Tandem Photoelectrochemical Cell Research Articles

Reduced synthesis temperatures of SrNbO2N perovskite films for photoelectrochemical fuel production

Perovskite oxynitrides, such as SrNbO2N, have been shown to be promising materials for photoanodes in tandem photoelectrochemical cells, due to their suitable bandgap and charge transport. However, thin film synthesis and characterization of oxynitride perovskites are challenging due to high processing temperatures that are incompatible with available substrates. In this work, we report on reduced synthesis temperatures of SrNbO2N perovskite thin films on Si substrates across a range of chemical compositions. Polycrystalline thin films with perovskite crystal structure are obtained by sputtering at ambient temperature and annealing at 550–600 °C. The perovskite structure has a relatively broad range of cation composition between 50 and 60% Sr with varying O/N ratio according to Rutherford backscattering spectrometry. The maximum photocurrent density was obtained at 55 cation % of Sr, which is slightly Sr-rich compared to the nominal SrNbO2N stoichiometry. This work shows the importance of considering cation and anion composition in studying oxynitride perovskites for solar fuel applications.

Progress in Ternary Metal Oxides as Photocathodes for Water Splitting Cells: Optimization Strategies

The conversion of solar energy into fuels and, specifically hydrogen, is a critical process for ensuring the sustainability of the future energy system. Among the different ways of converting solar energy into fuels and chemicals, photoelectrochemical tandem cells, containing two photoactive materials, stand out as they have the potential for direct solar‐to‐fuel conversion, thus minimizing cost. The implementation of photoelectrolysis for hydrogen generation is hindered by the lack of suitable electrode materials, particularly photocathodes. Among the candidates for photocathodes, ternary (and multinary) transition metal oxides have the advantage of lower cost and, potentially, higher stability than other metal compounds. Herein, the aim is to provide an overview of the current state of the art for ternary oxides (spinels, delafossites, perovskites, etc.), with a focus on the modification strategies that can optimize their behavior and applicability. Both copper‐based and iron‐based ternary oxides are the most studied and, currently, the most promising. Among the strategies being used for their optimization, doping, the deposition of underlayers and overlayers, and the use of cocatalysts are the most popular. However, it is apparent that both solar‐to‐hydrogen efficiencies and stability will need to be addressed in the future. Some guidelines in this respect are also provided.

Preparation of CuBi2O4 photocathodes for overall water splitting under visible light irradiation

In this study, the CuBi2O4 films were prepared by a facile drop-casting method for visible-light-driven water splitting, while the effect of film thickness and annealing temperature on the actual photoelectrochemical (PEC) performance were investigated. The annealing temperature was optimized as 550 °C. With increasing the layer number, the PEC performance was firstly increased and then decreased, reaching a maximum photocurrent density of 1.1 mA/cm2 at a layer number of 3 under a bias of 0 V vs. reverse hydrogen electrode (RHE). Besides, both the electrochemical impedance and photoluminescence (PL) spectra were collected to reveal the mechanism underlying the performance changes. It was found that the enhanced PEC performance was attributed to high specific surface area and broad spectral response of CuBi2O4 photocathodes. Moreover, a tandem PEC cell was constructed by combining CuBi2O4 with a ZnIn2S4 photoanode for unbiased solar water splitting. The photocurrent density at the joint point was 0.13 mA/cm2, corresponding to an unbiased solar water splitting efficiency of 0.056%. This work shows that CuBi2O4 has a good potential as a photocathode for solar-driven overall water splitting.

Efficient suppression of surface charge recombination by CoP-Modified nanoporous BiVO4 for photoelectrochemical water splitting

BiVO4 is a promising photoanode material for water splitting due to its substantial absorption of solar light as well as favorable band edge positions. However, the poor water oxidation kinetics of BiVO4 results in its insufficient photocurrent density. Herein, we demonstrate the use of CoP nanoparticles for facile surface modification of nanoporous BiVO4 photoanode in potassium borate buffer solution (pH 9.0), which can generate a tremendous cathodic shift of ~430 mV in the onset potential for photoelectrochemical water oxidation. In addition, a remarkable photocurrent density of 4.1 mA cm−2 is achieved at 1.23 V vs. RHE under AM 1.5G illumination. The photoelectrochemical measurement using sodium sulfite as a hole scavenger clearly shows that the greatly improved performances are attributed to the efficient suppression of interfacial charge recombination through loading of CoP catalyst. Moreover, the maximum surface charge injection yield can reach >81% at 1.23 V vs. RHE and the maximum IPCE of CoP/BiVO4 can reach 75.8% at 420 nm, suggesting the potential application of CoP-modified BiVO4 photoanode for overall solar water splitting in cost-effective tandem photoelectrochemical cells.

