Generation Of Solar Fuels Research Articles

Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion

BiOX (X = Br and Cl) have been widely used for photocatalytic environmental remediation and solar fuels generation. Here, we applied a new method for large scale preparation of BiOX ultrathin nanosheets. Samples were characterized by X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), BET surface area (BET), photoluminescence spectra (PL), UV–vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS). The BiOCl ultrathin nanosheets (BOC-U) and BiOBr ultrathin nanosheets (BOB-U) showed enhanced photocatalytic activity for CO2 conversion than bulk BiOCl and BiOBr.

(Invited) Atomic Layer Deposition: A Great Tool to Synthetize High Efficiency Electrodes for Solar Fuel Generation?

The sunlight’s intermittent nature is one of the issues limiting widespread harvesting of solar energy for power infrastructures. A leading approach is to store as chemical fuels the energy produced by this discontinuous renewable energy source. The development of photoelectrochemical cells (PECs) used to produce H2 and O2 from water photo-splitting has, therefore, attracted a great deal of interest. Nowadays, the efficiency of the photoelectrochemical cells (PEC) ranges from 12 to 18% depending on the materials and the type of cells (single- or multiple-junction) while the theoretical limit is 24.4 and 30% for tandem and multi-junction cells, respectively. High efficiency at a high cost has been shown from multi-junction cells but no sufficient improvements leading to a market compliant PEC have been reported yet. The photoelectrochemical technology remains at a low technology readiness level (TRL 1 to 2) and the main issue is the actual production cost of H2. To solve this question, it is mandatory to reduce significantly the costs and to increase the photoconversion efficiency. Many research groups are currently investigating different routes to fabricate PECs that could respond to the market demand. To improve efficiency, stability and price, one has, of course, to select a cost-effective photosensitive materials and the appropriate cell design. It has been recently shown that micro- or nanostructuring and/or surface functionalization of the photoelectrodes can lead to higher performances. Among the numerous approaches and techniques that have been used since nanosciences and nanotechnologies have emerged, atomic layer deposition (ALD) has recently demonstrated its high effectiveness to fabricate both two- and three-dimensional (2D, 3D) nano-objects. Energy storage and production are part of the fields of applications in which ALD has shown highly promising perspectives. It will be shown in this presentation that, during the last five years, ALD has been effectively integrated in various fabrication strategies of photoelectrodes. After a brief introduction on the various methods of surface micro- and nanostructuring methods, a short description of the ALD process will be presented. The goal of this presentation is to report the different types of uses of ALD in the field of solar fuel production: active materials, surface state passivation and corrosion protection. A special attention will be drawn on the recent results obtained in our laboratory where a rapid, inexpensive two-step method for structuring n-type (100) Si surfaces with micron-sized cavities, the process is based on the photoelectrochemical etching (PEE) of the Si surface and its subsequent alkaline etching. This method produces a layer of random macropores over a large area, which renders the Si surfaces antireflective over the visible spectrum. We demonstrate that such surfaces can be conformably coated by anatase TiO2 layers by atomic layer deposition (ALD) and that they can be used as stable photoanodes producing enhanced photocurrents under simulated sunlight with respect to their planar counterparts. These TiO2-protected Si microstructured surfaces were highly stable in strongly alkaline solutions and were used as photoanode for several hours under simulated sunlight. Such photoanodes surfaces showed 50 % photocurrent enhancements and ~400 mV negative shift of onset potential without any co-catalysts, demonstrating their high potential for solar energy conversion applications [3]. [1] N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. 103, 15729 (2006). [2] A. Fujishima and K. Honda, Nature 238, 37 (1972). [3] L. Santinacci, M. W. Diouf, M. K. S. Barr, B. Fabre, L. Joanny, F. Gouttefangeas, and G. Loget, ACS Appl. Mater. Interfaces, 8, 24810 (2016)

(Invited) High Performance Components for Solar-Driven Photoelectrochemical Fuel Generators

The use of sunlight to synthesize useful chemicals such as H2 and hydrocarbon fuels from components of air (water and CO2) has progressed rapidly due to the strong international effort to identify materials and device architectures that improve efficiency and stability. The targets are challenging: a solar fuels generation system must return at least as much chemical energy as the total energy required to fabricate and operate it, and last for a decade or more. Success requires addressing corrosion, membranes properties, integrated semiconductor/catalyst systems and performance under natural conditions involving large temperature swings and diurnal operation. In this talk I will describe studies conducted in JCAP directed toward achieving these targets. Considerations for solar water splitting and CO2 reduction systems will be compared.

