Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

The need for better energy storage and conversion devices has never been so urgent in order to enable the rapid deployment of renewable energies and reduce our use of fossil fuels. While batteries spurred the spread of portable electronics, their limited energy density hampers their use for large scale applications. Hydrogen has long been envisioned as a viable energy carrier owing to its very large energy density, nevertheless its electrochemical production by electrolysis, the most efficient carbon-free production route at large scale, greatly suffers from the slow kinetics associated with the oxygen evolution reaction (OER). The key challenges that need to be addressed to improve the OER kinetics are well-spotted and researchers eagerly pushed to better understand the reaction leading to numerous progresses since our early vision of this reaction. Hence, while some works were devoted to finding physical descriptors capable of describing the OER activity following a Sabatier principle, recent developments in the field point towards the complexity of such reaction. Indeed, a substantial body of evidence now points towards the involvement of the bulk chemistry of the most active transition metal oxides in the OER mechanism. Hence, we recently found that bulk oxygen atoms are evolved under OER conditions for cobalt-based perovskites materials,¹ eventually triggering a new mechanism into which chemical steps and proton exchange are rate limiting. We could then demonstrate that the origin for the often observed activity-stability relationship for OER electrocatalysts is nested into the existence of a common intermediate which is reactive surface oxygen in the form of oxyl-group.² Overall, the line becomes blurrier between heterogeneous and homogeneous catalysis when using transition metal oxides as OER catalysts for which complex surface dynamics are at play.³ Hence, efforts must be paid to understanding and stabilizing these intermediates in order to break this relationship and further enhance the stability of OER electrocatalysts. We will therefore discuss in this talk our recent efforts at designing chemical approach to counterbalance the chemical reactivity of the most active transition metal oxides through a combined crystallographic and electrolyte engineering approach.

The increasing replacement of fossil fuels by renewable energies from wind and sun requires large energy storage capabilities, because of the only intermittent availability of these sources. Such big capacities can only be provided by the storage in chemical bonds, the simplest being the hydrogen molecule formed by direct electrochemical water splitting. For this purpose, expensive and rare electrocatalysts made from Platinum series metals will have to be replaced by cheaper, more abundant and hazard-free materials. Nickel metal and its oxides are known since many years for their good activity for water splitting.

In our studies Nickeloxide nanoparticles and thin films are prepared by electrochemical deposition as well as by magnetron sputtering. The composition is optimised towards the achievement of an optimum activity for the hydrogen evolution reaction (HER) as well as the oxygen evolution reaction (OER). In order to gain a deeper understanding of the critical parameters for the catalytic activity during the electrochemical reaction, we investigate the chemical composition by XPS and SEM before and after electrochemical testing as well as by in-operando Raman-spectroscopy. The achieved activities are comparable to Platinum for the HER and to RuO2 for the OER. They depend strongly on the chemical composition of the catalyst, which in turn is heavily influenced by the chosen preparation method. Furthermore, we investigate the stability of our catalysts in relation to their activity and composition. The highly active Nickel (oxide/hydroxide) mixture for the HER degrades over time by the complete transformation to the less active pure Nickel-dihydroxide compound. For the OER the pre-treatment of the Nickel compound is of extreme importance in order to form a large amount of the catalytically active NiO(OH) species on the surface during the electrochemical reaction. These nanoparticles show a nearly constant activity for a testing period of 26 hours at a current density of 10 mA/cm2.

The study of electrocatalysts for water splitting is pivotal to improving the efficiency of fuel production. This is an attractive field since electrocatalysts can be used: (1) as part of a photoelectrode, for direct sunlight to fuel transformation, or (2) can be used in an electrolyser to transform the electricity generated through a solar panel to produce a fuel. In a complete system it is thought that water oxidation is the limiting process. Therefore, a lot of effort is devoted to finding a better catalyst for this reaction in order to enhance the final performance of the system.

Recently, iron and nickel oxyhydroxides have been appointed as good candidates for the water oxidation reaction in basic conditions. In this context, interest for understanding the origin of their behaviour has grown in recent years. In 2014, Boettcher and co-workers published for the first time that the high activity towards water oxidation of NiOOH was due to Fe incorporation from the electrolyte.[1] Since then, there has been a lot of effort to determine which species are involved in the catalytic reaction, with no consensus reached. Some experiments suggest the presence of Fe(IV) during the catalysis,[2] while on the contrary, other works cannot detect such iron species and suggest that nickel centres are the active sites.[3] Despite this debate, some structural differences have been found due to iron incorporation which leads to an improved performance. However, the mechanistic and kinetic analysis of metal oxide based electrocatalysts, in general, is hampered by the non-ideal nature of these materials. Most of these electrocatalysts present different redox states and a non-planar and dense structure. Thus the interpretation of the traditional electrochemical techniques, such as tafel plots, is more complicated and sometimes not possible.[4]

In this work, we used spectroelectrochemical methods to analyse three different samples: Pure FeOOH, a mixed FeOOHNiOOH sample composed of different layers, and finally Ni(Fe)OOH with spontaneous Fe incorporation. From these measurements, we are able to determine the active species and study the kinetics under catalytic conditions to yield a basic mechanistic picture.

