Recent advance on structural design of high-performance Pt-based nanocatalysts for oxygen reduction reaction

This study is led by Dr. Hong-Wen Huang (Hunan University). With the increasingly serious environmental and energy problems, there is an urgent need to develop from a traditional single fossil fuel system to an advanced fuel system with diversified and renewable sources. Hydrogen, as a zero-carbon energy carrier, is a promising alternative fuel to meet the future energy demands, which can be efficiently converted into the clean electricity via the proton exchange membrane fuel cells (PEMFCs). Compared with other energy generation devices, PEMFCs hold incomparable advantages such as high energy conversion efficiency, low pollution, adjustable power generation energy, and high working reliability. However, the development of PEMFCs still suffers from many challenges, especially the high cost of the usage of Pt metal as the catalyst.

Due to the low dissociation barrier of H-H bond, anodic hydrogen oxidation reaction (HOR) in PEMFCs often shows a low overpotential, which is only about 5 mV with anode Pt loading at 0.05 mg cm−2. In contrast, cathodic oxygen reduction reaction (ORR) has a long reaction path involving four electron-proton transfer processes and a high energy barrier with much higher overpotential up to 300–400 mV at same Pt loading. As consequence, it is particularly important to develop remarkable catalysts to accelerate the sluggish ORR kinetics and reduce the cost of PEMFCs. With insights from theoretical calculations on the tightly packed surface of various metals, Pt possesses the optimal adsorption free energy of oxygen intermediate (ΔEo) close to the top site of volcano plot, endowing it with ideal catalytic ORR activity. However, with focus on irreplaceability of Pt, the scarcity and high cost severely limit its application. How to optimize the geometrical and ligand structure of Pt catalyst to improve its ORR activity and reduce loading amount is particularly critical at present.

Herein, this review summarizes recent significant progress on the development of Pt-based ORR electrocatalysts, which can be divided into two directions, increasing activity per site and increasing the number of active sites. Before this part, the ORR mechanism is discussed here. In particular, d-band center theory is introduced to explain the relationship between adsorption energy and d-band center. Further, the regulation principle of d-band center is listed here as ligand effect and geometric effect, which provide a guide for designing catalysts with high intrinsic activity. In the end, the current challenges and future prospects of Pt-based catalysts for ORR are provided.

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https://doi.org/10.1016/j.asems.2022.100030