Photoelectrochemical Water Splitting Cells Research Articles

Surface-Attached Molecular Catalysts on Visible-Light-Absorbing Semiconductors: Opportunities and Challenges for a Stable Hybrid Water-Splitting Photoanode

Hybrid photoanodes for water-splitting photoelectrochemical cells (WS-PECs) consist of a visible-light-absorbing semiconductor (VLASC) with a molecular water-oxidation catalyst (mWOC) attached and ...

Cyclic voltammetry - A promising approach towards improving photoelectrochemical activity of hematite

Understanding the effect of electrochemical reaction on a particular material at the nanoscale level is essential for the application of solar conversion technology. Herein, electrochemical cyclic voltammetry (CV) treatment was applied on the α-Fe2O3 thin films by varying the number of cycles between 0 and 100. The thorough investigation provides insight into the modifications on the properties of the CV-treated films. The performance of pristine and CV-treated α-Fe2O3 as photoanodes are evaluated in the photoelectrochemical (PEC) water splitting cell. Consecutive CV scans up to the optimum number of cycles (50 cycles) promote the photocurrent density, where the photocurrent value at 0.6 VAg/AgCl becomes about 1.5 times higher than the pristine α-Fe2O3. This remarkable enhancement in PEC performance is due to the synergistic effect between the change of crystal orientation, the enhancement of surface oxygen vacancies and the increase of free charge carrier density. Our findings provide valuable information of possible pathway for improving the photoelectrochemical performance of a photoanode.

Enhanced visible light absorption of TiO2 nanorod photoanode by NiTiO3 decoration for high-performance photoelectrochemical cells

TiO2 nanorod (NR) and NiTiO3 (NTO) enhanced TiO2 NR photoanodes were studied for visible light driven water splitting in photoelectrochemical cells (PECs). The TiO2 and NTO/TiO2 growth on the FTO substrates were synthesised via the hydrothermal method. The Ni2+ content was the control factor to fabricate various different NTO/TiO2 heterostructures. The structure and surface morphology of NTO formation on TiO2 surfaces were investigated via SEM and TEM. Chemical absorption species and chemical bonding at interfaces were investigated by EDS and XPS spectroscopy. The optical absorption of the materials was studied by UV–visible and PL spectroscopy. Details on the influencing parameters on the electrochemical and photoelectrochemical properties were discussed. In this study, the 0.25NTO/TiO2 photoanode exhibited the best performance. This was because the electron lifetime, charge transfer resistance, donor density, and band gap improved.

Electrochemical Oxidation of Metal-Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of Fe2TiO5 Photoanodes.

A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Because catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type Fe2TiO5 photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water-splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in Fe2TiO5. Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe2TiO5 has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of Fe2TiO5. The combined experimental and theoretical results presented in this study greatly improve our understanding of Fe2TiO5 for use as a photoanode.

Advanced Semiconductor-Electrocatalyst Systems for Photoelectrochemical Hydrogen and Oxygen Production in Microgravity Environment

