Operating fuel cells with an electrically rechargeable liquid fuel

Abstract

Fuel cells are electrochemical energy conversion devices as well as clean power sources as they offer practicable and smooth means to directly convert chemical energy, available in fuels, to electrical energy. The promising capability of fuel cell technology, particularly direct liquid fuel cells, have piqued increasing research interest over the years. However, the slow reaction kinetics of conventional liquid fuels on pricey metal catalysts at the anode has largely hindered the commercial market penetration of liquid fuel cells. To address this issue, a novel system which employs an electrically rechargeable liquid fuel (e-fuel) for renewable energy storage and power generation was recently proposed. The e-fuel can be obtained from diverse range of electroactive materials and not only metal ions. An e-fuel solution, which contains vanadium ions dissolved in sulphuric acid, is therefore employed to operate fuel cells. Unlike the conventional liquid fuels, the vanadium-based e-fuel possesses superior reactivity on carbon-based electrodes, consequently eliminating catalyst materials at the anode and improving the cost effectiveness of fuel cells. In addition, the e-fuel possesses low freezing point which allows the operation of fuel cells at wide temperature range. As a result of the fascinating properties and excellent potential of this e-fuel, this thesis is focused on the experimental and numerical investigations of e-fuel cells while exploring various oxidants at the cell cathode. Firstly, the performance of the e-fuel cell is experimentally examined with the use of air as oxidant, while taking the use of oxygen as performance benchmark. The cell attains an open-circuit voltage (OCV) of 1.25 V, which is comparable to an OCV of 1.26 V obtained when pure oxygen is employed as oxidant. A peak power density of 168.3 mW cm-2, at room temperature, is obtained with the use of air as oxidant. Furthermore, an energy efficiency of 26.4 % is achieved by the e-fuel cell. Secondly, to further boost the performance of the e-fuel cell and more importantly realize its application in airtight environments, hydrogen peroxide (H2O2) is considered as oxidant, in lieu of gaseous oxygen or ambient air. The novel e-fuel/H2O2 fuel cell exhibits an impressive peak power density of 1456.0 mW cm-2 at 60 ℃, which is 70 % higher than the use of oxygen as oxidant. A maximum current density exceeding 3000 mA cm-2 is also achieved by the cell. Furthermore, the cell achieved a stable performance after it was refueled 10 times under a constant current discharge. Thirdly, a mathematical model to describe the underlying theoretical and working principle of the e-fuel cells is presented. The model is first developed for the e-fuel cells using pure oxygen as oxidant. Afterwards, the modeling framework is extended to the e-fuel cells that employ hydrogen peroxide as oxidant, where mixed potential is considered in the cathodic reactions. Lastly, numerical simulation of the model is conducted to predict and optimize the performance of the e-fuel cells.