Replacing passive ion-exchange membranes, like Nafion, with membranes that use light to drive ion transport would allow membranes in photoelectrochemical technologies to serve in an active role. Toward this, we modified perfluorosulfonic acid ionomer membranes with organic pyrenol-based photoacid dyes to sensitize the membranes to visible light and initiate proton transport. Covalent modification of the membranes was achieved by reacting Nafion sulfonyl fluoride poly-(perfluorosulfonyl fluoride) membranes with the photoacid 8-hydroxypyrene-1,3,6-tris(2-aminoethylsulfonamide). The modified membranes were strongly colored and maintained a high selectivity for cations over anions. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ionexchange measurements together provided strong evidence of covalent bond formation between the photoacids and the polymer membranes. Visible-light illumination of the photoacid-modified membranes resulted in a maximum power-producing ionic photoresponse of ∼100 μA/cm 2 and ∼1 mV under 40 Suns equivalent excitation with 405 nm light. In comparison, membranes that did not contain photoacids and instead contained ionically associated Ru II −polypyridyl coordination compound dyes, which are not photoacids, exhibited little-to-no photoeffects (∼1 μA/cm 2 ). These disparate photocurrents, yet similar yields for nonradiative excited-state decay from the photoacids and the Ru II dyes, suggest temperature gradients were not likely the cause of the observed photovoltaic action from photoacid-modified membranes. Moreover, spectral response measurements supported that light absorption by the covalently bound photoacids was required in order to observe photoeffects. These results represent the first demonstration of photovoltaic action from an ion-exchange membrane and offer promise for supplementing the power demands of electrochemical processes with renewable sunlight-driven ion transport.

Water electrolysis using a catholyte and anolyte at different pH values requires a bipolar membrane to sustain the pH difference and 1.23 V to electrolyze water. Bipolar membranes that separated concentrated aqueous acid and base exhibited an open-circuit potential consistent with the Nernst equation and rapid transport of protons and hydroxide ions. When excess supporting electrolyte was added to both solutions the membrane potential was measured to be ∼0 V, which suggested that water electrolysis occurred at 1.23 V and therefore, protons and hydroxide ions were not the majority ionic charge carriers. Monopolar ion-exchange membranes attenuate mixing of fuel species while bipolar ion-exchange membranes (BPMs) also attenuate mixing of ions. BPMs consist of a cation-exchange layer (CEL) and an anion-exchange layer (AEL), and sustain pH differences across the membrane even during the passage of large reverse-bias currents, where electric-field-enhanced water dissociation generates the majority of mobile ions ( Figure 1). 1 Each proton or hydroxide ion that migrates from (to) the CEL/AEL interfacial region is consumed (replenished) by proton-coupled-electron-transfer reactions at the electrodes. Differences in pH are useful for electrochemical technologies that incorporate materials that are inherently unstable in a single pH electrolyte. 2 Unlü, Zhou, and Kohl reported use of a BPM in a polymerelectrolyte-membrane H 2 /O 2 fuel cell. 3 The overall thermodynamic potential for water formation (i.e. H 2 oxidation and O 2 reduction; 1.23 V) remained the same as when a monopolar ion-exchange membrane was used. Mallouk and colleagues and Freund, Lewis, and colleagues independently demonstrated that this behavior also occurred when liquid electrolytes were used. 4,5 They showed that BPMs wetted by aqueous acidic electrolyte on the CEL side and alkaline electrolyte on the AEL side supported and maintained pH differences across the membrane. In general, their reported theories and results were similar to those ofÜnlü, Zhou, and Kohl; however, use of a liquid electrolyte allowed the potential difference across the membrane to be measured selectively using four-electrode electrochemical techniques, 6 which are analogous to solid-state four-point-probe methods. 7 The open-circuit (resting) potential measured across the membrane (E BPM ) when wetted by 1 M acid and base and using two saturated calomel electrodes (SCEs) was reported to be E BPM_SCE ≈ 0.8 V, 5 which is consistent with the value of 0.83 V calculated using a Nernst-like equation, where E ws is the potential for water electrolysis. Several studies have characterized BPMs wetted by aqueous acid on the CEL side and base on the AEL side; 4,5,8,9 however, no studies have reported electrochemical behavior when additional supporting electrolyte was present. These experimental conditions are important because supporting electrolyte is likely necessary in solar fuels devices that utilize two different, but non-extreme, pH conditions where the ionic strength is low. Data un...

