面向低成本纯水电解制氢技术的单原子化金属复合物析氢催化剂研究进展

Research Progress on Single Metal Atom Catalysts for Hydrogen Production by PEM Water Electrolysis with Lower Costs

纯水电解制氢技术是合理利用可再生能源实现电能-氢能相互转化的一个有效途径,对于实现中国的能源结构转型和“双碳”战略有着重要作用。纯水电解制氢的主要实施方式是采用酸性质子交换膜的电解槽,但是其催化剂中所需的大量贵金属组分导致了高昂的制造成本,严重限制了其未来的发展。单原子化金属复合物催化剂的研发与应用是有效降低催化剂中贵金属用量的一个重要策略,开发适应酸性环境、高反应活性、稳定可靠的贵金属甚至非贵金属体系单原子催化剂(SACs)成为研究和产业化的共同热点。本文在介绍单原子催化体系的研发历程和在酸性析氢反应(HER)过程中应用现状的基础上,分为贵金属体系和非贵金属体系两大类分别进行综述,具体阐述了单原子复合催化剂的制备方法、组分结构、电化学性能以及构效关系,最后对面向更低成本的单原子化析氢催化剂的发展前景和应用推广趋势进行了总结和展望。

Hydrogen energy was regarded as an ideal solution for the zero-carbon emission chain of the whole energy application lifecycle. The intermittent power supplies generated by solar, wind, tidal or other forms could be converted to hydrogen gas through the water electrolysis technology. And by storage or transportation of the gas, hydrogen would be converted again to be high-quality electric energy with fuel cell technology whenever or wherever it was needed. Water electrolysis was regarded as an effective medium for the conversion from renewable electricity to hydrogen, which played a vital role on the realization of energy structure transformation and “double carbon” strategy of China. There were two main types of water electrolysis, alkaline or proton exchange membrane(PEM).The alkaline water electrolysis technology was classic and already commercialized. However, the alkaline water electrolysis still had considerable drawbacks. For example, it was necessary to ensure a specific power load(at least >15% of the rated power) during the operation of alkaline water electrolysis stacks, otherwise it would cause serious damage to the service life of the stacks. In addition, the alkaline water electrolysis stacks required a long restart time after shutdown, which generally took 0.5~1 h. Another technical problem lay on that the hydrogen produced by alkaline water electrolysis was not strictly “green”. The impurities in the alkaline electrolyte were brought out with produced hydrogen which still need the high-cost post-treatment and purification process to achieve the required hydrogen purity for fuel cell application. These problems made it difficult for alkaline water hydrogen production technology to take advantage of the renewable electric energy with rapid fluctuation characteristics. Another emerging and promising commercial practice of water electrolysis was by applying electrolyzer stacks with acid-type PEM. Compared with the alkaline water electrolysis strategy, PEM electrolysis had a lot of merits such as higher dynamic response speed, larger output current density and enhanced system efficiency.However, the membrane electrode assembly(MEA) in PEM electrolysis stacks required platinum(Pt) based catalysts as the cathodes and iridium(Ir) based catalysts as the anodes. The large amount of noble metals required in MEA lead to expensive investment costs, which seriously limited the future development of PEM electrolyzers. On the issue of reducing the cost of noble metal catalysts, the effective strategies consisted of the fully utilization of noble metal atoms in the catalysts, or the exploration of new types of noble-metalfree catalysts with high activity and stability in acidic systems through the structural design of new generation catalytic materials and novel catalytic mechanisms. Driven by this goal, researchers had proposed a variety of novel material design, preparation and application routes, including nano/micro-morphology control, component structure optimization or catalytic three-phase interface design.Among them, single-atomic catalysis was recognized as an important strategy to reduce the consumption of noble metals. The development of stable and reliable single atom catalysts with noble-metal or even noble-metal-free components showing high reaction activity in acidic environment had become a common hotspot both for research and commercialization. This paper introduced the research and development of single atom catalysts and their application on the acidic hydrogen evolution reaction(HER). For HER process in PEMtype hydrogen production, due to the harsh and strong acid reaction environment, the precious platinum metal-based materials such as Pt black or Pt/C seemed to be almost the only choice in order to satisfy both the activity and service life. Although a considerable amount of platinum was now adequate to drive HER reaction at a low overpotential thanks to the design and development of catalytic materials, it was still necessary to reduce the use of precious metals in the catalyst to at least one tenth of the current quantity considering the economic cost for the water electrolysis demonstration or commercial application under megawatt(MW) or even gigawatt(GW) levels in addition to the scarcity of precious metal resources. Compared with traditional Pt black and Pt/C, the Pt single-atom catalysts had proved to exhibit ultra-high mass specific activity and reaction efficiency, which demonstrated the promising potential to replace the classic catalyst system based on Pt nanoparticles. The single atom catalysts with no noble metal components represented the ultimate objective for the catalysts applied in commercial PEM electrolyzers. Although significant breakthroughs had been made with the novel transition metal Fe, Co, Mo as single atom active sites and porous carbon or MoS2 as the supports, there was still a big gap on the overall HER performance compared to Pt single-atom catalysts, especially on the service life. The synthesis method, catalyst components, electrochemical performance and structure-activity relationship were also analyzed in details by two categories of noble-metal or noble-metal-free single atom catalysts. Finally, the summary and perspective on the development of single metal atom catalysts for HER with lower costs were illustrated.