Oxygen reduction reaction (ORR) is an important electrochemical process for renewable energy conversion and storage applications such as fuel cells and metal-air batteries. ORR is sluggish in kinetics and requires a large amount of platinum group metal (PGM)-based catalysts to facilitate its slow reaction rate. Application of precious metals raises the cost and decreases the competitivity of these devices in the market. To address this challenge, PGM-free ORR catalysts have been intensively investigated as an alternative to replace the PGM-based catalysts and to promote the deployment of ORR-related applications. In particular, the biomass holds promising potential to be used as the precursor material for PGM-free ORR catalysts. This pathway has gained more and more attention in recent years. In this review, recent advances regarding biomass-derived ORR catalysts are summarized with a focus on the rational design of both active sites and porous structures which are the two key factors in determining ORR performance of catalysts. At the end, the perspectives of development of biomass-derived catalysts is discussed.

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Undoped and C-doped (C: Mg, Ni, Mn, Co, Cu, Cr) ZnO nanorods were synthesized by a hydrothermal method at temperatures as low as 60 °C. The effect of doping on the morphology of the ZnO nanorods was visualized by taking their cross section and top SEM images. The results show that the size of nanorods was increased in both height and diameter by cation doping. The crystallinity change of the ZnO nanorods due to each doping element was thoroughly investigated by an x-ray diffraction (XRD). The XRD patterns show that the wurtzite crystal structure of ZnO nanorods was maintained after cation addition. The optical Raman-active modes of undoped and cation-doped nanorods were measured with a micro-Raman setup at room temperature. The surface chemistry of samples was investigated by x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy. Finally, the effect of each cation dopant on band-gap shift of the ZnO nanorods was investigated by a photoluminescence setup at room temperature. Although the amount of dopants (Mg, Ni, and Co) was smaller than the amount of Mn, Cu, and Cr in the nanorods, their effect on the band structure of the ZnO nanorods was profound. The highest band-gap shift was achieved for a Co-doped sample, and the best crystal orientation was for Mn-doped ZnO nanorods. Our results can be used as a comprehensive reference for engineering of the morphological, structural and optical properties of cation-doped ZnO nanorods by using a low-temperature synthesis as an economical mass-production approach.