Electrochemical reduction in ionic liquids: Applications for fuel cells and photovoltaics

Neat proton-containing salt electrolytes, or protic ionic liquids ((PILs), were made as alternatives for fuel cell operation over conventional water based acid electrolytes. Ion conduction can occur in PIL electrolytes consisting solely of ions without a solvent. Since these new proposed electrolytes operate in the absence of water, they can operate at temperatures "above 140°C and thereby alleviate difficult water management issues, such as the complexities of 2-phase fluidics and humidifiers in electrochemical reactors. This new class of non-aqueous electrolytes directly addresses the poor electrode kinetics of oxygen reduction at the cathode by effectively removing bulk water from the equation as a reactant with metal and a cause of oxide passivation of the metal electrode. Therefore, the catalysis of oxygen reduction on the metal electrode is not limited by the potential of metal-oxide reduction to metal but can take place at significantly lower overpotentials on the catalytically active metal electrode. This is due in part to the suppression of oxide formation on the metal electrode.;Previous work has demonstrated superior short-term fuel cell performance of PILs over aqueous electrolytes, but over the long-term they are oxidatively unstable. Ten fluorinated protic ionic liquids were synthesized and examined to address this stability problem. Cyclic voltammetry experiments of platinum in these fluorinated PIL electrolytes have shown that oxygen reduction at the cathode occurs at lower overpotentials (nearly 150mV in some cases) than aqueous based electrolytes. Fuel cell polarization tests for one of the PILs (2-fluoropyridinium triflate) exhibits a superior cell performance (>15%) over aqueous concentrated (85%) phosphoric acid over all current densities. Conductivity measurements coupled with viscosity measurements are demonstrated as a convenient way to characterize the mechanism of proton conduction through the electrolyte.;During the course of this study, it became apparent that electrochemically stable ionic liquids could be well adapted for use in other emerging fields. Preliminary work demonstrated the electrodeposition of silicon in an ionic liquid is feasible which could lead to lower cost photovoltaic cells.