Abstract
Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability—milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.
Highlights
Proton exchange membrane (PEM) fuel cells, the ones using hydrogen at the anode and oxygen or air at the cathode, namely H2 PEM fuel cells, can generate electricityFor PEM fuel cells, a typical polarization curve is shown in Fig. 1 [1]
The majority of platinum group metals (PGM) catalysts are consumed at cathode because of the much more sluggish kinetics of cathodic oxygen reduction reaction (ORR) than the coupled anodic hydrogen oxidation reaction
This paper mainly focuses on the PGM-free catalysts for the cathode of PEM fuel cells. (Without specific emphasis, the discussed catalysts in the following contents are referring to the cathode catalysts.) Section 2 presents the main challenges for PGM-free catalysts in terms of performance and durability/stability; Sects. 3 and 4 demonstrate the recent progress of high-performance and highly durable/stable PGM-free catalysts; in Sect. 5, the gaps between the current status and the future perspectives are identified
Summary
Proton exchange membrane (PEM) fuel cells, the ones using hydrogen at the anode and oxygen or air at the cathode, namely H2 PEM fuel cells, can generate electricity. The theoretical output voltage of PEM fuel cells is 1.23 V if the hydrogen and oxygen are fed at the anode and cathode, respectively. The fuel crossover is primarily determined by the membrane; the other three polarizations (i.e., kinetics, ohmic, and concentration) are all closely relevant to the catalysts. The use of PGM catalysts leads to the high cost and hinders the deployments of fuel cells in EVs. The majority of PGM catalysts are consumed at cathode because of the much more sluggish kinetics of cathodic oxygen reduction reaction (ORR) than the coupled anodic hydrogen oxidation reaction. Decreasing the usage of PGM catalysts and even replacing them with PGM-free alternatives for cathode are two promising strategies for PEM fuel cell applications. This paper mainly focuses on the PGM-free catalysts for the cathode of PEM fuel cells.
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