Grooved electrodes for high-power-density fuel cells

The urgent need to replace fossil fuels with clean energy sources is driving the development of Proton Exchange Membrane Fuel Cells (PEMFCs). PEMFCs offer a promising alternative to internal combustion engines for transportation, providing on-demand electricity with zero local carbon emissions. Using renewable hydrogen produced via water electrolysis, PEMFCs can eliminate the transport sector's reliance on fossil fuels. Heavy-duty trucking presents a particularly attractive application for PEMFCs, due to the relatively smaller number of refueling stations required and less stringent capital cost requirements compared to light-duty vehicles. Electrifying heavy-duty vehicles (HDVs) with batteries is challenging due to the demands for long range, fast refueling, low weight, and small size—factors that make PEMFCs a leading contender. However, commercialization requires significant improvements in durability, efficiency (fuel economy), and capital cost reduction. Developing improved electrode materials is crucial; however, the arrangement and interface of these materials within the electrode significantly impact achieving high power density, durability, and efficiency. Increasing power density—and consequently reducing stack size—is a key path to lowering capital costs, as it reduces material costs while also benefiting from lower volume and weight. This paper explores an improved electrode design that facilitates faster transport and more effective material use to achieve higher power density, durability, and efficiency, ultimately leading to reduced capital costs.

Conventional PEMFC electrodes consist of a carbon-supported platinum catalyst (Pt/C) and ionomer, mixed and deposited as a porous electrode. This top-down process results in a randomized structure with tortuous ionomer and pore networks, limiting mass transport and lowering catalyst utilization. The structure of today's commercial PEMFCs remains largely unchanged from the design pioneered over 30 years ago. To address the limitations of conventional electrode structures, several studies have explored alternative electrode architectures using advanced micro- and nanofabrication techniques, inkjet printing, template-based patterning, and thin-film deposition. While these approaches often improve mass activity, transport, or durability, the advantages tend to be limited to specific operating conditions and have not been consistently demonstrated across the wide range of conditions encountered in transportation applications. Specifically, enhanced performance at higher voltages (≥0.7 V) and under lower relative humidity conditions is essential for HDV applications.

This study introduces the grooved electrode, an alternative electrode design with high-ionomer-content ridges for facile H+ transport separated by grooves (void channels) that facilitate O2 diffusion. Grooved electrodes were fabricated by depositing a Pt/C and ionomer mixture onto a patterned silicon template, followed by transfer to a Nafion membrane. The resulting electrodes exhibited a well-defined groove and ridge structure, confirmed by nanoscale computed X-ray tomography and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS). Electrodes with groove dimensions of 2 μm/6 μm, 1.5 μm/4 μm, and 1 μm/3 μm (groove width/period) were tested, with grooves oriented perpendicular to flow channels. The performance of flat and grooved electrodes with varying ionomer-to-carbon (I/C) ratios was evaluated using polarization curves under different relative humidity (RH) conditions. Oxygen and proton transport resistances were measured using limiting current methods and electrochemical impedance spectroscopy (EIS). Multiphysics modeling was employed to simulate local reactant and product transport and predict the oxygen reduction reaction (ORR) rate distribution. Durability was assessed using an accelerated stress test (AST) involving 500 cycles from 1.0 to 1.5 V in H2/N2. Finally, adaptive machine learning (ML) integrated with multiphysics simulations was used to explore a wider design space of grooved electrode structures and predict potential performance improvements.

The grooved electrodes demonstrated significantly enhanced performance compared to optimized flat electrodes across a range of operating conditions. They exhibited peak performance at a higher I/C ratio (1.2) than flat electrodes (0.9), showcasing improved performance at relevant voltages (≥0.7 V) and low RH conditions crucial for HDV applications. The improvements were more pronounced under low RH conditions where both O2 and H+ transport are limiting. The grooves effectively reduced O2 transport resistance, even at high I/C ratios, as evidenced by sheet resistance, O2 transport resistance (Ro2), and O2 mass transport resistance (RMT) measurements. Multiphysics modeling confirmed that the grooves promoted a more uniform ORR rate throughout the electrode, improving catalyst utilization and reducing transport losses. The benefits of the grooved architecture were particularly evident after durability testing (following AST), where grooved electrodes showed significantly higher end-of-test (EOT) performance than flat electrodes, despite similar carbon loss. This was attributed to the grooves maintaining effective O2 transport pathways even in degraded electrodes with collapsed pore structures. Machine learning analysis predicted further performance improvements (up to 60% compared to the flat baseline electrode) with wider and deeper grooves, suggesting a path for future optimization. However, this would require improved fabrication techniques to handle higher aspect ratio electrode ridges and address potential mechanical stability concerns.

The results demonstrate that the grooved electrode design successfully addresses the conflicting needs of O2 and H+ transport in PEMFC electrodes. By spatially separating the transport pathways, the grooved electrode allows for high ionomer content to enhance H+ transport without severely compromising O2 transport. The consistent performance enhancement observed across a wide range of operating conditions, including low RH, highlights the practical relevance of this design for HDV applications. The improved durability observed after AST further strengthens the case for the grooved electrode's potential to enhance PEMFC lifetime and reduce costs. The findings from multiphysics modeling and machine learning provide valuable insights into the underlying mechanisms and suggest promising avenues for future optimization.

This study introduced a novel grooved electrode architecture for PEMFCs, demonstrating significant performance and durability advantages over conventional flat electrodes. The unique design effectively manages H+ and O2 transport, leading to improved reaction rate uniformity and reduced transport losses. The enhanced performance under low RH conditions and improved durability after AST highlight the potential of grooved electrodes for practical HDV applications. Future work should focus on developing improved fabrication techniques to realize the predicted performance gains from wider and deeper grooves, while carefully considering the mechanical stability of the electrode structure.

The current fabrication method using silicon templates presents limitations in creating wider and deeper grooves. The mechanical stability of the grooved electrodes might decrease with increasing ridge aspect ratios, requiring further investigation. The multiphysics model, while validated, still involves simplifications and assumptions that could affect the accuracy of predictions. The machine learning predictions should be considered as guidance for future experimental work, and further verification is needed.