Solar Water Splitting Cells Research Articles

Electrocatalytic Oxygen Evolution over Nano Scaled Amorphous Ni−Fe Particles in Alkaline Electrolyte

Electrocatalytic water oxidation plays the key role as efficient and stable platform in some of the promising renewable energy conversion and storage devices, such as water electrolyzers, solar water-splitting cells, and lithium-air batteries. The electrocatalytic oxygen evolution reaction (OER) at the anode often couples with electron or photoelectron CO2 reductionand, H2 evolution reactions at the cathode for the above devices but limits the performance of the entire system because of the sluggish OER kinetics, insufficient reaction sites, or low stability of precious metal catalysts. Series of different transition metallic catalysts were investigated in order to significantly increase the activity of OER catalysts and efficiently improve the performance of energy conversion and storage systems. Ni-Fe system is found to be the most promising bimetallic catalytic system with robust stability, much lower cost, and considerably higher activity than the IrO2, the well-known best OER catalyst. Nano scaled catalysts with high electrochemical surface area (ECSA) not only exhibits a promising feasibility to effectively increase the performance of OER catalysts, but also to be simply utilized in renewable energy conversion and storage devices Herein, we reported 4 nm Ni−Fe nanoparticles (NiyFe1−yOx/C) featuring amorphous structures prepared via a solution-phase nanocapsule method for active and durable OER electrocatalysts in alkaline electrolyte. The Ni−Fe nanoparticle catalyst containing 31% Fe (Ni0.69Fe0.31Ox/C) shows the highest activity, exhibiting a 280 mV overpotential at 10 mA cm−2 (equivalent to 10% efficiency of solar-to-fuel conversion) and a Tafel slope of 30 mV dec−1 in 1.0 M KOH solution. The achieved OER activity outperforms NiOx/C and commercial Ir/ C catalysts and is close to the highest performance of crystalline Ni−Fe thin films reported in the literature. In addition, a Faradaic efficiency of 97% measured on Ni0.69Fe0.31Ox/C suggests that carbon support corrosion and further oxidation of nanoparticle catalysts are negligible during the electrocatalytic OER tests. Ni0.69Fe0.31Ox/C further demonstrates high stability as there is no apparent OER activity loss (based on a chronoamperometry test) or particle aggregation (based on TEM image observation) after a 6 h anodization test. The high efficiency and durability make these supported amorphous Ni−Fe nanoparticles potentially applicable in the (photo)electrochemical cells for water splitting to make H2 fuel or CO2 reduction to produce usable fuels and chemicals.

(Invited) Improving Charge Transport in TiO2 nanoparticle Based Hybrid Photoanodes through Spatial Control of Graphene Nanoribbon Assembly

Two-dimensional (2-D) graphene nanomaterials have spurred immense research interests ranging from water splitting solar cells to other energy storage devices because of their tantalizing morphological, nano-mechanical, and electrical properties. However, they tend to be delegated to secondary roles for establishing conductive pathways, as unwanted aggregation detrimentally decreases the fill factor (FF). In this study, we demonstrate the co-assembly of geometrically engineered graphene nanoribbons (GNRs) with TiO2 nanoparticles (T-GNR) at an optimized weight ratio effectively alleviates the aforementioned constraints while synergistically improving junctions (VOC ), electrical contact (JSC ) and FF in photoelectrochemical (PEC) cells. Uniform and controllable-sized GNRs help define the domain of TiO2 nanoparticles well below the upper limit of charge carrier transport length (300 nm), thus effectively eliminating unwanted grain boundaries that are known to act as electron traps. Tip sonication is used to systematically decrease the size of individual GNRs in order to determine a geometric conformation that favors charge transport around TiO2 nanoparticles. Finally, we present the increasing temperature dependence on photocurrent density based on the elimination of defect and surface oxidation from annealing. Through this study, we identified a large gain in photocurrent density of a 3-layered (3L) tip-sonicated T-GNR assembly at 200°C, 72.6% higher than pure TiO2 nanoparticles while simultaneously improving the junction properties and open circuit potential. It also introduced a FF of 66.6% as compared with a pure layer of TiO2 FF of 52.4%.