Solar-Driven Reduction of 1 atm CO2 to Formate at 10% Energy-Conversion Efficiency by Use of a TiO2-Protected III-V Tandem Photoanode in Conjunction with a Bipolar Membrane and a Pd/C Cathode Electrocatalyst

A solar-driven CO2 reduction (CO2R) device that consists of a tandem InGaP/GaAs/TiO2/Ni photoanode in 1.0 M KOH for oxygen evolution reaction (OER), a Pd/C nanoparticles dark cathode in bicarbonate electrolyte at pH=8.0 for CO2R reaction and a bipolar membrane exhibited a solar-to-fuel (STF) conversion efficiency of 10%. The Pd/C nanoparticles showed low overpotential (< 100 mV) and high faradic efficiency (> 94%) for CO2 reduction to formate at 8.5 mA cm-2. The bipolar membrane exhibited low formate crossover and high transference number for proton and hydroxide at the steady-state pH gradient. The effective coupling between two different electrolytes that optimize the performance of CO2R reaction and OER provides new cell design spaces for efficient solar-fuel generation.

On the measured optical bandgap values of inorganic oxide semiconductors for solar fuels generation

This article focuses on variation in the values for the optical bandgaps (Eg) experimentally reported for fourteen oxide semiconductors that have been studied for solar photoelectrochemical and photocatalytic environmental remediation applications. Both binary and ternary compounds have been included in this study. The variance in the reported values is much more severe for the ternary compounds relative to the binary counterparts. Factors related to semiconductor doping and size quantization have been carefully removed from playing a role in the presented data. Instead the variability is attributed to intrinsic errors associated with the extraction of Eg values from spectral data (for example, via Tauc plots), and to other sample-related factors such as surface functional groups and compound non-stoichiometry. Finally, suggestions for future, follow-up study are also given.

Biology leads the race to turn sunlight into fuels

A bacterium can be harnessed to do the job, but can that process be scaled up?

A Pyridine Alkoxide Chelate Ligand That Promotes Both Unusually High Oxidation States and Water-Oxidation Catalysis.

Water-oxidation catalysis is a critical bottleneck in the direct generation of solar fuels by artificial photosynthesis. Catalytic oxidation of difficult substrates such as water requires harsh conditions, so the ligand must be designed both to stabilize high oxidation states of the metal center and to strenuously resist ligand degradation. Typical ligand choices either lack sufficient electron donor power or fail to stand up to the oxidizing conditions. Our research on Ir-based water-oxidation catalysts (WOCs) has led us to identify a ligand, 2-(2'-pyridyl)-2-propanoate or "pyalk", that fulfills these requirements. Work with a family of Cp*Ir(chelate)Cl complexes had indicated that the pyalk-containing precursor gave the most robust WOC, which was still molecular in nature but lost the Cp* fragment by oxidative degradation. In trying to characterize the resulting active "blue solution" WOC, we were able to identify a diiridium(IV)-mono-μ-oxo core but were stymied by the extensive geometrical isomerism and coordinative variability. By moving to a family of monomeric complexes [IrIII/IV(pyalk)3] and [IrIII/IV(pyalk)2Cl2], we were able to better understand the original WOC and identify the special properties of the ligand. In this Account, we cover some results using the pyalk ligand and indicate the main features that make it particularly suitable as a ligand for oxidation catalysis. The alkoxide group of pyalk allows for proton-coupled electron transfer (PCET) and its strong σ- and π-donor power strongly favors attainment of exceptionally high oxidation states. The aromatic pyridine ring with its methyl-protected benzylic position provides strong binding and degradation resistance during catalytic turnover. Furthermore, the ligand has two additional benefits: broad solubility in aqueous and nonaqueous solvents and an anisotropic ligand field that enhances the geometry-dependent redox properties of its complexes. After discussion of the general properties, we highlight the specific complexes studied in more detail. In the iridium work, the isolated mononuclear complexes showed easily accessible Ir(III/IV) redox couples, in some cases with the Ir(IV) state being indefinitely stable in water. We were able to rationalize the unusual geometry-dependent redox properties of the various isomers on the basis of ligand-field effects. Even more striking was the isolation and full characterization of a stable Rh(IV) state, for which prior examples were very reactive and poorly characterized. Importantly, we were able to convert monomeric Ir complexes to [Cl(pyalk)2IrIV-O-IrIVCl(pyalk)2] derivatives that help model the "blue solution" properties and provide groundwork for rational synthesis of active, well-defined WOCs. More recent work has moved toward the study of first-row transition metal complexes. Manganese-based studies have highlighted the importance of the chelate effect for labile metals, leading to the synthesis of pincer-type pyalk derivatives. Beyond water oxidation, we believe the pyalk ligand and its derivatives will also prove useful in other oxidative transformations.

Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface

Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a spectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide-based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to compare directly the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen.

Tierui Zhang.

"Guaranteed to make me laugh is … the movie A Chinese Odyssey, which is loosely based on the 16th century novel Journey to the West by Wu Cheng'en. If I could be a piece of lab equipment I would be … a light source that efficiently drives the generation of solar fuels in the presence of photocatalysts." This and more about Tierui Zhang can be found on page 164.

Evaluation of flow schemes for near-neutral pH electrolytes in solar-fuel generators

Operating conditions and design principles for the efficient operation of solar-fuel generators with active flow of near-neutral pH electrolytes.

A comparison of the chemical, optical and electrocatalytic properties of water-oxidation catalysts for use in integrated solar-fuel generators

Thein situoptical properties and electrocatalytic performance of representative water-oxidation catalyst have been considered.

Metal-Free Carbon-Based Nanomaterial Coatings Protect Silicon Photoanodes in Solar Water-Splitting.

The decreasing cost of silicon-based photovoltaics has enabled significant increases in solar electricity generation worldwide. Silicon photoanodes could also play an important role in the cost-effective generation of solar fuels, but the most successful methods of photoelectrode passivation and performance enhancement rely on a combination of precious metals and sophisticated processing methods that offset the economic arguments for silicon. Here we show that metal-free carbon-based nanomaterial coatings deposited from solution can protect silicon photoanodes carrying out the oxygen evolution reaction in a range of working environments. Purified semiconducting carbon nanotubes (CNTs) act as a hole extraction layer, and a graphene (Gr) capping layer both protects the CNT film and acts as a hole exchange layer with the electrolyte. The performance of semiconducting CNTs is found to be superior to that of metallic or unsorted CNTs in this context. Furthermore, the insertion of graphene oxide (GO) between the n-Si and CNTs reduces the overpotential relative to photoanodes with CNTs deposited on hydrogen-passivated silicon. The composite photoanode structure of n-Si/GO/CNT/Gr shows promising performance for oxygen evolution and excellent potential for improvement by optimizing the catalytic properties and stability of the graphene protective layer.

Photo-assisted self-optimizing of charge-carriers transport channel in the recrystallized multi-heterojunction nanofibers for highly efficient photocatalytic H2 generation

Rational arrangement of nanosized semiconductor components in the multi-heterojunction photocatalyst can lead to the formation of a high-speed transport channel for charge-carriers separation and migration, which provides a promising way to achieve excellent photocatalytic efficiency for solar fuels generation. Herein, we develop a photo-assisted self-optimization strategy to re-build the charge-carriers transport channel in the copper species nanocrystals/TiO2 electrospun nanofibers with the organic hydrogen-carrier molecules as the photo-reactants. As a result of the dynamics difference of the excitons migration during the photocatalytic process, the binary CuO/TiO2 heterojunction nanofibers are recrystallized to form the quaternary Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers that exhibits a higher rate constant (∼1.7×108S−1) for the interfacial electron-transfer than the former nanofibers (∼0.7×108S−1) due to the improved charge-carriers transport channel. In this way, a 40-fold enhanced H2 generation rate was observed on the recrystallized nanofibers photocatalyst during the photocatalytic decomposition of formic acid as compared to the pure TiO2 nanofibers. Our work presents an available paradigm to skillfully use a transient photo-physicochemical process to mildly engineer a high-quality charge-carriers transport channel in the hetero-nanophotocatalyst for realizing an optimal photocatalytic performance.

(Keynote) Artificial Photosynthesis: Progress, Science Outlook and Technology Prospects

The design of highly efficient devices that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system. In the past five years, considerable progress has been made in scientific discovery of key materials and mechanisms needed to realize artificial solar fuels generators, and advances modeling and design have generated a now quite widely embraced conceptual paradigm for a solar fuels generator. While we still lack sufficient knowledge to design solar-fuel generation systems with the ultimate efficiency, scalability, and sustainability to be economically viable, considerable advances have been made, particularly for water-splitting solar fuels devices. I will describe work done in this area by the Joint Center for Artificial Photosynthesis. I will also survey outstanding scientific challenges to realization of artificial photosynthesis for generation of fuels and chemicals by reduction of carbon dioxide and discuss JCAP's approach and recent advances in carbon dioxide reduction.

