Tandem Photoelectrochemical Cell Research Articles

Photoelectrochemical CO2 reduction in tandem photoelectrode cells: Interpretation of apparent photocurrents

Photoelectrochemical reactors harness solar energy to accelerate heterogeneous chemical reactions and reduce power consumption for a given reaction rate. The tandem photoelectrochemical cell design presented here employs a light penetrable TiO2 photoanode||Cu2O photocathode configuration that enables carbon dioxide turnover at limited cell voltages. During polarization tests, the tandem configuration demonstrated a photocurrent density of 0.2 mA/cm2 at a cell voltage of -2.6 V. That is 2-fold higher than the photocurrent density obtained with a solo photoanode (Cu||TiO2) and five orders of magnitude higher compared to the solo photocathode (Cu2O||Ni) design. Moreover, electrolysis under illumination (photoelectrolysis) showed a current density increase of 3.9 mA/cm2 with respect to electrolysis in the dark. However, this photocurrent density was merely apparent and was caused by light-triggered changes in the surface morphology and phase of Cu2O, which in turn affected the catalytic response. Moreover, the apparent partial ethylene photocurrent density and total partial current density during photoelectrolysis was 44 %, the highest among the detected products. That selectivity is attributed to the combined effect of Cu2O photoreduction and the associated local pH rise at the electrode-electrolyte interface.

Solar to hydrogen conversion by a 25 cm2-photoelectrochemical cell with upscaled components

An innovative tandem photoelectrochemical cell with a scaled-up active area from 0.25 to 25 cm2 based on low-cost and non-critical raw materials was employed to produce green hydrogen by water splitting. It uses a photoanode/membrane/photocathode tandem cell configuration in which a hematite-based photoanode is layered on a fluorine-doped tin oxide glass for the oxygen evolution reaction, CuO is deposited on a hydrophobic gas diffusion layer as the photocathode for the hydrogen evolution reaction and an anion exchange membrane is used as the electrolyte and gas separator. The solid membrane's low hydrogen and oxygen crossover guarantees the operational stability of the tandem photoelectrochemical cell and the efficient separation of water-splitting products. Significant efforts have been focused on scaling up the cell. It has allowed obtaining, for the first time, a 25 cm2 unit cell prototype. Appropriate design approaches have been considered to optimise water/gas management, current collection, gas-tightness and clamping. The adopted architecture allowed for the reduction of the bias-potential from −1.23 V, generally employed to investigate PEC cells, up to −0.6/-0.4 V 20 h-durability tests demonstrated good resistance to corrosion, showing a constant photocurrent. Efficiency at −0.6 V was about 0.2%. Selective hydrogen production was demonstrated by mass spectrometric analysis.

Unassisted Solar Syngas Production by a Molecular Dye-Cobalt Catalyst Assembly in a Tandem Photoelectrochemical Cell

Unassisted Solar Syngas Production by a Molecular Dye-Cobalt Catalyst Assembly in a Tandem Photoelectrochemical Cell

Scalability and Investigation of the Geometrical Features and Shapes of a Tandem Photo-Electrolysis Cell Based on Non-Critical Raw Materials

Tandem photoelectrochemical cells (PECs) are devices useful for water splitting (WS) with the production of oxygen at the photoanode (PA) and hydrogen at the photocathode (PC) by adsorbing more than 75% of the solar irradiation; a portion of the UV/Vis direct solar irradiation is captured by the PA and a diffused or transmitted IR/Vis portion by the PC. Herein, Ti-doped hematite (PA) and CuO (PC) were employed as abundant and non-critical raw semiconductors characterised by proper band gap and band edge banding for the photoelectrochemical WS and absorption of sunlight. The investigation of inexpensive PEC was focused on the scalability of an active area from 0.25 cm2 to 40 cm2 with a rectangular or square shape. For the first time, this study introduces the novel concept of a glass electrode membrane assembly (GEMA), which was developed with an ionomeric glue to improve the interfacial contact between the membrane and photoelectrodes. On a large scale, the electron–hole recombination and the non-optimal photoelectrodes/electrolyte interface were optimized by inserting a glass support at the photocathode and drilled fluorine tin oxide (FTO) at the photoanode to ensure the flow of reagents and products. Rectangular 40 cm2 PEC showed a larger maximum enthalpy efficiency of 0.6% compared to the square PEC, which had a value of 0.37% at a low bias-assisted voltage (−0.6 V). Furthermore, throughput efficiency reached a maximum value of 1.2% and 0.8%, demonstrating either an important effect of the PEC geometries or a non-significant variation of the photocurrent within the scalability.

