Date Published: December 05, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Qian Ren, Hui Wang, Xue‐Feng Lu, Ye‐Xiang Tong, Gao‐Ren Li.
The oxygen reduction reaction (ORR) is the core reaction of numerous sustainable energy‐conversion technologies such as fuel cells and metal–air batteries. It is crucial to develop a cost‐effective, highly active, and durable electrocatalysts for ORR to overcome the sluggish kinetics of four electrons pathway. In recent years, the carbon‐based electrocatalysts derived from metal–organic frameworks (MOFs) have attracted tremendous attention and have been shown to exhibit superior catalytic activity and excellent intrinsic properties such as large surface area, large pore volume, uniform pore distribution, and tunable chemical structure. Here in this review, the development of MOF‐derived heteroatom‐doped carbon‐based electrocatalysts, including non‐metal (such as N, S, B, and P) and metal (such as Fe and Co) doped carbon materials, is summarized. It furthermore, it is demonstrated that the enhancement of ORR performance is associated with favorably well‐designed porous structure, large surface area, and high‐tensity active sites. Finally, the future perspectives of carbon‐based electrocatalysts for ORR are provided with an emphasis on the development of a clear mechanism of MOF‐derived non‐metal‐doped electrocatalysts and certain metal‐doped electrocatalysts.
In order to meet the urgent requirements for sustainable and renewable power supplies in the booming electronics industry, numerous endeavors have been made in the development of high‐efficiency energy conversion and storage devices.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 The oxygen reduction reaction (ORR) plays a key role in the progress of advanced electrochemical energy conversion systems, including proton exchange membrane fuel cells (PEMFCs), metal–air batteries, direct alcohol fuel cells (DAFCs), etc. Nevertheless, these energy systems have their own limitations, including the sluggish kinetics of ORR, high cost and poor stability because of the aggregation/dissolution of Pt nanoparticles (NPs) in the Pt‐based electrocatalysts of these energy conversion systems.5, 12, 13, 14, 15, 16 Developing highly efficient electrocatalysts for ORR is a crucial step to accelerate the commercialization of these advanced energy storage and conversion devices. In this connection, a series of relevant explorations have been performed in both electrocatalysts and support materials for ORR, including that: (i) alloying Pt with other transition metals to increase specific activity and reduce the cost of electrocatalysts;17, 18, 19, 20, 21, 22 (ii) developing highly active nonprecious metal or metal oxide electrocatalysts with low cost;23, 24, 25, 26 (iii) searching new‐type support materials to increase active site centers.27, 28, 29, 30, 31, 32, 33 Recently, the number of researches related to nonprecious metal electrocatalysts has greatly increased because of popular price, regulable morphological structure and high electrocatalytic activity compared with noble metal electrocatalysts, such as transition metal oxides,34, 35, 36, 37, 38, 39, 40 metal phosphides,41, 42, 43, 44 metal sulfides,45, 46, 47, 48 as well as carbon‐based materials.49, 50, 51, 52 Among these advanced electrocatalysts, porous carbons have been widely applied in many research fields as electrode materials and catalytic supports for energy storage devices, fuel cells, adsorbents, and drug delivery carriers, etc. They feature plentiful attractive properties including high surface area, high conductivity, low cost, abundant porosity, and excellent corrosion resistance, which can be considered as the most ideal supports or Pt‐free electrocatalysts for ORR in fuel cells.53, 54, 55, 56 There are various methods to obtain highly porous carbon‐based electrocatalysts, such as direct carbonization of polymeric aerogels, electrospinning fiber technique, pyrolysis of organic precursors with physical or chemical activation, and nanocasting with sacrificial solid templates (such as zeolites and mesoporous silicas).57, 58, 59 Although the activated porous carbons possess a high surface area, the disordered structures caused by broad pore size distribution may limit their availability. In consequence, it is essential to explore a proper precursor for the preparation of metal‐free carbon‐based electrocatalysts with high specific surface areas, large pore volumes, and proper chemical stabilities for ORR. MOFs as the novel precursors of highly porous carbon‐based electrocatalysts have attracted great concerns.60, 61, 62, 63, 64, 65, 66 Their unique intrinsic structures provide an opportunity to obtain higher surface areas (≈10 000 m2 g−1) than other porous materials, such as zeolites and activated carbon, etc.67 Distinguished from the harsh operating conditions and high energy consumption of traditional synthetic strategy, the carbon‐based materials from carbonized MOFs as electrocatalysts offer numerous advantages: (i) The structure, composition, and function of MOFs possess flexible tunabilities because they can be modularly designed according to the targeted properties by self‐assembly of metal ions/clusters and bridging organic ligands68, 69; (ii) It is easy to implement heteroatom‐doped carbon materials with different nonmetals or metal elements, and this can be attributed to the ultrahigh surface area, various pore size distribution, and ordered porous structure of MOFs, which are easy to adsorb organic molecules.70, 71, 72 In 2008, Xu and co‐workers demonstrated the first example that they employed MOF‐5 framework as a template to obtain nanoporous carbon, which displayed high electrochemical performance as the electrode material for electrochemical double‐layered capacitor (EDLC) due to the high surface area and hydrogen adsorption capacity of nanoporous carbon derived from MOF‐5.70
