Photoelectrochemical Water Splitting Cells Research Articles

Nitrogen and cobalt co-doped zinc oxide nanowires – Viable photoanodes for hydrogen generation via photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has been considered as a promising and environmentally benign approach for efficient and economic hydrogen generation by utilization of solar energy. Development of semiconductor materials with low band gap, high photoelectrochemical activity and stability has been of particular interest for a viable PEC water splitting system. In this study, Co doped ZnO, .i.e., (Zn0.95Co0.05)O nanowires (NWs) was selected as the composition for further co-doping with nitrogen by comparing solar to hydrogen efficiency (SHE) of ZnO NWs with that of various compositions of (Zn1−xCox)O NWs (x = 0, 0.05, 0.1). Furthermore, nanostructured vertically aligned Co and N-doped ZnO, .i.e., (Zn1−xCox)O:N NWs (x = 0.05) have been studied as photoanodes for PEC water splitting. An optimal SHE of 1.39% the highest reported so far to the best of our knowledge for ZnO based photoanodes was obtained for the co-doped NWs, (Zn0.95Co0.05)O:N - 600 NWs generated at 600 °C in ammonia atmosphere. Further, (Zn0.95Co0.05)O:N-600 NWs exhibited excellent photoelectrochemical stability under illumination compared to pure ZnO NWs. These promising results suggest the potential of (Zn0.95Co0.05)O:N-600 NWs as a viable photoanode in PEC water splitting cell. Additionally, theoretical first principles study conducted explains the beneficial effects of Co and N co-doping on both, the electronic structure and the band gap of ZnO.

Chemical bias of electrochemical and photoelectrochemical water splitting using a hydrogel separator

Abstract Resorcinol-formaldehyde hydrogels are shown to be adequate separators in electrochemical and photoelectrochemical water splitting cells. Combined with concentrated buffer electrolytes, they allow the maintenance of relatively stable pH gradients between basic anolytes and acidic catholytes. The water splitting potential at a current density of 3.0 mA/cm 2 applied between two Pt electrodes and a pH bias of 8.1 units retained a value of 2.7 V for several hours. Using iron foam/hematite as photoanode and Pt as cathode, the water splitting photocurrents at an applied potential of 1.23 V reached values of 0.40 and 0.06 mA/cm 2 in the presence and absence, respectively, of this chemical bias.

Immobilization of a Molecular Ruthenium Catalyst on Hematite Nanorod Arrays for Water Oxidation with Stable Photocurrent.

Photoelectrochemical (PEC) cells for light-driven water splitting are prepared using hematite nanorod arrays on conductive glass as the photoanode. These devices improve the photocurrent of the hematite-based photoanode for water splitting, owing to fewer surface traps and decreased electron recombination resulting from the one-dimensional structure. By employing a molecular ruthenium co-catalyst, which contains a strong 2,6-pyridine-dicarboxylic acid anchoring group at the hematite photoanode, the photocurrent of the PEC cell is enhanced with high stability for over 10 000 s in a 1 m KOH solution. This approach can pave a route for combining one-dimensional nanomaterials and molecular catalysts to split water with high efficiency and stability.

