(722a) Solar Fuel Generation at Near-Neutral pH Conditions: Operational Advantages and Disadvantages

An efficient, long lasting, and cheap solar fuel generator is an alluring goal to support future energy demands. Solar fuel generator is a photoelectrochemical cell (PEC) where a photovoltaic (PV) cell is in direct contact with the water-splitting electrochemical cell. The available materials for the PV cells and catalysts can make a solar fuel generator that can simultaneously satisfy two out of the three characteristics mentioned above. For example, either efficiency or durability can be compromised to make a less expensive PEC. The efficiency and durability of the device are strongly coupled and dependent on i) the interaction of photoelectrodes with the liquid electrolyte and ii) the overall ionic resistances in the electrolyte. The photoelectrodes can corrode in extreme pH conditions and are only stable in a finite pH range. Therefore, the stability and longevity of the photoelectrodes require PEC operation in near neutral pH solutions.

The near-neutral pH electrolysis requires more energy to compensate for potential losses due to pH gradients, electrodialysis and ohmic resistance. The ohmic losses and concentration polarization can be minimized by increasing ionic strength and buffer capacity of the electrolyte, respectively. The supporting electrolyte in polyprotic acid and/or buffer solutions shows a positive effect on limiting current densities, whereas it has a negative effect on limiting current densities in monoprotic acid solutions. The major source of solution losses in neutral pH devices is due to pH swings at the electrodes which can be as high as 200 mV at 20 mA/cm² current density. Another major contributor to overall potential losses is the OER overpotential which can be > 320 mV for most OER catalysts. This talk will also discuss the effect of convection and mixing on potential losses and gas crossover in the device.