Characterizing CO2 Reduction Mechanism Using Advanced Synchrotron-Based X-Ray Spectroscopy Techniques

Generation of solar fuels through photo-catalytic reduction of CO2 could simultaneously address both the need for sustainable and affordable energy sources and the necessity of reducing global CO2 footprint. Discovering and designing new catalysts that have both high activity and selectivity for CO2 reduction is the critical barrier to overcome in order for solar fuel generation processes to commercialize. So far, copper is the sole material known to efficiently catalyze conversion of CO2 to considerable amounts of C2 hydrocarbon products over time. However, CO2 reduction on Copper exhibits a high overpotential plus the added disadvantage of producing a mixture of several products including hydrogen.1,2 Thus, it is of vital importance to understand the so far ambiguous and poorly understood catalytic CO2 reduction mechanism on copper. In this work, a combination of various in-situ X-ray spectroscopy techniques are utilized to probe the electrode/electrolyte interface and provide insight into the reduction reaction.X-ray absorption spectroscopy (XAS) provides an element-specific probe of the conduction band via a core-level excitation into unoccupied electronic states and can reveal both oxidation state and details of the electronic structure. Operando Grazing incidence XAS at beamline 11-2 of SSRL is utilized to study the electrode surface at the metal K- and L-edges. A specially designed 3D printed flow cell maintainsa 300 micron liquid layer above the catalyst surface, enabling XAS characterization of the catalyst surface during electrochemistry in grazing incidence mode. The grazing incidence geometry provides a probe depth of about 5 nm into the catalyst, hence providing a highly surface sensitive spectroscopic tool. Preliminary measurements were conducted on the relatively inert surface of AuPd alloy catalyst. This is the first step in determining the feasibility of the study before examining the more complicated copper system. Distinct reversible shifts are observed in spectroscopic features with applied potential, which are believed to be due to H+ intercalation into Pd phase. (Figure 1) Further, operando soft X-ray spectroscopy measurements at C-edge are conducted at beamline 8.0.1 of ALS to detect intermediate carbon species. Electron yield soft XAS data are collected at a depth of 1 nm above the electrode surface in a specially designed in-situ electrochemical flow cell3. Strong 1s to π* transition in C edge spectra provides fingerprint for different products. First principles DFT calculations are conducted to characterize features obtained through these XAS measurements. (Figure 2) Combination of these spectroscopic techniques with electrochemical and theoretical calculations will paint a comprehensive picture of the exact CO2RR mechanism on Cu surface, which then can be used to design next generation photo-catalysts.

Towards the Development of Practical Solar-Fuels Devices: Silicon-Based Solar-Hydrogen Generators with 14.2% Solar-to-Hydrogen Efficiency

The implementation of practical solar-fuel technologies can trigger a significant increase in the use of renewables in our energy ecosystem. These technologies can directly capture and store solar energy in the form of energy rich molecules, which could be used at a later stage as fuels for transportation or electricity generation. Storing solar-energy into fuels can also serve as a mean of seasonal energy storage, so that excess solar energy in the summer can be used during the periods of low solar irradiation in the winter. Also, solar-fuel generators can be operated in centralized facilities without disrupting the operation of the electricity grid. The spatio-temporal decoupling of the energy capture and utilization that solar-fuel technologies provide makes them an attractive renewable energy storage solution. For solar-hydrogen generators to be implemented, they need to be able to operate robustly, stably and continuously for prolonged periods of time – in the order of several years to decades. They also need to produce fuels in a cost-effective way and the energy produced over their lifetime needs to exceed the energy inputted into their manufacturing and operation. In this presentation, we describe a system agnostic approach to assess promising designs and components of solar-hydrogen generators from a technoeconomic perspective. Using this framework, we evaluate the effects of device design and material selection on the cost of the generated hydrogen. The results presented here provide insights and device engineering directions that would lead to cost-optimized solar-hydrogen generators. Our findings demonstrate that in order to minimize hydrogen production costs, optimized devices will need to be manufactured with water splitting components that are significantly smaller in area than photovoltaic units (by a factor of 10-100’s). Additionally, the analysis points out that devices based on silicon (Si) photovoltaics (either thin-film or crystalline Si cells) could reach hydrogen production costs that approach those of hydrogen from fossil fuels resources. As the cost of hydrogen is primarily driven by the light-absorbing units, further economic gains can be obtained by improving the overall solar-to-hydrogen efficiencies of devices. These efficiency improvements can be achieved by the implementation of Silicon heterojunction (SHJ) solar cells into solar-hydrogen devices. SHJ cells present higher open-circuit voltages (VOC) than crystalline Si cells and can reach levels above 700 mV. Their high VOC values are mainly due to an interfacial passivation with a thin (~5 nm) film of hydrogenated intrinsic amorphous silicon between the c-Si wafer and the oppositely doped emitter, forming the p-n junction. Thanks to the high VOC of SHJ cells, we demonstrate that modules of three cells in series can provide enough potential to drive the water splitting reaction at high current densities in electrolysis units. Devices based on SHJ modules and Nafion-based membrane-electrode assemblies with platinum and iridium oxide electrocatalysts show stable performance over 100 hrs of operation at an unprecedented SHE of 14.2%. The same efficiency level is demonstrated for devices operated under alkaline conditions using microstructured nickel electrodes; representing the highest SHE achieved with earth-abundant components to date. As both the SHJ cells and electrolysis units implemented in this study are commercially viable, easily scalable and have long lifetimes, the devices presented here have the potential to disruptively accelerate the deployment of cost effective solar-hydrogen generators.

