Core-Shell Structure for Oxygen Reduction Reaction: Comparison
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With the deterioration of the ecological environment and the depletion of fossil energy, fuel cells, representing a new generation of clean energy, have received widespread attention. This review summarized recent progress in noble metal-based core–shell catalysts for oxygen reduction reactions (ORRs) in proton exchange membrane fuel cells (PEMFCs). The novel testing methods, performance evaluation parameters and research methods of ORR were briefly introduced. The effects of the preparation method, temperature, kinds of doping elements and the number of shell layers on the ORR performances of noble metal-based core–shell catalysts were highlighted. The difficulties of mass production and the high cost of noble metal-based core–shell nanostructured ORR catalysts were also summarized. Thus, in order to promote the commercialization of noble metal-based core–shell catalysts, research directions and prospects on the further development of high performance ORR catalysts with simple synthesis and low cost are presented.

With the deterioration of the ecological environment and the depletion of fossil energy, fuel cells, representing a new generation of clean energy, have received widespread attention. Noble metal-based core–shell catalysts for oxygen reduction reactions (ORRs) in proton exchange membrane fuel cells (PEMFCs). The novel testing methods, performance evaluation parameters and research methods of ORR were briefly introduced. The effects of the preparation method, temperature, kinds of doping elements and the number of shell layers on the ORR performances of noble metal-based core–shell catalysts were highlighted. The difficulties of mass production and the high cost of noble metal-based core–shell nanostructured ORR catalysts were also summarized. Thus, in order to promote the commercialization of noble metal-based core–shell catalysts, research directions and prospects on the further development of high performance ORR catalysts with simple synthesis and low cost are presented.

  • core–shell nanostructure
  • oxygen reduction reaction
  • electrocatalyst
  • noble metal-based

1. Introduction

With the development and progress of science and technology, the exploitation and utilization of fossil fuels has reached its peak [1,2,3,4,5,6,7,8,9][1][2][3][4][5][6][7][8][9]. However, fossil fuel is a kind of non-renewable energy. Due to continuous exploitation, the content of fossil fuels in the earth is decreasing. Moreover, the generated gas after the combustion of fossil fuels has caused serious pollution to the environment, resulting in a large number of natural disasters. Therefore, the development of a new type of sustainable clean energy and efficient utilization system is particularly important to solve the current problems such as environmental pollution and energy shortages [10,11,12,13,14,15,16,17,18,19,20][10][11][12][13][14][15][16][17][18][19][20]. Different from the traditional internal combustion engine, which converts the chemical energy of fuel molecules into mechanical energy via combustion, a fuel cell is a device that converts the chemical energy of fuel molecules directly into electric energy through an electrochemical reaction. It is considered as one of the candidates for the next generation of energy because of its high energy conversion efficiency, its environmentally friendly characteristics, high energy density and so on [21,22][21][22]. However, the oxygen reduction reaction (ORR) at the cathode has become the biggest limiting factor for proton exchange membrane fuel cells (PEMFCs). Due to its slow kinetics, the application of fuel cells depends on a large number of platinum (Pt) catalysts. Due to the low reserves, Pt is extremely expensive, which means cathode catalysts account for about half of the commercial production cost of fuel cells. Therefore, reducing the loading of Pt and improving the efficiency of catalysts represent a very important research area in the field of fuel cells [23,24,25][23][24][25].
The core–shell nanomaterials are composed by a shell (outer material) and core (inner material). The core–shell structural noble metal-based nanomaterials are generated when a noble metal is used as the shell. Among many ORR catalysts, core–shell structural noble metal-based nanomaterials are more advanced. Core–shell structural noble metal-based catalysts have the following two main advantages: on the one hand, it uses the relatively cheap core material as the carrier and to support the noble metal material in the outer layer, which is able to greatly reduce the preparation cost of the catalyst. On the other hand, the synergy effect of the core and shell can improve the ORR activity and stability of the catalyst in a complex reaction environment. The synergy effect is mainly determined by the following three kinds of effects: (1) the ligand effect: the different composition of core and shell leads to electron transfer and the change of band structure; (2) the strain effect: the tension or compression of the noble metal shell lattice causes the change of surface adsorption energy; (3) the geometric effect: the different three-dimensional structure of surface atoms can affect the electrochemical properties. Through the combination of these effects and theoretical calculation, core–shell structural noble metal-based nanomaterials with better ORR properties can be reasonably designed and prepared [26,27,28][26][27][28].
In recent years, a large number of core–shell structural noble metal-based ORR catalyst with ultra-high activity have been successfully designed and synthesized. The attention has gradually shifted to the economic benefits such as saving costs and simplifying the preparation process. So, efforts to create a simpler and batch-operable preparation method which can decrease the dosage of noble metals are underway. For example, the traditional preparation method of the underpotential deposition (UPD) method requires a potentiostat to accurately control the reaction potential. Additionally, the reaction environment is harsh which is not suitable for batch production [29,30][29][30]. A synthesis strategy of the spontaneous deposition of copper without electrochemical equipment, reductants and stabilizers was designed [31]. The above strategy not only greatly reduces the requirement of reaction conditions, but also improves the ORR performances of the catalysts. In order to reduce the consumption of noble metals, a series of investigations were carried out on the thickness of the shell. Additionally, costs can be reduced by decreasing the thickness of the shell. A series of highly efficient catalysts with only monolayer shells were obtained, which exhibited ultra-high stability [32,33][32][33]. The ORR performances of core–shell structural noble metal-based catalysts can be affected by many factors. Hence, in order to design core–shell structural noble metal-based catalysts that are more suitable for commercial production, more information about the factors affecting ORR performance needs to be elucidated.

