Proton exchange membrane fuel cells (PEMFCs) are a promising technology for clean energy conversion due to their high efficiency, low operating temperatures, and rapid start-up capabilities. A critical component of PEMFCs is the electrocatalyst, which facilitates the oxygen reduction reaction (ORR) at the cathode and the hydrogen oxidation reaction (HOR) at the anode. Nanomaterials, particularly nanostructured catalysts, play a pivotal role in enhancing the performance, durability, and cost-effectiveness of PEMFCs. This article focuses on the application of platinum nanoparticles, alloyed catalysts, and carbon-supported catalysts in PEMFCs, addressing their synthesis, catalytic activity, durability, and challenges such as catalyst poisoning and cost reduction.

Platinum nanoparticles are the most widely used catalysts in PEMFCs due to their exceptional catalytic activity for both ORR and HOR. The synthesis of platinum nanoparticles typically involves chemical reduction methods, where precursors such as chloroplatinic acid are reduced using agents like sodium borohydride or ethylene glycol. The size and distribution of platinum nanoparticles significantly influence their catalytic performance. Smaller nanoparticles, typically in the range of 2-5 nm, exhibit higher surface area-to-volume ratios, leading to increased active sites for catalytic reactions. However, excessively small nanoparticles may suffer from agglomeration or dissolution during operation, reducing their durability. Uniform dispersion of platinum nanoparticles on conductive supports is crucial to maximize their utilization and prevent degradation.

Alloyed catalysts, such as platinum-cobalt (Pt-Co) and platinum-nickel (Pt-Ni), have emerged as alternatives to pure platinum catalysts due to their enhanced ORR activity and reduced platinum loading. The synthesis of alloyed catalysts often involves co-reduction of metal precursors or thermal treatment of pre-formed nanoparticles. The electronic and geometric effects induced by alloying platinum with transition metals modify the d-band center of platinum, optimizing the adsorption energy of oxygen intermediates and improving ORR kinetics. For instance, Pt-Co catalysts have demonstrated up to three times higher mass activity compared to pure platinum. However, the durability of alloyed catalysts can be compromised by the leaching of non-precious metals under acidic conditions, leading to performance degradation over time. Strategies such as surface segregation or core-shell structures have been explored to mitigate this issue.

Carbon-supported catalysts are another critical class of nanomaterials in PEMFCs, where platinum or alloyed nanoparticles are dispersed on high-surface-area carbon materials like Vulcan XC-72 or graphene. The carbon support not only provides electrical conductivity but also stabilizes the nanoparticles against agglomeration. The synthesis of carbon-supported catalysts often involves impregnation methods, where metal precursors are deposited onto the carbon support followed by reduction. The interaction between the nanoparticles and the carbon support influences the catalytic activity and durability. For example, nitrogen-doped carbon supports can enhance the anchoring of platinum nanoparticles and improve their stability. However, carbon corrosion under high-potential conditions remains a challenge, leading to catalyst detachment and performance loss.

The catalytic activity of nanomaterials in PEMFCs is closely related to their size, shape, and composition. Smaller nanoparticles generally exhibit higher specific activity due to the increased number of active sites, but their stability may be compromised. Shape-controlled synthesis, such as producing cubic or octahedral nanoparticles, can expose specific crystal facets with higher intrinsic activity for ORR. Compositional tuning, as in the case of alloyed catalysts, can further optimize the binding energy of reaction intermediates. The distribution of nanoparticles on the support also plays a critical role; uneven distribution can lead to localized high current densities and accelerated degradation.

Durability is a major concern for nanostructured catalysts in PEMFCs. Mechanisms of degradation include nanoparticle agglomeration, dissolution, and detachment from the support. Electrochemical cycling, high potentials, and harsh operating conditions exacerbate these processes. Accelerated stress tests have shown that platinum nanoparticles can grow from 3 nm to over 6 nm after thousands of cycles, resulting in reduced active surface area. Alloyed catalysts face additional challenges due to the leaching of non-precious metals. Strategies to enhance durability include the use of more stable supports like graphitized carbon or metal oxides, as well as the design of core-shell structures where a stable core protects the active shell.

Catalyst poisoning, particularly by carbon monoxide (CO) in the hydrogen feed, is another significant challenge. CO adsorbs strongly on platinum sites, blocking active sites for HOR and reducing fuel cell performance. Alloyed catalysts, such as platinum-ruthenium (Pt-Ru), exhibit improved CO tolerance due to the bifunctional mechanism, where ruthenium promotes the oxidation of CO at lower potentials. However, the incorporation of ruthenium can reduce the ORR activity, necessitating a balance between CO tolerance and overall catalytic performance.

Cost reduction is a critical driver for the development of nanostructured catalysts in PEMFCs. Platinum is expensive and scarce, accounting for a significant portion of the fuel cell cost. Strategies to reduce platinum loading include the use of ultra-low-loading catalysts, non-precious metal catalysts, and atomically dispersed platinum sites. Ultra-low-loading catalysts with high mass activity can achieve comparable performance to conventional catalysts with significantly less platinum. Non-precious metal catalysts, such as iron-nitrogen-carbon (Fe-N-C) materials, have shown promise for ORR but still lag behind platinum in terms of activity and durability. Atomically dispersed platinum catalysts, where single platinum atoms are anchored on a support, maximize platinum utilization and minimize loading.

In summary, nanomaterials are indispensable for advancing the performance and viability of PEMFCs. Platinum nanoparticles, alloyed catalysts, and carbon-supported catalysts each offer unique advantages and challenges. The synthesis methods, nanoparticle size, distribution, and composition critically influence catalytic activity and durability. Addressing challenges such as catalyst poisoning and cost reduction requires innovative approaches, including alloying, support modification, and alternative materials. Continued research and development in nanostructured catalysts are essential to realizing the full potential of PEMFCs for clean energy applications.