Light harvesting proteins for solar fuel generation in bioengineered photoelectrochemical cells.
ABSTRACT: The sun is the primary energy source of our planet and potentially can supply all societies with more than just their basic energy needs. Demand of electric energy can be satisfied with photovoltaics, however the global demand for fuels is even higher. The direct way to produce the solar fuel hydrogen is by water splitting in photoelectrochemical (PEC) cells, an artificial mimic of photosynthesis. There is currently strong resurging interest for solar fuels produced by PEC cells, but some fundamental technological problems need to be solved to make PEC water splitting an economic, competitive alternative. One of the problems is to provide a low cost, high performing water oxidizing and oxygen evolving photoanode in an environmentally benign setting. Hematite, ?-Fe2O3, satisfies many requirements for a good PEC photoanode, but its efficiency is insufficient in its pristine form. A promising strategy for enhancing photocurrent density takes advantage of photosynthetic proteins. In this paper we give an overview of how electrode surfaces in general and hematite photoanodes in particular can be functionalized with light harvesting proteins. Specifically, we demonstrate how low-cost biomaterials such as cyanobacterial phycocyanin and enzymatically produced melanin increase the overall performance of virtually no-cost metal oxide photoanodes in a PEC system. The implementation of biomaterials changes the overall nature of the photoanode assembly in a way that aggressive alkaline electrolytes such as concentrated KOH are not required anymore. Rather, a more environmentally benign and pH neutral electrolyte can be used.
Project description:In this Perspective, we introduce intensity modulated photocurrent/voltage spectroscopy (IMPS and IMVS) as powerful tools for the analysis of charge carrier dynamics in photoelectrochemical (PEC) cells for solar water splitting, taking hematite (?-Fe2O3) photoanodes as a case study. We complete the picture by including photoelectrochemical impedance spectroscopy (PEIS) and linking the trio of PEIS, IMPS and IMVS, introduced here as photoelectrochemical immittance triplets (PIT), both mathematically and phenomenologically, demonstrating what conclusions can be extracted from these measurements. A novel way of analyzing the results by an empirical approach with minimal presumptions is introduced, using the distribution of relaxation times (DRT) function. The DRT approach is compared to conventional analysis approaches that are based on physical models and therefore come with model presumptions. This work uses a thin film hematite photoanode as a model system, but the approach can be applied to other PEC systems as well.
Project description:Utilizing photoelectrochemical (PEC) cells to directly collecting solar energy into chemical fuels (e.g., H<sub>2</sub> via water splitting) is a promising way to tackle the energy challenge. ?-Fe<sub>2</sub>O<sub>3</sub> has emerged as a desirable photoanode material in a PEC cell due to its wide spectrum absorption range, chemical stability, and earth abundant component. However, the short excited state lifetime, poor minority charge carrier mobility, and long light penetration depth hamper its application. Recently, the elegantly designed hierarchical macroporous composite nanomaterial has emerged as a strong candidate for photoelectrical applications. Here, a novel 3D antimony-doped SnO<sub>2</sub> (ATO) macroporous structure is demonstrated as a transparent conducting scaffold to load 1D hematite nanorod to form a composite material for efficient PEC water splitting. An enormous enhancement in PEC performance is found in the 3D electrode compared to the controlled planar one, due to the outstanding light harvesting and charge transport. A facile and simple TiCl<sub>4</sub> treatment further introduces the Ti doping into the hematite while simultaneously forming a passivation layer to eliminate adverse reactions. The results indicate that the structural design and nanoengineering are an effective strategy to boost the PEC performance in order to bring more potential devices into practical use.
Project description:Hematite has a great potential as a photoanode for photoelectrochemical (PEC) water splitting by converting solar energy into hydrogen fuels, but the solar-to-hydrogen conversion efficiency of state-of-the-art hematite photoelectrodes are still far below the values required for practical hydrogen production. Here, we report a core-shell formation of gradient tantalum-doped hematite homojunction nanorods by combination of hydrothermal regrowth strategy and hybrid microwave annealing, which enhances the photocurrent density and reduces the turn-on voltage simultaneously. The unusual bi-functional effects originate from the passivation of the surface states and intrinsic built-in electric field by the homojunction formation. The additional driving force provided by the field can effectively suppress charge-carrier recombination both in the bulk and on the surface of hematite, especially at lower potentials. Moreover, the synthesized homojunction shows a remarkable synergy with NiFe(OH)<sub>x</sub> cocatalyst with significant additional improvements of photocurrent density and cathodic shift of turn-on voltage. The work has nicely demonstrated multiple collaborative strategies of gradient doping, homojunction formation, and cocatalyst modification, and the concept could shed light on designing and constructing the efficient nanostructures of semiconductor photoelectrodes in the field of solar energy conversion.
Project description:Converting solar energy into sustainable hydrogen fuel by photoelectrochemical (PEC) water splitting is a promising technology to solve increasingly serious global energy supply and environmental issues. However, the PEC performance based on TiO2 nanomaterials is hindered by the limited sunlight-harvesting ability and its high recombination rate of photogenerated charge carriers. In this work, layered SnS2 absorbers and CoOx nanoparticles decorated two-dimensional (2D) TiO2 nanosheet array photoelectrode have been rationally designed and successfully synthesized, which remarkably enhanced the PEC performance for water splitting. As the result, photoconversion efficiency of TiO2/SnS2/CoOx and TiO2/SnS2 hybrid photoanodes increases by 3.6 and 2.0 times under simulated sunlight illumination, compared with the bare TiO2 nanosheet arrays photoanode. Furthermore, the TiO2/SnS2/CoOx photoanode also presented higher PEC stability owing to CoOx catalyst served as efficient water oxidation catalyst as well as an effective protectant for preventing absorber photocorrosion.