Efficient artificial photosynthesis systems are currently realized as catalyst- and surface-functionalized photovoltaic tandem and triple junction devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible.We recently demonstrated in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. Current limiting factors such as the absence of macroconvection processes were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography (SNL), generating so-called catalytic ‘hot-spots’ on the electrode surface which prevent gas bubble coalescence [3,4]. We found that the J-V characteristics of the half-cell and the overall device efficiency in microgravity environment are significantly affected by alterations in the electrocatalyst nanotopography [5]. By varying the shape and distance of catalytic ‘hot-spots’ on the electrode surface, we could control the gas bubble radius upon detachment from the electrode surface and the light absorption properties of the semiconductor in free fall. Shadow nanosphere lithography can therefore be used as a prosperous tool to develop custom-tailored electrocatalyst nanostructures of high fidelity on a light-absorbing semiconductor surface for an optimized device performance in microgravity and terrestrial applications.[1] Young J. L., Steiner M. A., Döscher H., France R. M., Turner J. A., Deutsch T. G. (2017). “Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures”, Nat. Energ. 2. (17028).[2] Cheng W. H., Richter M. H., May M. M., Ohlmann J., Lackner D., Dimroth F., Hannappel T., Atwater H. A., Lewerenz H. J (2018). “Monolithic Photoelectrochemical Device for 19% Direct Water Splitting”, ACS Energy Lett. 3, 8, 1795-1800.[3] Brinkert K., Richter M. H., Akay Ö., Liedtke J., Gierisig M., Fountaine K. T., Lewerenz H. J. (2018). Efficient Solar Hydrogen Production in Microgravity Environment. Nat. Commun. 9 (2527).[4] Patoka P., Giersig M. (2011). “Self-assembly of latex particles for the creation of nanostructures with tunable plasmonic properties”, J. Mater. Chem. 21, 16783-16796.[5] Brinkert K., Richter M. H., Akay Ö., Giersig M., Fountaine K. T., Lewerenz H.-J. (2018). “Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography”, Faraday Discuss. 208, 523-535.

Cu2O/InGaN heterojunction thin films with enhanced photoelectrochemical activity for solar water splitting

We study Cu2O/InGaN heterojunction thin films with different thicknesses of Cu2O layer as a photoanode in photoelectrochemical (PEC) water splitting cell. Results show that the bandgap energy of Cu2O/InGaN heterojunction thin films is 2.60–2.72 eV and, according to Vegard’s law, the indium content of the InGaN thin film is 22%. Electrochemical impedance spectroscopy shows the charge-transfer resistance value of about 0.4 kΩ for the optimized sample revealing enhanced charge separation and transfer at the interface. A maximum photocurrent density of 0.16 mA cm−2 at 0.5 V vs. Ag/AgCl was obtained for the Cu2O/InGaN heterojunction thin films with an overall thickness of 250 nm. The obtained value is 4.2 and 3.2 times higher than that of pure InGaN and Cu2O thin films photoanodes, respectively. We showed charge separation mechanism in the Cu2O/InGaN heterojunction photoelectrodes. According to our model, gradient energy bandgap reduce the recombination rate of photo-induced electron-hole pairs, and significantly enhance the PEC performance.

Establishing best practices to model the electronic structure of CuFeO2 from first principles

The cuprous delafossite, ${\mathrm{CuFeO}}_{2}$, has received significant attention in recent years as a potential photocathode material in photoelectrochemical water-splitting cells. Presented herein is an investigation of the electronic structure of ${\mathrm{CuFeO}}_{2}$ in the framework of density functional theory. We have benchmarked three of the most popular formulations for the treatment of the electron exchange and correlation interactions, highlighting their strengths and weaknesses in predicting electronic structures compatible with the available spectroscopic measurements. Although some features are correctly reproduced by the simplest approach, which is based on the generalized gradient approximation, this fails in describing the fundamental semiconducting character of the material. The introduction of the fully self-consistent Hubbard $U$ correction in the exchange correlation functional accounts explicitly for the on-site Coulomb interaction among localized $d$ electrons, thereby opening a gap in the band structure. However, our results indicate that the $U$ correction disrupts the crystal-field splitting of the ${t}_{2g}$ and ${e}_{g}$ states, resulting in an inaccurate description of the conduction-band edge. We provide a qualitative and quantitative analysis to explain why the ${t}_{2g}$ and ${e}_{g}$ states behave differently when the Hubbard correction is switched on. We find that best practice for accurate, yet computationally viable, simulations of CFO makes use of hybrid functionals, where the fraction of exact exchange is not arbitrarily selected but tuned according to the static dielectric constant of the material. In this case, theoretical predictions are found to be in excellent agreement with experimental results.

