Proton-exchange membrane fuel cells (PEMFCs) are efficient and clean hydrogen energy technologies for transportation and stationary applications. Highly active and durable low-cost cathode catalysts for the oxygen-reduction reaction (ORR) under challenging acidic environments are desperately needed to address the cost and durability issues of PEMFCs. The most promising platinum group metal (PGM)-free catalysts for the ORR in acidic media are atomically dispersed and nitrogen-coordinated metal site catalysts denoted as M–N–C, M = Fe, Co, or Mn. Due to significant efforts in the past few decades, these catalysts have demonstrated much-improved ORR activity and promising initial fuel cell performance approaching traditional Pt/C catalysts. However, the insufficient long-term stability (up to 5000 h) under PEMFC operation represents a primary technical barrier to making current PGM-free catalysts less viable yet in PEMFCs. In this Account, we highlight recent advances in synthesizing efficient PGM-free catalysts for the ORR in PEMFCs, emphasizing effective strategies to improve mass and intrinsic activity and the possible degradation mechanisms. In particular, a chemical doping method based on the zeolitic imidazolate framework (ZIF)-8 represents the key to developing efficient M–N–C catalysts containing atomically dispersed and nitrogen-coordinated single metal active sites (i.e., MN₄). The newly acquired understanding of the formation mechanism of MN₄ active sites during the thermal activation and its correlation to catalytic properties guide the rational catalyst design rather than relying on current trial-and-error approaches. Considerable efforts have further been invested in increasing the active site density and enhancing intrinsic activity by regulating carbon-phase structures and the local coordination environment. These highly active catalysts usually suffer from significant activity loss during the ORR. Therefore, breaking the activity–stability trade-off is the key to simultaneously achieving activity and stability in one catalyst, which is discussed on the basis of our recent successes in regulating local carbon structures surrounding active single metal sites. Significant research efforts toward understanding the degradation mechanisms and improving the lifetime of PGM-free catalysts are still crucial for viable applications in the future. Novel electrode designing strategies are needed to translate the PGM-free catalysts’ ORR activity to solid-state electrolyte-based membrane electrode assemblies (MEAs) with robust three-phase (i.e., gas–liquid–solid) interfaces for efficient charge and mass transports for performance improvement. On the basis of our effort at the University at Buffalo supported by ElectroCat Consortium associated with U.S. DOE’s Hydrogen and Fuel Cell Technologies Office, we provide a perspective on PGM-free cathode catalysts concerning remaining bottlenecks and future opportunities, aiming to inspire the community in both mechanistic understanding and technological development.

中文翻译：

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用于质子交换膜燃料电池的无 PGM 氧还原催化剂开发：挑战、解决方案和前景

质子交换膜燃料电池 (PEMFC) 是用于运输和固定应用的高效清洁氢能技术。迫切需要用于在具有挑战性的酸性环境下进行氧还原反应 (ORR) 的高活性和耐用的低成本阴极催化剂，以解决 PEMFC 的成本和耐用性问题。在酸性介质中用于 ORR 的最有希望的无铂族金属 (PGM) 催化剂是原子分散和氮配位的金属位点催化剂，表示为 M-N-C，M = Fe、Co 或 Mn。由于过去几十年的重大努力，这些催化剂已显示出大大提高的 ORR 活性和有希望接近传统 Pt/C 催化剂的初始燃料电池性能。然而，PEMFC 操作下的长期稳定性不足（长达 5000 小时）是使当前的无 PGM 催化剂在 PEMFC 中不太可行的主要技术障碍。在本文中，我们重点介绍了在 PEMFC 中为 ORR 合成有效的无 PGM 催化剂方面的最新进展，强调了提高质量和内在活性的有效策略以及可能的降解机制。特别是，基于沸石咪唑酯骨架 (ZIF)-8 的化学掺杂方法是开发含有原子分散和氮配位单金属活性位点的高效 M-N-C 催化剂（即 MN 强调改善质量和内在活性的有效策略以及可能的降解机制。特别是，基于沸石咪唑酯骨架 (ZIF)-8 的化学掺杂方法是开发含有原子分散和氮配位单金属活性位点的高效 M-N-C 催化剂（即 MN 强调改善质量和内在活性的有效策略以及可能的降解机制。特别是，基于沸石咪唑酯骨架 (ZIF)-8 的化学掺杂方法是开发含有原子分散和氮配位单金属活性位点的高效 M-N-C 催化剂（即 MN₄）。_{对MN 4}形成机制的新认识热活化过程中的活性位点及其与催化性能的相关性指导合理的催化剂设计，而不是依赖于当前的试错法。通过调节碳相结构和局部配位环境，进一步投入了大量努力来增加活性位点密度和增强内在活性。这些高活性催化剂在 ORR 期间通常会遭受显着的活性损失。因此，打破活性-稳定性权衡是在一种催化剂中同时实现活性和稳定性的关键，这是基于我们最近在调节活性单金属位点周围的局部碳结构方面取得的成功而进行的讨论。为了解降解机制和提高无 PGM 催化剂的寿命而进行的重大研究对于未来的可行应用仍然至关重要。需要新的电极设计策略来将无 PGM 催化剂的 ORR 活性转化为具有稳健三相（即气-液-固）界面的固态电解质膜电极组件 (MEA)，以实现有效的电荷和质量传输用于性能改进。基于我们在布法罗大学所做的努力，得到了与美国能源部氢和燃料电池技术办公室相关的 ElectroCat 联盟的支持，我们提供了关于剩余瓶颈和未来机会的无 PGM 阴极催化剂的观点，旨在激励社区在这两个方面机械理解和技术发展。