Lead–halide-based hybrid organic–inorganic perovskite materials (APbX3) have recently garnered increased attention among researchers worldwide as promising thin-film photovoltaic materials. Laboratory-scale solar cells incorporating APbX3 materials as the light absorber have demonstrated impressive efficiencies of > 20%, but commercialization of solar cells based on solution-cast materials and with only low-temperature processing steps is in-part limited by toxicity and instability of the photoactive materials even under ambient conditions. Specifically, APbX3 contains toxic lead and is not stable in ambient conditions in part because AX dissociates into two water-soluble, low-boiling point species.

Therefore, the goal of my doctoral thesis project was to replace the organic monocationic salts in perovskite materials with non-volatile and less water-soluble analogs in the form of organic dicationic salts. Using this rationale, I have demonstrated the use of novel perovskite-like bismuth–halide and copper–halide materials as the photoactive layer in solar cells, which contain hexanediammonium dications (HDA2+) that serve as the organic crystal fastener. I have elucidated structure–property relationships of these and related materials with different organic linkers through measurement of crystal structure, powder and thin-film characterization of the materials, and long-term thermal and moisture stability. I have also studied these materials using ultrafast spectroscopy techniques to elucidate energy-transport and charge-transport dynamics of these materials as thin films. Efficient photovoltaic devices featuring a dicationic metal–halide material that does not contain lead may offer a more environmentally-friendly, stable alternative to the APbX3 photovoltaic devices that currently dominate emerging photovoltaic technology research.

Charged polymers deposited on electrocatalysts are known to alter electrochemical redox reaction selectivity. Interpretation of this multistep process, however, requires knowledge of fundamental thermodynamic and species transport properties, which many times are difficult to measure. To bridge this capability gap, herein we report a suite of electroanalytical methods that together allow us to quantify the Donnan electric potential difference at polymer|liquid interfaces, the chemical potential of protonic species at electrode surfaces – as pH or pOH, ionic transport properties within polymer layers, and electrical double layer capacitances at electrocatalyst|polymer interfaces. Our work reveals that coupled aqueous and polymer properties control ion transport flux and the extent of specific adsorption of ions to electrocatalyst surfaces. Moreover, our methods are broadly applicable and straightforward to implement. We expect them to aid electrochemists in understanding and controlling ionic properties that influence electrochemical reaction selectivity.

This thesis investigates the thermodynamic and kinetic properties of reversible excited-state photoacids using steady-state spectroscopic techniques and explores the relationship between the Brønsted-Lowry excited-state acidities (thermodynamics) and the rate constants (kinetics) for excited-state proton-transfer reactions using driving-force-dependent kinetic theories. Chapter 1 presents methods to synthesize donor-π-acceptor reversible excited-state weak photoacids whose ground-state acidity is ideal to deprotonate water. The excited-state acidities (pKa* values) determined using the Hill equation analysis of steady-state photoluminescence data were close to the ground-state acidities (pKa values) but differed by several orders of magnitude from pKa* values calculated using the Förster cycle analysis. Initial efforts to resolve these discrepancies were aimed at understanding the pathways that result in the loss of excitation energy using time-dependent density function theory. However, non-ideal Hill coefficient values and ultrafast transient absorption spectroscopy data confirmed that the most dominant factors contributing to the determination of incorrect pKa* values using Hill equation analysis were the short lifetimes of the electronic excited-state species and the absence of a chemical quasi-equilibrium for the ESPT reaction to hydroxide, which is often the case for weak photoacids. Chapter 2 reports the experimental determination of driving-force-dependent rate constants for intermolecular excited-state proton-transfer reactions from reversible excited-state weak photoacids to a series of proton-accepting quenchers with varied and known pKa values using the Stern–Volmer equation. This chapter also discusses the dependence of pseudo-pKa* values of weak photoacids obtained using the Hill or Henderson-Hasselbalch equation on their excited-state lifetimes. Using a driving-force-dependent kinetic theory developed by Marcus and Cohen based on the bond-energy–bond-order, the true pKa* of these photoacids was determined from data measured using readily-available steady-state photoluminescence spectroscopy. Chapter 3 discloses the reasons for discrepancies in the previously reported values of intrinsic activation energies for proton-transfer reactions obtained from driving-force-dependence studies and sets forth guidelines for correctly applying driving-force-dependent kinetic theories to proton-transfer reactions involving reversible excited-state photoacids. Chapter 4 discusses the applicability of a modified Stern–Volmer equation to extract kinetic information from steady-state photoluminescence data for reversible excited-state strong photoacids with longer excited-state lifetimes of both protonated and deprotonated excited-state species than the excited-state lifetimes of weak photoacids. In such cases, the experimentally observed overall excited-state proton-transfer rate constants have significant contributions from rate constants for the forward and the reverse reactions, which can be deconvoluted using a modified Stern–Volmer equation.