Dye-Sensitized Photocathodes Assembly and Tandem Photoelectrochemical Cells for CO2 Reduction

Dye-Sensitized Photocathodes Assembly and Tandem Photoelectrochemical Cells for CO₂ Reduction

Bias‐Free Solar‐to‐Hydrogen Conversion in a BiVO4/PM6:Y6 Compact Tandem with Optically Balanced Light Absorption

The high voltage required to overcome the thermodynamic threshold and the complicated kinetics of the water splitting reaction limit the efficiency of single semiconductor‐based photoelectrochemistry. A semiconductor/solar cell tandem structure has been theoretically demonstrated as a viable path to achieve an efficient direct transformation of sunlight into chemical energy. However, compact designs exhibiting the indispensable optimally balanced light absorption have not been demonstrated. In the current work, we design and implement a compact tandem providing the complementary absorption of a highly transparent BiVO4 photoanode and a PM6:Y6 solar cell. Such bandgap combination approaches the optimal to reach the solar‐to‐hydrogen (STH) conversion upper limit for tandem photoelectrochemical cells (PECs). We demonstrate that, by using a photonic multilayer structure to adequately balance sunlight absorption among both tandem materials, a 25% increase in the bias‐free STH conversion can be achieved, setting a clear path to take compact tandem PECs to the theoretical limit performance.

A Multi-Pyridine-Anchored and -Linked Bilayer Photocathode for Water Reduction.

The development of efficient photocathodes is of critical importance for the constructions of promising tandem photo-electrochemical cells. Most known dye-sensitized photocathodes are prepared with the conventional carboxylic or phosphonic acid anchors and require the presence of other terminal linking groups to connect catalysts; they suffer from high synthetic difficulty and low adsorption stability in aqueous media. Here, a compact bilayer photocathode has been prepared by using a pyrene-based photosensitizer with multiple terminal pyridine moieties as both the anchoring and linking groups to connect a Co hydrogen-evolution catalyst to the NiO substrate. The catalyst and dye molecule are assembled in a layer-by-layer manner on NiO through the metal-pyridine coordination. This photocathode exhibits good dye adsorption stability in aqueous media. A stable cathodic photocurrent of 70 μA cm-2 was achieved, with H2 being generated at the photocathode under the visible-light irradiation. The Faraday efficiency of H2 evolution was estimated to be 9.1 %. Transient absorption spectral studies suggest that the interfacial hole transfer occurs within a few picoseconds. The integration of the organic photosensitizer with pyridine anchoring and linking groups is expected to provide a simple method for the fabrication of stable and efficient photocathodes.

Development of BiOBr/TiO2 nanotubes electrode for conversion of nitrogen to ammonia in a tandem photoelectrochemical cell under visible light

Ammonia (NH3) is one of the important chemicals for human life. The demand for ammonia is expected to increase every year. Conventionally, the fixation process of N2 to produce NH3 in the industrial sector is carried out through the Haber−Bosch process, which requires extreme temperature and pressure conditions that consume a high amount of energy and emit a considerable amount of CO2. Therefore, it is necessary to develop alternative technology to produce ammonia using environmentally friendly methods. Many studies have developed the photo-electrochemical conversion of nitrogen to ammonia in the presence of semiconductor materials, but the resulting efficiency is still not as expected. In this research, the development of the tandem system of Dye-Sensitized Solar Cell - Photoelectrochemistry (DSSC - PEC) was carried out for the conversion of nitrogen to ammonia. The DSSC cell was prepared using N719/TiO2 nanotubes as photoanode, Pt/FTO as cathode, and electrolyte I-/I3-. The DSSC efficiency produced in this research was 1.49%. PEC cell at the cathode and anode were prepared using BiOBr/TiO2 nanotubes synthesized by the SILAR (Successive Ionic Layer Adsorption and Reaction) method. The resulting ammonia levels were analyzed using the phenate method. In this study, ammonia levels were obtained at 0.1272 µmol for 6 hours of irradiation with an SCC (Solar to Chemical Conversion) percentage of 0.0021%.

