Optimization of PTFE Content in Catalyst Layer of Gas Diffusion Electrode for Alkaline Fuel Cell

Abstract

Alkaline fuel cell (AFC) is historically 1st successfully applied fuel cell type. The main benefit of AFC is still possibility to use non platinum based catalysts at ambient conditions. With increase of demand for platinum metals for PEM water electrolysis and fuel cell this benefit would become more significant. Therefore it has sense to focus on AFC development.Gas diffusion electrodes (GDE) are crucial part in numerous technologies. Its main function is to enable contact of gaseous reactant with electron conductor and ionic conductor. Enlargement of contact area of mentioned three phase boundary lead to increase of electrode performance. In the case of liquid electrolyte like potassium hydroxide in AFC it is crucial to balance penetration of liquid into pores of electrode and to keep part of electrode unflooded. It must be realized in catalytic layer of GDE. Variation of temperature change electrolyte viscosity and vapor pressure both influences flooding of electrode.Beside suitable structure GDE for AFC must be stable in strong alkaline solutions. Standard GDE construction is based on combination of hydrophilic a hydrophobic materials to reach desired wettability. Also, the preparation method influences the porosity of the catalytic layer. GDE consist of three layers: catalytic, diffusion and supporting. The gas/liquid interface must be realized in catalytic layer. It is usually composed of catalyst and binder. Catalyst is chosen with respect to electrochemical reaction running on electrode thus binder must deliver desired hydrophobicity. Most typical binders are perfluorinated polymers like PTFE polytetrafluoroethylene (PTFE) with excellent chemical resistivity. Nickel, manganese, platinum, silver etc. are widely used in alkaline systems like catalysts. The most important property of the catalytic layer is the ratio of catalyst to PTFE which provides suitable hydrophobicity. The diffusion layer (GDL) is usually made of a mixture of carbon blacks/graphite and PTFE. The support layer is typically made of nickel foam or carbon paper.This work aims to optimize the ratio of PTFE to platinum catalyst in the catalytic layer. The commercial catalyst HiSPEC 40 wt. % Pt/C was used and Pt load was set to 0.5 mg/cm2. Electrodes were prepared by ink spraying technique on commercial GDL Sigracet® BC28. The ink with different ratios of PTFE to catalyst was applied to prepare GDE with PTFE content from 5 wt. % to 40 wt. %. Electrode baking under inert atmosphere was the last preparation step to increase mechanical stability.As the first characteristic the contact angle was measured. The droplet of water was placed on catalyst layer surface. The contact angle of droplet to GDE surface represents the level of hydrophobicity. A high level of hydrophobicity brings about low inundation GDE and bad contact between liquid, gas and catalyst. On the other hand, if GDE has low hydrophobicity, liquid can inundate the catalytic layer easily. As it was found already 5% of PTFE makes GDE surface hydrophobic. Higher loading of PTFE lead only to small angle increase. But significant decrease was found after GDE operation and PTFE loading over 15% is necessary to prevent further hydrophobicity decrease.In the second step prepared GDE were tested in the laboratory AFC. Pure hydrogen and pure oxygen were dosed to GDE chambers. 25% KOH solution was used as electrolyte. The I-V curve at the beginning of cell assembly, performance in time and I-V curve at the end of experiment was measure. The power of AFC with prepared electrodes improves if the hydrophobicity of GDE decreases. However, the penetration of liquid through the GDE was observed in low hydrophobic GDE in a long time testing alkaline fuel cell. Also the gradual decrease of cell performance was observed due to the flooding of the catalytic layer. After stop of operation and repeated start the performance of the cell again improved but no to the origin level. Only if the PTFE content was over 40% the performance was identical for several repeated experiments.The importance of PTFE loading in catalyst layer composition was demonstrated and its influence on AFC performance. The low loading of PTFE in GDE shows the highest performance but with fast drop. For longtime operation it is necessary to use PTFE leading at least 40%. Acknowledgment Financial support of this research by the Technological Agency of the Czech Republic under the project No TK02030001 “Research and development of advanced flow energy storage technologies” is gratefully acknowledged.