Abstract
Electrocatalysts used for oxygen reduction and oxygen evolution reactions are critical materials in many renewable-energy devices, such as rechargeable metal-air batteries, regenerative fuel cells, and water-splitting systems. Compared with conventional electrodes made from catalyst powders, oxygen electrodes with a freestanding architecture are highly desirable because of their binder-free fabrication and effective elimination of catalyst agglomeration. Among all freestanding electrode structures that have been investigated so far, fibrous materials exhibit many unique advantages, such as a wide range of available fibers, low material and material-processing costs, large specific surface area, highly porous structure, and simplicity of fiber functionalization. Recent advances in the use of fibrous structures for freestanding electrocatalytic oxygen electrodes are summarized, including electrospun nanofibers, bacterial cellulose, cellulose fibrous structures, carbon clothes/papers, metal nanowires, and metal meshes. After detailed discussion of common techniques for oxygen electrode evaluation, freestanding electrode fabrication, and their electrocatalytic performance, current challenges and future prospects are also presented for future development.
Highlights
With increasing demands for clean and sustainable energy sources, the development of low-cost and efficient renewable energy technologies has received enormous attention from both industrial and academic sections in recent years
2.2.8 Turnover Frequency (TOF) TOF is always used to compare oxygen revolution reaction (OER) performance of different electrocatalysts. It is calculated from the following equation: TOF = (j × A)/(4 × F × m) where j is the current density at a given overpotential, A refers to the area of the working electrode, F represents the Faraday constant (96485 C/mol) and m means the number of moles of the active materials
Except for getting nitrogen source from NH3 gas, Ye et al reported a novel “vein-leaf” carbon structure through simple calcination of a mixture of urea and Bacterial cellulose (BC), in which urea acted as both the nitrogen source and the precursor for the layered graphitic carbon nitride.[113]. Such 3D network was favourable for electron transfer and mass diffusion during an oxygen reduction reaction (ORR) process. Another N-doped CNF catalyst was synthesized by carbonizing PPy coated BC, and it was demonstrated that the pyridinic-N (2.95%) played a vital role in enhancing ORR performance.[114]
Summary
With increasing demands for clean and sustainable energy sources, the development of low-cost and efficient renewable energy technologies has received enormous attention from both industrial and academic sections in recent years. The use of organic solvent for paste preparation can pose safety hazards, with serious environmental and health concerns.[10] In this regard, fabricating freestanding electrocatalytic oxygen electrode from available low-cost material to avoid the pre-preparation of electrocatalyst powders and the use of any organic binder and solvent, while at the same time, to achieve high catalytic activity and durability is of practical importance to the future development of metal-air batteries and fuel cells.[11]. The basic oxygen electrochemistry and evaluation criteria will be firstly introduced, followed by detailed discussion on the fabrication and electrocatalytic performance of various types of freestanding oxygen electrodes obtained from different fibrous sources (electrospun nanofibers, bacterial cellulose, cellulose fibrous structures, CFP, CC, metal nanowires and metal meshes, etc.). The challenges and prospects are discussed for future development in the field
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