Combined metal ferrite oxide photoanodes and photocathodes for unassisted sunlight-driven tandem photoelectrochemical cells

Combined metal ferrite oxide photoanodes and photocathodes for unassisted sunlight-driven tandem photoelectrochemical cells

Bi2S3‐Cu3BiS3 Mixed Phase Interlayer for High‐Performance Cu3BiS3‐Photocathode for 2.33% Unassisted Solar Water Splitting Efficiency

To realize practical solar hydrogen production, a low-cost photocathode with high photocurrent density and onset potential should be developed. Herein, an efficient and stable overall photoelectrochemical tandem cell is developed with a Cu3 BiS3 -based photocathode. By exploiting the crystallographic similarities between Bi2 S3 and Cu3 BiS3 , a one-step solution process with two sulfur sources is used to prepare the Bi2 S3 -Cu3 BiS3 blended interlayer. The elongated Bi2 S3 -Cu3 BiS3 mixed-phase 1D nanorods atop a planar Cu3 BiS3 film enable a high photocurrent density of 7.8mA cm-2 at 0V versus the reversible hydrogen electrode, with an onset potential of 0.9 VRHE . The increased performance over the single-phase Cu3 BiS3 thin-film photocathode is attributed to the enhanced light scattering and charge collection through the unique 1D nanostructure, improved electrical conductivity, and better band alignment with the n-type CdS layer. A solar-to-hydrogen efficiency of 2.33% is achieved under unassisted conditions with a state-of-the-art Mo:BiVO4 photoanode, with excellent stability exceeding 21h.

Carbon-Protected BiVO4—Cu2O Thin Film Tandem Cell for Solar Water Splitting Applications

Carbon-protected BiVO4 photoanode and Cu2O photocathode tandem photoelectrochemical (PEC) system has been explored to reduce surface recombination and enhance the stability of the photoelectrodes. In addition to the carbon layer, the electrodeposited FeOOH nanolayer and drop-casted MoS2 co-catalyst layer on the photoanode and photocathode, respectively improve the reaction kinetics. The optimized photoanode (Mo-BiVO4/C/FeOOH) and photocathode (Cu2O/C/MoS2) produces current densities of ~1.22 mA cm−2 at 1.23 V vs. RHE and ~−1.48 mA cm−2 at 0 V vs. RHE, respectively. The obtained photocurrent is higher than bare photoelectrodes without a carbon layer. Finally, a tandem cell has been constructed, and an unassisted current density of ~0.107 mA cm−2 is obtained for a carbon-protected BiVO4–Cu2O tandem PEC cell at zero bias. The improved stability and enhanced photocurrent of the carbon protective layer are attributed to its better charge transfer resistance and minimized surface defects. Carbon protective layer can be a viable option to improve the stability of photoelectrodes in aqueous media.

A tandem photoelectrochemical cell based on Cu2-xTe nanocrystals for solar energy conversion to hydrogen

In this work we present the design, assembly and characterization of a tandem photoelectrochemical (PEC) cell based on two different crystallographic phases of sub-stoichiometric copper telluride nanocrystals (NCs). The first one, a pseudo-cubic phase, pc-Cu 2-x Te, is characterized by positive photocurrents, while the second one, a hexagonal phase, h-Cu 2-x Te, favors negative ones. Taking advantage of the different optoelectronic properties of the two Cu 2-x Te structures, we prepared a PEC cell composed of a hybrid pc-Cu 2-x Te/TiO 2 photoanode, with TiO 2 acting as a light absorber and electron selective layer, and a h-Cu 2-x Te/CuI photocathode, with CuI behaving as a photo-absorber and hole selective layer. The tandem PEC cell shows a photocurrent density of ∼0.5 mA/cm 2 when measured in a 2-electrode configuration without any co-catalyst. Finally, to test the PEC cell performance for the hydrogen evolution reaction (HER), a thin film of Pt was deposited on top of the photocathode and ∼7 μmol of hydrogen were obtained at 0.6 V in a 1-h experiment, reaching a photocurrent of 1 mA/cm 2 with no losses. • Investigating the optoelectronic properties of different Cu2-XTe crystallographic phases and using them as photoelectrodes for the first time. • Combining Cu2-XTe with TiO2 and CuI to build fully working hybrid photoelectrodes. • Building a functional Tandem PEC cell for hydrogen evolution reaction (HER).

An Organic Semiconductor Photoelectrochemical Tandem Cell for Solar Water Splitting

AbstractPhotoelectrochemical cells employing organic semiconductors (OS) are promising for solar‐to‐fuel conversion via water splitting. However, despite encouraging advances with the half reactions, complete overall water splitting remains a challenge. Herein, a robust organic photocathode operating in near‐neutral pH electrolyte by careful selections of a semiconducting polymer bulk heterojunction (BHJ) blend and organic charge‐selective layer is realized. The optimized photocathode produces a photocurrent density of >4 mA cm−2 at 0 V vs the reversible hydrogen electrode (VRHE) for solar water reduction with noticeable operational stability (retaining ≈90% of the initial performance over 6 h) at pH 9. Combining the optimized BHJ photocathode with a benchmark BHJ photoanode leads to the demonstration of a large‐area (2.4 cm2) organic photoelectrochemical tandem cell for complete solar water splitting, with a predicted solar‐to‐hydrogen (STH) conversion efficiency of 0.8%. Under unassisted two‐electrode operation (1 Sun illumination) a stabilized photocurrent of 0.6 mA and an STH of 0.3% are observed together with near unity Faradaic efficiency of H2 and O2 production.