Most solar energy conversion technologies generate power through transport of energized electron particles; however, the physics that describes these technologies only requires that the particles be charged and not specifically that they are electrons. My research group studies solar energy conversion technologies that generate power from sunlight through ion transport. In my presentation I will report on my research group’s recent demonstration of ion transport against concentration gradients driven by solar illumination of dye-sensitized ion-exchange materials. Mechanistically, visible light was used to drive endergonic excited-state proton transfer from a covalent photoacid-functionalized polymer membrane. 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. As a model system for ion-channels in polymer-electrolyte ion-exchange membranes, dye-sensitized conical nanopores in poly(ethylene terephthalate) (1 – 108 pores/cm2) were investigated. Remarkably, in a region occupied by ~20 zeptoliters (~2 x 10-20 L) of aqueous electrolyte, electrochemistry was used to determine the number of binding groups of the dyes and the ground-state pK a of the dyes, and fluorescence microscopy was used to determine the conditions where excited-state proton transfer occurred. These data were consistent with a hypothesis that pK a values of the photoacids in the ground-state and excited-state were significantly smaller than those measured for dye molecules in solution, likely due to incomplete screening of surface charges in the confined nanopores. Dye-sensitized Nafion monopolar ion-exchange membranes and bipolar ion-exchange membranes were also investigated. Under sunlight-simulated illumination these materials were found to exhibit photovoltaic action, i.e. generation of a photocurrent and a photovoltage. Bipolar membranes are a class of polymeric ion-exchange materials that consist of a monopolar cation-exchange membrane that is in intimate contact with a monopolar 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. This body of work represents an underexplored solar energy conversion process that is being pioneered by my research group to operate via a mechanism similar to that in semiconductor pn-junctions. The applicability and practicality of these materials as standalone ionic photoelectrochemical devices will also be presented.

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.

Ion-conducting polymers are passive. They respond to electric biases that are applied across them by transporting ions via migration. However, there is no reason these materials cannot be made active, such that upon absorption of light, ions are driven in directions pre-defined by built-in potential distributions in the materials and collected via selective contacts to generate power. This is exactly how traditional solid-state solar cells work and the physics that dictates this behavior only requires that the mobile carriers be charged, like ions, and not that they specifically be electrons and/or holes. My research group studies solar energy conversion technologies that generate power from sunlight absorption through ion transport. In my presentation I will report on my research group’s recent demonstration of ion transport against concentration gradients that was driven by solar illumination of dye-sensitized ion-conducting materials. Mechanistically, visible light was used to drive endergonic excited-state proton transfer from a covalent photoacid-modified cation-conductive membrane. Photoacid molecules 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 selective contact for protons such that absorption of light resulted in photovoltaic action, i.e. a photocurrent and a photovoltage. Bipolar membranes are another class of ion-conductive polymers that were used to demonstrate similar photo-activity. They consist of a monopolar cation-conductive polymer that is in intimate contact with a monopolar anion-conductive polymer. The physics that describes the ion-equilibration processes within bipolar membranes resembles that which occurs during equilibration of semiconductor pn-junctions. As a model system for ion-channels in phase-segregated ion-conducting polymers, dye-sensitized conical nanopores in poly(ethylene terephthalate) (1 – 108 pores/cm2) were investigated. Remarkably, in a region occupied by ~20 zeptoliters (~2 x 10-20 L) of aqueous electrolyte, electrochemistry and fluorescence microscopy were used to determine the photoacidity of the surface-bound dye molecules, which changed from the values found in bulk solution, likely as a result of surface charges in the confined nanopores. This body of work represents an underappreciated solar energy conversion process that is being pioneered by my research group to operate via a mechanism similar to that of semiconductor pn-junctions. The applicability and practicality of these materials as standalone devices for desalination of salt water will also be presented.