(644c) Solar-Driven Electrochemical Water-Splitting at Near-Neutral pH Conditions – Operating Strategies and Their Limitations

The operation of solar fuel generators at the near-neutral pH conditions can offer enhanced stability to the catalysts and semiconductors. However, such systems can have lower energy-conversion efficiency due to higher solution losses including ohmic and Nernstian. The majority of potential losses in the near-neutral pH electrolytes originate from the pH gradients at the electrodes as well as from the kinetic overpotential of the oxygen evolution reaction (OER). The losses due to pH gradients can be as high as 450 mV, even greater than the OER overpotential. Although the supporting electrolyte can decrease the solution losses, it significantly reduces migration causing the limiting current density to decrease. The pH buffers can minimize the pH gradients and can provide ionic conductivity to the electrolyte, so the current density can be supported primarily by the buffer species. However, obtaining a current density of 10 mA cm^-2 in the near-neutral pH electrolytes requires membrane-free, bubble convected cell and/or an electrode separation distance of < 1 mm, which still produces total voltage losses of ~180 mV, while additionally making robust separation of the product gas streams difficult in practice. The addition of an ion exchange membrane in the neutral pH electrolytes increases the pH changes at the electrodes, making anode and cathode more acidic (pH = 2) and alkaline (pH = 10), respectively. This talk will discuss advantages and limitation of various active and passive schemes for the operation of solar-driven water-splitting systems at the near-neutral pH conditions.