Nonmetallic Electrocatalysts for Oxygen Reduction Reactions: Comparison
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Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by 斐 张.

As a key role in hindering the large-scale application of fuel cells, oxygen reduction reaction has always been a hot issue and nodus. Nonmetallic catalysts have a low cost, good stability and resistance in oxygen reduction reactions. 

  • nonmetallic electrocatalysts
  • oxygen reduction reaction
  • carbon materials

1. Introduction

Fuel cells and metal-air batteries are clean, efficient and safe batteries that convert chemical energy directly into electric energy, and they have the advantages of a high energy density, safety and environmental protection compared with traditional energy sources [1,2][1][2]. Currently, Pt-based cathode catalysts are widely used as the main catalyst of precious metals; however, high costs and limited reserves become major obstacles to the commercialization of fuel cells [3,4,5][3][4][5]. In addition, the fuel usually contains an amount of CO gas, easily making a Pt-based catalyst susceptible to poisoning [6,7,8][6][7][8]. Direct methanol fuel cells are a type of fuel cells that use methanol as the anode active substance directly. In these cells, methanol can penetrate the cathode through an ion exchange membrane, leading to catalyst poisoning and deactivation. Hence, studying the methanol tolerance of catalysts is of significance in practical applications. Therefore, improving the utilization rate of platinum, reducing its consumption and developing new non-platinum catalysts with low prices have become the main research directions of electrocatalysts for low-temperature fuel cells in recent years [9,10,11][9][10][11].
The development of low-cost, highly reactive non-precious metal catalysts can reduce the cost of fuel cells and promote large-scale commercial applications [7,12,13,14,15][7][12][13][14][15]. The surface chemical properties of one-dimensional titanates give them broad prospects in catalysis and energy storage [16]. Recent studies show that nonmetallic catalysts have a relatively high ORR catalytic activity and stability, and they are expected to replace Pt-based catalysts [6,17,18,19,20,21][6][17][18][19][20][21]. For ORR catalysts, the following basic conditions must be met: (1) good electrical conductivity; (2) high specific surface area; (3) abundant active sites [22]. Therefore, heteroatomic-doped nano-carbon materials are the most studied among non-metallic catalysts, including single-atom (such as N, S, P and B) doped carbon materials and multi-doped (such as N/S, N/B, N/P and N/P/S) carbon materials [6,12,23,24,25,26,27][6][12][23][24][25][26][27].

2. Carbon Materials

2.1. Carbon Nanotubes

Carbon nanotubes are one-dimensional materials with a special structure (nanoscale radial dimensions, micrometer-scale axial dimensions and sealed ends at both ends of the tube). They are well known for their excellent mechanical strength, stability and conductivity and large specific surface areas, making them highly favored in the field of electrochemistry as electrode materials [28]. The tube ends and defects of carbon nanotubes have a certain catalytic activity and can directly catalyze oxygen reduction. The surface modification of carbon nanotubes can significantly improve their catalytic performance, and the current approach for catalyzing oxygen reduction mainly involves functionalizing their surface groups and doping them with oxygen or nitrogen atoms to promote the oxygen reduction reaction [29].

2.2. Graphene

Graphene is a novel carbon material composed of a single atomic layer of sp2 hybridized carbon atoms. Due to its large specific surface area, excellent conductivity, high electron transfer efficiency and good chemical stability, it can be used as a promising catalyst support material for the cathodic oxygen reduction reaction in fuel cells [30,31,32][30][31][32]. Graphene doping with heteroatoms (such as N, P, S, etc.) can not only change the charge distribution and electronic properties in the graphene structure but also create new active centers on its surface, thereby enhancing its electrocatalytic activity [33,34][33][34]. In addition, the doped atoms are covalently bound to the carbon atoms, and the effect of graphene doping will not fade away, even in a long-term battery environment, ensuring the stability and practical value of doping as a cathodic ORR catalyst.

