ABSTRACT
The efficient and cost-effective production of green hydrogen is essential to decarbonize heavily polluting sectors such as transportation and heavy manufacturing industries such as metal refining. Polymer electrolyte membrane water electrolysis (PEMWE) is the most promising and rapidly maturing technology for producing green hydrogen at a scale and on demand. However, substantial cost reduction by lowering precious metal catalyst loadings and efficiency improvement is necessary to lower the cost of the produced hydrogen. Porous transport layers (PTLs) play a major role in influencing the PEMWE efficiency and catalyst utilization. Several studies have projected that the use of microporous layers (MPLs) on PTLs can improve the efficiency of PEMWEs, but very limited literature exists on how MPLs affect anodic interfacial properties and oxygen transport in PTLs. In this study, for the first time, we use X-ray microtomography and innovative image processing techniques to elucidate the oxygen flow patterns in PTLs with varying MPL thicknesses. We used stained water to improve contrast of oxygen in PTLs and demonstrate visualization of time averaged oxygen flow patterns. The results show that PTLs with MPLs significantly improve interfacial contact by almost 20% as compared to single layer sintered PTL. For the single layer PTL without MPL, the pore volume utilization for oxygen flow is low and the oxygen follows a viscous fingering flow regime. With MPLs, the pore volume utilization is higher, and the number of oxygen transport pathways is increased significantly. MPLs were also shown to suppress capillary fingering and transition oxygen flow to the viscous fingering regime, which has been proven to decrease site masking effects. Finally, durability tests showed the least voltage degradation for thin MPL and thicker MPLs run into mass transport limitations. Based on these findings, PTL/MPL design optimization strategies are proposed for enabling low catalyst loadings and improving durability.
ABSTRACT
The kinetics of the oxidation of NO by O(2) was studied on 1 cm diameter single crystals, Pt(111) and Pt(321), at atmospheric pressure. The surface of the (321) crystal is composed of 20% kink, 20% step, and 60% terrace atoms and simulates small 1-3 nm size Pt particles on supported catalysts, while the (111) surface simulates the most stable plane found on large, >5 nm, particles. The turnover rates (TORs), that is, rate normalized by the exposed platinum, on the two single crystals differ by less than a factor of 2 over the range of conditions studied and are also similar to the TOR on a supported catalyst with an average particle size of 9 nm. Both surfaces show a dynamic kinetic behavior as evidenced by a change in the apparent activation energy and reaction orders as a function of reaction conditions. The oxygen coverage after initial rate experiments on Pt(111) was 0.6 monolayer (ML) on average which is similar to that measured previously by in situ X-ray photoelectron spectroscopy (XPS) under similar conditions. This oxygen overlayer, which is likely controlled by the relative presence of NO and NO(2), inhibits O(2) dissociation but lowers the binding energy of reactants enough to allow the catalysis. Long-term stability studies on Pt(111) correlate catalyst deactivation with irreversibly bound oxygen on the surface at coverages over 1 ML, as measured after reaction. Ex situ Auger electron spectroscopy (AES) and XPS results suggest that the surface defect sites on Pt(321) begin to oxidize relative to atoms on the (111) plane at lower NO(2) to NO ratios.
