Mechanisms Of Parasitic Reactions Induced By Reactant Starvation In Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have been recognized as one of themost promising power sources for automobiles, stationary power plants and electronicdevices. The widespread commercialization of PEMFCs, however, is still hindered byvarious challenges, among which insufficient durability is an important issue. Reactantstarvation is one of the most critical issues that can exert detrimental impacts on thedurability of PEMFCs, as it can induce some parasitic reactions like carbon corrosion,platinum and ruthenium dissolution and hydrogen evolution inside the cell which caninduce cell degradation. Therefore, tremendous research efforts have been made in thepast decade with regard to this issue, most of which focused on material research, such asthe development of anti-corrosion carbon materials. Little attention, however, has beenpaid to the mass transport phenomena inside the cell which is the essential cause for thereactant starvation. To bridge this research gap, we conduct both experimental andnumerical studies in this thesis with aim to elucidate the mechanisms of species transportinside the cell at the reactant starvation conditions. The main findings of this thesis aresummarized as follows:1. Research regarding the carbon corrosion phenomenon in the cathode electrode ofthe H2-PEMFC induced by localized hydrogen starvation: We developed a numericalmodel to investigate the behavior of a H2-PEMFC at localized hydrogen starvationcondition induced by anode channel clogging. Special attention is paid to the mechanismsof hydrogen transport in the diffusion medium within the flooded region, which are foundto be characterized by in-plane convection at the fringe of the flooded area and bydiffusion as it flows deep inside. The convection effect is induced by the simultaneousconsumption of hydrogen and water vapor, and can be significantly weakened if hydrogenis diluted by nitrogen, leading to wider area with hydrogen starvation. Another part of thisresearch focuses on the spatial distribution of carbon corrosion rate within the hydrogenstarvation region. It is found that the effect of in-plane proton conduction has dramaticimpact on the distribution of carbon corrosion rate. Meanwhile, it is also found that the maximum carbon corrosion rate is solely determined by oxygen crossover rate at high cellvoltages, but is influenced by the kinetics of oxygen reduction reaction in the anode at lowcell voltages.2. Research regarding the voltage reversal and carbon corrosion phenomenon in agalvanostatically discharging H2-PEMFCs induced by gross hydrogen starvation: Wedeveloped a numerical model to investigate the behavior of a single H2-PEMFC atgalvanostatic operation with insufficient hydrogen supply. Special attention is paid to thecurrent and potential distributions, especially along the through-plane direction. It is foundthat most of hydrogen at this condition is oxidized in a narrow region close to thefuel-inlet, and the anode area before hydrogen depletion can be characterized into anactivation limited region and a mass-transport limited region. Meanwhile, an unexpectedhydrogen evolution phenomenon is discovered in the cathode electrode adjacent to thefuel inlet, owing to the imbalance between the localized ultrahigh hydrogen oxidationcurrent density in the anode the the lower limiting current density of oxygen reductionreaction in the adjacent cathode. Moreover, the evolved hydrogen gas is also found to beoxidized nearby due to the steep variation of electrode potential in the cathode catalystlayer, indicating that hydrogen evolution, hydrogen oxidation and oxygen reduction occurssubsequently within the micro-scale thickness of cathode catalyst layer, adding to thecomplexity of the coupled phenomena in the voltage-reversed single cell.3. Research regarding the parasitic hydrogen evolution in a direct methanol fuel cell(DMFC) induced by oxygen starvation in the cathode: The first part of this work centerson modeling the behavior of a DMFC at open-circuit condition with low air flow rate(AFR). In accordance with previous experimental observations, the modeling resultsindicate that the cell splits into an upstream galvanic region (GR) and a downstreamelectrolytic region (ER) at low AFRs. Through analyzing the transport phenomena aroundthe interface between the GR and ER, we find that a localized DMFC arises near thisinterface due to the significant in-plane proton conduction effect induced by the abruptrise of ionic potential near this interface. Moreover, the effect of cathode flooding on thecurrent distribution at this condition is also highlighted. It is found that both theopen-circuit potential and the current distribution vary significantly with different extentof cathode flooding at the same AFR, which explains the reason why previous researchersget quite different current distributions at the same experimental condition. The secondpart of this work focuses on the behavior of a DMFC at galvanostatic operation withdifferent AFRs. The voltage of a single DMFC is measured experimentally with differentAFRs, and it is found that the AFR can be characterized into three different ranges basedon the voltage-AFR dependence. The cell voltage varies little in range1where the AFR is high, but decreases dramatically with the AFR in range2, and becomes negative in range3.Numerical modeling is then conducted to investigate the cell characteristics at differentAFR ranges. It is found that the entire cell acts as a normal DMFC with the AFR in range1, but turns into bi-functional mode in range2with parasitic hydrogen evolution from theanode in the downstream region due to partial oxygen depletion. Extremely high currentdensity arises near the air inlet when the cell potential approaches zero, which leads tohigh anode potential and thus induce serious ruthenium dissolution. When the AFRdecreases to range3, hydrogen evolution starts to occur in the cathode, and the evolvedhydrogen can be re-oxidized upstream, leading to coexistence of oxygen reduction,methanol oxidation and hydrogen oxidation near the inlet region.