2.3. Graphdiyne

Graphdiyne is a two-dimensional planar carbon material formed by connecting benzene rings through 1,3-diynyl bonds. Its sp2 and sp hybridized carbon structures give it high π-conjugation, a regularly ordered porous structure and tunable electronic properties, making it potentially useful in the field of energy storage. The unique sp-C in GDY provides specific sites for designing chemical reactions, enabling controlled doping. The presence of diynyl bonds in GDY creates positively charged carbon atoms, promoting the adsorption and activation of oxygen and facilitating its decomposition. The presence of sp-hybridized carbon atoms in GDY allows for the further regulation of the charge distribution through the doping of heteroatoms (such as N, S, B, P, etc.), generating new hybrid forms and improving electrocatalytic activity.

3. Single-Atom Doped Carbon Materials

3.1. Nitrogen Doping

The process of nitrogen (N) doping carbon materials is relatively simple and easy to achieve, and the ORR catalytic activity and stability of N-doped carbon materials are high. Therefore, among all kinds of heteroatomic-doped carbon materials, N-doped carbon materials are the most studied [9,35,36,37][9][35][36][37]. The doping structure of nitrogen atoms plays a leading role in the performance of the catalyst. N-doped carbon materials have five bonding forms, which are pyridine nitrogen, pyrrole nitrogen, graphite nitrogen, nitrile nitrogen and nitrogen oxide [38]. It is not clear which N-doped carbon material has the best activity [39].  When the N doping concentration is high, it is easy to form pyridine nitrogen and pyrrole nitrogen with a two-dimensional planar structure; thus, the plane-conjugated large π bond structure can be preserved. The electrical conductivity of N-doped graphene materials is higher, so the catalytic activity of ORR is better. The direction of preparing high-performance N-doped carbon materials is to form pyridine nitrogen and pyrrole nitrogen with a two-dimensional planar structure and reduce or inhibit graphite nitrogen with a three-dimensional uneven structure [41][40]. The N content and specific surface area of doped carbon materials have an important effect on the catalytic performance. The template method can increase the specific surface area of the catalyst. Carbon nanotubes (CNTS) have many advantages, such as an intrinsic sp hybrid structure, excellent electrical conductivity, high specific surface area and good chemical stability, which have attracted wide attention [43][41]. Therefore, the doping of heterogeneous elements in CNTs can optimize the surface electronic structure and surface charge distribution of the material, enhance the adsorption of O2, and also introduce more defects (edges, vacancy, etc.) to promote ORR catalysis [44][42]. Nitrogen-containing precursors come from a wide range of sources and have a controllable structural morphology, so N-doped carbon nanotubes (N-CNTs) have attracted widespread attention. Dai et al. prepared electron-rich N-doped sp2 hybrid vertical carbon nanotube arrays (VA-NCNTs), and the ORR activity was close to that of P/C under alkaline conditions [45][43]. As N atoms possess a higher electronegativity than C atoms (the N electronegativity is 3.04; the C electronegativity is 2.55), a theoretical calculation shows that N doping makes adjacent c atoms positively charged, which is conducive to the adsorption of O2 for promoting ORR activity [46][44]. The form and content of N bonding have a crucial influence on the catalytic performance. Generally speaking, in N-doped carbon materials, N has three bonding structures, which are pyridine nitrogen (398.6 eV), pyrrole nitrogen (400.6 eV) and graphite nitrogen (401.6 eV).  According to the reported research, for the ORR catalytic property with the chemical compositions of N-doped carbon samples, the catalytic activity order of different N species is pyridinic-N > pyrrolic-N > graphitic-N > oxidized-N > C (carbon) [59,60][45][46]. Although N-doped carbon materials still face some problems such as unclear catalytic mechanisms and a lower catalytic performance than that of commercial Pt/C catalysts in an acidic medium, they are still one of the non-metallic catalysts with potential development for fuel cells [38,48,61,62,63,64][38][47][48][49][50][51].