Hydrogen plasma-treated 1D/3D TiO2 nanorod array photoanode for efficient photoelectrochemical water splitting

The rutile phase titanium oxide (TiO2) based photoanode exhibits less energy conversion efficiency, as compared to anatase phase TiO2 photoanode. Enhancing the performance of the rutile phase TiO2 photoanode in a photoelectrochemical cell for water splitting is a significant challenge. In this study, we have grown the rutile phase 1D/3D nanorod array structure by using a one-step hydrothermal process. To enhance the surface properties of 1D/3D TiO2 nanorod array films, the as-grown films were treated in a hydrogen plasma atmosphere created using an electron cyclotron resonance (ECR) plasma system. The crystalline phases of the untreated and Hydrogen plasma treated films are confirmed by XRD and Raman studies. We found that, hydrogen plasma treatment creates Ti3+ sites and oxygen vacancies, leading to an increment in optical absorbance and reducing the band gap from 3.17 to 2.89 eV. Additionally, a significant enhancement in photocurrent density (0.369 mAcm−2 at 1 V versus Ag/AgCl) was observed in hydrogen plasma-treated 1D/3D TiO2 nanorod array photoanodes compared to untreated films (0.034 mAcm−2 at 1 V). The enhancement is attributed to the increase in carrier concentration induced by oxygen vacancy formation and the improved electron-hole pair separation. Further, the hydrogen plasma-treated 1D/3D TiO2 nanorod array photoanode exhibited high Photostability for long time duration.

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.

Unusual Photoactive Water Oxidation Activity of Pt/PtOx Cocatalyst Decorated Crystalline α‐Fe2O3 Nanostructures: Exposed Facets Dependent Reactivity

AbstractA key challenge remains the preparation of an active and robust water oxidation photocatalyst with abundant earth materials. As a useful to improve photocatalytic efficiency, the development of α‐Fe2O3 nanocrystals with different exposed facets and interfaces has attracted enormous attention. But literature survey shows that α‐Fe2O3 nanomaterials were mainly used in photoelectrochemical cell for water splitting as anode materials. In this work, highly efficient Pt/PtOx cocatalyst decorated α‐Fe2O3 shaped‐photocatalysts were derived as photocatalyst for white light‐induced O2 generation from water without any electric potential. The maximum apparent quantum efficiency of photooxidation reaction was reached 55.33 % by Pt/PtOx decorated bitruncated‐dodecahedron shaped α‐Fe2O3 and around 30.5, 2.3, 1.28 times better than the Pt/PtOx decorated nanocuboid, nanorod and bitruncated‐octahedron shape respectively. The Pt/PtOx decorated bitruncated‐dodecahedron shaped α‐Fe2O3 display an unusual photooxidation activity with a high TOF (1.96×10−2 s−1) due to presence of destabilized and unsaturated Fe centre containing high energy {104} and {100} facets. The present findings clearly demonstrate that the selective O2 evolution from water can possible without any sacrificial agent.

Fabrication of Heterostructured Fe2TiO5–TiO2 Nanocages with Enhanced Photoelectrochemical Performance for Solar Energy Conversion

AbstractPhotocatalysts with well‐designed compositions and structures are desirable for achieving highly efficient solar‐to‐chemical energy conversion. Heterostructured semiconductor photocatalysts with advanced hollow structures possess beneficial features for promoting the activity towards photocatalytic reactions. Here we develop a facile synthetic strategy for the fabrication of Fe2TiO5–TiO2 nanocages (NCs) as anode materials in photoelectrochemical (PEC) water splitting cells. A hydrothermal reaction is performed to transform MIL‐125(Ti) nanodisks (NDs) to Ti–Fe–O NCs, which are further converted to Fe2TiO5–TiO2 NCs through a post annealing process. Owing to the compositional and structural advantages, the heterostructured Fe2TiO5–TiO2 NCs show enhanced performance for PEC water oxidation compared with TiO2 NDs, Fe2TiO5 nanoparticles (NPs) and Fe2TiO5–TiO2 NPs.

Fabrication of Heterostructured Fe2 TiO5 -TiO2 Nanocages with Enhanced Photoelectrochemical Performance for Solar Energy Conversion.

