(Invited) Sunlight-Driven Ionic Power Generation from Bipolar Ion-Exchange Membranes Functionalized with Photoacids

Abstract

Bipolar membranes are a class of polymeric ion-exchange materials that consist of a cation-exchange membrane that is in intimate contact with an anion-exchange membrane. They are unique among the ion-exchange membranes in that they that separate and maintain pH differences across the membrane even during passage of ionic current. Moreover, the physics of ion equilibration processes within these membranes resembles that which occurs during equilibration of semiconductor pn-junctions. In my presentation I will report on my research group’s recent results from measurements of the electrochemical behavior of bipolar membranes under conditions relevant to solar fuels devices. We identified a condition where the energy required to electrolyze water was seemingly less than 1.23 V, which we showed was due to transport of ions other than protons and hydroxide ions. This subtlety will be explained in great detail to clarify misunderstandings in the interpretation of current–voltage behavior of bipolar membranes. My research group has also recently successfully demonstrated ionic power generation through solar light harvesting in dye-sensitized ion-exchange membranes. Visible light illumination of photoacid-functionalized Nafion membranes drove endergonic excited-state proton transfer and a photovoltage and photocurrent resulted. Photoacid molecules convert the energy in light into a change in the chemical potential of protons via a weakening of protic functional groups on the photoacid, i.e. a drop in its pK a. Many photoacids absorb visible light poorly and so a secondary research thrust on this project is to develop new visible-light-absorbing photoacids. Toward this, we have developed the first Ir-based inorganic coordination compound photoacids and quantum dot photoacids. Visible-light excitation of these molecules resulted in clear observation of effective photoacid behavior. Calculation of the excited-state pK a and ongoing work to comprehend the analysis of photoacid behavior in complex systems will be covered. The applicability and practicality of bipolar membranes in solar fuels devices and as standalone ionic photoelectrochemical devices will also be discussed. As most of these materials, techniques, and analyses are new to the artificial photosynthesis community, we plan to carefully present the results of our studies and provide ample explanations. Collectively, this body of work represents several new directions in artificial photosynthesis research and development that are being pioneered by my research group.