Electrochemical Studies On Membrane Electrode Assembly In Direct Methanol Fuel Cells

Direct methanol fuel cell (DMFC) is viewed as a promising power source forvarious portable devices due to its simplicity, convenience and higher energydensity of the system. The cell performance largely depends on the properties ofmembrane electrode assembly (MEA), which is an essential part of DMFC. In thisthesis, the electrochemical properties of DMFC under operating condition werestudied by using electrochemical impedance spectra (EIS). The pre-treatment onMEA and the fine-tuning of electrode component on DMFC performance wereinvestigated systemtically. The main causes of cell performance degradation wereanalyzed during its long-term operation, and the recovery and mitigationstrategies were also proposed. The accelerated durability test (ADT) technique forcatalyst and MEA was explored. In addition, the effects of chloride ion as apotential fuel impurity on methanol electro-oxidation and single cell performancewere investigated.The EIS results showed that charge transfer resistances and the inductance atlow frequency decreased with the working voltage decreasing at differenttemperatures, whereas the internal resistance increased. The charge transferresistance and mass transport resistance of oxygen reduction reaction increasedwith the increment of methanol permeation.The MEA conditioning is important to maximize the cell performance. Theconditioning process is accompanied by time-dependant variation of interfacialproperties and microstructure of catalyst layer. The experiment results indicatedthat the single cell could reach its maximum performance by discharging at lowercurrent in relative short time. It is found that the Pt-Ru loading in the anodecatalyst layer over an "upper limit"does not significantly improve theperformance of DMFC. The"upper limit"of the Pt-Ru loading is dependant on theoperating temperature. The cell performance degradation was mainly caused by catalyst deactivationand delamination between the Nafion membrane and catalyst layer. The catalystagglomeration was mainly caused by the electric field forces. The performancedegradation can be timely mitigated or partly recovered by using a potentialsweep, which is a good method to eliminate the adsorbed intermediated species onthe catalyst surface from methanolelectro-oxidation. In addition, the effects of Cl-anions as a potential fuel impurity on DMFCwere investigated. Trace amounts of Cl-anions in methanol solution considerablydegraded cell performance, which was not resumed after washing with hot water.It is implied that high purity conditions are required for preparation of the MEAand the humidified feed-streams for DMFC.