(601g) Photocatalysis for Z-Scheme Solar Water Splitting Using New Reactor Designs

Particle suspension reactors for solar water splitting can be an economical alternative to photovoltaic-driven electrolysis. One design resembles Natureâ??s Z-scheme where together two photocatalyst nanoparticle reactor beds that are adjoined side-by-side and on the meters length scale, drive overall solar water splitting.^1,2 Electronic charge is mediated between the beds by a dissolved redox shuttle that undergoes oxidation or reduction at the particles and transports between the beds through a nanoporous material. 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 beds are 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 therefore reducing or even eliminating the need for forced convection.

In my presentation I will report on our teamâ??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 Beerâ??Bougerâ??Lambert law was applied to obtain the spatial light-intensity field and volumetric reaction rates were obtained by coupling solid-state photodiode expressions with Butlerâ??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 nanomaterials over many size 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 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. This was significant because it allowed, for the first time, in situ photoelectrochemical characterization of particle(s). We also synthesized, characterized, and evaluated the photo(electro)chemical performance of BiVO₄ and Rh-modified SrTiO₃ nanocrystallites as mesoporous thin films and particles in model reactors, and evaluated the transport properties of several redox shuttles. The BiVO₄ and Rh-modified SrTiO₃ were grown by sol gel methods and characterized by XRD, XPS, transmission-mode or diffuse-reflectance ultravioletâ??visible electronic absorption spectroscopy, He II UPS, DLS (as particles), Raman, and photoelectrochemical performance. These nanocrystallites exhibited some of the best performance reported to date.

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, 1983â??2002.