Cooperative Catalytic Effect of ZrO2 and α-Fe2 O3 Nanoparticles on BiVO4 Photoanodes for Enhanced Photoelectrochemical Water Splitting.

Photoelectrochemical water splitting with metal oxide semiconductors offers a cost-competitive alternative for the generation of solar fuels. Most of the materials studied so far suffer from poor charge-transfer kinetics at the semiconductor/liquid interface, making compulsory the use of catalytic layers to overcome the large overpotentials required for the water oxidation reaction. Herein, we report a very soft electrolytic synthesis deposition method, which allows remarkably enhanced water oxidation kinetics of BiVO4 photoanodes by the sequential addition of Zr and Fe precursors. Upon a heat treatment cycle, these precursors are converted into monoclinic ZrO2 and α-Fe2 O3 nanoparticles, which mainly act as catalysts, leading to a five-fold increase of the water oxidation photocurrent of BiVO4 . This method provides a versatile platform that is easy to apply to different semiconductor materials, fully reproducible, and facile to scale-up on large area conductive substrates with attractive implications for technological deployment.

A nickel iron diselenide-derived efficient oxygen-evolution catalyst.

Efficient oxygen-evolution reaction catalysts are required for the cost-effective generation of solar fuels. Metal selenides have been reported as promising oxygen-evolution catalysts; however, their active forms are yet to be elucidated. Here we show that a representative selenide catalyst, nickel selenide, is entirely converted into nickel hydroxide under oxygen-evolution conditions. This result indicates that metal selenides are unstable during oxygen evolution, and the in situ generated metal oxides are responsible for their activity. This knowledge inspired us to synthesize nanostructured nickel iron diselenide, a hitherto unknown metal selenide, and to use it as a templating precursor to a highly active nickel iron oxide catalyst. This selenide-derived oxide catalyses oxygen evolution with an overpotential of only 195 mV for 10 mA cm−2. Our work underscores the importance of identifying the active species of oxygen-evolution catalysts, and demonstrates how such knowledge can be applied to develop better catalysts.

Highly Fluorinated Ir(III)-2,2':6',2″-Terpyridine-Phenylpyridine-X Complexes via Selective C-F Activation: Robust Photocatalysts for Solar Fuel Generation and Photoredox Catalysis.

A series of fluorinated Ir(III)-terpyridine-phenylpyridine-X (X = anionic monodentate ligand) complexes were synthesized by selective C-F activation, whereby perfluorinated phenylpyridines were readily complexed. The combination of fluorinated phenylpyridine ligands with an electron-rich tri-tert-butyl terpyridine ligand generates a "push-pull" force on the electrons upon excitation, imparting significant enhancements to the stability, electrochemical, and photophysical properties of the complexes. Application of the complexes as photosensitizers for photocatalytic generation of hydrogen from water and as redox photocatalysts for decarboxylative fluorination of several carboxylic acids showcases the performance of the complexes in highly coordinating solvents, in some cases exceeding that of the leading photosensitizers. Changes in the photophysical properties and the nature of the excited states are observed as the compounds increase in fluorination as well as upon exchange of the ancillary chloride ligand to a cyanide. These changes in the excited states have been corroborated using density functional theory modeling.

An Alternative Reaction Pathway for Iridium-Catalyzed Water Oxidation Driven by Cerium Ammonium Nitrate (CAN)

The generation of solar fuels by means of a photosynthetic apparatus strongly relies on the development of an efficient water oxidation catalyst (WOC). Cerium ammonium nitrate (CAN) is the most commonly used sacrificial oxidant to explore the potentiality of WOCs. It is usually assumed that CAN has the unique role to oxidatively energize WOCs, making them capable to offer a low-energy reaction pathway to transform H2O to O2. Herein, we show that CAN might have a much more relevant and direct role in WO, mainly related to the capture and liberation of O–O-containing molecular moieties.