Water-soluble graphene in agar gel as electrolyte for magnesium-air battery / Liew Siaw Ying

Liew , Siaw Ying (2020) Water-soluble graphene in agar gel as electrolyte for magnesium-air battery / Liew Siaw Ying. Masters thesis, University of Malaya.

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Abstract

Presently, ever-increasing demands on energy and the global environmental issues have given the impetus to investigate the new battery systems which are inexpensive, efficient, having high performance, and also green to the environment. Among all the battery technologies, magnesium-air (Mg-air) battery is an attractive battery to be developed owing to the interesting properties of the Mg anode. Mg has high specific energy (6.46 kWh kg-1) and reactivity, apart from its high abundance, lower cost, low toxicity, and comparatively safe to handle in the atmosphere. Nevertheless, Mg-air battery is still not being widely investigated as compared to the other batteries. Electrolyte is one of the vital components in Mg-air battery. Mg corrodes readily in conventional aqueous electrolytes, resulting in battery self-discharge. Moreover, the aqueous electrolytes used may leak and evaporate through the open cell structure. Therefore, a gel polymer electrolyte with the corrosion inhibition property could be a new electrolyte material for Mg-air battery. Water-soluble graphene (WSG) was successfully synthesised through simplified Hummers’ method followed by chemical reduction (with the addition of ammonia, NH3). WSG was incorporated into agar (a natural polymer) as an environment-friendly gel polymer electrolyte for Mg-air battery. Continuous efforts have been focused on the improvement in electrochemical performance of Mg-air battery via the preparation of different WSG-AGAR gel electrolytes. Detailed investigations on different parameters, for instance, synthesis condition of WSG, agar concentration, WSG concentration, and types of electrolyte were conducted in order to produce the optimal WSG-AGAR gel electrolyte. It was found that the incorporation of 0.1% w/v WSG-7 in 3.5% w/v sodium chloride (NaCl), entrapped in 3% w/v agar gel exhibited the greatest electrochemical performance due to the optimisation between high ionic conductivity and sufficient anodic corrosion resistance. The optimal gel electrolyte had an ionic conductivity of 9.40 × 10-2 S cm-1. The discharge capacity and energy density of assembled Mg-air battery with respect to the mass of Mg anode consumed during discharging can reach up to 1303.94 mAh g-1 and 1820.70 mWh g-1, respectively, at the constant current density of 11.11 mA cm-2. The incorporation of WSG-7 (optimal WSG) in agar gel electrolyte had demonstrated the improvement in ionic conductivity by 32.96% and discharge capacity by 58.34% as compared to that without electrolyte additive. The Mg-air battery with the optimal WSG-AGAR gel electrolyte was further discharged at different current densities. The peak discharge capacity and energy density with respect to the mass of Mg anode consumed during discharging were achieved at the current density of 1.11 mA cm-2, with the value of 1632.74 mAh g-1 and 2432.78 mWh g-1, respectively. The performance of the assembled Mg-air battery with this economical, inherently safe, and environmentally benign biopolymer electrolyte was notable.

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