Institute

At the Institute for Solar Fuels we develop new materials and devices for the production of chemical fuels from cheap and abundant resources, such as water and CO2, using sunlight. Our current efforts are focused on photo-electrochemical water splitting. Towards this end, we develop deposition processes and synthesis routes for thin film and nanostructured semiconductors and catalysts, and we investigate the fundamental processes of charge generation, separation, and transfer in the bulk and at the interfaces of these materials. Of particular interest is the role of defects, which we aim to control by developing thermal treatments, passivation layers, and doping strategies. Our experimental toolbox includes a range of thin film deposition techniques, electrochemistry and photo-electrochemistry, time-resolved spectroscopy on fs – s time scales, and synchrotron-based methods under operando conditions.

Institute for Solar Fuels

News and Recent Publications

Structural transformations in MoSx, a highly efficient hydrogen evolution catalyst

We show that molybdenum sulfide (MoSx) films made with reactive magnetron sputtering can evolve hydrogen at current densities of ‑10 mA/cm2 at an overpotential of only 180 mV in 0.5 M H2SO4. The material is deposited at room temperature and does not contain any noble metals. Moreover, the deposition technique can be easily scaled up to large areas. The initially amorphous films consist of [Mo3S13]2- and [Mo3S12]2- species which are interconnected via [S2]2- and S2- ligands. During electrochemical activation, these clusters are converted into layer-type MoS2-x with a large number of catalytically-active molybdenum atoms at the edges of nano-sized islands.  

  

Xi, P. Bogdanoff, K. Harbauer, P. Plate, C. Höhn, J. Rappich, B. Wang, X. Han, R. van de Krol, S. Fiechter, “Structural transformation identification of sputtered amorphous MoSx as efficient hydrogen evolving catalyst during electrochemical activation”, ACS Catal. 9, 2368-2380 (2019)

A 50 cm2 solar water splitting device

Scale-up to areas beyond 1 cm2 is a major challenge for photoelectrochemical (PEC) water splitting devices. In this paper, we demonstrate a 50 cm2 pH-neutral, bias-free tandem device based on cobalt phosphate-coated tungsten-doped BiVO4 photoanodes placed in front of series-connected silicon heterojunction solar cells. In this first demonstration of its kind, we show a solar-to-hydrogen efficiency of 2.1% and a lifetime of more than 7 hours. The corresponding small-area efficiency for the same device is 5.5%. We quantify the various losses in the system and found that mass transport of H+/OH- reactant species is the main bottleneck in this pH-neutral demonstrator.

 

I.Y. Ahmet, Y. Ma, J.W. Jang, T. Henschel, B. Stannowski, T. Lopes, A. Vilanova, A. Mendes, F.F. Abdi, R. van de Krol, “Demonstration of a 50 cm2 BiVO4 Tandem Photoelectrochemical-Photovoltaic Water Splitting Device”, Sust. Energy Fuels (2019)

How defects change the interaction of water with single-crystalline surfaces

Studying the initial interaction of water with a semiconductor surface can help to understand many properties decisive for its performance as photoabsorber for solar water splitting. The contact with water can lead to corrosion or surface passivation which then leads to or prevents losses by surface charge carrier recombination. In this study, we show that the slightest deviation from ideal surfaces in  the form of terrace edges or trace carbon contamination can change the interaction of the solid with water both qualitatively and quantitatively. These results highlight challenges for the interpretation of experimental data and their comparability to idealised electronic structure models.

 

M. M. May, H. Stange, J. Weinrich, T. Hannappel, O. Supplie, "The impact of non-ideal surfaces on the solid–water interaction: a time-resolved adsorption study" SciPost Physics 6, 58/1-13 (2019)

 

Characterization of single crystalline BiVO4 surfaces

The power of single crystal surface studies stems from their atomically well-defined nature. In this paper, we structurally, chemically and electronically characterized the most stable (010) surface of rare, Mo-doped BiVO4 single crystals. Using Low Energy Electron Diffraction (LEED), and synchrotron-based X-ray spectroscopies (X-ray absorption, photoelectron spectroscopy and resonant photoemission) we find that the (010) surface is structurally similar to a bulk termination of a BiVO4 crystal along [010] direction. Further, the extra electron from the Mo dopant localizes in vanadium sites and gives rise to a small polaron state 0.9 eV above the valence band maximum. This study provides a basis for future work aimed at a molecular-level understanding of BiVO4/aqueous electrolyte interfaces used in solar water splitting devices.

 

M. Favaro, R. Uecker, S. Nappini, I. Píš, E. Magnano, H. Bluhm, R. van de Krol D. E. Starr, “Chemical, Structural, and Electronic Characterization of the (010) Surface of Single Crystalline Bismuth Vanadate” J. Phys. Chem. C 123, 8347 - 8359 (2019)

Copper oxide photocathodes: tr-2PPE reveals location of efficiency loss

While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. In this paper, we used femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses in high-quality Cu2O single crystals provided by our partners from the California Institute of Technology (Caltech). We observed that the presence of copper vacancies can lead to ultrafast trapping of conduction band electrons within one picosecond preventing excited carriers to reach the surface at conduction band levels. Hence, we could directly reveal the origin and mechanism of photovoltage losses in Cu2O resulting from bulk recombination processes into bulk defect states.