Photoelectrochemical cells that utilize bipolar ion-exchange membranes (BPMs) can support and maintain a pH difference across the membrane. Redox-inactive salts are often also incorporated in these cells and it is unclear if salts have an effect on the electrostatics within the cell and perceived thermodynamics of the overall redox chemistry. In this thesis, commercial BPMs wetted by various electrolytes were investigated, e.g. in the presence of acid, base, and/or salt. The electrochemical behavior of the BPMs was assessed by measuring the current density vs potential behavior. The results suggest that potential differences measured across BPMs may not always reflect the pH of the bulk electrolyte and therefore may require additional information in order to justify the overall free energy required for the redox reactions.

The second half of this thesis was dedicated to the development of an artificial light-driven ion pump using BPM materials. An activated Nafion precursor membrane was functionalized with newly developed photoacid molecules and sandwiched between an anion-exchange membrane and a cation-exchange membrane. The photoelectrochemical properties of the resulting photoacid-functionalized BPM were measured and a photovoltaic response was observed. These initial findings provide a solid foundation for future research into customized BPMs to be used as artificial light-driven ion pumps.

This thesis describes the first demonstration of a quantum dot photoacid. Photoacids are a class of molecules with a protic bond that is weakened upon photo-excitation. CdSe quantum dots were synthesized and were made to have mixed-ligand surfaces with a majority of water-solubilizing sulfonate functional groups and fewer acidic electron-donor functional groups, notably 4-mercaptophenol (MPh). Sulfonate groups are very acidic and therefore remain charged over a wide range of pH values. Through judicious choice of synthesis and ligand-exchange conditions, water-solubilizing 2-mercaptoethanesulfonate (MES) was introduced in addition to an equal concentration of protic and dipolar MPh groups. The electronic states of surface-bound MPh groups are such that they can serve as electron-transfer donors to excited-state CdSe quantum dots. The challenge to characterize the photoacidity of this class of chromophores is that the charge-separated CdSe– and MPh+ state does not efficiently emit photons. Therefore, a recently developed electrochemical impedance spectroscopy technique was studied in great detail to understand the cause of observed changes in impedance and then utilized to definitively demonstrate that this new class of quantum dot dyes were photoacidic. This light-driven energy conversion process is uniquely suited to convert light into ionic power, which can be used for direct desalination of saltwater.

Dye-sensitized solar cells are a potential low-cost alternative to silicon solar cells. A current limitation to DSSCs is that most dyes used in these cells have bandgaps that are too large for ideal sunlight absorption. In this thesis, cis-Os(dcbpy)_{2}I_2 dye, a sensitizer with an absorption onset near 1.2 eV is introduced that has significantly better light-absorbing capability in the near-infrared spectral regions compared to benchmark RuN3, and related, dyes. To understand the effect of overpotential required to drive iodide/triiodide redox chemistry on the performance of DSSCs containing RuN3 or OsI_2 dyes bound to TiO_2, their performance was examined with varied ratios of [I^-] to [I_3^-]. Measurements showed that the maximum light-to-electrical power conversion efficiency for RuN3 DSSCs occurred at an electrolyte potential of 0.35 V corresponded to 6.3 times more I^- than I_3^-. Whereas for OsI_2 DSSCs, an insignificant amount of power was generated due to slow electron-transfer kinetics between the redox shuttle and OsI_2. It was proposed that the addition of dmFc to the electrolyte could improve OsI_2 DSSCs performance by directly catalyzing I^- oxidation. To validate this hypothesis, foot-of-the-wave analysis was applied to the cyclic voltammetry responses of TiO_2 photoelectrodes dyed with OsI_2. It demonstrated that dmFc catalyzed I_3^- reduction faster than I^- oxidation. Photoelectrochemical measurements unexpectedly showed that at the presence of dmFc in the electrolyte of OsI_2 DSSCs, electrons transferred from excited OsI_2 to the electrolyte solution, instead of injecting into the TiO_2 film, making these films photocathodic even though TiO_2 is n-type.