A promising ternary sulfide bidirectional p-n heterojunction for unassisted tandem photoelectrochemical cells.

There has been great demand for high-efficiency, low-cost, and abundant photoelectrode materials for photoelectrochemical (PEC) systems. Here, we studied the PEC performance of ternary sulfide photoelectrodes (ZnIn2S4 and CuInS2) with the same nanostructure and applied bidirectional p-n heterojunctions with energy levels that were well matched to the unassisted tandem PEC cell device. Moreover, ZnIn2S4/CuInS2 and CuInS2/ZnIn2S4 were used as a photoanode and photocathode, respectively, in the unassisted tandem PEC cell device, which achieved a relatively high photocurrent density of 0.06 mA cm-2. This work provides new ideas for the design and manufacture of high-efficiency unassisted tandem PEC cell devices.

Pure organic quinacridone dyes as dual sensitizers in tandem photoelectrochemical cells for unassisted total water splitting.

Pure organic dye QAP-C8 based on quinacridone (QA) with octyl side chains as the donor and pyridine dicarboxylic acid (PDA) as the acceptor was first used in both the photoanode and the photocathode of photoelectrochemical cells. A tandem device with QAP-C8 as the photosensitizer realized overall water splitting and showed a STH of 0.11% under neutral pH conditions without an external bias.

An Unassisted Tandem Photoelectrochemical Cell Based on p- and n-Cu2O Photoelectrodes

By investigating photoelectrochemical (PEC) performance of p and n-type Cu2O, we do an exploratory study on dual-photoelectrode tandem PEC cell consisting of p-Cu2O/Pt photocathode and n-Cu2O/Co–Pi photoanode for the first time. Benefiting from electron capture (Pt) and hole transfer (Co–Pi) layers, the photoelectrodes have better charge separation efficiency and faster charge transfer kinetics. The p-Cu2O/Pt||n-Cu2O/Co–Pi PEC cell finally achieves an unassisted photocurrent density of 0.09 mA/cm2 after illumination for 1000 s, which preliminarily confirms the viewpoint of unassisted overall water splitting. We then anticipate this work can provide a novel platform for the design and fabrication of various PEC energy conversion devices with excellent performance.

Dry Hydrogen Production in a Tandem Critical Raw Material-Free Water Photoelectrolysis Cell Using a Hydrophobic Gas-Diffusion Backing Layer

A photoelectrochemical tandem cell (PEC) based on a cathodic hydrophobic gas-diffusion backing layer was developed to produce dry hydrogen from solar driven water splitting. The cell consisted of low cost and non-critical raw materials (CRMs). A relatively high-energy gap (2.1 eV) hematite-based photoanode and a low energy gap (1.2 eV) cupric oxide photocathode were deposited on a fluorine-doped tin oxide glass (FTO) and a hydrophobic carbonaceous substrate, respectively. The cell was illuminated from the anode. The electrolyte separator consisted of a transparent hydrophilic anionic solid polymer membrane allowing higher wavelengths not absorbed by the photoanode to be transmitted to the photocathode. To enhance the oxygen evolution rate, a NiFeOX surface promoter was deposited on the anodic semiconductor surface. To investigate the role of the cathodic backing layer, waterproofing and electrical conductivity properties were studied. Two different porous carbonaceous gas diffusion layers were tested (Spectracarb® and Sigracet®). These were also subjected to additional hydrophobisation procedures. The Sigracet 35BC® showed appropriate ex-situ properties for various wettability grades and it was selected as a cathodic substrate for the PEC. The enthalpic and throughput efficiency characteristics were determined, and the results compared to a conventional FTO glass-based cathode substrate. A throughput efficiency of 2% was achieved for the cell based on the hydrophobic backing layer, under a voltage bias of about 0.6 V, compared to 1% for the conventional cell. For the best configuration, an endurance test was carried out under operative conditions. The cells were electrochemically characterised by linear polarisation tests and impedance spectroscopy measurements. X-Ray Diffraction (XRD) patterns and Scanning Electron Microscopy (SEM) micrographs were analysed to assess the structure and morphology of the investigated materials.