Photoelectrochemical water splitting promoted with a disordered surface layer created by electrochemical reduction.

The recent discovery of colored TiO2 indicated that the disordered surface layer over the TiO2 particles/photoelectrodes is beneficial for higher photocatalytic performance; however, the role of the disordered surface TiO2 layer is not well understood. Here, we report an electrochemical strategy for tuning the surface structure of TiO2 nanorod arrays (NRAs) and try to understand the role of the disordered surface TiO2 layer. Photoelectrodes of TiO2 NRAs with a disordered shell were prepared by an electrochemical reduction method. The photocurrent of the NRAs with a disordered shell can reach as high as ∼1.18 mA/cm(2) at 1.23 V, which is 2.2 times of that of the pristine TiO2 NRAs. Our results show that the surface disordered layer not only improves the bulk charge separation but also suppresses the charge recombination at the electrode/electrolyte interface, acting as an efficient water oxidation cocatalyst of photoelectrochemical cell for solar water splitting.

Fabrication of Hierarchical ZnO/Si Nanowire Structure for Photoelectrochemical Cells

We demonstrate the fabrication of three dimensional hierarchical ZnO/Si nanowire core-shell structure, which is used as a high surface area photoelectrode in photoelectrochemical cells for solar water splitting. This structure is fabricated via a two-step solution process: fabricating vertical aligned silicon nanowires via metal assisted etching method and then growing ZnO nanowires on the silicon nanowires via hydrothermal method. Transmission electron microscopy, scanning electron microscopy, spectrometer and X-ray diffractmeter are employed to characterize the hierarchical structure. The surface area of the hierarchical structure increases significantly compared to the simplex nanowire structure, which can substantially promote the photoelectrochemical reactions. The core-shell characteristic is beneficial to the charge carrier separation and the silicon nanowire backbones can work as the charge transfer channels, which contribute to the efficiency improvement. The experimental results indicate that the photocurrent density and the maximum conversion efficiency of the hierarchical structure photoelectrode have increased obviously, confirming that this hierarchical structure is more suitable for the application of photoelectrode than the ZnO nanowires/planar silicon structure.

Recent theoretical progress in the development of photoanode materials for solar water splitting photoelectrochemical cells

The search for earth-abundant materials that can be used in solar water splitting cells remains an important goal for affordable and environmentally friendly methods for energy conversion and storage.

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

Laboratory demonstrations of spontaneous photoelectrochemical solar water splitting cells are reviewed. Reported solar-to-hydrogen conversion efficiencies are as high as 18%. Reported operational lifetimes are relatively short, with few demonstrations exceeding one week.

Assessing the utility of bipolar membranes for use in photoelectrochemical water-splitting cells.

Membranes are important in water-splitting solar cells because they prevent crossover of hydrogen and oxygen. Here, bipolar membranes (BPMs) were tested as separators in water electrolysis cells. Steady-state membrane and solution resistances, electrode overpotentials, and pH gradients were measured at current densities relevant to solar photoelectrolysis. Under forward bias conditions, electrodialysis of phosphate buffer ions creates a pH gradient across a BPM. Under reverse bias, the BPM can maintain a constant buffer pH on both sides of the cell, but a large membrane potential develops. Thus, the BPM does not present a viable solution for electrolysis in buffered electrolytes. However, the membrane potential is minimized when the anode and cathode compartments of the cell contain strongly basic and acidic electrolytes, respectively.

2D ZnIn(2)S(4) nanosheet/1D TiO(2) nanorod heterostructure arrays for improved photoelectrochemical water splitting.