3.2. Boron Doping

Since the boron atom is less electronegative than the C atom, the addition of boron will carry a partial positive charge, while the surrounding C part carries a negative charge [65][52]. Therefore, the electron-deficient properties of boron can be used to modify pure carbon materials and be used in non-metallic oxygen reduction catalysts. The 2 pz vacant orbital of the B element can conjugate with the delocalized π orbital of carbon to activate the delocalized π electron, strengthen the sp2 hybrid structure of graphite and improve the ORR catalytic activity of B-doped carbon materials [66][53]. Under alkaline conditions, the ORR catalytic activity of BCNTs increased with the increase in the B content. The improvement of the ORR catalytic performance was due to the conjugation of the 2 pz orbital of B with the orbital of carbon, which was catalyzed by a two-electron process, similar to N-doped carbon nanotubes. Compared to those of NCNTs catalysts, the stability and CO toxicity resistance of BCNTs catalysts were also good. However, the ORR catalytic activity of BCNTs was lower than that of commercial Pt/C catalysts and even lower than that of NCNTs catalysts under acidic conditions. Suo et al. made boron-doped carbon catalysts by chemical vapor deposition and high-temperature annealing [68][54]. 

3.3. Phosphorus Doping

Theoretical calculations have confirmed that the involvement of phosphorus can effectively promote oxygen reduction catalysts. Peng et al. prepared phosphorus-doped graphite flake materials, which showed high electrocatalytic activity, stability and methanol resistance in alkaline dielectrics [73][55]. Phosphorus-doped multi-walled carbon nanotubes were obtained by adding ferrocene to the original system, and the diameter and size of the nanotubes could be controlled by the pyrolysis temperature. The catalytic activity of carbon nanotubes containing a small amount of phosphorus exceeds that of commercial carbon-loaded platinum under alkaline conditions, but the role of residual metal has yet to be investigated. Moreover, Yang used the SBA-15 template to prepare phosphorus-doped mesoporous carbon materials, ensuring that the catalyst only acts as phosphorus and thus avoids the effect of metal on the oxygen reduction activity [74][56]. X-ray photoelectron spectroscopy (XPS) confirmed that phosphorus mainly exists in the form of P-O and P-C in the catalyst doped with phosphorus. The doping of P has significantly improved the catalytic activity of pure carbon materials and is more stable and more resistant to toxicity than commercial Pt/C. Although the ORR catalytic activity of B- and P-doped carbon catalysts is not as good as that of commercial Pt/C catalysts, they have better stability and anti-CO poisoning ability than Pt/C catalysts, which is of certain value in the research of non-metallic catalysts [77][57].

3.4. Sulfur Doping

In addition to studying doping N, B or P, the sulfur atom, which is more electronegative than B and P and similar to C, has also received much attention. Zhai et al. prepared S-doped reduced graphene oxide (S-RGO) nanosheets using dimethyl sulfone and GO as raw materials by a simple method [78][58]. Compared with the commercial 20% Pt/C catalyst, S-RGO has better corrosion resistance to methanol, stability and CO poisoning resistance. Sun et al. prepared S-doped carbon microspheres with a micropore structure by the in situ doping method, with a specific surface area of more than 503 m2/g [79][59]. The ORR catalytic activity and stability of S-doped carbon microspheres are better than those of undoped carbon microspheres. The reason is that the crystal structure and specific surface area of microspheres are changed by S doping into an appropriate carbon lattice, and the content of S plays a key role in the catalytic activity of the ORR of doped carbon microspheres [80][60]. 

4. Binary-Doped Carbon Materials

In addition to doping carbon materials with a single element, the binary doping of carbon materials can also be performed using synergies between heteroatoms, such as nitrogen, boron, nitrogen, sulfur, nitrogen and phosphorus double-doping. The electronegativity of B, P, S and N elements is different from that of N elements. The synergistic effect of these elements with nitrogen and the unique electronic structure can be used to improve the current carbon materials and achieve multi-doping. At present, most of the carbon materials doped with heteroatoms, especially nanotubes and graphene, have been prepared by chemical vapor deposition under vacuum, which is complicated and expensive [88][61].

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