Understanding Direct Borohydride – Hydrogen Peroxide Fuel Cell Performance with a Calibrated Numerical Model
In the H2O2-DBFC shown in Fig. 1, a Nafion 117 membrane separates parallel flow channels. The channel walls are graphite electrodes coated with electro-deposited catalysts. Fuel (1-50 mM NaBH4 / 2 M NaOH) is oxidized at the Au anode and oxidizer (10-40 mM H2O2 / 1 M H2SO4) is reduced at the Pd:Ir cathode, while Na+ and H2O cross the membrane.
The steady-state finite-difference model includes diffusion, migration, and advection in the flow channels, transport across the membrane, multiple charge transfer reactions at each electrode to yield mixed potentials, and chemical parasitic reactions at each electrode. It is formulated in MATLAB and solved by a modified Newton solver in SUNDIALS.
Model features were chosen to reflect experimental observations. For example, experiments showed that low cathode potential caused by oxidizer-lean stoichiometry can lead to H+ reduction at the cathode, therefore this reaction was included in the model. The model was calibrated to experiments by fitting predicted polarization curves to measured curves. Electrode reaction rates were modeled with global expressions that summarized many intermediate steps, and the fitted parameters were the rate constants in those global expressions. Predicted vs. measured polarization curve comparisons showed good agreement (R2 ≥ 0.920) over the fuel cell operating space. The predicted and measured limiting current densities agreed, indicating transport was modeled accurately.
Results from the calibrated model show compact concentration boundary layers in both channels, which limit reactant utilization and current density. Transport limits are most pronounced at the cathode because BH4- has higher diffusivity than H2O2, BH4- transport is aided by migration, and BH4- can provide up to 8e- while H2O2 only consumes up to 2e-. Decreasing inlet concentrations, flow rates, and cell voltage result in lower losses to parasitic side reactions as these changes decrease reactant concentrations near the electrodes, which favors charge transfer reactions. Peak conversion efficiency coincides with peak power density because thermodynamic efficiency and parasitic reaction rates both decrease (relative to charge transfer reaction rates) with increasing current density.
This work shows that advection, migration and diffusion must be included in H2O2-DBFC models, and that global reactions are sufficient to capture the essential features of complex H2O2-DBFC reactions.
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