We report the fabrication of 2D ZnIn2S4 nanosheet/1D TiO2 nanorod heterojunction arrays by a facile hydrothermal process and their use as photoelectrodes in a photoelectrochemical (PEC) cell for high-performance solar water splitting. The morphology, microstructure, and phase of pristine TiO2 and 2D ZnIn2S4 nanosheet/1D TiO2 nanorod heterojunction arrays were characterized in detail. PEC measurements showed that 2D/1D heterojunction arrays offered enhanced photocurrent density (3 times higher than that of pristine TiO2), negatively shifted onset potential from 0.05 to -0.53 V, and high light on/off cycle stability. Electrochemical impedance investigation attested to a significant improvement of the interface electron transfer kinetics in this heterojunction, thus facilitating electron-hole separation, transfer, and collection, which resulted in enhanced PEC properties.

Keeping semiconductors safe from harm

Water Splitting Solar cells harvest the energy of sunlight to create electricity, but electricity is hard to store. Solar cells could also be used to make hydrogen from water, which can be stored as a fuel. Separating water into hydrogen and oxygen, however, presents challenges, especially if this is done directly by illuminating the anode that oxides water. Under the acidic or alkaline conditions needed for practical devices, semiconducting anode materials corrode during operation. Hu et al. now show that amorphous titanium dioxide coatings can protect semiconductors from alkaline corrosion while still allowing light through. Science , this issue p. [1005][1] [1]: /lookup/doi/10.1126/science.1251428

A Bismuth Vanadate–Cuprous Oxide Tandem Cell for Overall Solar Water Splitting

Through examination of the optoelectronic and photoelectrochemical properties of BiVO4 and Cu2O photoelectrodes, we evaluate the feasibility of a BiVO4/Cu2O photoanode/photocathode tandem cell for overall unassisted solar water splitting. Using state-of-the-art photoelectrodes we identify current-matching conditions by altering the photoanode active layer thickness. By further employing water oxidation and reduction catalysts (Co-Pi and RuOx, respectively) together with an operating point analysis, we show that an unassisted solar photocurrent density on the order of 1 mA cm(-2) is possible in a tandem cell and moreover gain insight into routes for improvement. Finally, we demonstrate the unassisted 2-electrode operation of the tandem cell. Photocurrents corresponding to ca. 0.5% solar-to-hydrogen conversion efficiency were found to decay over the course of minutes because of the detachment of the Co-Pi catalyst. This aspect provides a fundamental challenge to the stable operation of the tandem cell with the currently employed catalysts.

Hematite nanostructuring using electrohydrodynamic lithography

Tailoring hematite thin film nanostructure is particularly interesting since this oxide's function is closely related to its structure, for example when implemented as a photoanode in water splitting solar cells. In this study, electrohydrodynamic destabilization was designed to grow hematite nanodroplets with morphologies controlled by a master electrode. A polymer/iron salt film was destabilized by electrohydrodynamic destabilization and the resulting structures were pyrolysed to achieve crystalline α-Fe2O3 nanodroplets of 30 nm height and 70 nm radius. NEXAFS spectroscopy proved that the structures contain ferrihydrite, which is converted into hematite during pyrolysis, while the polymer was decomposed. Homogeneous nanoparticle precipitation in the bulk of the polymer, due to encapsulation of the iron precursor in the polymer matrix, is accounted for the good preservation of the structures. This study represents the first step towards the use of electrohydrodynamic destabilization for nanostructuring of hematite thin films, with a control over the feature size.

Quantitative Analysis and Visualized Evidence for High Charge Separation Efficiency in a Solid‐Liquid Bulk Heterojunction

In the past few decades, some novel low‐cost nanostructured devices have been explored for converting solar energy into electrical or chemical energy, such as organic photovoltaic cells, photoelectrochemical solar cells, and solar water splitting cells. Generally, higher light absorption and/or charge separation efficiency are considered as the main reasons for improved performance in a nanostructured device versus a planar structure. However, quantitative analysis and definite experimental evidence remain elusive. Here, using BiVO4 as an example, comparable samples with porous and dense structures have been prepared by a simple method. The porous and dense films are assembled into a solid‐electrolyte bulk and planar heterojunction, respectively. Some quantitative results are obtained by decoupling photon absorption, interfacial charge transfer, and charge separation processes. These results suggest that higher charge separation efficiency is mainly responsible for enhanced performance in a solid‐electrolyte bulk heterojunction. Moreover, we also present visualized evidence to show higher charge separation efficiency comes from a shorter photo‐generated hole diffusion distance in a bulk heterojunction. These results can deepen understanding charge transfer in a bulk heterojunction and offer guidance to design a more efficient low‐cost device for solar conversion and storage.