 

Selected as Editor’s Highlight in Nature Communications

 

HZB science highlight from 9.5.2019

 

M. Borgwardt, S. T. Omelchenko, M. Favaro, P. Plate, Ch. Höhn, D. Abou-Ras, K. Schwarzburg, R. van de Krol, H. A. Atwater, N. S. Lewis, R. Eichberger, D. Friedrich, “Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes”, Nature Communications 10, 2106 (2019)

 

Schematic illustration of time resolved two photon photoemission spectroscopy (tr-2PPE). Taken from the original article: https://www.nature.com/articles/s41467-019-10143-x. The article’s creative commons license is available here: http://creativecommons.org/licenses/by/4.0/

Improved Back-Contact for CuBi2O4 Photocathodes

P-type CuBi2O4 is a candidate absorber material for solar water splitting devices. The bandgap is nearly ideal, but the photocurrents are still far below the theoretical maximum. In this work we show that this can be improved by adding a hole-selective Cu-doped NiO contact layer. The Cu:NiO is inserted between the CuBi2O4 film and the FTO back contact and lowers the height of the potential barrier. This improves the photocurrent by 25%, resulting in photocurrents up to 2.83 mA/cm2 in the presence of a H2O2 electron scavenger. These are the highest photocurrents reported thus far for this material.

 

Song, P. Plate, A. Chemseddine, F. Wang, F.F. Abdi, M. Wollgarten, R. van de Krol, S.P. Berglund, “Cu:NiO as a hole-selective back contact to improve the photoelectrochemical performance of CuBi2O4 thin film photocathodes”, J. Mater. Chem. A 7, 9183-9194 (2019).

Advances in Photoelectrochemical Water Splitting

Edited by scientists from EPFL, NREL, and HZB, this book provides an overview of recent advances in photoelectrochemical water splitting. It starts by outlining the challenges in the field, followed by theoretical approaches toward materials screening and design. This is followed by chapters on identification on reaction intermediates, charge transfer, solution-processed photoelectrodes, nanoparticulate systems, and the use of bipolar membranes in water splitting devices. The final part is on the modeling of PEC devices and a techno-economic analysis of water splitting at various scales. The chapters are written by internationally reknowned experts in this exciting and fast-developing field.

 

 

 

“Advances in Photoelectrochemical Water Splitting – Theory, Experiment and Systems Analysis”, Edited by S.D. Tilley, S. Lany, and R. van de Krol.
RSC Publishing (2018).  ISBN: 978-1-78262-925-2

Pathways to electrochemical solar-hydrogen technologies

In this paper, more than 40 scientists join forces to discuss potential pathways for solar hydrogen technologies. This work is the result of a one-week focused workshop that all authors attended in the summer of 2016 at the Lorentz center in Leiden, the Netherlands. Various technical approaches, societal impact, economic drivers, technical challenges, and research opportunities are outlined, followed by both short- and long-term implementation scenarios.

 

S. Ardo, D. Fernandez Rivas, M. Modestino, V. Schulze Greiving, F.F. Abdi, E. Alarcon Llado, V. Artero, K. Ayers, C. Battaglia, J. Becker, D. Bederak, A. Berger, F. Buda, E. Chinello, B. Dam, V. Di Palma, T. Edvinsson, K. Fujii, H. Gardeniers, H. Geerlings, S. Mohammad, H. Hashemi, S. Haussener, F. Houle, J. Huskens, B.D. James, K. Konrad, A. Kudo, P. Patil Kunturu, D. Lohse, B. Mei, E.L. Miller, G. F. Moore, J. Muller, K.L. Orchard, T.E. Rosser, F. Saadi, J.W. Schüttauf, B. Seger, S.W. Sheehan, W.A. Smith, J. Spurgeon, M. Tang, R. van de Krol, P.C.K. Vesborg, P. Westerik, “Pathways to electrochemical solar-hydrogen technologies”, Energy Env. Sci. 11, 2768 (2018).

Review on advances in rational engineering of multinary oxides for water splitting

The group of Prof. Hongqiang Wang at Northwestern Polytechnical University in Xi’an, China, published an extensive review in Nano Energy on multinary metal oxide photoelectrodes and various approaches on how to improve the performance of these materials. We contributed to part of this interesting and comprehensive review.