References

[1]          L. Trotochaud, et al. JACS, 2014, 136, 6744-6753.

[2]          J. Y. C. Chen, et al. JACS, 2015, 137, 15090-15093.

[3]          M. Görlin, et at, JACS, 2017, 139, 2070-2082.

[4]          E. Pastor, et al, Nat. Commun., 2017, 8, 14280.

Photoelectrochemical water splitting is one of the most interesting alternatives to produce hydrogen in a clean way by solar energy conversion.(1) Despite the huge potential and great advances, new materials need to be developed in order to take this technology to a commercial level. At present, different materials as oxides, oxisulfides and metal chalcogenides are being investigated as photoelectrodes in photoelectrochemical cells. However, achieving high energy conversion efficiencies by using a single material is a very tricky objective. Therefore, hybrid materials are getting a lot of attention lately. (2)

In this work, we present a hybrid material formed by the heterojunction of a novel synthesized organic conductive polymer and TiO2 nanocrystals. The nanostructured conjugated porous polymer is based on dithiothiophene moiety (Nano-CMPDTT) and was synthesized by Sonogashira cross coupling reaction from precursors in mini-emulsion conditions. In order to elucidate the electronic structure and the ability of this material to be used as a photocatalyst, HOMO and LUMO positions were determined by cyclic voltammetry. The energy diagram shows an ideal position of the energy bands in order to use the synthesized polymer as an electron injector to TiO2 in photocatalytic reactions. TiO2 NCs and organic polymer suspensions have been deposited by spin coating in ITO glasses. The formed films have been characterized by X-ray diffraction, SEM, EDX and AFM. Photoelectrochemical measurements were performed in a three electrode cell configuration, using the hybrid material as the working electrode. The hybrid material presents an enhancement in photovoltages and photocurrents values. Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid material in comparison to the TiO2 nanocrystals alone. In fact, a decrease in the resistance associated with this phenomenon was found. This confirms that the presence of the polymer in the hybrid material increases the absorption of light, charge transfer and reduces electron-hole recombination, making this hybrid a good candidate to be used as a photoelectrode for the hydrogen evolution reaction. In fact, recent results show an improvement in the energy conversion efficiency by using this new hybrid material as electrode compared with regular TiO2.

[1] Z. Chen, H. N. Dinh, E. Miller, Photoelectrochemical Water Splitting (Springer New York, New York, NY, 2013; http://link.springer.com/10.1007/978-1-4614-8298-7), SpringerBriefs in Energy.

[2] M. P. Arciniegas et al., Self-Assembled Dense Colloidal Cu2Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

Transition metal oxides are amongst the most widely studied materials for solar water oxidation owing to their earth abundance, good aqueous stability and facile syntheses.¹ Understanding the dynamics of the photogenerated charge carriers in these materials is key to highlighting properties that need to be targeted to improve water splitting performance. With a narrower band gap than TiO₂, tungsten trioxide (WO₃) can absorb a larger proportion of the solar spectrum and has a deeper valence band energy to provide a large thermodynamic driving force for water oxidation.^2-3 WO₃ is also often reported to have high electrical conductivity, leading to its frequent implementation as an electron transporting layer in various heterojunction photoanodes.^2-4 This high conductivity is considered to be a result of a large density of charge carriers, caused by intrinsic oxygen vacancies which can act as n-type dopants.⁵ However, deviations from stoichiometry have also been suggested to introduce chemical defects that can result in increased trapping of charges which, rather than boost performance, can often introduce additional recombination pathways.⁶

Using transient diffuse reflectance spectroscopy and transient photocurrent measurements, I will discuss the dynamics of the photogenerated charges in WO₃ photoanodes, synthesised by chemical vapour deposition (CVD). The synthesis method generates heavily doped monoclinic WO_3-x needles, which are annealed in air at elevated temperature to remove most of the oxygen vacancies. These photoanodes exhibit an early photocurrent onset and a high faradaic efficiency (>85%) for water oxidation. Compared to other transition metal oxide photoanodes, we observed rapid water oxidation (>1 ms) but found the rate of electron extraction is significantly slower (>10 ms). We investigated the effect of oxygen vacancy states on electron transport and found that electron trapping in the needles is significant, proposing a trap-mediated mechanism of electron transport to the back contact. We then used ultrafast transient absorption spectroscopy to examine the bias dependence on bulk recombination processes. Finally, we altered the oxygen vacancy content to determine the overall effect of these defects on charge transport and performance.

