Investigation of a new bis(carboxylate)triazole-based anchoring ligand for dye-sensitised solar cell chromophore complexes

A novel anchoring ligand for dye-sensitised solar cell chromophoric complexes, 1-(2,2’-bipyrid-4-yl)-1,2,3-triazole-4,5-dicarboxylic acid (dctzbpy), is described. The new dye complexes [Ru(bpy)2(dctzbpy)][PF6]2 (AS16), [Ir(ppy)2(dctzbpy)][PF6] (AS17) and [Re(dctzbpy)(CO)3Cl] (AS18) were prepared in a two stage procedure with intermediate isolation of their diester analogues, AS16-Et2, AS17-Et2 and AS18-Et2 respectively. Electrochemical analysis of AS16-Et2, AS17-Et2 and AS18-Et2 reveal reduction potentials in the range -1.50 to -1.59 V (vs Fc+/Fc) which is cathodically shifted with respect to that of the model complex [Ru(bpy)2(dcbH2)]2+ (1) (Ered = -1.34 V, dcbH2 = 2,2’-bipyridyl-4,4’dicarboxylic acid). This therefore demonstrates that the LUMO of the complex is correctly positioned for favourable electron transfer into the TiO2 conduction band upon photoexcitation. The higher energy LUMOs for AS16 to AS18 and a larger HOMO-LUMO gap result in blue-shifted absorption spectra and hence reduced light harvesting efficiency relative to their dcbH2 analogues. Preliminary tests on TiO2 n-type and NiO p-type DSSCs have been carried out. In the cases of the Ir(III) and Re(I) based dyes AS17 and AS18 these show inferior performance to their dcbH2 analogues. However, the Ru(II) dye AS16 (η = 0.61 %) exhibits significantly greater efficiency than 1 (η = 0.1 %). In a p-type cell AS16 shows the highest photovoltaic efficiency (η = 0.028 %), almost three times that of cells incorporating the benchmark dye coumarin C343

Evaluation of the role that photoacid excited-state acidity has on photovoltage and photocurrent of dye-sensitized ion-exchange membranes

Light-driven ion pumps can be fabricated from ion-exchange membranes infiltrated with water as the protonic semiconductor. Absorption of visible light and generation of mobile charge carrier protons are accomplished using photoacids that are covalently bonded to the membranes. Prior results from our work suggest that the photoacid excited-state acidity is not large enough to result in significant yields for conversion of light into mobile protons. Herein we compare a series of photoacid-bearing membranes that are even stronger acids in their excited states, and we determine that excited-state acidity does not correlate with photovoltage. By assessing the photoresponse of a series of bipolar membranes fabricated by laminating a photoacid-bearing cation-exchange membrane to an anionexchange membrane, no clear trend was observed between net built-in electric potential and photovoltaic performance. This suggests that other properties dictate the effectiveness of these light-driven proton pumps

(Invited) Recent Progress in Fundamental Photoelectrochemical Studies Relevant to New Low-Cost Designs for Z-Scheme Solar Water Splitting Reactors

Particle suspension reactors for solar water splitting are capable of generating hydrogen at a cost that is competitive with hydrogen produced from steam methane reforming. One reactor design resembles Nature’s Z-scheme where two side-by-side and connected photocatalyst reactor beds together drive overall solar water splitting.1,2 The photocatalyst in each reactor bed also performs a half-reaction with a redox shuttle, i.e. oxidation or reduction, and therefore to prevent complete depletion of the redox shuttle, the electrolyte must transport between the beds via a nanoporous gas separator. While this design facilitates separation of the H2 and O2 reaction products, and therefore can circumvent formation of an explosive mixture of gases, transport of the redox shuttle between the beds requires active pumping which equates to additional capital expenditures that result in a near doubling of the reactor cost.1,2 Our team is evaluating the feasibility of new reactor designs where the beds are simply stacked. This generates a true tandem light-absorbing reactor where the theoretical maximum solar-to-hydrogen conversion efficiency is over 50% larger than a side-by-side or single light-absorber design. Each bed is projected to be < 10 cm tall and therefore, this design greatly decreases the distance required for redox shuttle transport, reducing or even eliminating the need for pipes, pumps, and forced convection. In my presentation I will report on our team’s recent progress on this design. We used finite-element numerical methods to model and simulate in two dimensions the transient mass transport processes, light absorption, and electrochemical kinetics in the proposed reactor. Model results suggest that a reactor operating at a ~4% solar-to-hydrogen conversion efficiency can operate indefinitely without complete loss of the active form of the redox shuttle at any location in the reactor beds. Experimentally, we are investigating and characterizing photocatalyst nanomaterials over many length scales, from single nanoparticles (~10 nm to ~1 µm in diameter) to mesoporous thin films (~10 µm thick) to laboratory-scale prototype particle-suspension reactors (on the scale of feet). On the single-particle level we jammed and covalently bound TiO2 nanoparticles into a single nanopore in a plastic sheet, wetted the particles with liquid electrolyte on both sides, and measured photovoltages that resulted from optical excitation of the particles. This was significant because it allowed, for the first time, in situ photoelectrochemical characterization of nanoparticle(s). We also synthesized, characterized, and evaluated the photo(electro)chemical performance of BiVO4, WO3, and Rh-doped SrTiO3 nanocrystallites as mesoporous thin films and as particles in model reactors, and evaluated the transport properties of several redox shuttles. As a model system, H2-evolving Rh-doped SrTiO3 was evaluated as a mesoporous electrode and in particle form factor in the absence and presence of the ferric and/or ferrous iron redox shuttle. Based on this suite of results, we demonstrated that in the presence of FeII the limiting rate of reduction of FeIII is attenuated and the rate of catalysis for H2 evolution is enhanced. This is important because both halves of the iron redox shuttle are present in the reactor bed containing Rh-doped SrTiO3 and therefore this suggests that reduction of H+ can outcompete reduction of at least some FeIII under certain circumstances. Collectively, our efforts represent strides toward achieving a high-level of techno-economic viability in solar water splitting reactors. Acknowledgments: This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Incubator Program under Award No. DE-EE0006963 and Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. References: (1) B. D. James, G. N. Baum, J. Perez and K. N. Baum, Technoeconomic Analysis of Photoelectrochemical (PEC) Hydrogen Production, Directed Technologies Inc., (US DOE Contract no. GS-10F-009J), Arlington, VA, 2009. (2) B. A. Pinaud, J. D. Benck, L. C. Seitz, A. J. Forman, Z. Chen, T. G. Deutsch, B. D. James, K. N. Baum, G. N. Baum, S. Ardo, H. Wang, E. Miller, and T. F. Jaramillo, Energy & Environmental Science, 2013, 6, 1983–2002

