation technologies; The formation mechanism of the special morphology of the catalyst and the catalytic mechanism were studied by means of advanced technologies such as in-situ technology. The performance and practical application prospects of these catalysts in fuel cells were studied.

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