Dynamic behaviour of fuel cells

Abstract

This thesis addresses the dynamic behaviour of proton exchange membrane fuel cells (PEMFCs) and alkaline fuel cells (AFCs). For successful implementation in automotive vehicles and other applications with rapidly varying power demands, the dynamic behaviour of the fuel cell is critical. Knowledge of the load variation requirements as well as the response time of the cell at load change is essential for identifying the need for and design of a buffer system.

The transient response of a PEMFC supplied with pure hydrogen and oxygen was investigated by load step measurements assisted by electrochemical impedance spectroscopy and chronoamperometry. Using an in-house designed resistance board, the uncontrolled response in both cell voltage and current upon step changes in a resistive load was observed. The PEMFC was found to respond quickly and reproducibly to load changes. Two transient processes limiting the fuel cell response were identified: i) A cathodic charge transfer process with a potential dependent response time and ii) a diffusion process with a constant response time. The diffusion transient only appeared at high current densities and was offset from the charge transfer transient by a temporarily stable plateau. Transient paths were plotted in the V-i diagram, matching a predicted pattern with overshooting cell voltage and current during a load step.

The transient response of a PEMFC was measured for various cathode gas compositions and gas utilisations (fraction of supplied reactant gas which is consumed in the fuel cell reaction). For a PEMFC operated on pure hydrogen and oxygen, the cell voltage response to current steps was fast, with response times in the range 0.01-1 s, depending on the applied current. For a PEMFC supplied with air as cathode gas, an additional relaxation process related to oxygen transport caused a slower response (appr. 0.1-2 s depending on the applied current). Response curves up to appr. 0.01 s were apparently unaffected by gas composition and utilisation and were most likely dominated by capacitive discharge of the double layer and reaction with surplus oxygen residing in the cathode. The utilisation of hydrogen had only a minor effect on the response curves, while the utilisation of air severely influenced the transient PEMFC response. Results suggested that air flow rates should be high to obtain rapid PEMFC response.

The load-following capability of a single PEMFC was studied by measuring the cell voltage response to a sinusoidal current load with large amplitudes. Effects on the cell voltage response when varying the DC value, amplitude and frequency of the current load were recorded. The load-following capability of the PEMFC was excellent in the operating range where changes in cell voltage were dominated by ohmic losses. No hysteresis in the cell voltage response was observed in this range for frequencies up to 1-10 Hz. In the operating range where changes in cell voltage were dominated by activation losses, hysteresis appeared at lower frequencies (>0.1 Hz) due to sluggish response in the voltage range near open circuit voltage. The increased mass transport limitation imposed when supplying the PEMFC with air caused hysteresis to appear at lower frequencies than for oxygen (above 0.1 Hz, compared to 1-10 Hz for oxygen).

The dynamic behaviour of an AFC supplied with pure oxygen and hydrogen was investigated by load step measurements assisted by electrochemical impedance spectroscopy (EIS). Load step measurements were carried out using an in-house designed resistance board which gave step changes in a purely resistive load. Resistive load steps between various operating points along the polarisation curve were carried out and the corresponding transient response in cell voltage and current was measured. The transient cell response consisted of an initial ohmic drop followed by a relaxation towards the new steady state. The observed response was slower at higher cell voltages. Measured response times varied on a time scale of appr. 10 ms to 10 s, depending on the initial and final voltages. Results from EIS measurements suggested that the potential dependent response time stemmed from the charge transfer reaction at the cathode. Transient response curves were plotted in the V-i diagram and shown to follow a pattern determined by the load resistance and ohmic resistance of the AFC. Results showed that when supplied with pure oxygen and hydrogen, the AFC responded sufficiently fast for automotive applications.

An iso-thermal, one-dimensional, transient model of an AFC cathode was developed, based on mass balances for oxygen and ionic species and floodedagglomerate theory. Model results show the coupled effects of oxygen diffusion, ion transport and propagation of local electrode potential on the response in current density to a cathodic step in electrode potential. For a set of base case parameters, oxygen diffusion and potential propagation, with characteristic time constants of 0.30 and 0.11 ms, respectively, dominated the current response up to appr. 1 ms, while the slower ion diffusion with time constant 5.0 s controlled the final relaxation towards steady state at appr. 60 s. A smaller agglomerate radius and electrode thickness and a smaller double layer capacitance gave faster electrode response. For a cathodic step in electrode potential, an overshoot in faradaic current appeared around 0.5 ms. This overshoot was related to an initially higher oxygen concentration in the agglomerates, but was masked by capacitive current for base case parameters. Simulated response in oxygen concentration profiles suggested that the potential dependent response time found in previous studies can be related to consumption of surplus oxygen in the catalyst layer.