Photocatalysts with well-designed compositions and structures are desirable for achieving highly efficient solar-to-chemical energy conversion. Heterostructured semiconductor photocatalysts with advanced hollow structures possess beneficial features for promoting the activity towards photocatalytic reactions. Here we develop a facile synthetic strategy for the fabrication of Fe2 TiO5 -TiO2 nanocages (NCs) as anode materials in photoelectrochemical (PEC) water splitting cells. A hydrothermal reaction is performed to transform MIL-125(Ti) nanodisks (NDs) to Ti-Fe-O NCs, which are further converted to Fe2 TiO5 -TiO2 NCs through a post annealing process. Owing to the compositional and structural advantages, the heterostructured Fe2 TiO5 -TiO2 NCs show enhanced performance for PEC water oxidation compared with TiO2 NDs, Fe2 TiO5 nanoparticles (NPs) and Fe2 TiO5 -TiO2 NPs.

Effect of citric acid on the synthesis of ZnWO4/ZnO nanorods for photoelectrochemical water splitting

ZnO nanorods (NRs) and ZnWO4/ZnO heterostructure, using TiO2 seed layers, were synthesized by two steps hydrothermal method and annealed at 450 °C for 2 hr. The results showed that the ZnWO4 was grown on the hexagonal ZnO NRs and change to hierarchically structure when the citric acid content was increased. All samples were applied as photoanode for photoelectrochemical (PEC) water splitting cell. As the result, the 50ZnW/ZnO sample showed the best performance of the PEC water splitting process than others. From the Electrochemical Impedance Spectroscopy (EIS) study, the Nyquist plot of 25ZnW/ZnO and 50ZnW/ZnO samples showed smaller arc compared to ZnO NRs. This is signified that the ZnWO4/ZnO heterostructure has greater charge transfer between the photoanode and electrolyte than the pristine ZnO. Hence, the modified ZnWO4 on the ZnO surface enhances the performance of ZnO NRs based PEC water splitting cell for the UV light hydrogen fuel generation.

Light management in TiO2 thin films integrated with Au plasmonic nanoparticles

The light-harvesting properties of metal oxide thin films can be remarkably increased by the introduction of plasmonic nanostructures, leading to higher efficiencies in photovoltaic or photoelectrochemical devices. In the prototypical material combination, Au-TiO2, nano- and mesoscale porosity of TiO2 is desirable to improve not only the light-harvesting, but also the available surface area for chemical reactions. Moreover, great attention has been given to the control of size and shape of Au nanoparticles (NPs) to tune the overall optical properties of the film. In this work, we investigate the optical properties of composite Au-TiO2 films exhibiting remarkable light scattering properties. TiO2 is characterized by a tree-like hierarchical morphology produced by pulsed laser deposition, and two different configurations for Au integration, namely Au on top and at the bottom of TiO2, are explored while varying the size of Au NPs. The hierarchical oxide morphology allows to achieve superior scattering properties after the combination with Au NPs with respect to films obtained from a commercial paste deposition. Both the Au-top and Au-bottom configurations enable to tune the plasmonic properties of Au NPs. Specifically, outstanding scattering properties are exhibited by the composite TiO2 film grown on top of large (∼100 nm) Au NPs. These results show the potential interest of employing such integrated films as photoanodes in dye-sensitized or perovskite-based solar cells, or in photoelectrochemical cells for water splitting. An analogous approach can be employed for alternative materials, both considering the plasmonic structures as well as the semiconductor layer.