Probing the dye-semiconductor interface in dye-sensitized NiO solar cells.

The development of p-type dye-sensitized solar cells (p-DSSCs) offers an opportunity to assemble tandem photoelectrochemical solar cells with higher efficiencies than TiO2-based photoanodes, pioneered by O'Regan and Grätzel [Nature 353, 737-740 (1991)]. This paper describes an investigation into the behavior at the interfaces in p-DSSCs, using a series of BODIPY dyes, BOD1-3. The three dyes have different structural and electronic properties, which lead to different performances in p-DSSCs. We have applied photoelectron spectroscopy and transient absorption spectroscopy to rationalize these differences. The results show that the electronic orbitals of the dyes are appropriately aligned with the valence band of the NiO semiconductor to promote light-induced charge transfer, but charge-recombination is too fast for efficient dye regeneration by the electrolyte. We attribute this fast recombination, which limits the efficiency of the solar cells, to the electronic structure of the dye and the presence of Ni3+ recombination sites at the NiO surface.

Decoupling Strategy for Enhanced Syngas Generation from Photoelectrochemical CO2 Reduction.

SummaryPhotoelectrochemical CO2 reduction into syngas (a mixture of CO and H2) provides a promising route to mitigate greenhouse gas emissions and store intermittent solar energy into value-added chemicals. Design of photoelectrode with high energy conversion efficiency and controllable syngas composition is of central importance but remains challenging. Herein, we report a decoupling strategy using dual cocatalysts to tackle the challenge based on joint computational and experimental investigations. Density functional theory calculations indicate the optimization of syngas generation using a combination of fundamentally distinctive catalytic sites. Experimentally, by integrating spatially separated dual cocatalysts of a CO-generating catalyst and a H2-generating catalyst with GaN nanowires on planar Si photocathode, we report a record high applied bias photon-to-current efficiency of 1.88% and controllable syngas products with tunable CO/H2 ratios (0–10) under one-sun illumination. Moreover, unassisted solar CO2 reduction with a solar-to-syngas efficiency of 0.63% is demonstrated in a tandem photoelectrochemical cell.

Enhanced Photoelectrochemical Water Oxidation from CdTe Photoanodes Annealed with CdCl2

AbstractMost CdTe photoanodes and photocathodes show positive and negative photocurrent onset potentials for water oxidation and reduction, respectively, and are thus unable to drive photoelectrochemical (PEC) water splitting without external applied biases. Herein, the activity of a CdTe photoanode having an internal p‐n junction during PEC water oxidation was enhanced by applying a CdCl2 annealing treatment together with surface modifications. The resulting CdTe photoanode generated photocurrents of 1.8 and 5.4 mA cm−2 at 0.6 and 1.2 VRHE, respectively, with a photoanodic current onset potential of 0.22 VRHE under simulated sunlight (AM 1.5G). The CdCl2 annealing increased the grain sizes and lowered the density of grain boundaries, allowing more efficient charge separation. Consequently, a two‐electrode tandem PEC cell comprising a CdTe‐based photoanode and photocathode split water without any external bias at a solar‐to‐hydrogen conversion efficiency of 0.51 % at the beginning of the reaction.

Enhanced Photoelectrochemical Water Oxidation from CdTe Photoanodes Annealed with CdCl2

CdTe absorbs sunlight up to 830 nm and has the potential to promote efficient photoelectrochemical (PEC) water splitting. However, most CdTe photoanodes and CdTe photocathodes show positive and negative photocurrent onset potentials for water oxidation and reduction, respectively, and are thus unable to drive PEC water splitting without external applied biases. In this work, we enhanced the activity of a CdTe photoanode having an internal p-n junction during PEC water oxidation by applying a CdCl 2 annealing treatment together with surface modifications. The resulting CdTe photoanode generated photocurrents of 1.8 and 5.4 mA cm -2 at 0.6 and 1.2 V RHE , respectively, with a photoanodic current onset potential of 0.22 V RHE under simulated sunlight (AM 1.5G). The CdCl 2 annealing increased the grain sizes in this material and lowered the density of grain boundaries, allowing for more efficient charge separation. Consequently, a two-electrode tandem PEC cell comprising a CdTe-based photoanode and photocathode split water without any external bias at a solar-to-hydrogen conversion efficiency of 0.51% at the beginning of the reaction.

