Roel van de Krol

Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode

Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n(+)-n homojunction. This concept is demonstrated with a low-cost, spray-deposited and non-porous tungsten-doped bismuth vanadate photoanode in which carrier-separation efficiencies of up to 80% are achieved. By combining this state-of-the-art photoanode with an earth-abundant cobalt phosphate water-oxidation catalyst and a double- or single-junction amorphous Si solar cell in a tandem configuration, stable short-circuit water-splitting photocurrents of ~4 and 3 mA cm(-2), respectively, are achieved under 1 sun illumination. The 4 mA cm(-2) photocurrent corresponds to a solar-to-hydrogen efficiency of 4.9%, which is the highest efficiency yet reported for a stand-alone water-splitting device based on a metal oxide photoanode.

Solar hydrogen production with nanostructured metal oxides

The direct conversion of solar energy into hydrogen represents an attractive but challenging alternative for photo-voltaic solar cells. Several metal oxide semiconductors are able to split water into hydrogen and oxygen upon illumination, but the efficiencies are still (too) low. The operating principles of photo-electrochemical devices for water splitting, their main bottlenecks, and the various device concepts will be reviewed. Materials properties play a key role, and the advantages and pitfalls of the use of interfacial layers and dopants will be discussed. Special attention will be given to recent progress made in the synthesis of nanostructured metal oxides with high aspect ratios, such as nanowire arrays, which offers new opportunities to develop efficient photo-active materials for solar water splitting.

Photoelectrochemical hydrogen production

Part I: Basic Principles: Introduction.- Principles of Photoelectrochemical Cells.- Photoelectrochemical Measurements.- Part II: Materials Properties and Synthesis: Nanostructured alpha-Fe2O3 Photoanodes.- Mixed Metal Oxide Photoelectrodes and Photocatalysts.- Combinatorial Identification and Optimization of New Oxide Semiconductors.- Part III: Devices and Device Characterization.- Multijunction Approaches to Photo-electrochemical Water Splitting.- Part IV: Future Perspectives.- Economic and Business Perspectives.- Emerging Trends in Water Photoelectrolysis.

Efficient BiVO4 Thin Film Photoanodes Modified with Cobalt Phosphate Catalyst and W-doping

Bismuth vanadate (BiVO4) thin film photoanodes for light‐induced water oxidation are deposited by a low‐cost and scalable spray pyrolysis method. The resulting films are of high quality, as indicated by an internal quantum efficiency close to 100 % between 360 and 450 nm. However, its performance under AM1.5 illumination is limited by slow water oxidation kinetics. This can be addressed by using cobalt phosphate (Co‐Pi) as a water oxidation co‐catalyst. Electrodeposition of 30 nm Co‐Pi catalyst on the surface of BiVO4 increases the water oxidation efficiency from ≈30 % to more than 90 % at potentials higher than 1.2 V vs. a reversible hydrogen electrode (RHE). Once the surface catalysis limitation is removed, the performance of the photoanode is limited by low charge separation efficiency; more than 60 % of the electron‐hole pairs recombine before reaching the respective interfaces. Slow electron transport is shown to be the main cause of this low efficiency. We show that this can be remedied by introducing W as a donor type dopant in BiVO4, resulting in an AM1.5 photocurrent of ≈2.3 mA cm−2 at 1.23 V vs. RHE for 1 % W‐doped Co‐Pi‐catalyzed BiVO4.

Efficient Water‐Splitting Device Based on a Bismuth Vanadate Photoanode and Thin‐Film Silicon Solar Cells

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2% under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten-bismuth vanadate (W:BiVO4 ) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO4 and the nc-Si:H cell.