 

J. Jian, G. Jiang, R. van de Krol, B. Wei, H. Wang, Recent Advances in Rational Engineering of Multinary Semiconductors for Photoelectrochemical Hydrogen Generation, Nano Energy 51, 457-480 (2018)

FeVO4 photoanodes for solar water oxidation

Together with our colleagues from Nanyang Technological University in Singapore, we explored FeVO4 as a potential photoanode material for water splitting. With a bandgap of 2.07 eV this material is a good visible light absorber, but we find that the material suffers from low carrier mobilities. Intriguingly, doping the material with molybdenum enhances both the carrier mobility and lifetime by a factor of 3. Despite this improvement, extensive nanostructuring is likely needed to obtain practical photocurrent densities.

 

Zhang, Y. Ma, D. Friedrich, R. van de Krol, L.H. Wong, F.F. Abdi, Elucidation of the opto-electronic and photoelectrochemical properties of FeVO4 photoanodes for solar water oxidation, J. Mater. Chem. A 6, 548-555 (2018)

Perspectives on the photoelectrochemical storage of solar energy

In this paper, we argue that water splitting will be a central challenge for any fossil fuel-free energy infrastructure that relies on liquid or gaseous chemical fuels. We discuss potential advantages of integrated ‘direct’ photoelectrolysis vs. PV-driven electrolysis and outline several key challenges and research needs in the field.

 

 

R. van de Krol and B.A. Parkinson, “Perspectives on the Photoelectrochemical Storage of Solar Energy, MRS Energy Sustain. 4, E13 (2017)

Gradient Doping improves charge separation in CuBi2O4 photocathodes

p-Type CuBi2O4 is a candidate material for photoelectrochemical water splitting. By deliberately introducing a gradient in the Cu vacancy concentration, the charge separation efficiency in sprayed CuBi2O4 photocathodes could be significantly enhanced. This resulted in an AM1.5 photocurrent of 2.5 mA/cm2 in the presence of an electron scavenger, a new record for this material. Together with our colleagues from the University of Zurich, we also developed a CdS/TiO2/Pt protection layer to prevent (photo)corrosion. These protected photocathodes showed a photocurrent density of -1.0 mA/cm2 at 0 V vs. RHE and evolve hydrogen with a Faradaic efficiency of 91%.

 

F. Wang, W. Septina, A. Chemseddine, F.F. Abdi, D. Friedrich, P. Bogdanoff, R. van de Krol, S.D. Tilley, S.P. Berglund, "Gradient self-doped CuBi2O4 with highly improved charge separation efficiency", J. Am. Chem. Soc. 139, 15094 (2017)

Hydrogen treatment extends charge carrier lifetime in metal oxide photoelectrodes

Photoelectrodes based on metal oxides have only shown relatively low efficiency, due to poor carrier transport properties and a large concentration of point defects that may act as performance killers. By treating the material at moderate temperatures (300°C) in a hydrogen atmosphere, the photoactivity can be significantly improved. Together with our collaborators, we showed that hydrogen goes into the BiVO4 lattice, where it passivates defects and increases the carrier lifetime by more than a factor of two.

 

J.W. Jang, D. Friedrich, Ss Müller, M. Lamers, H. Hempel, S. Lardhi, Z. Cao, M. Harb, L. Cavallo, R. Heller, R. Eichberger, R. van de Krol, and F. F. Abdi*, Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment”, Adv. Energy Materals, 1701536 (2017)

 

Read press release at HZB website here

Operando study of light-induced reactions at BiVO4 surface

We investigated light-induced modifications of the BiVO4/electrolyte interface. A BiVO4 sample was kept in contact with a phosphate buffer solution while the interface was probed using AP-HAXPES. Measurements were performed at open circuit potential, under dark and light conditions (1 Sun). We find that under illumination bismuth phosphate forms on the BiVO4 surface leading to an increase in negative charge and a re-distribution of the aqueous ions near the interface. The bismuth phosphate layer may act to passivate surface states observed in complementary photoelectrochemical measurements. Finally, we find that such changes are reversible upon returning to dark conditions.

 

M. Favaro,* F. F. Abdi, M. Lamers, E. J. Crumlin, Z. Liu, R. van de Krol, D. E. Starr*, “Light-induced Surface Reactions at the Bismuth Vanadate/Potassium Phosphate Interface”, J. Phys. Chem. B, (accepted, 2017)

High oxygen evolution activities for electrodeposited α-Mn2O3 films

The generation of hydrogen by solar-driven electrochemical water splitting is a possible approach to store renewable energies as a non-fossil fuel in large quantities. To achieve this goal, earth-abundant and highly active catalysts for both half reactions, the hydrogen and the oxygen evolution reaction (OER), have to be identified and developed for a mass market. This kind of catalysts should be applicable in photoelectrochemical water splitting devices, by being deposited as co-catalysts on the surface of suitable photoelectrodes.
Under this aspect, highly porous manganese(III) oxide layers have been developed as OER electrodes showing low over voltages (340mV at 10mA/cm2 current density).

 

M. Kölbach, S. Fiechter, R. van de Krol, P. Bogdanoff, "Evaluation of electrodeposited α-Mn2O3 as a catalyst for the oxygen evolution reaction", Catalysis Today, 290, p. 2-9, (2017)