• Energy Research

Abstract Membrane is one of the most important components in proton exchange membrane fuel cells (PEMFCs), which determines the transport phenomena, performance, and durability. With the rapid development of novel membranes, many transport coefficients in membranes applied in numerical studies are outdated due to the lack of experimental data for new membranes. In this review, the fundamentals of commercially available membranes are scrutinized, followed by the fundamental working mechanisms. A detailed examination of the transport phenomena within the membranes, including transport mechanisms, mathematical description, and experimental methods, is conducted for protonic conduction, electro-osmosis drag, diffusion, hydraulic permeation, and gas crossover, which are urgently needed for theoretical and numerical studies. It is found that various empirical or analytical correlations have been established to predict the transport coefficients of the membranes. However, empirical models may not be accurate for all types of membranes since there is no sufficient experimental data for a solid correlation and validation. The experimental methods reviewed in the present study can be applied for new membranes, which is essential to quantify the transport phenomena and its further impact on cell performance and durability. The key transport-phenomena-related factors that affect the performance and failure modes of membranes are also reviewed in this study, which helps to develop strategies in improving membranes’ performance and durability during operation. This review deepens the understanding of the short-term and long-term performance of the membrane in PEMFCs and provides important insights into the further design of novel membranes.

Abstract For polymer electrolyte membrane (PEM) fuel cells, the importance of mass transport property, gas permeability, in gas diffusion layer (GDL) is widely recognized with less attention being paid to catalyzed electrode (GDL with a catalyst layer). In this study, the contribution of the catalyst layer to the overall gas permeability of the electrode is experimentally investigated for different catalysts with a range of Pt loadings at various temperatures for air, oxygen and nitrogen gases. Results indicate that the gas permeability of the GDLs can be reduced by 58–77% with the presence of a catalyst layer. For the constant Pt loadings, the electrodes with higher Pt/C ratios (e.g., 60% Pt/C) show larger gas permeability than those with lower ratios (e.g., 30% Pt/C) due to their smaller thicknesses and higher porosity. Similarly, for the electrodes with the same type of catalysts, the gas permeability is higher for lower Pt loadings. Further, the effective gas permeability of the catalyst layers alone is about two orders of magnitude smaller than that of the GDLs. Additionally, operating at higher temperatures slightly enhances the permeability. Oxygen gas has a higher permeability than air and nitrogen, but the differences are small. These results highlight the importance of catalyst layer, hence the Pt loadings and Pt/C ratios, in determining the mass transport throughout the entire electrode in PEM fuel cells.

Abstract Electrode structure determines the rate of transport and electrochemical reactions and is significantly affected by the catalyst deposition method. In this study, the effect of catalyst deposition is investigated on the pore structure, mass transport, and operating performance of the catalyzed electrodes prepared by the methods of catalyst coated on membrane (CCM) and catalyst coated on substrate (CCS). The result indicates that the CCS electrode is thinner, yielding larger porosity, smaller geometric pore surface area, smaller diffusion and permeation resistivity, and lower cell performance. The maximum power density of the CCS electrodes is only about 4% smaller than that of the CCM electrodes at high Pt loadings (0.4 mg·cm−2), while it is as much as 60% less than that of the CCM counterparts at low Pt loadings (0.1 mg·cm−2). The significant performance drop for the low-Pt-loading CCS electrodes is due to the relatively low surface area in the catalyst layers resulted from catalyst penetration into the pores of the gas diffusion layer, even though the mass transfer resistivity is smaller than their CCM counterparts. The CCS method is therefore unsuitable for low-Pt-loading electrodes (

Microstructure changes of the catalyst layers in proton exchange membrane fuel cells (PEMFCs) lead to significant performance degradation and durability limitations, especially under dynamic loading condition; and they are investigated experimentally in this study simulating the effect of wet-dry cycles through water intrusion-evaporation and water flow-through-dehydration experiments, respectively. It is found that the cycling of water intrusion-evaporation processes significantly contributes to the growth of agglomerates as well as the formation of pinholes and cracks, causing irreversible losses of active surface areas and catalytic activity; in contrast, the cycling of water flow-through-dehydration experiments enlarges the large pores but changes very little the agglomerate sizes. This is because water tends to flow through the path of least resistance, that is, the large pores in the catalyst layers, leaving the other parts of the catalyst layers with relatively small pores less affected. These observations provide experimental evidence of microstructure changes and their forms for a better understanding of degradation in PEMFCs, especially under dynamic operating conditions.

