Modified activated carbon-based cathodes using cobalt tetramethoxyphenylporphyrin (CoTMPP) and graphene for zinc-air battery

This study proposed on the improvement of the performance of activated carbon (AC)-based electrodes by electrocatalyst doping and modifying the carbon support system for zinc-air battery (ZAB). Electrode or specifically known as air-cathode in ZAB is the key component that contributing to its performance. The catalyst was doped into carbon matrix following by pyrolysis process. Besides catalyst doping, a carbon support material also play the crucial role on the electrocatalytic performance. Combinations of two different carbon materials not only increase the specific surface area but also improve the electric conductivity. At first, different weight percent (wt.%) loading of cobalt tetramethoxyphenylporphyrin (CoTMPP) catalyst (AC-CoTMPP) were prepared using simple ultrasonication technique. The microstructure effects of CoTMPP addition on AC matrix or electrode materials were analysed using X-Ray Diffractometry (XRD), Fourier Transformed Raman spectroscopy (FT-RAMAN) and Field Emission Scanning Electron Microscopy (FESEM). Meanwhile the electrochemical performances of prepared electrodes involved cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) conducted at room temperature (25 ± 1 °C) using three-electrode system. The optimized experimental results from the CoTMPP composition were studied to determine the effect of heat treatment on structural and electrocatalytic activity of the AC-CoTMPP composites and electrodes at elevated temperatures known as pyrolysis process. The reduction peak current value of AC electrode with optimized content of CoTMPP (25 wt.% of CoTMPP loading and heat treatment of 800 °C) is −0.0455 mA. Although catalytic activity of heat-treated AC doped CoTMPP has shown an improvement, the obtained reduction peak current is still lower than that prepared electrode using platinum on carbon (Pt/C) catalyst (−0.0553 mA). Therefore, the proposed carbon-carbon composite through the combination of AC and graphene (Gr) as supports materials has become an effective strategy to enhance the structural morphology and electrochemical activity. This is due to the fact that Gr-AC composite not only enlarges the effective surface area of GrAC composite electrode but also provides a highly conductive graphitized surface thus increasing electrical conductivity. The reduction peak current and effective surface area of GrAC composite electrode has increased by 330 % and 108 % than that pure AC electrode, respectively. Subsequently, the electrode employing GrAC composite as support material doped CoTMPP catalyst has been explored to obtain the optimized structural and electrochemical properties. The heat treated GrAC-CoTMPP-25 wt.% electrode has the highest reduction peak current with −0.0653 mA, which are 36 % and 9 % higher than that heat-treated AC-CoTMPP-25 wt.% and GrAC electrodes, respectively. The Zn-air battery utilizing heat-treated GrAC-CoTMPP as cathode has the highest energy density with value of 122.65 Wh/g. The obtained air-cathode materials (heat treated GrAC-CoTMPP) outperforms commercial Pt/C in oxygen reduction reaction with greater reduction peak current value up to 13 %. When applied in ZAB, a high open- circuit voltage, excellent energy density, and satisfactory stability are achieved, implying that heat treated GrAC-CoTMPP cathode material has potential to replace platinum based catalyst.