Key Takeaways & Executive Findings
- •• Mechanisms of bulk and local oxygen transport in cathode catalyst layers (CCLs) of PEMFCs are systematically presented. • State-of-the-art strategies to mitigate oxygen transport resistance in CCLs are reviewed, including novel structure design, carbon supports design, and ionomer design. • New directions for oxygen transport development in anode catalyst layers (ACLs) of PEMWEs are inspired by PEMFCs. • The review highlights the critical role of pore structure and catalyst–ionomer agglomerates in oxygen transport for both PEMFCs and PEMWEs.
Abstract
Urgent requirements of the renewable energy boost the development of stable and clean hydrogen, which could effectively displace fossil fuels in mitigating climate changes. The efficient interconversion of hydrogen and electronic is highly based on polymer electrolyte membrane fuel cells (PEMFCs) and water electrolysis (PEMWEs). However, the high cost continues to impede large-scale commercialization of both PEMFC and PEMWE technologies, with the expense primarily attributed to noble catalysts serving as a major bottleneck. The reduction of Pt loading in PEMFCs is essential but limited by the oxygen transport resistance in the cathode catalyst layers (CCLs), while the oxygen transport in anode catalyst layers (ACLs) in PEMWEs also being focused as the Ir/IrOx catalyst reduced. The pore structure and the catalyst–ionomer agglomerates play important roles in the oxygen transport process of both PEMFCs and PEMWEs due to the similarity of membrane electrode assembly (MEA). Herein, the oxygen transport mechanism of PEMFCs in pore structure and ionomer thin films in CCLs is systematically reviewed, while state-of-the-art strategies are presented for enhancing oxygen transport and performance through materials and structural design. The deeply research opens avenues for exploring similar key scientific problems in oxygen transport process of PEMWEs and their further development.
1. Introduction
To meet the energy demand without fossil fuels and accelerating greenhouse gas emissions, it is imperative to increase uptake of clean energy. Hydrogen offers a broad range of benefits, including high calorific value, which is 2–3 times that of petroleum, stable energy output, wide range of sources and zero pollution [1]. It is believed that the decarbonization of the global economy in the upcoming years would heavily rely on the pivotal role of hydrogen. The entire “hydrogen-electrode” industry chain aims to achieve optimal utilization of hydrogen, encompassing upstream production, midstream storage, and downstream usage [2].
The fuel cell, which finds widespread applications in stationary, transportation, and portable sectors, is considered to be one of the most rapidly advancing and well-established forms of hydrogen utilization. It is regarded as a promising power device that directly converts the chemical energy stored in hydrogen to electrical energy through electrochemical reactions, including alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and proton exchange membrane fuel cell (PEMFC). Among them, PEMFCs are receiving the most attention especially for transportation and portable charging because of their highly efficient energy conversion, low operating temperature, and strong reliability [3, 4].
On the other hand, water electrolysis represents a reverse process for generating hydrogen from water through electrolytic reactions, providing an environmentally friendly alternative approach for producing green hydrogen. The primary advantage of green hydrogen lies in its ability to enable maximum utilization of electrical energy, thereby optimizing the use of low-quality electrical sources such as solar, wind, or even nuclear power, enhancing compatibility with dynamic current loads, and balancing the volatility when integrating renewable energy resources. With the significant increase in the installed capacity of renewable energy, the production cost of green hydrogen will be further reduced, and hydrogen production technology from renewable energy will gradually become the mainstream. Water electrolysis devices could be divided into alkaline liquid water electrolysis (ALWEs), alkaline membrane water electrolysis (AMWEs), proton exchange membrane water electrolysis (PEMWEs), and solid oxide water electrolysis (SOECs). The high efficiency, low maintenance cost, fast dynamics, and high hydrogen purity of PEMWEs have garnered increasing attention [5].
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Huiyuan Li, Shu Yuan, Jiabin You, Congfan Zhao, Xiaojing Cheng, Liuxuan Luo, Xiaohui Yan, Shuiyun Shen, Junliang Zhang (2025). Revealing the Oxygen Transport Challenges in Catalyst Layers in Proton Exchange Membrane Fuel Cells and Water Electrolysis. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01719-y
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Frequently Asked Questions
What are the main challenges of oxygen transport in PEM fuel cells?
The main challenges include high oxygen transport resistance in the cathode catalyst layers, particularly due to the presence of ionomer thin films and pore structure, which limit the reduction of Pt loading and increase cost.
How does the pore structure affect oxygen transport in catalyst layers?
The pore structure influences the effective diffusivity of oxygen. Optimized pore size distribution and connectivity can reduce transport resistance, while agglomerates of catalyst and ionomer can create local barriers.
What strategies are proposed to mitigate oxygen transport resistance?
Strategies include novel structural designs of the catalyst layer, engineering carbon supports with tailored porosity, and modifying ionomer properties to enhance oxygen permeability and distribution.
Why is oxygen transport important in water electrolysis?
In PEM water electrolysis, oxygen transport in the anode catalyst layer affects the efficiency and performance, especially when reducing the loading of Ir-based catalysts. Understanding and improving oxygen transport can lower costs and enhance hydrogen production.
What is the significance of this review for the hydrogen economy?
This review provides insights into overcoming oxygen transport limitations in both fuel cells and electrolyzers, which is crucial for reducing the cost and improving the efficiency of hydrogen technologies, thereby supporting the transition to a clean energy economy.
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