A large-scale industrial application of proton exchange membrane fuel cells (PEMFCs) greatly depends on both substantial cost reduction and continuous durability enhancement. However, compared to effects of material degradation on apparent activity loss, little attention has been paid to influences on the phenomena of mass transport. In this review, influences of the degradation of key materials in membrane electrode assemblies (MEAs) on oxygen transport resistance in both cathode catalyst layers (CCLs) and gas diffusion layers (GDLs) are comprehensively explored, including carbon support, electrocatalyst, ionomer in CCLs as well as carbon material and hydrophobic polytetrafluoroethylene (PTFE) in GDLs. It is analyzed that carbon corrosion in CCLs will result in pore structure destruction and impact ionomer distribution, thus affecting both the bulk and local oxygen transport behavior. Considering the catalyst degradation, an eventual decrease in electrochemical active surface area (ECSA) definitely increases the local oxygen transport resistance since a decrease in active sites will lead to a longer oxygen transport path. It is also noted that problems concerning oxygen transport caused by the degradation of ionomer chemical structure in CCLs should not be ignored. Both cation contamination and chemical decomposition will change the structure of ionomer, thus worsening the local oxygen transport. Finally, it is found that the loss of carbon and PTFE in GDLs lead to a higher hydrophilicity, which is related to an occurrence of water flooding and increase in the oxygen transport resistance.

Concentrating active Pt atoms in the outer layers of electrocatalysts is a very effective approach to greatly reduce the Pt loading without compromising the electrocatalytic performance and the total electrochemically active surface area (ECSA) for the oxygen reduction reaction (ORR) in hydrogen-based proton-exchange membrane fuel cells. Accordingly, a facile, low-cost, and hydrogen-assisted two-step method is developed in this work, to massively prepare carbon-supported uniform, small-sized, and surfactant-free Pd nanoparticles (NPs) with ultrathin ~3-atomic-layer Pt shells (Pd@Pt_3L NPs/C). Comprehensive physicochemical characterizations, electrochemical analyses, fuel cell tests, and density functional theory calculations reveal that, benefiting from the ultrathin Pt-shell nanostructure as well as the resulting ligand and geometric effects, Pd@Pt_3L NPs/C exhibits not only significantly enhanced ECSA, electrocatalytic activity, and noble-metal (NM) utilization compared to commercial Pt/C, showing 81.24 m²/g_Pt, 0.710 mA/cm², and 352/577 mA/mg_NM/Pt in ECSA, area-, and NM-/Pt-mass-specific activity, respectively; but also a much better electrochemical stability during the 10,000-cycle accelerated degradation test. More importantly, the corresponding 25-cm² H₂-air/O₂ fuel cell with the low cathodic Pt loading of ~ 0.152 mg_Pt/cm²_geo achieves the high power density of 0.962/1.261 W/cm²_geo at the current density of only 1,600 mA/cm²_geo, which is much higher than that for the commercial Pt/C. This work not only develops a high-performance and practical Pt-based ORR electrocatalyst, but also provides a scalable preparation method for fabricating the ultrathin Pt-shell nanostructure, which can be further expanded to other metal shells for other energy-conversion applications.

Undoubtedly, it is imperative to figure out two stubborn issues concerning low electronic conductivity and sluggish lithium ion diffusion to promote the practical application of Li₂FeSiO₄ materials in lithium-ion battery (LIB) cathode. Herein, we report an innovative and simple strategy that combines a hydrothermal process with subsequent annealing to synthesize highly uniform Li₂FeSiO₄/C hollow nanospheres. During the hydrothermal process, polystyrene nanospheres are employed not only as the template but also, more tactfully, as carbon source to form amorphous carbon layers, which will function to enhance the electronic conductivity and restrict particle aggregations. The use of the LIB Li₂FeSiO₄/C hollow nanospheres as a LIB cathode delivers a desired stable capacity at each rate stage, and even at a high rate of 10 C, the hollow nanosphere cathode can present a specific discharge capacity as high as 50.5 mAh·g^-1. After 100 cycles, the capacity retentions at 1 and 10 C remain as high as 93% and 72%, respectively. The superior electrochemical performance is believed to be related to special architectures of the Li₂FeSiO₄/C hollow nanosphere cathode.

Undoubtedly, there remains an urgent prerequisite to achieve significant advances in both the specific capacity and cyclability of Li-O₂ batteries for their practical application. In this work, a series of unique three-dimensional (3D) α-MnO₂/MWCNTs hybrids are successfully prepared using a facile lyophilization method and investigated as the cathode of Li-O₂ batteries. Thereinto, cross-linked α-MnO₂/MWCNTs nanocomposites are first synthesized via a modified chemical route. Results demonstrate that MnO₂ nanorods in the nanocomposites have a length of 100–400 nm and a diameter ranging from 5 to 10 nm, and more attractively, the as-lyophilized 3D MnO₂/MWCNTs hybrids is uniquely constructed with large amounts of interconnected macroporous channels. The Li-O₂ battery with the 3D macroporous hybrid cathode that has a mass percentage of 50% of α-MnO₂ delivers a high discharge specific capacity of 8, 643 mAh·g⁻¹ at 100 mA·g⁻¹, and maintains over 90 cycles before the discharge voltage drops to 2.0 V under a controlled specific capacity of 1, 000 mAh·g⁻¹. It is observed that when being recharged, the product of toroidal Li₂O₂ particles disappears and electrode surfaces are well recovered, thus confirming a good reversibility. The excellent performance of Li-O₂ battery with the 3D α-MnO₂/MWCNTs macroporous hybrid cathode is ascribed to a synergistic combination between the unique macroporous architecture and highly efficient bi-functional α-MnO₂/MWCNTs electrocatalyst.