An Integrated 1-Dimensional Model of a Photoelectrochemical Cell for Water Splitting

A one-dimensional model of a photoelectrochemical cell for solar water splitting has been developed, with applicability to both “wired” and “wireless” designs. The model of the light absorber handles electron and hole transport. The model of the electrolyte accounts for mass transport through regions of aqueous solution, including stagnant diffusion layers and bulk regions to address mixing due to bubbles, natural convection, or other sources. A polymer membrane may be present in the electrolyte. The models of the light absorber and the electrolyte are integrated through the reactions taking place at the interface between them. Charge transfer from the semiconductor to the solution is handled using a kinetic model involving reactions between the species in both the light absorber and the electrolyte. A simplified model is also presented for use when concentration gradients in the electrolyte are negligible. The simplified model captures the effect of the electrolyte in the boundary conditions for the light absorber. The model is validated against current-potential data for a hydrogen-evolving light absorber with varying degrees of simulated solar illumination. The model then shows how overall solar-to-hydrogen yield depends on the efficiency of the light absorber and the areal fraction of absorbers in a membrane separator.

Boron-Doped Diamond (BDD) Coatings Protect Underlying Silicon in Aqueous Acidic Media–Application to the Hydrogen Evolution Reaction

Silicon has potential application as a functional semiconductor electrode in proposed solar water splitting cells. It is abundant and has excellent photovoltaic attributes, however it is extremely susceptible to corrosion, even in the dark, resulting in the formation of an electrochemically passive oxide upon interaction with aqueous media. This work investigates the potential for conductive, inert and transparent boron doped diamond (BDD) coatings to protect p-type Silicon (p-Si). The stability and electrochemical performance of p-Si and p-Si|BDD were investigated using voltammetric techniques in 1M H2SO4, before and after long-term exposure to the acidic medium (up to 280hours) under no applied potential bias. Unprotected Si degraded very rapidly whilst BDD was shown to protect the underlying Si, as evident from I-V curves that indicated no increased resistance across the Si-diamond interface. Furthermore, BDD supported facile proton reduction at significantly lower onset potential for the hydrogen evolution reaction (up to -500mV vs. SCE) compared with bare Si cathode (-850mV vs. SCE). The activity of the BDD electrode/electrolyte interface was further improved by coating with platinum catalyst particles, to produce a p-Si|BDD|Pt strucure, which reduced the HER onset to nearly zero overpotential. Tafel analysis indicated that desirable electrochemical activity and stability were achieved for p-Si|BDD|Pt, making this a promising electrode for application in water splitting cells.

Thermodynamics of native point defects in α-Fe2O3: an ab initio study

The native point defects in Fe2O3 are theoretically investigated using ab initio methods based on the GGA + U formalism. We consider vacancies and interstitials of Fe and O atoms as well as the electron polaron as Fe(II) defects at the host Fe(III) site. The formation energies and charge transition levels are computed for each defect type with careful elimination of size effects of the supercell. It is found that the Fe interstitial and vacancy form donor and acceptor levels close to band edges, respectively, thereby allowing for charge carriers at room temperature. We determine the oxygen deficiency under high-temperature equilibrium conditions and find an excellent agreement with experiment. In the quenched condition, it is found that the Fermi level is pinned at ~0.5 eV below the conduction band minimum, which may limit the performance of Fe2O3 as a photoanode in solar water-splitting cells. Furthermore, the oxygen vacancy is mostly neutral and the Fe interstitial is responsible for electron carriers.

Electrochemical Synthesis of p-Type CuFeO2 Electrodes for Use in a Photoelectrochemical Cell.