Abstract Catalyst layer (CL) has a significant impact on the overall pore structure of the entire electrodes, thereby impacting the transport processes and the performance of polymer electrolyte membrane (PEM) fuel cells. In this study, the contribution of the CL to the entire electrode structure is experimentally investigated. The electrodes are prepared by using two types of catalysts with different platinum/carbon (Pt/C) ratios and Pt loadings and characterized by the method of standard porosimetry (MSP). The results show that for the same type of catalysts, as the Pt loading is increased, both the porosity and mean pore size of the electrode decrease, whereas the pore surface area increases. For a constant Pt loading, a lower Pt/C ratio results in a thicker electrode with a smaller porosity, smaller pore size, and larger pore surface area. The fractal dimension is found to be a good representative of the complexity of the pore structure of the electrode; a larger fractal dimension is detected for a higher Pt loading and a smaller Pt/C ratio.

Abstract Durability is one of the most significant technical barriers to successful commercialization of polymer electrolyte membrane (PEM) fuel cells for practical vehicular applications. It is determined by the aging (degradation) and malfunction of various components during the long-term operation. Therefore, understanding the mechanisms of degradation modes in different components is crucial to the development of high-performing and long-lasting PEM fuel cells. In this review article, the critical degradation modes in major cell components, including membranes, catalyst layers, gas diffusion layers, and distribution plates, are comprehensively reviewed and analyzed, and the potential causes are described. Advanced experimental techniques to investigate the PEM fuel cell degradation modes reported in literature include steady-state durability tests and accelerated stress tests (ASTs). The steady-state durability test is straightforward but time-consuming and costly; therefore, ASTs are often applied to accelerate durability testing. For comparable results among different research studies, the experimental protocols and conditions have to be consistent, and the details of these experimental techniques are systematically reviewed in this article. The experimental results with a focus on the degradation modes, degradation rate, and test time of the PEM fuel cells have been reported. Finally, in order to understand the root causes of degradation modes and to develop the mitigation strategies, ex-situ ASTs in literature have been reviewed, including the effects of cyclic temperature, humidity, water wet-dry, freeze-thaw, clamping stress, and vibration operations.

Abstract Catalyst layer structural changes in polymer electrolyte membrane fuel cells have significant impact on the cell performance and durability. In this study, ex-situ experiments are designed to investigate the effect of humidity and/or thermal cycles on the structural changes of catalyst layers. The relative humidity and temperature are controlled by an environmental chamber and the catalyst layer structure is characterized by scanning electron microscopy and optical microscopy. The experimental results indicate that crack growth and development, catalyst agglomerate detachment, and surface bulges are the main structural changes of the catalyst layers. Applying relative humidity and thermal cycling simultaneously causes the most significant crack growth, while applying thermal cycling alone causes no appreciable changes. This indicates that the absolute humidity is the key parameter for the crack growth. Through cyclic voltammetry analysis, it is shown that the electrochemical active surface area decreases from 64.1 m2 g−1 to 49.1 m2 g−1 after 500 combined relative humidity and thermal cycles. Analyses of electrochemical impedance spectroscopy show that the charge transfer resistance and ohmic resistance increase significantly after 500 combined relative humidity and thermal cycles, causing the cell performance degradation.

Abstract The performance of a proton exchange membrane (PEM) fuel cell is determined by many factors, including operating conditions, component specifications, and system design, making it challenging to predict its performance over a wide range of operating conditions. Existing fuel cell models can be complex and computationally demanding or may be over-simplified by neglecting many transport phenomena. Therefore, a high-fidelity and computationally efficient model is urgently needed for the model-based control of fuel cells. In this study, semi-implicit multi-physics numerical models have been established, taking the mass, momentum, reactants, liquid water, membrane water, electrons, ions, and energy in all fuel cell components into account. The developed 1D model is of high fidelity by incorporating the two-phase flow, non-isothermal effect, and convection, and is still computationally efficient. These models are validated against data from an auto manufacturer with good agreements, and the computing efficiency is evaluated on a modest laptop computer. The modeling results suggest that the two-phase flow model exhibits better prediction accuracy than the single-phase flow model when reactants are fully humidified, while under low humidity conditions, the two models present equivalent performance as liquid water does not exist in the fuel cell components. The results also suggest that the maximum convective/diffusive ratio of H2, O2, and vapor mass fluxes can be 12%, 5.3%, and 35%, respectively, which are ignored in most diffusion-dominant models. The developed models are computationally efficient, requiring only 0.56 s and 0.26 s to simulate a steady-state operation of fuel cells for the two- and single-phase flow models, respectively. This implies that the developed models are suitable for the control of PEM fuel cells.