Enhanced Photoelectrochemical Water Splitting at Hematite Photoanodes by Effect of a NiFe-Oxide co-Catalyst

Tandem photoelectrochemical cells (PECs), made up of a solid electrolyte membrane between two low-cost photoelectrodes, were investigated to produce “green” hydrogen by exploiting renewable solar energy. The assembly of the PEC consisted of an anionic solid polymer electrolyte membrane (gas separator) clamped between an n-type Fe2O3 photoanode and a p-type CuO photocathode. The semiconductors were deposited on fluorine-doped tin oxide (FTO) transparent substrates and the cell was investigated with the hematite surface directly exposed to a solar simulator. Ionomer dispersions obtained from the dissolution of commercial polymers in the appropriate solvents were employed as an ionic interface with the photoelectrodes. Thus, the overall photoelectrochemical water splitting occurred in two membrane-separated compartments, i.e., the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. A cost-effective NiFeOx co-catalyst was deposited on the hematite photoanode surface and investigated as a surface catalytic enhancer in order to improve the OER kinetics, this reaction being the rate-determining step of the entire process. The co-catalyst was compared with other well-known OER electrocatalysts such as La0.6Sr0.4Fe0.8CoO3 (LSFCO) perovskite and IrRuOx. The Ni-Fe oxide was the most promising co-catalyst for the oxygen evolution in the anionic environment in terms of an enhanced PEC photocurrent and efficiency. The materials were physico-chemically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Reversible electron storage in tandem photoelectrochemical cell for light driven unassisted overall water splitting

Photoelectrochemical (PEC) cell, representing as one of the most promising candidates to implement solar-driven unassisted overall water splitting, still remains a bottleneck in the construction of technology strategies for efficient solar energy utilization due to the intermittent nature of sunlight. Herein, we demonstrate an approach to realize round-the-clock hydrogen production by a solar rechargeable tandem PEC cell with two photoactive electrodes, where the photoanode is composed of a pseudocapacitive WO3 nanoflakes film sensitized with visible-light responsive zeolitic-imidazolate-framework-67 (ZIF-67), and the photocathode is constituted with a Z-scheme BiVO4-black phosphorus (BP) heterojunctions that can broaden the light absorption to near-infrared (NIR) region as well as maintain the strong redox ability to fasten hydrogen evolution reactions by the effective charge separation. Deliberately, the alignment of Fermi level between the as-prepared WO3-ZIF-67 photoanode and BiVO4-BP photocathode permits us to realize a tandem PEC cell with reversible electron storage property, enabling light-induced charge storing and on-demand release in the dark, for the application of unassisted overall water splitting. The formed tandem PEC cell shows a promising strategy for the conversion of solar power into hydrogen fuel by integrating pseudocapacitive materials into man-made photovoltaic cells, and provides a new guidance for the design of multi-functional PEC device.

An integrated thermoelectric-assisted photoelectrochemical system to boost water splitting

Common solar-driven photoelectrochemical (PEC) cells for water splitting were designed by using semiconducting photoactive materials as working photoelectrodes to capture sunlight. Due to the thermodynamic requirement of 1.23 eV and kinetic energy loss of about 0.6 eV, a photo-voltage of 1.8 V produced by PEC cells is generally required for spontaneous water splitting. Therefore, the minimum bandgap of 1.8 eV is demanded for photoactive materials in single-photoelectrode PEC cells, and the bandgap of about 1 eV for back photoactive materials is appropriate in tandem PEC cells. All these PEC cells cannot effectively utilize the infrared light from 1250 to 2500 nm. In order to realize the full spectrum utilization of solar light, here, we develop a solar-driven PEC water splitting system integrated with a thermoelectric device. The key feature of this system is that the thermoelectric device produces a voltage as an additional bias for the PEC system by using the temperature difference between the incident infrared-light heated aqueous electrolyte in the PEC cell as the hot source and unirradiated external water as the cold source. Compared to a reference PEC system without the thermoelectric device, this system has a significantly improved overall water splitting activity of 1.6 times and may provide a strategy for accelerating the application of full spectrum solar light-driven PEC cells for hydrogen production.