[1] G. Wang, et al. Energy Environl Sci, 2012, 5, 6180–6187.

[2] Z. Huang, et al. Advanced Materials, 2015, 27 (36), 5309-5327.

[3] X. Liu, et al. PCCP, 2012, 14 (22), 7894-7911.

[4] C.X. Kronawitter, et al. Energy Environl Sci 2011, 4 (10), 3889-3899.

[5] T. Zhu, et al. ChemSusChem, 2014, 7, 2974-2997

[6] M.B. Johansson, et al. J Phys Condens Matter, 2016, 28, 475802.

Organometallic complexes with reactive metal centers are promising candidates for photocatalysis. To improve their performance, insight into the nature of excited states and control thereof is crucial. Ultrafast x-ray spectroscopy is a powerful method to achieve mechanistic insight into processes at catalytic metal moieties and is particularly useful to detect light-induced changes in oxidation state and coordination geometry. It can also be applied to probe the essential charge transfer step to the catalyst in real-time, including the potential involvement of atomic rearrangements.
We recently developed a series of new Ru-metal photocatalysts, of which the RuPt derivative showed a H2 turnover number of 80 after 6 h of irradiation at 470 nm. The photodynamics of RuPt studied by femtosecond transient absorption are highly complex and differ significantly from the RuPd analogue [1], indicating an important role of the catalytic moiety and motivating ultrafast x-ray absorption studies performed at the Advanced Photon Source at Argonne. This work particularly focuses at light-induced redox processes at the Pt moiety and the timescales thereof. Another question to be answered involves the local structure of the catalytic moiety and possible temporal or permanent changes.
The earliest differential absorption spectrum at 30 ps indicates that the Pt moiety is already reduced at this timescale. The signal intensity partially decays on a timescale of ca. 930 ps. The observation of a differential absorption signal at timescales far beyond indicates branching. The spectra at late timescales can be modelled by a hexa-coordinated PtIV (or possibly PtIII) species. This intermediate species may be formed by oxidative addition of iodine, is long-lived (>10 microseconds) and ultimately recovers into the original ground state structure. Hence, activation of the catalyst by a single photon may lead to withdrawal of two electrons. This mechanism presents a new paradigm for H2 generation by supramolecular photocatalysts.

[1] Q. Pan, F. Mecozzi, J.P. Korterik, J.G. Vos, W.R. Browne, A. Huijser, "The Critical Role Played by the Catalytic Moiety in the Early-Time Photodynamics of Hydrogen-Generating Bimetallic Photocatalysts", ChemPhysChem 17, 2654-2659 (2016).   

Developing energy storage mechanisms is a crucial step to secure a sustainable energy economy. This is particularly important for solar power because there is a mismatch between the periods of peak energy harvesting and peak consumption. Photo-electrocatalytic energy storage has traditionally been dominated by inorganic systems. In particular, transition metal oxides are attractive due to their stability in liquid electrolytes and the remarkable catalytic flexibility of metallic centres. However, inorganic systems often underperform and only poor quantum yields are achieved, even when synthetic care is taken to eliminate defects and impurities.  In the dark, oxides like Fe₂O₃ and BiVO₄ are poor conductors due to the localization of charges on specific atomic sites forming small polarons. Under illumination, transport improves but the mechanisms behind it are unknown. Recent x-ray and terahertz studies have hinted at the existence of light-induced small polarons and their link with structural properties. ^1-3 However, these measurements lack device sensitivity and it is difficult to know if such states really influence performance. In this talk, I will present ultrafast optical studies with photocurrent detection of photoelectrochemical devices. These measurements are capable of probing light-initiated changes that control actual device activity.⁴ I will present data demonstrating that, in oxides such as efficient Fe₂O₃ thin-films,⁵ charges intrinsically self-trap within femtoseconds of photo-generation hindering transport. Importantly, I will show how in efficient systems, crossover between molecular-type, localised transport and band-like transport is critical and necessary for activity. This behavior is remarkably similar to that observed in molecular crystals and some of the strategies used to improve molecular semiconductors might apply to metal oxides. I will discuss the material implication of these findings and how it might be possible to achieve synthetic control of charge hopping and delocalisation rates in oxides.