Light-Harvesting Proteins and Biofilms on Iron Oxide Photoelectrodes

The so-called artificial photosynthesis has developed in at least two branches: semiconductor photoelectrochemistry with photoelectrochemical cells, and bio-molecular systems. Here we show bio-hybrid systems where metal oxide semiconductor photoelectrodes are fucntionalized at their surface with light harvesting proteins, specifically phycocyanin light antenna from blue-green algae. It has turned out in the early beginning of our studies that such funtionalization can yield a higher photocurrent and a higher hydrogen output. The actual underlying physico-chemical processes are not yet so well explored. We have therefore conducted electrochemical impedance studies on bio-hybride electrodes under physiological conditions such as to determine charge transfer processes. Moreover have we conducted x-ray photoemission studies on model systems in order to understand the valence band struc ture of the bio-electric interface and its charge transfer properties, particularly under dark conditions. The first bio-hybrid electrodes were used by Melvin Calvin in the late 1950s where he sublimated chlorophyll on aquadag graphite interdigital electrodes. This was done merely to study the bio-organic material. Helmut Tributsch' dye-sensitized solar cells (DSSC) in the early 1970s were ZnO electrodes, i.e. semiconductors further functionalized with natural absorbing dye molecules. Since, DSSCs have developed as a field in photovoltaic solar energy conversion technology. Meanwhile, photoelectrodes for solar water splitting in photoelectrochemical cells (PEC) evolved as an independent field. This field has recently emerged again and is virtually taking center stage. Also here, functionalization of traditional semicondcutor electrodes with bio-organic motifs is gaining more and more interest. Hydrogenase is a frequently used protein for the promotion of hydrogen evolution on photocathodes. We have recently coated the light harvesting antenna protein C-phycocyanin on iron oxide photoelectrodes. This approach was quite promising. We measured an increrased photocurrent density at the photoanode and an increased hydrogen evolution at the counter electrode. During our studies we were pointed to the question of charge transfer across the bio-electronic interface which is formed by biomolecules and metal oxides. Because the expressioan and purification of C-phycocyanin, which is extracted from cyanobacteria, is a costly and laborious process, we explored also the use of complete cyanobacteria on the photoelectrodes. The thylakoid membrane and cell membrane may pose electrical barriers between the biocatalytic or light harvesting components in the photosynthetic apparatus and the semicondcutor electrode underneath. It is therefore necessary to quantify this barrier. For this, we are performing electroanalytical experiments and x-ray spectroscopy experiments operando and in situ, in the latter case with an actual anabaena biofilm with ambient pressure XPS on a bio-electrochemica cell.[1] J. Ihssen, A. Braun, G. Faccio, K. Gajda-Schrantz, L. Thöny-Meyer; Light harvesting proteins for solar fuel generation in bioengineered photoelectrochemical cells; Current Protein and Peptide Science, 2014, 15 (4),374-384.[2] D. K. Bora, Y. Hu, S. Thiess, S. Erat, X. Feng, S. Mukherjee, G. Fortunato, N. Gaillard, R. Toth, K. Gajda-Schrantz, W. Drube, M. Grätzel, J. Guo, J. Zhu, E.C. Constable, D.D. Sarma, H. Wang, A. Braun, Between Photocatalysis and Photosynthesis: Synchrotron spectroscopy methods on molecules and materials for solar hydrogen generation, J. Electron Spectr. Rel. Phenom. 2013, 190 A, 93-105.[3] D.K. Bora, A.A. Rozhkova, K. Schrantz, P.P. Wyss, A. Braun, T. Graule, E.C. Constable, Functionalization of Nanostructured Hematite Thin-Film Electrodes with the Light-Harvesting Membrane Protein C-Phycocyanin Yields an Enhanced Photocurrent, Advanced Functional Materials 2012, 22 (3) 490-502.[4] D.K. Bora, A. Braun, E.C. Constable, “In rust we trust”. Hematite the prospective inorganic backbone for artificial photosynthesis, Energy Environ. Sci., 2013,6, 407-425.[5] A Braun, F. Boudoire, D. K. Bora, G. Faccio, Y. Hu, A. Kroll, B. S. Mun, S. T. Wilson, Biological components and bio-electronic interfaces of water splitting photo-electrodes for solar hydrogen production, Chem. Eur. J. 2015, 21(11), 4188-4199.

In situ measurement of photostability of CdSe/CdS/ZnO nanowires photoelectrode for photoelectrochemical water splitting

Quantum dot sensitized photoanodes have drawn much attention due to their high potential as efficient anodes for photoelectrochemical (PEC) water splitting or solar cells. However the photocorrosion of QDs is the crucial barrier for applications in these devices. The in situ analysis of photocorrosion is important in understanding its mechanism and also developing the possible solution for photocorrosion. In this study we have developed a novel, integrated analysis system for in situ measurements of photocorrosion and PEC performances. We have fabricated the CdSe/CdS/ZnO nanowire (NW) arrays on quartz crystal microbalance (QCM) as a platform for usage as PEC photoanodes and also mass analysis at the same time. The in situ measuring of photocurrents and mass changes were performed with continuous operation of PEC cells for CdSe/CdS/ZnO NWs photoanode. The study exhibited highly correlated tendency in photocurrent decrease and mass reduction, due to photocorrosion of CdSe/CdS/ZnO NWs. Also to improve the photostability of CdSe/CdS/ZnO NWs, applications of passivation and catalysts were studied and their effects were discussed. Our integrated in situ analysis system is highly applicable to various semiconductor sensitized systems.