The promise of solar water splitting as a sustainable route to renewable energy storage has motivated many device designs. One design that mimics natural photosynthesis is termed Z-Scheme. It consists of two artificial photosystems that each absorbs in different regions of the solar spectrum to drive H2 evolution via water reduction and O2 evolution via water oxidation. When these reactions occur in one compartment an explosive, and therefore dangerous, oxyhydrogen gas mixture results. A safer alternative is to separately compartmentalize each reaction, which then requires the use of an aqueous redox shuttle, such as Fe(III/II), to mediate charge between compartments. While safer, this strategy requires exquisite control over redox selectivity. Fe(III) reduction and Fe(II) oxidation are each more thermodynamically and kinetically favored over their desired counterparts of H2 evolution and O2 evolution, respectively. Overcoming this challenge requires new and innovative means to achieve exceptional redox reaction selectivity on the nanoscale. This is underscored by the poor photoactivity of state-of-the-art H2 evolving nanoparticles, such as SrTiO3 doped with transition metals and with metallic cocatalysts on their surface whose solar-to-hydrogen energy conversion efficiency is only of the order of 1%.In this dissertation I will present my efforts toward improving the solar-to-hydrogen energy conversion efficiency of doped SrTiO3 materials by targeting three key aspects: (1) dopant engineering for tuning overall photoactivity (2) cocatalyst photodeposition for enhanced H2 evolution rates and (3) ultrathin oxide coatings for redox reaction selectivity. I will also provide lessons learned from revitalizing home-built microwave and radiowave apparatuses for contactless photoconductivity measurements of these photocatalyst samples as well as the effects of exogenous buffers in facilitating proton binding and transport during water splitting reactions. In accordance with my involvement of each of the three aforementioned key aspects, from high to low, I will first describe results obtained for Rh-doped SrTiO3 photocatalyst particles coated with a conformal layer of ultrathin oxide coatings (i.e. TiOx or SiOx). These nanoparticles were used to unequivocally demonstrate enhanced reaction selectivity for visible-light-driven H2 evolution in the presence of significant amounts of facile-to-reduce aqueous Fe(III) oxidation products. Comparison of results using coatings consisting of amorphous TiOx versus SiOx, variable cocatalyst loading, and practical electrolyte conditions indicate the role of the ultrathin oxide coatings to selectively inhibit permeation of Fe(III), thus affording a higher selectivity for H2 evolution. Although our data suggest that permeation of Fe(II) through the coatings is also inhibited to some degree, thus attenuating desired rates of Fe(II) oxidation by photogenerated oxidizing equivalents, we believe that our approach and results validate a new platform for designing enhanced reaction selectivity into photocatalytic systems. Next, I will also present recent results that begin to shed light on the cause of the commonly observed, yet currently unexplained, delay in H2 evolution activity that is widely observed during photodeposition of Pt cocatalysts on these nanoparticles. Furthermore, I will discuss my attempts in identifying the locations and chemical character of dopants in SrTiO3 particles co-doped with Rh and La atoms using multiple instrumental techniques with the aim of providing design rules for doping to achieve high photocatalytic activity. My ability to glean new information into these technoeconomically important designs was supported by use of a mass spectrometry instrument for rapid detection of H2 and a home-built time-resolved microwave conductivity apparatus to measure photogenerated transient mobile charge carriers in wetted semiconductor nanoparticles. Finally, I will report my attempts to facilitate proton-transfer reactions during the OER using one of the best electrocatalysts, i.e. IrOx, by intelligently selecting appropriate buffers.

