Performance of Polymer Electrolyte Membrane Fuel Cells at Ultra-Low Platinum Loadings

Abstract

Proton exchange membrane fuel cells (PEMFCs) generate electricity from hydrogen, powering a range of applications while emitting nothing but water. Therefore, PEMFCs are regarded as an environmentally friendly alternative to internal combustion engines for the future. Nevertheless, the high cost and scarcity of platinum (Pt) sources prevent the widespread adoption of fuel cells. With the development of fuel cell manufacturing technology, current Pt utilization has increased to a relatively high level of 0.2 g·kW−1 in PEMFCs. However, according to the PGM market report from Johnson Matthey (2020), current Pt utilization in fuel cells is still too low to meet the need for its large-scale application in the automotive industry, unless the Pt utilization can be further reduced to an ultra-low level (0.01 g·kW−1). Therefore, higher Pt mass activity and higher Pt utilization must be realized in membrane electrode assemblies (MEA) to achieve ultra-low Pt loadings and a reduced Pt usage. Many key variables affect the performance of MEA, such as the activity of electrocatalysts, conductivity and distribution of ionomers, gas diffusion in carbon papers, and the thickness of the proton exchange membrane. For example, a wide variety of highly promising catalysts have been developed, such as shape-controlled Pt nanocrystals, Pt alloy/dealloys, core-shells, the synergetic effect of active supports, single atom/single-atom layer catalysts for improving the utilization of Pt, and anti-poisoning catalysts. However, the super-high activity of a Pt catalyst is elusive in a real fuel cell because of the lack of a fundamental understanding of the reaction interface structure and mass transfer properties in real cells. For instance, the recently developed Pt-Ni nanoframes that exhibited an extremely high mass activity of 5.7 A·mg−1 for the oxygen reduction reaction (ORR) in a liquid half-cell only showed about one-tenth the activity in a real fuel cell (0.76 A·mg−1 Pt at 0.90 V). To achieve widespread adoption of Pt in fuel cells, we urgently need to explore new combinations of electrocatalysts, ionomers, gas diffusion layers, and proton exchange membranes. Taking into account all these factors, recent advances have enhanced the performance of MEA, such as a neural-network-like catalyst structure for higher Pt utilization, a highly order-structured with vertically aligned carbon nanotubes as a highly ordered catalyst layer that exhibits higher mass transfer efficiency, a novel anti-flooding electrode, a higher oxygen permeability and ionic conductivity ionomer, and an ultrathin MEA with low Pt loading that exhibits higher fuel cell output efficiency. This review mainly focuses on the recent progress in fuel cell cathode performance at ultra-low Pt loadings. To achieve the ultimate goal of Pt utilization (0.01 g·kW−1), further efforts to accelerate this progress are urgently needed, including improving catalytic performance by using highly active and stable supports, decreasing the gas diffusion resistance, enhancing the water management in the catalytic layer, improving the anti-poisoning property, and establishing an integrated ultra-thin and low platinum film electrode.