(123b) High Performance Direct Methanol Fuel Cells Incorporating pH-Gradient-Enabled Microscale Bipolar Interfaces

Herein we report a direct methanol – hydrogen peroxide fuel cell (DMHPFC) operated with a sustained pH-gradient between an alkaline methanol fueled anode and an acidic hydrogen peroxide fed cathode. The DMHPFC produced ca 0.5 W cm^-2 peak power density at 0.85V with an open circuit voltage (OCV) of 1.65V. Direct methanol fuel cells (DMFCs^1,2) 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. 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/l³. So, 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. Further, methanol exhibits good electrochemical activity, is consistently available, is biodegradable and is relatively cheap⁴. However, DMFCs have not been a viable alternative to hydrogen fuel cells due to their low power density caused by the sluggish kinetics of both electrode reactions⁵. In this work, we significantly improve the kinetics of the methanol oxidation reaction (MOR) by moving to alkaline media from acidic media⁶. To address the inferior kinetics of the oxygen reduction reaction (ORR) at the cathode, hydrogen peroxide is used as the oxidant in place of oxygen^7,8. The theoretical potential for MOR in basic medium is lower than that in an acidic medium (-0.78 V vs. -0.02 V) and the theoretical potential for the reduction of hydrogen peroxide is higher than that for oxygen reduction (1.77 V vs. 1.23 V in acidic medium). This results in the highest theoretically possible cell voltage for cells with alkaline anodes and acidic cathodes.

We achieved sustained operation of the DMHPFC with an alkaline anode and an acidic cathode by incorporating our pH-gradient-enabled microscale bipolar interface (PMBI). We have previously demonstrated that the PMBI can sustain a pH-gradient between the anode and the cathode of a direct borohydride fuel cell (DBFC) and enable sustained high power operation^9–12. Translating the same technology to DMHPFC, we demonstrate that the judicious selection of the electrode pH and inter-electrode pH-gradients enabled by the PMBI leads to enhanced performance of CH₃OH/H₂O₂ fuel cells compared with CH₃OH/O₂ fuel cells. Similar to methanol, liquid fuels like ethanol, propanol, glycerol also have lower theoretical potential in alkaline medium. Our work paves the way for such other liquid fuel cells with hydrogen peroxide as the oxidant and enabled with PMBI resulting in significantly improved performance.

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