Effective Approaches for Designing Stable M-Nx /C Oxygen-Reduction Catalysts for Proton-Exchange-Membrane Fuel Cells

The large-scale commercialization of proton-exchange-membrane fuel cells (PEMFCs) is extremely limited by their costly platinum-group metals (PGMs) catalysts, which are used for catalyzing the sluggish oxygen reduction reaction (ORR) kinetics at the cathode. Among the reported PGM-free catalysts so far, metal-nitrogen-carbon (M-N_x /C) catalysts hold a great potential to replace PGMs catalysts for the ORR due to their excellent initial activity and low cost. However, despite tremendous progress in this field in the past decade, their further applications are restricted by fast degradation under practical conditions. Herein, the theoretical fundamentals of the stability of the M-N_x /C catalysts are first introduced in terms of thermodynamics and kinetics. The primary degradation mechanisms of M-N_x /C catalysts and the corresponding mitigating strategies are discussed in detail. Finally, the current challenges and the prospects for designing highly stable M-N_x /C catalysts are outlined.

An Initial Covalent Organic Polymer with Closed-F Edges Directly for Proton-Exchange-Membrane Fuel Cells

Covalent organic polymers (COPs) are a class of rising electrocatalysts for the oxygen reduction reaction (ORR) due to the atomically metrical control of the organic molecular components along with highly architectural robustness and thermodynamic stability even in acid or alkaline media. However, the direct application of pristine COPs as acidic ORR electrocatalysts, especially in device manner, e.g., in proton-exchange-membrane fuel cells (PEMFCs), remains a big challenge. Currently, the decoration toward electronic structures of active sites is considered a vital pathway to enhancing the acidic ORR activity of carbon-based electrocatalysts. Here, an initial F-decorated fully closed π-conjugated quasi-phthalocyanine COP (denoted as COP_BTC -F) is reported. The introduction of the closed-F edges stepwise drags more electrons from FeN₄ sites in COP_BTC -F into the catalyst margin, which weakens the occupied numbers of bonding orbitals between COP_BTC -F and OH* intermediates at the rate-determining step, exhibiting over five times intrinsic performance beyond the counterpart without F functionalities (termed as COP_BTC ). Significantly, the maximum power density utilizing COP_BTC -F as a cathode catalyst in PEMFCs is remarkably increased by an order of magnitude compared with COP_BTC , which is a stride forward among catalysts based on a pyrolysis-free conjugated-polymer network in device manner to date.

Enhanced catalytic performance of Pt by coupling with carbon defects

Defect engineering is a promising strategy for supported catalysts to improve the catalytic activity and durability. Here, we selected the carbon (C) matrix enriched with topological defects to serve as the substrate material, in which the topological defects can act as anchoring centers to trap Pt nanoparticles for driving the O₂ reduction reactions (ORRs). Both experimental characterizations and theoretical simulations revealed the strong Pt-defect interaction with enhanced charge transfer on the interface. Despite a low Pt loading, the supported catalyst can still achieve a remarkable 55 mV positive shift of half-wave potential toward ORR in O₂-saturated 0.1 M HClO₄ electrolyte compared with the commercial Pt catalyst on graphitized C. Moreover, the degeneration after 5,000 voltage cycles was negligible. This finding indicates that the presence of strong interaction between Pt and topological C defects can not only stabilize Pt nanoparticles but also optimize the electronic structures of Pt/C catalysts toward ORR.

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A novel method to obtain topologically defective porous carbon particles is proposed

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Pt nanoparticles are deposited on the topological carbon defects of porous carbon

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The interaction between Pt particles and carbon defects can adjust d-band center position

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Pt-DPC shows much better ORR performance than that of commercial Pt/C

Networking Pyrolyzed Zeolitic Imidazolate Frameworks by Carbon Nanotubes Improves Conductivity and Enhances Oxygen-Reduction Performance in Polymer-Electrolyte-Membrane Fuel Cells

A high-performance nonprecious-metal oxygen-reduction electrocatalyst is prepared via in situ growth of bimetallic zeolitic imidazolate frameworks on multiwalled carbon nanotubes (CNTs) followed by adsorption of furfuryl alcohol and pyrolysis. The networking boosts the conductivity and performance in a polymer electrolyte membrane fuel cell, yielding a maximal power density of 820 mW cm^-2 .