|DMFC dynamic behaviour current steps system analysis methanol oxidation flow field design|
This work contributes to understanding the dynamic behaviour of direct methanol fuel cells (DMFCs). It focuses on experimental and model-based analysis of the influence of dynamic cell current changes on the cell voltage. In a first step, cell voltage responses to cell current step changes were analysed experimentally. The voltage responses in most cases showed significant overshooting. Understanding such dynamic effects is not only essential for operation and control of DMFCs, but also for understanding the processes occurring inside the cell. A general mathematical criterion for overshooting could be developed using linear system analysis. Models of different complexity yielded information on the relevance of the physico-chemical phenomena governing the dynamic DMFC behaviour: Minor influence of the dynamics of methanol storage in the membrane. Minor influence of cathode and anode double layer charging/discharging. Significant effect of the reaction kinetics of methanol oxidation. Application of a simple, two step methanol oxidation mechanism could generate absolute overshoots of 10 % of the experimentally observed values. The underlying phenomenon was the influence of changes in the anode methanol concentration on the methanol transport to the cathode, and therefore on the cathode overpotential. The significant dependence of DMFC models on the methanol oxidation kinetics motivated an in-depth analysis of the anode reaction kinetics. The kinetics of methanol oxidation was studied experimentally at half cells using electrochemical impedance spectroscopy. The results were compared to simulations using various reaction kinetic formulations, and a kinetics was identified that is able to adequately predict the methanol oxidation experiments, and after integration into a DMFC model, also the experimentally observed dynamic behaviour of DMFCs. Typically, DMFCs are operated at low anode volume flow rates to increase the system efficiency. Low anode flow rates in turn lead to distinct methanol concentration differences within anode flow fields. To identify their influence on the DMFC performance, the hydrodynamic behaviour of three anode flow field designs was experimentally characterised and modelled using networks of ideally mixed reactors. Although the analysis identified large concentration differences over the active area for some designs, all DMFC model responses as well as the experimental cell voltage responses of two designs showed similar dynamic behaviour at medium to high anode flow rates. Model analysis attributed these results to the high insensitivity of the cell voltage to methanol concentration changes at high and moderate concentration levels. The DMFC models were also able to predict the experimentally observed dependences of the dynamic DMFC response on process parameters like current density and anode flow rate. They identified the state of the anode catalyst layer as the main governing factor. Nonetheless, comparison of experiment and simulation also suggested a significant influence of gaseous carbon dioxide on the cell behaviour for all designs. This effect can be crucial for exact model predictions of the steady state and dynamic behaviour of DMFCs. Extending the presented DMFC models with the influence of gaseous carbon dioxide on methanol transport should therefore be the next decisive step to a complete understanding of the dynamic behaviour of the DMFC. The models presented in this thesis are reduced models which are valid for the activation controlled and pseudo-Ohmic regions of the DMFC polarisation curve. They can be easily extended or adapted to different DMFC designs or DMFC stacks. The models as well as the results obtained can be applied in various fields, ranging from research and development to industrial application. Diagnostic tools for the state of a given DMFC, and controller design are among the possible options. The models are a potential basis for DMFC controller design due to their short simulation times. The design of reliable controllers is essential especially if DMFCs are to be used without or with minimised additional buffers, like a battery or supercapacitor. Nevertheless, the investigations given here also show that the effect of overshooting cell voltage may not be easily eliminated or used for improving the cell performance: The cell voltage overshooting is mainly determined by the anode reaction kinetics, i.e. by the interaction between the fast water decomposition and the slow decomposition of methanol and intermediates.