Controlled Growth of WO<sub>3</sub>-Loaded TiO<sub>2</sub> Nanotubes for Tandem Solar-Driven Water Splitting Cell

Nowadays, hydrogen production using solar-driven photoelectrochemical (PEC) water splitting has attracted considerable attention since the introduction of TiO2 photoelectrodes. However, TiO2 is not able to split water on its own because the cleavage requires more than 1.4 V or even up to 1.9 V, including the redox potential of water (1.23 V) and unavoidable over-potentials. Many semiconductors have been studied, but only a very few large band gap materials can generate enough photo-voltage to cleave water for a single photoelectrode PEC water splitting cell especially processing under solar illumination. In the present study, development of WO3-loaded TiO2 nanotubes (WTNT) is a possible solution to generate a voltage that is high enough to split the water while absorbing more light (photons) from a greater part of solar spectrum. Furthermore, WTNT offered several advantages over the pure TiO2 photoelectrode, such as excellent chemical and thermal stability, active at room temperature as well as responsive to UV and visible illumination. The paper concludes by presenting the comparison results between unmodified TiO2 and WTNT photoelectrode in term of morphology, phase, elemental analysis, electrical properties, and electrochemical properties for the tandem solar-driven water splitting cell.

Understanding the Influence of Structure on Activity and Stability in the Catalysis of the Oxygen Evolution Reaction (OER) Using Crystalline Oxides As a Platform

Oxides of manganese and cobalt have proved to be effective (electro)catalysts for the water oxidation half-reaction in water splitting. Recently, the research focus has mostly been on developing high surface area morphologies of catalysts with high specific activity that are potentially easily adaptable to photoelectrochemical cells for water splitting. However, it is still not completely understood why these catalysts show such high intrinsic activity, and many have amorphous structures, making it difficult to elucidate structure-function relationships. Using crystalline oxides as an experimental framework, we have elucidated the effects of structure on the activity and stability of cobalt and manganese oxides for the oxygen evolution reaction (OER). For instance, in two distinct polymorphs of lithium cobalt oxide (LiCoO2), we have found the cubic structure (Fd-3m, spinel-like), with a Co4O4 oxo-metallic cube motif, is highly active for water oxidation, whereas the layered polymorph (R-3m) with slabs of CoO2 (used for rechargeable Li ion batteries) is not. In fact, upon electrochemical processing, the layered polymorph undergoes a surface restructuring that we can monitor using high-resolution transmission electron microscopy (HRTEM). This has been shown to occur in both aqueous and non-aqueous electrolyte. After the transformation occurs, which can also be monitored by the electrochemical profile and corrosion of lithium, the OER activity of the layered material mirrors that of the cubic LiCoO2. However, it is unstable and eventually diminishes with repeated electrochemical cycling or prolonged electrolysis. In this way, we show that the underlying bulk crystal structure has influence over not just activity, but also stability, both equally important for commercial applications. Parallel to this work, we isolated discrete Co2O2, Co3O3, and Co4O4 molecular clusters in the same ligand sets, and found only the complete Co4O4 cubanes were catalytically active for water oxidation. This difference in activity correlates to the accessibility of a formal Co4+ oxidation state, which occurs at modest potentials only for the cubane, which compensates by delocalizing the hole across all cobalt and bridging oxos. The singly oxidized cubane is sufficient to produce O2 just in reaction with hydroxide. We believe this phenomenon relates back to the solid-state, and is in part the reason for the high activity in the cubic LiCoO2.