New Reactor Designs for Z-Scheme Solar Water Splitting Photocatalysis

Particle suspension reactors for solar water splitting can be an economical alternative to photovoltaic-driven electrolysis. One design resembles Nature’s Z-scheme where two photosystems work in concert to drive overall water splitting. A conceptual Z-scheme reactor has been reported where two compartments on the meter length scale are adjoined side-by-side, each containing photocatalyst particles that drive one half-reaction of overall water splitting, and are connected by a nanoporous material that allows mixing of the liquid electrolyte.1,2 Electronic charge is mediated between the compartments by a dissolved redox shuttle that undergoes oxidation or reduction at the particles. While this design facilitates product separation (i.e. separation of H2 and O2) and therefore circumvents formation of an explosive mixture of gases, active transport of the redox shuttle over these distances has been projected to account for about half of the capital cost of the reactor.1,2 Our team is evaluating the feasibility of new reactor designs where the compartments are stacked vertically. This generates a true tandem light-absorbing reactor where the theoretical maximum solar-to-hydrogen conversion efficiency is ~50% larger than a side-by-side or single light-absorber design. Because the compartments are expected to be ~10 cm tall, this design greatly decreases the distance required for redox shuttle transport therefore reducing or even eliminating the need for forced convection. In my presentation I will report on our team’s progress on this design. We used finite-element numerical methods to model and simulate in two dimensions the transient mass transport processes, light absorption, and electrochemical kinetics in the proposed reactor. The developed model provided insights into the influence of the reactor geometry and operating conditions on the overall performance. The Beer–Bouger–Lambert law was applied to obtain the spatial light-intensity field and volumetric reaction rates were obtained by coupling solid-state photodiode expressions with Butler–Volmer kinetics on the surface of the particles. Model results suggested that a reactor operating at a ~1% solar-to-hydrogen conversion efficiency can operate for greater than half a year without complete loss of redox shuttle at any location in the reactor. Experimentally, we investigated materials over many size scales, from single particles (~10 nm in diameter) to mesoporous thin films (~10 µm thick) to laboratory-scale prototype particle-suspension reactors (on the scale of feet). On the single particle level we used bipolar electrodeposition to create Janus-type particles consisting of model carbon particles with metal and metal-oxide electrocatalysts for H2 evolution and O2 evolution at the poles. We also jammed and covalently bound TiO2 nanoparticles into a single nanopore in a plastic sheet, wetted the particles with liquid electrolyte on both sides, and measured photovoltages that resulted from excitation of few particles. We also synthesized, characterized, and evaluated the photo(electro)chemical performance of BiVO4 and Rh-doped SrTiO3 nanocrystallites as mesoporous thin films and particles in model reactors, and evaluated the transport properties of several redox shuttles. Collectively, our efforts represent strides toward achieving a high-level of techno-economic viability in solar water splitting reactors. Acknowledgments: This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Incubator Program under Award No. DE-EE0006963 and Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. References: (1) B. D. James, G. N. Baum, J. Perez and K. N. Baum, Technoeconomic Analysis of Photoelectrochemical (PEC) Hydrogen Production, Directed Technologies Inc., (US DOE Contract no. GS-10F-009J), Arlington, VA, 2009. (2) B. A. Pinaud, J. D. Benck, L. C. Seitz, A. J. Forman, Z. Chen, T. G. Deutsch, B. D. James, K. N. Baum, G. N. Baum, S. Ardo, H. Wang, E. Miller, and T. F. Jaramillo, Energy & Environmental Science, 2013, 6, 1983–2002

The 2022 solar fuels roadmap

Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO2 reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field