Effect of Catalyst Layer Hydrophobicity on Fe−N−C Proton Exchange Membrane Fuel Cells

Breaking the Activity and Stability Trade-Off of Platinum-Free Catalysts for the Oxygen Reduction Reaction in Hydrogen Fuel Cells

Hydrogen fuel cells, which use hydrogen as fuel to generate electricity, hold great promises as future energy conversion devices for heavy-duty transport, due to their zero CO2 emissions, high energy conversion efficiency, and high power density. However, the adoption of hydrogen fuel cells has been slow due to their reliance on large amounts of costly and scarce platinum (Pt) for the oxygen reduction reaction. The replacement of Pt with Earth-abundant transition metals such as Fe, Co, Mn, and Sn with oxygen reduction reaction affinity has thus been a holy grail of electrocatalysis research. Pt-free catalysts must combine both high power density and high stability in hydrogen fuel cells to be considered viable alternatives to Pt. Despite promising progress on both fronts, a trade-off has emerged: Pt-free catalysts either achieve high power densities (≥1.5 W cm-2) but suffer from low stabilities (≥70% loss after 25 h) or more recently demonstrate improved stability (≤25% loss after 150 h), while delivering considerably lower power densities (<1 W cm-2) in hydrogen fuel cells. Herein, we summarize the recent progress in the synthesis of high power density M-N-C catalysts for hydrogen fuel cells and highlight the critical importance of uncovering the underlying mechanisms using operando methods. We then discuss the primary causes of catalyst degradation in hydrogen fuel cells and the most promising strategies to enhance the stability of the M-N-C catalysts. Finally, a roadmap is proposed to overcome the activity stability trade-off for Pt-free catalysts in hydrogen fuel cells.

Stabilizing Fe-N-C Catalysts as Model for Oxygen Reduction Reaction

The highly efficient energy conversion of the polymer-electrolyte-membrane fuel cell (PEMFC) is extremely limited by the sluggish oxygen reduction reaction (ORR) kinetics and poor electrochemical stability of catalysts. Hitherto, to replace costly Pt-based catalysts, non-noble-metal ORR catalysts are developed, among which transition metal-heteroatoms-carbon (TM-H-C) materials present great potential for industrial applications due to their outstanding catalytic activity and low expense. However, their poor stability during testing in a two-electrode system and their high complexity have become a big barrier for commercial applications. Thus, herein, to simplify the research, the typical Fe-N-C material with the relatively simple constitution and structure, is selected as a model catalyst for TM-H-C to explore and improve the stability of such a kind of catalysts. Then, different types of active sites (centers) and coordination in Fe-N-C are systematically summarized and discussed, and the possible attenuation mechanism and strategies are analyzed. Finally, some challenges faced by such catalysts and their prospects are proposed to shed some light on the future development trend of TM-H-C materials for advanced ORR catalysis.

Proton Exchange Membrane Fuel Cells (PEMFCs): Advances and Challenges

The study of the electrochemical catalyst conversion of renewable electricity and carbon oxides into chemical fuels attracts a great deal of attention by different researchers. The main role of this process is in mitigating the worldwide energy crisis through a closed technological carbon cycle, where chemical fuels, such as hydrogen, are stored and reconverted to electricity via electrochemical reaction processes in fuel cells. The scientific community focuses its efforts on the development of high-performance polymeric membranes together with nanomaterials with high catalytic activity and stability in order to reduce the platinum group metal applied as a cathode to build stacks of proton exchange membrane fuel cells (PEMFCs) to work at low and moderate temperatures. The design of new conductive membranes and nanoparticles (NPs) whose morphology directly affects their catalytic properties is of utmost importance. Nanoparticle morphologies, like cubes, octahedrons, icosahedrons, bipyramids, plates, and polyhedrons, among others, are widely studied for catalysis applications. The recent progress around the high catalytic activity has focused on the stabilizing agents and their potential impact on nanomaterial synthesis to induce changes in the morphology of NPs.