BiVO4 Photoelectrodes for Use in Solar Water Splitting

Many recent efforts have focused on the development of n-type semiconductors that can serve as efficient photoanodes for solar water oxidation. Desirable photoanodes should have a small bandgap to utilize a significant portion of visible light, and a valence band (VB) edge that is positive enough to provide sufficient overpotential for the water oxidation reaction. The position of the conduction band (CB) edge or the flatband potential of the photoanode is also important because it determines the potential of the photoexcited electrons that will be used for the cathode reaction (i.e., water reduction). If these electrons do not have sufficient overpotential to reduce water, an external bias needs to be provided, effectively reducing the cell efficiency. Bismuth vanadate (BiVO4) has recently emerged as a promising material for use as a photoanode in water splitting photoelectrochemical cells. It is because it absorbs a substantial portion of the visible spectrum (bandgap energy, ca. 2.4 eV) and has a favorable conduction band (CB) edge position very near the thermodynamic H2 evolution potential. Significant advancement in the understanding and construction of efficient BiVO4-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this presentation, we will discuss our recent efforts in improving the photoelectrochemical properties of BiVO4 photoanodes, which include an enhancement in photon absorption and charge transport properties. These efforts resulted in an applied bias photon-to-current efficiency (ABPE) for water splitting higher than 2% using all oxide-based electrodes.

Catalytic and photoelectrochemical performances of Cu–Zn–Sn–Se thin films prepared using selenization of electrodeposited Cu–Zn–Sn metal precursors

In this study, Cu2ZnSnSe4 (CZTSe) films are deposited onto the fluorine-doped-tin-oxide-coated glass substrate via the selenization of electrodeposited Cu–Zn–Sn metal precursors in an acidic solution with the applied potential of −0.9 V vs. an Ag/AgCl electrode. X-ray diffraction patterns reveal that the samples are the quaternary tetragonal CZTSe phase. The thicknesses and direct band gaps of the samples are in the ranges of 2.3 to 2.7 μm and 0.95 to 1.02 eV, respectively. All samples are p-type semiconductors with carrier density, mobility and flat-band potential in the ranges of 3.88 × 1017 to 1.37 × 1018 cm−3, 10.31 to 12.6 cm2 V−1 s−1 and −0.01 V to −0.08 V vs. Ag/AgCl reference electrode, respectively. The sample with [Cu]/[Zn + Sn] and [Zn]/[Sn] molar ratios of 0.87 and 0.66, respectively, has a maximum photo-enhanced current density of 0.41 mA cm−2 at an applied bias of −0.5 V vs. an Ag/AgCl electrode in 0.5 M H2SO4 solution under illumination. The best photo-conversion efficiency of dye-sensitized solar cells using CZTSe with [Cu]/[Zn + Sn] and [Zn]/[Sn] molar ratios of 0.87 and 0.66, respectively, as the counter electrode was 7.98%. The results show the high quality CZTSe films have potentials in applications of photoelectrochemical water splitting and dye-sensitized solar cells.

Nanostructured Fe2O3/TiO2 thick films: Analysis of structural and electronic properties

Thick films using starting nanopowders of TiO2 (anatase 99.7%) and α-Fe2O3 (hematite) mixed in the weight ratios 60:40 and 40:60 for thick film paste were fabricated by screen printing technology on an alumina substrate. Samples were sintered at temperatures of 800–950°C for 60min. Structural and electronic properties were analyzed by XRD and SEM analysis, UV/vis spectroscopy and current–voltage analysis in view of potential application as gas sensors and anodes in photoelectrochemical cells for hydrogen generation and water splitting. XRD analysis showed that at 800°C monoclinic pseudobrookite formed and anatase had transformed into rutile. Increase in sintering temperature lead to a gradual transformation of monoclinic pseudobrookite into the orthorhombic phase. SEM analysis showed that a homogenous small grain structure was obtained especially in the case of thick film samples with the 40:60 ratio of starting anatase and hematite sintered at 850°C.

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.

Porous C3N4 Nanolayers@N-Graphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution

Pt-free electrocatalysts for hydrogen evolution reaction (HER) with high activity and low price are desirable for many state-of-the-art renewable energy devices, such as water electrolysis and photoelectrochemical water splitting cells. However, the design and fabrication of such materials remain a significant challenge. This work reports the preparation of a flexible three-dimensional (3D) film by integrating porous C3N4 nanolayers with nitrogen-doped graphene sheets, which can be directly utilized as HER catalyst electrodes without substrates. This nonmetal electrocatalyst has displayed an unbeatable HER performance with a very positive onset-potential close to that of commercial Pt (8 mV vs 0 mV of Pt/C, vs RHE @ 0.5 mA cm(-2)), high exchange current density of 0.43 mA cm(-2), and remarkable durability (seldom activity loss >5000 cycles). The extraordinary HER performance stems from strong synergistic effect originating from (i) highly exposed active sites generated by introduction of in-plane pores into C3N4 and exfoliation of C3N4 into nanosheets, (ii) hierarchical porous structure of the hybrid film, and (iii) 3D conductive graphene network.