2. ORR Testing Technology

The ORR process involves multiple intermediates and multi-step reactions. Many factors, such as electrode potential, type of catalyst, crystal plane structure, reaction temperature and so on, could affect the corresponding electrochemical performances [34,35,36][34][35][36]. The lack of experimental verification methods for the reaction process results in accurate ORR mechanisms not being obtained [37]. However, in recent years, researchers have begun to describe the ORR mechanism by means of characterization [38,39,40][38][39][40] and theoretical calculation [41], which is a benefit in the design of catalysts that meet the requirements. After the desired structural catalysts are obtained, the ORR activity and durability of the catalysts can be analyzed by measuring the electrochemically active surface area (ECSA), Tafel slope, the onset potential (Eonset) and the half-wave potential (E1/2) [42,43,44,45,46][42][43][44][45][46]. Two testing techniques are described in detail below.

2.1. Testing Technology of Thin-Film Rotating Disk Electrode (TF-RDE)

In principle, the newly synthesized ORR catalyst should be tested and evaluated in the real working environment of the fuel cell, but this testing method is often not feasible in practice. On the one hand, the preparation of membrane electrode assembly (MEA) requires professional skills, complex expansion equipment and a large number of catalysts, so it is difficult to prepare a good MEA in the general laboratory. On the other hand, a rapid screening technique for performance is needed in the early stage of catalyst preparation. However, the complexity of MEA preparation cannot achieve the purpose of rapid screening. TF-RDE is a common laboratory technology based on commercial RDE technology (Figure 1a), which can test the ORR performance of milligram catalysts [47]. TF-RDE is supported by complete hydrodynamic equations and convection–diffusion equations. In the process of continuous improvement, it provides a fine method for membrane preparation technology and electrochemical parameter control [48,49][48][49]. Because of its simple and fast operation, TF-RDE is widely used in the laboratory. Additionally, there is a standard operation flow that can compare the performances of different catalysts to a certain extent [50,51][50][51]. However, it should be noted that there are still differences in the details of the experimental schemes and methods of TF-RDE, which may lead to different measured catalyst activities or different results.

2.2. Testing Technology of Gas Diffusion Electrode (GDE)

Although TF-RDE is practical for the initial screening of catalysts in the laboratory, some shortcomings of TF-RDE limit its ability to predict catalysts in complex battery environments. On the one hand, in the procedure of TF-RDE, the reaction gas (H2, O2 or air) is charged into the electrolyte. However, the low solubility of the gas in the electrolyte is very different from the gas concentration in practical application [52]. On the other hand, the catalyst layer used in TF-RDE is usually less than 1 μm, while the thickness is greater than 5 μm in actual fuel cells. In addition, the catalyst coverage area is also very different. So, there is a great difference in the catalyst activity measured by TF-RDE and MEA [53]. Therefore, GDE testing technology has been proposed, which is an intermediate technology between TF-RDE and MEA (Figure 1b–d) [54]. Through the improvement and supplement of GDE testing technology, the gap between laboratory catalyst testing and actual fuel cell catalyst testing can be narrowed under simple and fast conditions [55,56,57][55][56][57].
Nanomaterials 12 02480 g001
Figure 1. (a) The RDE configuration. (b) Radar charts of RDE. (c) The GDE configuration. (d) Radar charts of GDE and MEA. Reproduced with permission from [54]. Copyright © 2022, Nature.
GDE is a kind of electrode with a high active surface area and high catalytic activity according to the principle of fuel cells. The principle is to keep the gas and electrolyte in equilibrium in the micropores at atmospheric pressure. Then, the potential is controlled to cause an electrochemical reaction while a stable gas–liquid–solid three-phase interface is formed [58]. At present, GDE testing can check the relevant catalyst layer structure, current and potential range similarly to MEA testing technology. Additionally, it can ensure the comparability, speed and accuracy of half-cell experiments. Therefore, GDE test is considered to be an important step from TF-RDE test to MEA test in the prediction of catalyst application performance.

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