Project description:A BiVO4 photoanode with exposed (040) facets was prepared to enhance its photoelectrochemical performance. The exposure of the (040) crystal planes of the BiVO4 film was induced by adding NaCl to the precursor solution. The as-prepared BiVO4 photoanode exhibits higher solar-light absorption and charge-separation efficiency compared to those of an anode prepared without adding NaCl. To our knowledge, the photocurrent density (1.26 mA cm-2 at 1.23 V vs. RHE) of as-prepared BiVO4 photoanode is the highest according to the reports for bare BiVO4 films under simulated AM1.5G solar light, and the incident photon-to-current conversion efficiency is above 35% at 400 nm. The photoelectrochemical (PEC) water-splitting performance was also dramatically improved with a hydrogen evolution rate of 9.11 ?mol cm-2 h-1, which is five times compared with the BiVO4 photoanode prepared without NaCl (1.82 ?mol cm-2 h-1). Intensity-modulated photocurrent spectroscopy and transient photocurrent measurements show a higher charge-carrier-transfer rate for this photoanode. These results demonstrate a promising approach for the development of high-performance BiVO4 photoanodes which can be used for efficient PEC water splitting and degradation of organic pollutants.
Project description:Solar water splitting represents one of the most promising strategies in the quest for clean and renewable energy. However, low conversion efficiency, use of sacrificial agents, and external bias for current water splitting system limit its practical application. Here, a gold-sensitized Si/ZnOcore/shell nanowire photoelectrochemical (PEC) cell is reported for efficient solar water oxidation. We demonstrated gold-sensitized n-Si/n-ZnO nanowire arrays exhibited higher energy conversion efficiency than gold-sensitized p-Si/n-ZnO nanowire arrays due to the favorable energy-band alignment characteristics. Without any assistance from an external electrical source and sacrificial reagents, gold-sensitized n-Si/n-ZnO core/shell nanowire array photoanode achieved unbiased water splitting under simulated solar light illumination. This method opens a promising venue to cost-efficient production of solar fuels.
Project description:The exploration of photoanode materials with high efficiency and stability is the eternal pursuit for the realization of practically solar-driven photoelectrochemical (PEC) water splitting. Here we develop a deficient ternary metal sulfide (CdIn2S4) photoanode, and its PEC performance is significantly enhanced by introducing surface sulfur vacancies, achieving a photocurrent density of 5.73?mA?cm-2 at 1.23?V vs. RHE and 1 Sun with an applied bias photon-to-current efficiency of 2.49% at 0.477?V vs. RHE. The experimental characterizations and theoretical calculations highlight the enhanced effect of surface sulfur vacancies on the interfacial charge separation and transfer kinetics, which also demonstrate the restrained surface states distribution and the transformation of active sites after introducing surface sulfur vacancies. This work may inspire more excellent work on developing sulfide-based photoanodes.
Project description:Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45?V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.
Project description:Hematite (?-Fe2O3) is one of the most promising candidates as a photoanode materials for solar water splitting. Owing to the difficulty in suppressing the significant charge recombination, however, the photoelectrochemical (PEC) conversion efficiency of hematite is still far below the theoretical limit. Here we report thick hematite films (?1500?nm) constructed by highly ordered and intimately attached hematite mesocrystals (MCs) for highly efficient PEC water oxidation. Due to the formation of abundant interfacial oxygen vacancies yielding a high carrier density of ?1020?cm-3 and the resulting extremely large proportion of depletion regions with short depletion widths (<10?nm) in hierarchical structures, charge separation and collection efficiencies could be markedly improved. Moreover, it was found that long-lived charges are generated via excitation by shorter wavelength light (below ?500?nm), thus enabling long-range hole transfer through the MC network to drive high efficiency of light-to-energy conversion under back illumination.
Project description:Hydrogen production from water splitting using solar energy based on photoelectrochemical (PEC) cells has attracted increasing attention because it leaves less of a carbon footprint and has economic superiority of solar and hydrogen energy. Oxide semiconductors such as ZnO possessing high stability against photocorrosion in hole scavenger systems have been widely used to build photoanodes of PEC cells but under visible light their conversion efficiencies with respect to incident-photon-to-current conversion efficiency (IPCE) measured without external bias are still not satisfied. An innovative way is presented here to significantly improve the conversion efficiency of PEC cells by constructing a core-shell structure-based photoanode comprising Au@CdS core-shell nanoparticles on ZnO nanowires (Au@CdS-ZnO). The Au core offers strong electronic interactions with both CdS and ZnO resulting in a unique nanojunction to facilitate charge transfer. The Au@CdS-ZnO PEC cell under 400 nm light irradiation without any applied bias provides an IPCE of 14.8%. Under AM1.5 light illumination with a bias of 0.4 V, the Au@CdS-ZnO PEC cell produces H2 at a constant rate of 11.5 ?mol h-1 as long as 10 h. This work provides a fundamental insight to improve the conversion efficiency for visible light in water splitting.