Although integral to the operation of photoelectrochemical devices, few research groups develop membranes specifically engineered for photoelectrochemical applications. There is a great need for this, because the membrane electrolyte is generally the limiting factor in device sustainability and practicality due to limitations from ion crossover, alkaline instability, cost of perfluorinated membranes, and embrittlement in the presence of CO₂.

In my presentation I will report on my research group’s recent results on the (photo)electrochemical behavior of novel bipolar ion-exchange membranes. Bipolar membranes are a class of polymeric ion-selective materials that consist of a cation-conductive polymer that is in intimate contact with an anion-conductive polymer. They are unique among the ion-selective membranes in that they can separate and maintain pH differences across the membrane even during the passage of ionic current. This ability arises from the presence of an electric potential space-charge region that is formed during initial ion equilibration through generation, recombination, drift, and diffusion transport processes.

Using bipolar ion-exchange membranes, my research group identified a condition where the energy required to electrolyze water was seemingly less than 1.23 V. We showed that this was due to unsustainable transport of ions other than protons and hydroxides and lack of compete formation of the space-charge region. This subtlety is one of several that are absolutely critical to correctly interpreting the current–voltage behavior of bipolar membranes. With an understanding of how to interpret electrochemical data obtained using bipolar membranes, I will report on my research group’s recent demonstration of ionic power generation through solar light harvesting in novel dye-sensitized bipolar ion-exchange membranes. Visible light was used to drive endergonic excited-state proton transfer from a cation-conductive membrane covalently modified with photoacid molecules. Photoacids convert the energy in light into a change in the chemical potential of a proton via weakening of a protic functional group on the photoacid, i.e. a decrease in its pK_a. A cation-conductive membrane served as the proton-selective contact while an anion-conductive membrane served as the hydroxide-selective contact such that visible-light absorption resulted in photovoltaic action, i.e. a photocurrent and a photovoltage.

In the past half-decade, semiconductors that form the backbone of all photoelectrochemical devices have benefitted from nanostructuring, dye-sensitization, and other non-traditional innovations, yet no commercial photoelectrochemical technologies exist. Passive ion-exchange membranes that are critical to these devices have had few new developments; we believe that innovations in membrane science can help enable commercialization.