A new electrodeposition route was developed to prepare p-type CuFeO2 as a thin film-type electrode for use as a photocathode in a solar water splitting cell. The resulting p-CuFeO2 film has a bandgap energy of ca. 1.55 eV, with its conduction band edge located at a more negative potential than the reduction potential of water. Various photoelectrochemical properties of the p-CuFeO2 electrode were investigated, and its photoelectrochemical hydrogen production in 1 M NaOH solution was confirmed by gas chromatography. The incident photon-to-current conversion efficiency plot confirmed that p-CuFeO2 has the ability to utilize the entire range of visible light.

A perspective on solar-driven water splitting with all-oxide hetero-nanostructures

A perspective on the design of all-oxide heterostructures for application in photoelectrochemical cells for solar water splitting is provided. Particular attention is paid to those structures which possess nanoscale feature dimensionality, as structures of this type are most likely to utilize the benefits afforded by the formation of oxide heterojunctions and likely to show functional behavior relating to the interfacial region. In the context of this discussion, a novel hetero-nanostructure array, based on quantum-confined and visible light-active iron(III) oxide nanostructures and their surface modification with tungsten(VI) oxide, is introduced. The heterostructure architecture is designed to combine the functionality of the consituent phases to address the primary requirements for electrodes enabling the efficient generation of hydrogen using solar energy: visible light activity, chemical stability, appropriate bandedge characteristics, and potential for low-cost fabrication. Photoelectrochemical characterization for solar hydrogen/oxygen generation indicates the presence of unexpected minority carrier transfer dynamics within the oxide hetero-nanostructures, as observed additionally by ultrafast transient absorption spectroscopy.

Examining architectures of photoanode–photovoltaic tandem cells for solar water splitting

Given the limitations of the materials available for photoelectrochemical water splitting, a multiphoton (tandem) approach is required to convert solar energy into hydrogen efficiently and durably. Here we investigate a promising system consisting of a hematite photoanode in combination with dye-sensitized solar cells with newly developed organic dyes, such as the squaraine dye, which permit new configurations of this tandem system. Three configurations were investigated: two side-by-side dye cells behind a semitransparent hematite photoanode, two semitransparent dye sensitized solar cells (DSCs) in front of the hematite, and a trilevel hematite/DSC/DSC architecture. Based on the current-voltage curves of state-of-the-art devices made in our laboratories, we found the trilevel tandem architecture (hematite/SQ1 dye/N749 dye) produces the highest operating current density and thus the highest expected solar-to-hydrogen efficiency (1.36% compared with 1.16% with the standard back DSC case and 0.76% for the front DSC case). Further investigation into the wavelength-dependent quantum efficiency of each component revealed that in each case photons lost as a result of scattering and reflection reduce the performance from the expected 3.3% based on the nanostructured hematite photoanodes. We further suggest avenues for the improvement of each configuration from both the DSC and the photoanode parts.

Direct solar water splitting cell using water, WO3, Pt, and polymer electrolyte membrane

Abstract A solar water splitting cell composed of WO 3 , Polymer Electrolyte Membrane (PEM) and Pt was constructed for producing hydrogen from deionized water in sunlight. Spectral responsivity measurements under various temperatures and bias voltages were conducted for the cell using the Incident Photon to Current Efficiency (IPCE) method. For comparison, a known WO 3 Photo Electro Chemical (PEC) cell containing H 3 PO 4 electrolyte, WO 3 /H 3 PO 4 /Pt, was tested using the same test method. The WO 3 /PEM–H 2 O/Pt cell showed better Quantum Efficiency (QE) performance compared to that obtained from the cell with the chemical electrolyte. For the first time, spectral responsivity of photo water splitting process without bias power was unveiled in the new WO 3 cell, demonstrating the self-sustained photo electrolysis capability. Bias voltage effect on Solar to Hydrogen (STH) conversion efficiency was dramatic in the range from 0.2 V to 1.2 V and suppressions of STH were observed when high bias voltages were applied. In addition, a strong temperature effect on the energy conversion efficiency at high bias voltage was observed in the cell containing PEM–H 2 O, revealing that the STH at 54 °C is nearly five times that at 14 °C.