| Solid polymer electrolytes (SPE, typically Nafion) have been increasingly adopted in electrochemical techniques, such as fuel cells, water electrolyses, electrochemical syntheses, electrochemical sensors, etc., because of the attractive advantages of SPE, including compact device structure and being free from troublesome electrolyte leakage. A rather common feature of these applications is the SPE impregnated catalyst electrode. SPE impregnation is necessary to ensure a high utilization of the catalyst surface. However, there usually still is a part of catalyst surfaces not in contact with the SPE networks in the catalytic layer. Most of these catalyst surfaces are in contact with pure water or gas phase instead. Therefore, question is raised as to whether these catalyst surfaces can contribute to the targeted electrode reactions.For the catalyst surfaces not in contact with SPE networks to take part in the targeted electrode reactions, there are in principle two possible ways. One is the surface diffusion of adsorbed reaction intermediates. In this mechanism, the reactant is adsorbed and dissociated on these surfaces to form adsorbed reactive intermediates which are transported to the catalyst/SPE interfaces by surface diffusion and undergoes electrochemical reactions there. In this way, the ionic conductance is supported solely by the SPE network. Alternatively, the charge transfer reactions may take place on the sites of dissociative adsorption and a sufficient ionic conductance in water is required in a long or short distance, depending on the electrode structure. It is necessary, therefore, to study the surface diffusion of adsorbed species on catalyst surfaces and the ionic conductance in pure water. In this thesis, the ionic conductivity of pure water in miro-porous electrodes and the surface diffusion of adsorbed hydrogen and oxygen on Pt and Au were studied, respectively, and then the combined effects of surface diffusion and water conductance were investigated with a model SPE fuel cell. The major results are as follows.1. The reference electrode for pure water systems By thermodynamic reasoning, it was proved that the reversible hydrogen electrode (RHE) or dynamic hydrogen electrode (DHE) on a Nafion membrane can be used directly as a reference to measure the electrode in the pure water in contact with the Nafion membrane. The voltage reading thus obtained is exactly the potential of the test electrode with respect to the RHE in pure water, without the need for a true RHE in pure water. This approach makes the electrochemical experiments simple for pure water systems and was adopted throughout the thesis works.2. Ionic conductivity of pure water in micro-porous electrodesA special device was designed to measure the ionic conductivity of pure water in micro-porous electrodes as a function of electrode potential and three kinds of electrodes were measured, i.e., gold powder electrodes, platinum black electrodes and carbon powder electrodes. The experimental results were compared to the theoretical prediction based on the Gouy-Chapman-Stern model for the electrical double layer.The experimental results of gold powder electrodes were found to be in reasonable agreement with the theoretical prediction. The ionic conductivity changed little in the potential region of about 0.5V round the potential of zero charge (PZC), but increased quickly as the potential moved further apart from PZC either to the negative or to the positive direction. For example, at 0.1V (vs. RHE) the conductivity may reach 1 mS/cm, which is about a percent of the conductivity of fully wetted Nafion membrane.The situation of platinum black electrodes conformed to the theoretical prediction in the potential region positive to PZC but the ionic conductance was found to be much higher than the predicted for potentials below 0.4V (vs. RHE). This abnormal behavior was explained by the quick diffusion of adsorbed hydrogen (the underpotential deposited hydrogen, UPD-H) in conjunction with accompanying anodic oxidation and cathodic deposition of UPD-H on the two sides of the micro-porous electrode, respectively.The ionic conductance of water in XC-72 carbon powder electrodes was found to be greater than prediction and was interpreted by the presence of ionizable surface groups. Besides, the ionic conductance increased on an electrochemical treatment at a high potential (1.6V vs. RHE), attributable to the increases in ionizable surface groups and/or total electrochemical surface area. 3. Surface diffusion studies using cyclic voltammetryThe surface diffusion of UPD-H was studied using the powder microelectrode technique. The surface diffusion of UPD-H was clearly demonstrated by the cyclic voltammetry (CV) of a Pt black powder microelectrode under water and pressed on a Nafion membrane on the other side of which a counter electrode was attached. The CV current thus obtained for the weakly adsorbed UPD-H was found to be about two thirds of that obtained in sulfuric acid solutions, but the current of rest part of CV curve was markedly smaller than that in sulfuric acid. For a low loading Pt/C (5wt%Pt/C), in which continuous Pt surface networks were believed to be absent, the CV of the powder microelectrode pressed on Nafion membrane still showed UPD-H charge but much smaller than in sulfuric acid, indicating that the overflow of UPD-H from Pt to carbon surfaces occurred and adsorbed hydrogen could diffusion over carbon surfaces but the overflow-diffusion process was slower than the surface diffusion on pure Pt surfaces.The CV of a porous Au powder electrode in contact with Nafion membrane showed two cathodic current peaks attributable to the diffusion-reduction of two different surface oxygen species.4. Test of a model PEMFC with an additional Nafion-free catalyst layerIn order to test the combined effects of surface diffusion and water conductance for the catalyst surfaces not in contact with Nafion networks in an electrode, measurements were conducted on a model PEMFC in which one electrode was overlaid with an additional Nafion-free Pt black layer. In a dry Ar atmosphere, the additional layer showed little contribution to CV current but markedly increased CV current was observed in humid Ar. The increased CV current with almost unchanged current peak potential could not be explained by the water conductance in porous electrodes, thus proved that both adsorbed hydrogen and adsorbed oxygen can diffuse over Pt surfaces in a wet atmosphere but much less in dry condition, if any.The performance of the FC electrode with the additional Nafion-free Pt black layer was found to be much improved in comparison to the intrinsic electrode, both as hydrogen anode and oxygen cathode. The IR drop for such increased current was estimated on the basis of water conductivity improvement due to double layer charge and was found to be much too large to match the experimental findings. Therefore, the increased performance due to the additional Nafion-free catalyst layer must rely on the surface diffusion of reactive intermediates which were produced on the Nafion-free Pt surfaces and diffused to Pt/Nafion interfaces to undergo charge transfer reactions there.5. Comments on the utilization of catalyst surfaces not in contact to Nafion networksAssuming that the Pt surfaces not in direct contact with the Nafion networks are bridged by water channels to the Nafion networks, one may estimate the tolerable maximum size of the Nafion-free catalyst agglomerates, according to the targeted volume current density and the IR drop tolerance in an agglomerate. For example, for targeted current density 1000A/cm3 and IR drop tolerance 1 mV, the Nafion-free agglomerates should be not larger than 0.1μm in diameter. However, the utilization of catalyst surface is not only affected by the IR drop within the electrode but also by the eXChange current density of the targeted reaction. If the eXChange current density at Pt/water interface is appreciably lower than that for Pt/Nafion interface, the Pt surface in contact with water will be little useful even without IR drop.When the Nafion-free catalyst surfaces are active for dissociative adsorption of the reactants and surface diffusion available, the Nafion-free surface is able to contribute to current generation for fuel cell electrodes only if the Nafion-contacted surfaces are short in the ability for dissociative adsorption. For a well optimized catalyst electrode, an additional Nafion-free catalyst layer would not help at all.6. PZC in pure waterAs a by-product, the PZC values for Pt and Au in pure water were estimated from the experimental curves of ionic conductance versus potential to be 0.19(vs. SHE) and 0.5V(vs. SHE), respectively.7. Unusually large Frumkin effects in pure waterThe CV of Pt in pure water showed an abnormal anodic current peak round 0.7V (vs. RHE), which was attributed to the UPD-H oxidation retarded by severe Frumkin effect near PZC. |
