(417e) Analysis of Mixed Potential at the Cathode of a Direct Methanol-Hydrogen Peroxide Fuel Cell for Enhanced Performance

Direct methanol fuel cells (DMFCs) are promising energy conversion devices that can generate electricity from the electrochemical oxidation of methanol. However, the use of traditional oxidants, such as oxygen, can lead to various challenges, such as catalyst poisoning and incomplete fuel oxidation. Hydrogen peroxide (H₂O₂) has emerged as an attractive alternative oxidant for DMFCs due to its high energy density and non-toxic nature. The use of H₂O₂ as an oxidant in fuel cells offers several advantages over traditional oxygen-based systems, including improved safety, reduced corrosion, increased efficiency and use in O₂ free environment.

Direct methanol hydrogen peroxide fuel cells (DMHPFC) are an important alternative to hydrogen-fed polymer electrode membrane fuel cell (PEMFCs) and anion exchange membrane fuel cells (AEMFCs) due to methanol’s high energy density compared to that of hydrogen and superior kinetics of hydrogen peroxide reduction reaction as compared to Oxygen. A unit volume of Hydrogen gas stored at a pressure of approximately 69 MPa corresponds to volume-specific energy density of roughly 2.1 kWh/l while 39% volume of aqueous methanol and 41% volume of aqueous hydrogen peroxide corresponds to volume-specific energy density of 9.2 kWh/l. So, a DMHPFC’s energy density is approximately four times higher than hydrogen fuel cell’s energy density and is comparable to gasoline which contains roughly 9.2 kWh/l of available bond energy. We have achieved sustained operation of the DMHPFC with a peak power density of 0.8 W cm^-2 and a power density of > 0.5 W cm^-2 at 1.0 V. However, the system exhibits a relatively low Faradaic efficiency of 50% due to various overpotentials.

In DMHPFCs, the cathode overpotential is critical and determines the overall performance of the fuel cell. The cathode reaction involves hydrogen peroxide reduction, which is a complex process because of the parasitic evolution of O₂ by the decomposition of H₂O₂. The O₂ evolution results in a mixed potential at the cathode due to the occurrence of both the Hydrogen peroxide reduction reaction, (HPRR) (E₀ = 1.77 V versus standard hydrogen electrode [SHE]) and the oxygen reduction reaction (ORR) (E₀ = 1.23 V versus SHE), lowering the overall cell potential. The O₂ surface coverage reduces the available sites for the HPRR, effectively deactivating the catalyst. Electrocatalysts exhibiting a combination of high HPRR activity and selectivity (by inhibiting H₂O₂ decomposition, and hence inhibiting ORR would result in higher faradic efficiency and have been the subject of sustained interest. Another challenge at the cathode is the crossover of methanol from the anode to the Pt-based Hydrogen peroxide cathode. This mixed potential due to methanol crossover can have a significant impact on the performance of the cathode and the overall efficiency of the DMHPFC.

Understanding and controlling the mixed potential at the cathode in DMHPFCs is crucial for optimizing the cell performance and improving its efficiency. The mixed potential at the cathode depends on various factors such as the concentration of methanol and hydrogen peroxide, pH, temperature, membrane, and electrode materials. In this work, we will provide an overview of the mixed potential theory and its significance in the DMHPFCs. We will also discuss the factors influencing the mixed potential at the cathode and implications for the cathode reaction, and strategies for mitigating its effects.