Advanced Nanostructured Conjugated Microporous Polymer Application in a Tandem Photoelectrochemical Cell for Hydrogen Evolution Reaction.

Solar energy conversion through photoelectrochemical cells by organic semiconductors is a hot topic that continues to grow due to the promising optoelectronic properties of this class of materials. In this sense, conjugated polymers have raised the interest of researchers due to their interesting light-harvesting properties. Besides, their extended π-conjugation provides them with an excellent charge conduction along the whole structure. In particular, conjugated porous polymers (CPPs) exhibit an inherent porosity and three-dimensional structure, offering greater surface area, and higher photochemical and mechanical stability than their linear relatives (conjugated polymers, CPs). However, CPP synthesis generally provides large particle powders unsuitable for thin film preparation, limiting its application in optoelectronic devices. Here, a synthetic strategy is presented to prepare nanostructures of a CPP suitable to be used as photoelectrode in a photoelectrochemical (PEC) cell. In this way, electronic and photoelectrochemical properties are measured and, attending to the optoelectronic properties, two hybrid photoelectrodes (photoanode and photocathode) are designed and built to assemble a tandem PEC cell. The final device exhibits photocurrents of 0.5mA cm-2 at a 0.7V in the two electrode configuration and the hydrogen evolution reaction is observed and quantified by gas chromatography, achieving 581µmol of H2 in a one-hour reaction.

Efficient charge separation and transport in a tandem solar cell with photoconducting Se sub-microtubes and AgBiS2 quantum dots

This article describes a tandem photoelectrochemical cell that captures and converts visible to near infrared light to deliver a power conversion efficiency of ∼ 7%. In this tandem device the photocathode was prepared from p-type nickel oxide sensitized with silver bismuth sulfide (AgBiS2) quantum dots and was paired with a photoanode based on cadmium sulfide-sensitized titania. Trigonal-selenium sub-microtubes (t-Se s-μT) were anchored to the photoanode and served two roles: (1) enhancement of the conductivity under illumination (with holes as majority carriers); (2) co-sensitization provided more electron-hole pairs that increased the device performance by 1.5-fold. The separate p-type (AgBiS2/NiO−Ni) and n-type (TiO2/CdS/Se−Ni) solar cells were characterized as well as the (p/n) tandem cell architecture with a quasi-solid polysulfide/silica gel electrolyte under 1 sun illumination (AM 1.5G). The results provide scientific insights into the proposed charge flow mechanism via favourable energy level alignment in this unique device.

Chemically Stable Semitransparent Perovskite Solar Cells with High Hydrogen Generation Rates Based on Photovoltaic–Photoelectrochemical Tandem Cells

Photovoltaic (PV)‐assisted photoelectrochemical (PEC) tandem cells with elevated hydrogen (H2) production rates are a practical approach for carbon‐dioxide‐free, green H2 production. A semitransparent single‐cell‐based wide‐bandgap perovskite solar cell (PSC) coupled with an Si photocathode provides sufficient potential for H2 generation when combined with a sulfate oxidation reaction. While energetically favorable ZnO as an electron transport layer (ETL) increases the V OC to 1.19 V for mixed‐halide perovskite, phase decomposition is induced when Br ions contacted the ZnO ETL. The SnO2 interlayer shows improved passivation, superior operational stability, and excellent performance among the various atomic layer deposited metal oxides tested. Furthermore, the resulting semitransparent PSC demonstrates reproducibility of its enhanced PV parameters (i.e., V OC 1.17 ± 0.01 V, FF = 76.78 ± 1.39%, and PCE = 11.95 ± 1.13%) due to better interface quality. The precise calculation of light absorption from both PV and Si for the overall tandem device leads to optimized light harvesting in the top and bottom electrodes, maximizing H2 production. Overall, the PV‐PEC device incorporated with a chemically stable semitransparent top PSC and bottom Si photocathode allows to accomplish stable H2 production at 11.1 mA cm−2 under unbiased conditions.

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.