Plasmon-enhanced photoelectrochemical water splitting by InGaN/GaN nano-photoanodes

It has been extensively focused on the development of photoelectrochemical (PEC) solar water splitting cells due to the urgent need for clean energy. In this study, InGaN/GaN multiple quantum wells (MQWs) nanorods (NRs) PEs are purposely designed with coupled plasmonic metal (Ag–Au core–shell nanowires) for high efficiency PEC water splitting, aiming for enhancing visible light absorption and carrier kinetic energy. By means of self-organized Nickel island patterning and ICP dry etching with proper depths, the photocurrent density of InGaN/GaN nano-photoanodes in NaCl solution can be significantly enhanced from 0.74 mA cm−2 to 1.4 mA cm−2 at 1.5 V versus RHE (reversible hydrogen electrode). Furthermore, by decorating plasmonic metal into the InGaN/GaN NR arrays, the PEC water splitting performance can reach a maximum photocurrent of 1.77 mA cm−2 at 1.5 versus RHE. Such improvement is mainly attributed to the synergistic effects of the visible light absorption of MQWs and the generation of surface plasmon resonance (SPR). The approach of plasmon-enhanced nanostructure with the III-nitride based nano-PEs will accelerate the developments of visible light photocatalysts for PEC solar cells.

Influence of Dye Sensitizers on Charge Dynamics in SnO2 Nanoparticles Probed with THz Spectroscopy

Using dye-sensitized water-splitting photoelectrochemical cells is a promising way to satisfy an increasing demand for energy without significant production of greenhouse gases. These cells commonl...

In situ textured carbon nitride photoanodes with enhanced photoelectrochemical activity by band-gap state modulation

As promising photoelectrodes in photoelectrochemical water splitting cells, the photoelectrochemical activity of oxygen-doped CN films can be enhanced by a band-gap state modulation strategy to effectively improve the charge carrier separation.

Visible light-driven simultaneous water oxidation and quinone reduction by a nano-structured conjugated polymer without co-catalysts.

In artificial photosynthesis, chemists are aiming to borrow principles from natural photosynthesis to develop photoelectrochemical cells (PEC) for water splitting. The water plastoquinone photo-oxidoreductase enzyme, also known as photosystem II, uses light to perform the four-electron, four-proton oxidation of water to dioxygen and stores reducing equivalents in reduced forms of quinones which are ultimately used in dark reactions for the synthesis of energy-rich molecules. We report a nano-structured semiconducting conjugated polymer based on poly(diphenylbutadiyne) (nano-PDPB) and its photocatalytic activities towards the water oxidation reaction under visible light irradiation when dispersed in water in the absence of any sacrificial agents or co-catalysts. Charge recovery at the nano-PDPB directly or delayed in time was exemplified by the reduction of quinone acting as a hydrogen reservoir. In the absence of quinones as electron acceptors H2O2 formation was detected, stemming from the partial reduction of O2.

Large scale compatible fabrication of gold capped titanium dioxide nanoantennas using a shadowing effect for photoelectrochemical water splitting

In this paper, a visible light driven plasmonic based photoelectrochemical water splitting (PEC-WS) cell is designed with an elegant two-step fabrication route. First, titanium dioxide (TiO2) nanowires (NWs) were synthesized using the hydrothermal method. Then, angled deposition was used to selectively coat the tips of the NWs yielding Au-capped TiO2 NWs with multiple sizes and shapes. The provided randomness leads to a multi-resonant system where the superposition of these resonance modes leads to an overall broadband absorption. The excited localized surface resonance (LSPR) modes contribute to the performance enhancement of the cell via near field effects and a hot electron injection mechanism. Moreover, these nanotips can trigger the formation of Fabry-Pérot (FP) cavity modes. The combination of the above-mentioned mechanisms leads to a high performance visible light driven plasmonic cell. At an applied potential of 1.23 V vs RHE, a photocurrent value as high as 82 μA/cm2 is acquired for the plasmonic based photoanode. The proposed design strategy is a large scale compatible route with no material restriction. Therefore, vast variety of semiconductor-metal pairs can be fabricated to obtain highly efficient water splitting cell for hydrogen generation.

Thermodynamic Stability and Native Point Defects of CuFeO2 Photocathodes in Dry and Electrochemical Environments

CuFeO2 has been recently identified as a promising photocathode material for photoelectrochemical water splitting cells. In spite of the first encouraging results, improvements in the catalytic act...