Durability Investigation of Low Pt-Loaded PEM Fuel Cells with Different Catalyst Layer Morphologies

Abstract

Low-temperature Polymer Electrolyte Membrane Fuel Cells (PEMFCs) emerge as the most promising fuel cell technology for transportation. Despite the growing interest in fuel cell electric vehicles (FCEVs) motivated by their rapid refueling, high power density, and long-range, their commercialization lags behind that of battery electric vehicles (BEVs) and internal combustion engine vehicles (ICVs). Apart from the lack of hydrogen infrastructure, other primary bottleneck lies in the cost-intensive production of essential components, particularly the dependence on noble metals such as platinum (Pt) to achieve high catalytic activity in the catalyst layer (CL). Simultaneously, meeting durability requirements for long-term performance and stability proposes another challenge. Therefore, enhancing the durability of PEM fuel cell systems without compromising performance and cost presents a significant challenge which stems from the trade-off relationships among these factors. The current target set by the U.S. Department of Energy (DOE) is achieving a durability of 8,000 hours of operation time, meaning 15,000 miles of driving, with less than 10% performance degradation using a 0.1 mg/cm2 Pt-loaded catalyst.The CL is a complex, heterogeneous structure composed of multiple components with different functionalities, and the electrochemical reaction takes place at the interfaces of the components. Several requirements are essential for the CL, including the presence of highly-dispersed Pt sites with high catalytic activity, a continuous path for efficient proton transport and electron conduction, and a through-connected pore network facilitating gas transport and water removal. Extensive research has been directed toward reducing Pt loading in the CL , motivated by the high cost and limited availability of Pt. At low Pt-loadings, researchers detailed the increased oxygen transport resistance within the CL, which is attributed to the reduction of active sites, as well as the heightened flux of reactants near each active site [1]. Therefore, optimizing the CL, both in terms of mass transport and reaction rate, as well as meeting economic and technical requirements, poses a significant challenge. While the impact of CL compositions on the cell performance is studied previously [2][3], systematic investigations are necessary to reveal the effect of CL characteristics on the electrode degradation. The understanding of performance losses during degradation processes is necessary for optimizing the CL for long-term operation and reducing Pt-loading in the cathode without compromising cell durability.This work aims on tailoring the impact of CL characteristics on electrode degradation for low Pt-loaded cells. To achieve this, five different samples with distinct CL features including Pt-loading, CL thickness, Pt/C ratio, and dilution ratio are fabricated. The samples are fabricated using the catalyst-coated membrane approach to suppress interfacial resistances between the membrane and the ionomer in the CL. In addition, a novel ultrasonic deposition method is employed to develop uniform and ultra-thin CLs enabling precise control of the CL thickness. With the use of an accelerated stress test (AST) ranging within 0.6-1 V in an air/H2 environment, the performance loss of the samples at the beginning-of-life (BOL), end-of-life (EOL) and at different stages during the applied AST is investigated. This experimental study serves as a foundational exploration into comprehending the individual impacts of CL compositions on electrode degradation and establishes the framework for the design of durable low Pt-loaded PEM fuel cells.