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.

Biological components and bioelectronic interfaces of water splitting photoelectrodes for solar hydrogen production.

Artificial photosynthesis (AP) is inspired by photosynthesis in nature. In AP, solar hydrogen can be produced by water splitting in photoelectrochemical cells (PEC). The necessary photoelectrodes are inorganic semiconductors. Light-harvesting proteins and biocatalysts can be coupled with these photoelectrodes and thus form bioelectronic interfaces. We expand this concept toward PEC devices with vital bio-organic components and interfaces, and their integration into the built environment.

Molybdenum phosphosulfide: an active, acid-stable, earth-abundant catalyst for the hydrogen evolution reaction.

Introducing sulfur into the surface of molybdenum phosphide (MoP) produces a molybdenum phosphosulfide (MoP|S) catalyst with superb activity and stability for the hydrogen evolution reaction (HER) in acidic environments. The MoP|S catalyst reported herein exhibits one of the highest HER activities of any non-noble-metal electrocatalyst investigated in strong acid, while remaining perfectly stable in accelerated durability testing. Whereas mixed-metal alloy catalysts are well-known, MoP|S represents a more uncommon mixed-anion catalyst where synergistic effects between sulfur and phosphorus produce a high-surface-area electrode that is more active than those based on either the pure sulfide or the pure phosphide. The extraordinarily high activity and stability of this catalyst open up avenues to replace platinum in technologies relevant to renewable energies, such as proton exchange membrane (PEM) electrolyzers and solar photoelectrochemical (PEC) water-splitting cells.

Pt-free tandem molecular photoelectrochemical cells for water splitting driven by visible light.

Photoelectrochemical (PEC) cells using molecular catalysts to split water into hydrogen and oxygen have been investigated intensively during the past years. However, the high-cost of Pt counter electrodes and instability of molecular PEC cells hinder the practical applications. We describe in this article a Pt-free tandem molecular PEC cell, for the first time, employing molecular ruthenium- and cobalt-catalysts with strong dipicolinic acid anchoring groups on the respective photoanode and photocathode for total water splitting. The Pt-free tandem molecular PEC cell showed an effective and steady photocurrent density of ca. 25 μA cm(-2) for water splitting driven by visible light without external bias. This study indicates that tandem molecular PEC cells can provide great potential to the Pt-free devices for light driven total water splitting.

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.

CdS quantum dot sensitized p-type NiO as photocathode with integrated cobaloxime in photoelectrochemical cell for water splitting

Abstract CdS sensitized NiO electrode was used as the photoactive cathode in a photoelectrochemical cell for water splitting, avoiding the use of a sacrificial electron donor. Photocurrent increment under visible light irradiation was observed after integration of [Co(dmgH) 2 (4-Me-py)Cl] ( 1 ) to the photocathode, suggesting 1 could accept electrons from photoexcited CdS for water reduction and NiO could move the holes in the valence band of CdS to anode for water oxidation.

Unveiling Two Electron-Transport Modes in Oxygen-Deficient TiO2 Nanowires and Their Influence on Photoelectrochemical Operation.

Introducing oxygen vacancies (VO) into TiO2 materials is one of the most promising ways to significantly enhance light-harvesting and photocatalytic efficiencies of photoelectrochemical (PEC) cells for water splitting among others. However, the nature of electron transport in VO-TiO2 nanostructures is not well understood, especially in an operating device. In this work, we use the intensity-modulated photocurrent spectroscopy technique to study the electron-transport property of VO-TiO2 nanowires (NWs). It is found that the electron transport in pristine TiO2 NWs displays a single trap-limited mode, whereas two electron-transport modes were detected in VO-TiO2 NWs, a trap-free transport mode at the core, and a trap-limited transport mode near the surface. The considerably higher diffusion coefficient (Dn) of the trap-free transport mode grants a more rapid electron flow in VO-TiO2 NWs than that in pristine TiO2 NWs. This electron-transport feature is expected to be common in other oxygen-deficient metal oxides, lending a general strategy for promoting the PEC device performance.