In recent years, nonmetal‐doped carbon‐based electrocatalysts have demonstrated an enormous research prospect for the development of ORR, especially nitrogen (N)‐doped carbon‐based catalysts. Researchers observed that the carbon catalysts containing nitrogen species, involving carbon nanotubes (CNTs),77, 78 graphene,79, 80 and hollow carbon spheres,81 exhibited good electrical conductivity and high selectivity toward ORR as an excellent nonmetal electrocatalyst candidate for ORR.82, 83, 84 This can be ascribed to the intrinsic electronic properties originating from the conjugation between graphene π‐system and nitrogen lone pair electrons.85 In 2009, Dai and co‐workers first engineered an aligned nitrogen‐containing carbon nanotube arrays as a metal‐free electrocatalyst for four‐electron oxygen reduction process in alkaline fuel cell, which showed a good catalytic activity, low overpotential and good electrochemical tolerance.86
Among the numerous heteroatom‐doped carbon electrocatalysts, the transition metal (such as Fe, Co, Cu, Ni, Mn, etc.) and N co‐doped carbon catalysts can achieve high ORR catalytic activity owing to their unique electronic structures and synergetic effects between diverse active species. However, besides Co or Fe‐doped carbon electrocatalysts, MOF‐derived other metal, such as Cu, Ni, and Mn, doped carbon electrocatalysts are rarely published in recent years,7, 123, 124, 125 likely due to their poor catalytic activity and stability compared with Fe and Co‐doped catalysts.126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136 Hence, MOF‐derived Fe and Co‐doped carbon catalysts are mainly summarized in this review. The ORR activity of Co phthalocyanine complexes were firstly discovered by Jasinski in 1964,137 and then the concept was proposed by Yeager and co‐workers who handled the surfaces of carbon materials with nitrogen groups to bind transition metal species to form M–Nx structure.102 Because of the similar M–Nx catalytic sites, the M–N–C ORR electrocatalysts, which are usually obtained via a pyrolysis process of composite precursors containing metal ion, nitrogen, and carbon, have been extensively investigated.138, 139, 140 Different from most of the complex classical synthetic procedures, MOFs, as the solid precursors to form the M–Nx moieties doped carbon electrocatalysts, possess three main advantages: (i) it is capable to form hierarchical pores by the pyrolysis of MOFs141, 142, 143, 144, 145; (ii) the high homodispersity of active metal centers in carbon matrices is beneficial for enhancing graphitization, as well as further promoting the electrocatalytic activity146, 147, 148, 149; (iii) the electrocatalytic properties can be regulated and controlled through a rational design of MOF precursor’s composition and structure, or introducing guest molecules.150, 171, 172, 186, 190 In addition, in virtue of the protection of graphitic carbon shells around the metallic nanoparticles, the active metal centers in the carbon matrices of MNx/C structure are relatively stable. In view of three phase interfaces (gas, liquid, and solid) in the electrocatalytic process for ORR, it is imperative to prepare electrocatalysts with a hierarchically porous structure and uniformly distributed high‐density active sites to compensate low catalytic activity and avoid overcommitting the electrocatalysts, thus ensuring a high ORR performance.151, 152
Heteroatom‐doped carbon materials as efficient ORR electrocatalysts hold great promise to replace noble metal electrocatalysts for advanced electrochemical energy conversion systems including proton exchange membrane fuel cells, metal–air batteries, etc. Doping different elements, fabricating multidimensional structures and constructing of hierarchical pores have been widely employed to design superior ORR electrocatalysts. The fabrication of MOF‐derived heteroatom‐doped carbon electrocatalysts usually owns the advantages of mild reaction conditions, convenient operating process, and low cost. Furthermore, benefiting from the unique and adjustable structure of MOF precursors, the obtained electrocatalysts possess ultrahigh specific surface area, hierarchical pores structure, and high‐density active sites with a good dispersity, which will provide fast mass and proton transfers as well as the enhanced catalytic activity of ORR. The general strategy for the fabrication of nonmetal heteroatom‐doped carbon electrocatalysts from MOFs is performed through introducing guest molecules or a direct carbonation of MOF precursors containing heteroatoms. Unfortunately, they might suffer from a small amount of metal residue, thus resulting in ambiguous catalytic mechanism for metal‐free carbon‐based electrocatalysts of ORR. It has been discovered that the introduction of the additional carbon into the carbonation process of host MOFs is not only conduce to completing removal of residual metals or their compounds, but also enhances the graphitization degree of ultimate carbon‐based materials. Even so, the related research is still in the preliminary stage. Therefore, for the nonmetal heteroatom‐doped carbon electrocatalysts, the future research efforts will be advocated to solve the issue of metal residues and explore the catalytic mechanism of pure metal‐free carbon electrocatalysts derived from MOFs. In addition, the multiplex heteroatom‐doped carbon electrocatalysts could further improve electrocatalytic activity of ORR, which can be attributed to the synergistic effects among the various heteroatoms that can generate a large asymmetrical spin and optimize the charge density into carbon skeletons.
The authors declare no conflict of interest.