Resistance and polarization losses in aqueous buffer-membrane electrolytes for water-splitting photoelectrochemical cells

The recent development of inexpensive catalysts for the oxygen evolution reaction has suggested that efficient photoelectrochemical cells (PECs) might be constructed from terrestrially abundant materials. Because these catalysts operate in aqueous buffer solutions at neutral to slightly basic pH, it is important to consider whether electrolytic cells can have low series loss under these conditions. Water-splitting or fuel-forming PECs will likely require porous separators or electrolyte membranes to separate the cathode products from oxygen produced at the anode. For this reason we analyze the individual potential losses in electrolytic systems of buffer solutions and commercially available anion- and cation-exchange membranes. Potentiometric analysis and pH measurements were employed to measure the potential losses associated with solution resistance, membrane resistance, and pH gradient formation at the current density (25 mA cm^-2) expected for efficient PECs. The membrane pH gradient is the most problematic source of loss in these systems, but monoprotic buffers can minimize the pH gradient by diffusion of the neutral acidic or basic form of the buffer across the membrane. These results suggest that water-splitting PECs can be viable with properly chosen membrane-buffer combinations.