Hierarchically SrTiO3@TiO2@Fe2O3 nanorod heterostructures for enhanced photoelectrochemical water splitting

In this particular work, the fabrication of SrTiO3@TiO2@ Fe2O3 nanorod heterostructure has been demonstrated via hydrothermal growth of SrTiO3 cubic on the rutile TiO2 nanorod as a template and later sensitized with Fe2O3 for photocatalytic solar hydrogen production in a tandem photoelectrochemical cell and dye-sensitized solar cell (DSSC) module. The photocatalytic solar hydrogen production of this heterostructure was optimized by controlling the amount of Sr and Fe on the surface of photocatalyst. The details of the influencing parameters on the physicochemical and photoelectrochemical properties are discussed. It was found that the morphology and quality of the fabricated materials were greatly manipulated by the concentration of Sr and Fe. The optimized 0.025 M SrTiO3@TiO2@ Fe2O3 heterostructure exhibited a higher photoconversion efficiency with a long electron lifetime, low charge transfer resistance and large donor density at the electrode and electrolyte interface. This composite has significantly improved the photocatalytic hydrogen production, yielding 716 μmol/cm2 of maximum accumulative hydrogen. These results show that morphology rendering and manipulation of energy band alignment is crucial in creating efficient heterojunctions for excellent contributions in photocatalytic applications.

Assessment of a W:BiVO4-CuBi2O4Tandem Photoelectrochemical Cell for Overall Solar Water Splitting.

We assess a tandem photoelectrochemical cell consisting of a W:BiVO4 photoanode top absorber and a CuBi2O4 photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO4 photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi2O4 photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuOx is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi2O4 rather than proton reduction. Protecting the CuBi2O4 photocathode with a CdS/TiO2 heterojunction and adding RuOx as a cocatalyst prevents photocorrosion and allows for photoelectrochemical production of hydrogen at potentials ≤0.3 V vs RHE. A tandem photoelectrochemical cell composed of a W:BiVO4/CoPi photoanode and a CuBi2O4/CdS/TiO2/RuOx photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO4 and conduction band of CuBi2O4 are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes, that is, the relatively low quasi-Fermi level splitting within BiVO4 and CuBi2O4. This work is the first experimental demonstration of hydrogen production from a BiVO4-CuBi2O4-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi2O4 photocathodes.

Semi-transparent quaternary oxynitride photoanodes on GaN underlayers.

Conformal atomic layer deposition (ALD) technique is employed to make semi-transparent TaOxNy, providing the possibility to build semi-transparent oxy(nitride) heterojunction photoanodes on conductive substrates. A generalized approach was developed to manufacture semi-transparent quaternary metal oxynitrides on conductive substrates beyond semi-transparent binary Ta3N5 photoanodes aiming for wireless tandem photoelectrochemical (PEC) cells.

How photocorrosion can trick you: a detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production.

The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li0.03Cu0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of LixCu1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu2O grows as distinct crystallites on the surface of LixCu1-xO instead of forming a dense layer. Additionally, there is no obvious Cu2O gradient inside the films, as Cu2O seems to form on all LixCu1-xO nanocrystals exposed to water. The application of a thin Ti0.8Nb0.2Ox coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of LixCu1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes.

Unassisted solar water splitting using a Cu2O/Ni(OH)2-ZnO/Au tandem photoelectrochemical cell

A tandem photoelectrochemical (PEC) cell consisting of a Cu2O/Ni(OH)2 photocathode and a ZnO/Au photoanode was investigated for unassisted solar water splitting. It was found that the two photoelectrodes were optically matched in UV and visible range, according to the absorbed luminous flux. Compared with the bare Cu2O photocathode, the Cu2O/Ni(OH)2 photocathode had larger photocurrent and smaller onset potential, because the Ni(OH)2 electrocatalysts reduced charge recombination and the overpotential for hydrogen evolution. The tandem PEC cell achieved a photoconversion efficiency of about 0.20% under 1 sun illumination. The results indicate that metal oxide semiconductors synthesized via facile and scalable solution methods are promising photoelectrode materials for unassisted solar water splitting.