The electrochemical cell was set up for each anodic and cathodic environment with the BPM oriented in reverse bias (cation exchange layer on the cathode side). IR-compensated, threeelectrode CV measurements were performed on each half-cell reaction, with the Ag/AgCl reference electrode was placed in the working electrode cell. A summary list of the relevant reactions is provided in Table . The data gathered from the CV measurements were used to predict two-electrode cell potential for each reaction pair using a LLA. Two electrode chronopotentiometry (CP) experiments were performed at a cathode current density (j) of -10.0 mA cm -2 for the noted extended periods to compare actual cell potential vs. LLA prediction. This current density was chosen as representative of rates targeted for photoelectrochemical solar-tohydrogen systems. The two-electrode CP experiments were performed for hydrogen generation in acid (denoted as aHER), and in seawater (sHER), paired with OER and GEOR in alkaline conditions with the anode and cathode compartments separated by a BPM. The CO2 reduction reaction in a CO2-purged sodium bicarbonate catholyte was paired with GEOR using a BPM, and with OER in a sodium bicarbonate solution (bOER) separated by a Selemion membrane.