Efficient Plasma Route to Nanostructure Materials: Case Study on the Use of m-WO3 for Solar Water Splitting

One of the main challenges in developing highly efficient nanostructured photoelectrodes is to achieve good control over the desired morphology and good electrical conductivity. We present an efficient plasma-processing technique to form porous structures in tungsten substrates. After an optimized two-step annealling procedure, the mesoporous tungsten transforms into photoactive monoclinic WO3. The excellent control over the feature size and good contact between the crystallites obtained with the plasma technique offers an exciting new synthesis route for nanostructured materials for use in processes such as solar water splitting.

Hetero-type dual photoanodes for unbiased solar water splitting with extended light harvesting

Metal oxide semiconductors are promising photoelectrode materials for solar water splitting due to their robustness in aqueous solutions and low cost. Yet, their solar-to-hydrogen conversion efficiencies are still not high enough for practical applications. Here we present a strategy to enhance the efficiency of metal oxides, hetero-type dual photoelectrodes, in which two photoanodes of different bandgaps are connected in parallel for extended light harvesting. Thus, a photoelectrochemical device made of modified BiVO4 and α-Fe2O3 as dual photoanodes utilizes visible light up to 610 nm for water splitting, and shows stable photocurrents of 7.0±0.2 mA cm−2 at 1.23 VRHE under 1 sun irradiation. A tandem cell composed with the dual photoanodes–silicon solar cell demonstrates unbiased water splitting efficiency of 7.7%. These results and concept represent a significant step forward en route to the goal of >10% efficiency required for practical solar hydrogen production.

Electrical and optical properties of TiO2 in accumulation and of lithium titanate Li0.5TiO2

Changes in the optical absorption and electrical conductivity of dense and mesoporous anatase TiO2 films were measured in situ as a function of electrode potential during electrochemical lithium intercalation. A special two-electrode geometry was used for the conductivity measurements, in which the contacts were separated by a small gap bridged by the TiO2. When electrons are injected, an accumulation layer is formed and the conductivity increases several orders of magnitude. A monotonic increase of the optical absorption with wavelength confirms the presence of (partially) delocalized electrons. Insertion of lithium ions results in the formation of the Li0.5TiO2 phase and a decrease of the overall conductance. The specific conductivity of the Li0.5TiO2 phase is (9.1±0.2) S/cm, significantly lower than that of Li-doped anatase TiO2. This is corroborated by the absorption spectrum of Li0.5TiO2, which shows two pronounced peaks around 440 and 725 nm and no characteristic free-electron features. At potential...

Principles of Photoelectrochemical Cells

In this chapter, the basic principles of photoelectrochemical water splitting are reviewed. After a brief introduction of the photoelectrochemical cell and the electrochemical reactions involved, the electronic structure and properties of semiconductors are discussed. The emphasis is on metal oxide semiconductors, and special attention is given to the presence of ionic point defects in these materials. This is followed by a closer look at the semiconductor/electrolyte interface. The energy conversion efficiency and different definitions of the quantum efficiency are treated next. The chapter concludes with a brief outline of the material’s requirements and challenges facing the development of highly efficient photoelectrodes.

α-Fe2O3 films for photoelectrochemical water oxidation – insights of key performance parameters

We report the deposition of ultra-thin α-Fe2O3 (hematite) films on fluorine-doped tin oxide (FTO) substrates using radio frequency (RF) sputtering, and the investigation of their photoelectrochemical (PEC) performance towards water oxidation. By varying the deposition pressure and time, the film microstructure and morphology could be optimized. The best hematite films having a thickness of about 50 nm exhibited a photocurrent density of 0.59 mA cm−2 at U = 1.23 V vs. RHE and 1.92 mA cm−2 at U = 1.85 V using a tungsten halogen lamp of 40 mW cm−2 light intensity in the wavelength range from 300 to 600 nm. These values are comparable or even higher than those ever measured hematite films (undoped and having no co-catalyst deposited on top of the electrode). Further measurements were explored to investigate the limiting factors in our films for possibly approaching their predicted PEC properties. A detailed analysis reveals that a slow water oxidation reaction and a trapping of charges on the surface, especially at the potential below 1.4 V, are obviously the reasons for the limited PEC performance.