This dissertation focuses on the interrogation of the photoacid sensitization (Förster) cycle to understand mechanisms for light-to-protonic energy conversion. I define protonic species as those capable of undergoing proton-transfer reactions, including H2O(l), H+(aq), OH-(aq), and Brønstead–Lowry (conjugate) acids/bases. The central theme of this work is relating operating principles between traditional electronic photovoltaics (electron/holes) and protonic photovoltaics (OH-/H+) including charge carrier generation and conduction with the goal of developing design strategies for efficient light-to-ionic power conversion. This is achieved by investigating ground-state proton transfer processes in the Förster cycle using electrochemical and spectroscopic techniques and leveraging this knowledge to understand photo-initiated ion transport in ion-exchange membranes.Transient absorption spectroscopy studies of aqueous photoacids provided insight into the protonic species generated during the Förster cycle and the inefficiencies of current state-of-the-art photoacids for generating large photoresponses in photoacid-modified ion-exchange membranes. An abundance of ultrafast spectroscopy data exist that revealed mechanistic details for excited-state proton transfer, but there was limited information regarding the mechanisms for the subsequent ground-state proton transfer. Proton acceptance in the excited-state and proton donation in the ground-state from H2O(l) in the Förster cycle results in the transient generation of H+ and OH-, analogous to photogeneration of electrons and holes in electronic photovoltaics. An obstacle for realizing this ideal mechanism is kinetics. These were analyzed via a systematic study using various photoacids by varying the concentration of deprotonated photoacids and protons, photoacid ground-state acidity, solution acidity, and total photoacid concentration. Photoacids were covalently bonded to ion-exchange membranes to assess the impact that their photochemical mechanisms had on their ability to exhibit photovoltaic action. Existing methodology for four-probe electrochemical measurements was revamped to produce reproducible signals and temporally stable baselines. The key factor was eliminating the introduction of external electrolyte from reference electrodes filled with saturated internal electrolyte by replacing those electrodes with reference electrodes immersed directly into the electrolyte in contact with the photoacid-modified ion-exchange membranes. This introduced experimental constraints that required physical modification of the experimental apparatus as well as deconvolution of the Nernst electrode potential from the membrane potential. Implementation of this new setup resulted in the observation of a “reverse” photovoltage, meaning that it was opposite in sign to photovoltages measured in traditional electronic photovoltaics and that observed in our group in all prior studies of these systems. A series of control experiments was performed, and a mechanism coined “electrolyte crossover induced bulk membrane polarization” was hypothesized to explain this reverse photovoltage.

This thesis explores mechanisms of photovoltaic action, bulk current conduction and electric-field-enhanced and chemically catalyzed water dissociation in bipolar membranes, polymeric devices that, when exposed to water and when pretreated to have only H+ and OH‒ as the mobile species, abide by the physics and mimic the behavior of semiconductor pn-junctions. The first study entails the fabrication of a protonic solar cell consisting of a polymeric bipolar membrane sensitized to visible light with covalently bound photoacid dye molecules. Electrodes were applied to either face of the membrane and fed with humidified H2 to form RHEs as part of MEAs that allowed interconversion of electronic and protonic signals. Photon absorption by photoacids covalently bound within the bipolar MEA resulted in proton transfer to water and/or hydroxide. Classical solar cell current-voltage curves and Mott-Schottky analyses in the dark and under illumination showed “reverse” photovoltaic action, consistent with a light-induced loss of protonic mobile charge carriers or dynamic processes in these new polymeric materials. The second study concerns similar bipolar membrane-electrode assemblies (MEAs) but rather than containing photoacidic dyes, the junction is coated with an intermediate polymer that contains organic proton-transfer catalysts, specifically phosphonic acid groups, that are akin to Shockley‒Read‒Hall recombination centers in traditional semiconductors and can enhance the rate of water dissociation at small applied potentials. The report described herein is the first to utilize a polymer scaffold, poly(2,6-dimethyl-1,4-phenylene oxide), to anchor organic functional groups with controlled pKa values in the bipolar membrane. The champion bipolar membrane in the study yielded a two order of magnitude enhancement in the rate of water dissociation compared to the state-of-the-art commercial metal oxide-containing bipolar membrane. A thickness dependence study was performed by varying the dimensions of the phosphonic acid catalyst layer, and insight into the mechanism of water dissociation current and current conduction in the BPM was gained. A numerical model was also developed using the Law of Mass Balance and Poisson-Nernst-Planck transport physics to rationalize experimental trends. The third study utilizes finite-element modeling to identify design principles for a floating reactor capable of direct oceanic carbon capture. A thermodynamically rigorous model was developed to include a web of 13 chemical reactions at play in the system, namely proton-transfer processes between water, dissolved inorganic carbon species, and catalyst species that we propose to functionalize the reactor with to enhance the rate of CO2 capture. Analyses have identified the range of liquid flow rates and vacuum levels at which the system should operate to achieve substantial carbon capture and means to enhance performance using catalysts for specific reaction steps.

Models of solar fuels devices that are completely electrochemically-mediated and consist of ensembles of optically thin light absorbers were used to calculate their maximum theoretical solar-to-fuel efficiencies. These models are based on the thermodynamic principles of detailed balance and blackbody radiation, semiconductor device physics, and simple catalysis. The maximum efficiency of an electrochemically-mediated tandem water splitting device was calculated and the crucial dependence of this efficiency on the redox shuttle thermodynamic potential was elucidated. A novel model of a stack of optically thin, radiatively coupled light-absorbers that each independently perform solar fuels reactions was developed to demonstrate the substantial gains in solar-to-fuel efficiency that are possible when using ensembles of small light-absorbers. The results presented herein can be used by researchers to develop high-efficiency, low-cost solar fuels materials and devices.