1D Spatially Resolved Model of Alloyed Catalyst Degradation in LT-PEMFC

Abstract

Low-temperature fuel cells with proton exchange membrane (LT-PEMFC) are among the most promising energy devices for future decarbonizaion of heavy-duty long-distance transport. Fuel cells use hydrogen as the energy source and air as oxidizer to produce electricity in catalyzed electrochemical process, with only side-products being heat and water. Despite some clear advantages over other energy sources, i.e. batteries, their adoption is still somewhat limited by some prominent factors such as high price and limited durability. Both of these factors are closely related to the use of specific catalyst materials needed to ensure high efficiency and power density of hydrogen fuel cells. Due to its high catalytic activity, Pt is traditionally used as a catalyst material, dispersed in form of nanoparticles on a highly porous carbon matrix ensuring good accessibility of hydrogen and air to the catalyst surface and efficient removal of water.Due to harsh local conditions in the catalyst layer, i.e. high temperatures of up to 80° C, high humidity and high electric potentials, catalyst nanoparticles are exposed to various degradation processes. Electrochemical corrosion of carbon support can result in detachment of particles and their consequent agglomeration, which results in a decrease of specific area suitable for electrochemical reactions. Despite high redox potential of Pt (1.115 V vs. SHE), high electric potential in fuel cell cathode can also lead to electrochemical dissolution of catalyst particles, which is additionally accelerated by their small size. Dissolved catalyst can either redeposit to larger particles, resulting in net growth of catalyst particles (so-called Ostwald ripening), or diffuse in proton exchange membrane, where it is reduced by crossover hydrogen. Both processes result in a loss of electrochemically active surface area in the catalyst layer.To reduce the amount of Pt used and thus lower the price, new types of catalyst materials are being developed, using alloys of Pt with other metals, such as Co, Cu or Ni, which also increases the specific electrochemical activity of the material. [1] Since alloying metals have lower redox potential compared to Pt, alloyed catalyst are more susceptible to dissolution, which represents a significant challenge in their wider adoption. Degradation is somewhat mitigated by preconditioning of catalyst nanoparticles during which Pt shell is formed around alloy particle core, resulting in higher stability of core-shell catalysts. Despite clear advantages, the use of alloyed catalyst might introduce some new risks in terms of fuel cell durability. Dissolution of alloying metal introduces ions of transition metals into the membrane, where they could serve as a catalyst in chemical reactions leading to the chemical degradation of the membrane [2]. Metal ions can also block the active sites in proton exchange membrane and thus reduce its proton conductivity, while diffusion from cathode to anode catalyst layer and deposition on catalyst surface can present a risk of anode catalyst surface passivization [3].To improve the understanding of aforementioned processes, we propose a 1-dimensional spatially and temporally resolved degradation model of alloyed catalyst degradation in the membrane-electrode assembly (MEA). The model describes the changes of cathode core-shell catalyst structure as a redistribution of particle core sizes and shell thicknesses due to electrochemical surface oxidation, dissolution and redeposition. To provide realistic internal states of the fuel cell during operation, the degradation model is coupled with advanced spatially and temporally resolved model of the fuel cell operation [4]. Dissolution model represents a source of Pt and alloying ions in the catalyst layer, which is coupled to 1D model of ion diffusion in the catalyst layer and the membrane, taking into account the specifics of transport in different MEA components. The transport model is further coupled with potential ion sinks, such as reduction in the presence of hydrogen, interaction with the proton exchange membrane and redeposition on the anode.Preliminary results indicate that Pt ions diffusion results in spatial inhomogeneities of catalyst layer degradation. The location and mechanism of metal ion deposition depends on the redox potential of metal and can occur in either membrane or anode catalyst layer. The model shows great promise in developing a better understanding of degradation processes of alloyed catalyst materials and could in future guide a development of better operation strategies for fuel cell use, mitigating their degradation and improving their lifetime.[1] E. Antolini et al., J. Power Sources. 160 (2006) 957–968.[2] M. Strlič et al., Acta Chim. Slov. 50 (2003) 619–632.[3] A. Han et al., Int. J. Hydrogen Energy. 45 (2020) 25276–25285.[4] A. Kregar et al., Appl. Energy. 263 (2020) 114547