References:

¹Carneiro L. et al. Nat Mater. 2017, 16, 8, 819 

²Biswas S. et al. Nano Lett. 2018,18,1228

³Butler  K.T. et al. J.Mat.Chem.A, 2016,4,18516

⁴Bakulin A. et al. J.Phys.Chem. Lett. 2016, 7, 2, 250

⁵Steier L. et al. ACS Nano, 2015,9,11775

Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a primary target for artificial photosynthesis,¹ which requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. We will report on our contribution to the development of various series of catalysts for H₂ evolution,^2-4 including the reinvestigation of amorphous molybdenum sulfide⁵ and to the establishment of methodologies towards the rational benchmarking of their catalytic activity. Besides, we will also describe our effort towards the combination of such catalysts with various photoactive motifs for the preparation of photoelectrode materials^6-10 that can be implemented into photoelectrochemical (PEC) cells for water splitting.

References

1.         N. Queyriaux, N. Kaeffer, A. Morozan, M. Chavarot-Kerlidou and V. Artero, J. Photochem. Photobiol. C, 2015, 25, 90-105.

2.         D. Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons, J. Pécaut, S. Demeshko, F. Meyer, M. Orio, V. Artero and C. Duboc, Nat. Chem., 2016, 8, 1054-1060.

3.         N. Kaeffer, M. Chavarot-Kerlidou and V. Artero, Acc. Chem. Res., 2015, 48, 1286–1295.

4.         T. N. Huan, R. T. Jane, A. Benayad, L. Guetaz, P. D. Tran and V. Artero, Energy Environ. Sci., 2016, 9, 940-947.

5.         P. D. Tran, T. V. Tran, M. Orio, S. Torelli, Q. D. Truong, K. Nayuki, Y. Sasaki, S. Y. Chiam, R. Yi, I. Honma, J. Barber and V. Artero, Nat. Mater., 2016, 15, 640-646.

6.         J. Massin, M. Bräutigam, N. Kaeffer, N. Queyriaux, M. J. Field, F. H. Schacher, J. Popp, M. Chavarot-Kerlidou, B. Dietzek and V. Artero, Interface Focus, 2015, 5, 20140083.

7.         N. Kaeffer, J. Massin, C. Lebrun, O. Renault, M. Chavarot-Kerlidou and V. Artero, J. Am. Chem. Soc., 2016, 138, 12308−12311.

8.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, C. Laberty-Robert, S. Campidelli, R. de Bettignies, V. Artero, S. Palacin and B. Jousselme, Energy Environ. Sci., 2013, 6, 2706-2713.

9.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, R. Cornut, V. Artero and B. Jousselme, ACS Appl. Mater. Interfaces, 2015, 7, 16395–16403.

10.       A. Morozan, T. Bourgeteau, D. Tondelier, B. Geffroy, B. Jousselme and V. Artero, Nanotechnology, 2016, 27, 355401.

One of the most promising future sources of alternative energy involves water-splitting photoelectrochemical cells (PECs) – a technology that could potentially convert sunlight and water directly to a clean, environmentally-friendly, and cheap hydrogen fuel. Practical PEC-mediated hydrogen production requires robust and highly efficient semiconductors, which should possess good light-harvesting properties, a suitable energy band position, stability in harsh condition, and a low price. Despite great progress in this field, new semiconductors that entail such stringent requirements are still sought after. Over the past few years, graphitic carbon nitride (g-CN) has attracted widespread attention due to its outstanding electronic properties, which have been exploited in various applications – including in photo- and electro-catalysis, heterogeneous catalysis, CO2 reduction, water splitting, light-emitting diodes, and PV cells. gCN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, to date, only a few reports regarded the utilization of g-CN in PECs, due to the difficulty in acquiring a homogenous g-CN layer on a conductive substrate and to our lack of basic understanding of the intrinsic layer properties of g-CN. In this talk I will introduce new approaches to grow g-CN layers with altered properties on conductive substrates for photoelectrochemical application. The growth mechanism as well as their chemical, photophysical, electronic and charge transfer properties will be discussed.

While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic photocatalysts are currently gaining substantial momentum - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerising suitable monomers in defined ratios.¹ This tunability has sparked intense research interest in organic photocatalysts,^2,3 however, the fundamental understanding of photoinduced processes in these systems has stayed behind the rapid development of new materials. Some parallels can be drawn to organic photovoltaics where comparable materials are used, but especially the aqueous environment makes polymer photocatalysts distinct from other applications. To understand what dictates their performance and how structurally similar polymers can exhibit very different degrees of hydrogen evolution activity,⁴ photophysical processes in these materials require further investigation.

The combined study presented here is the first in-depth investigation of hydrogen evolution activity of linear conjugated polymers and combines materials development with spectroscopic characterisation and computational modelling. We investigate a series of polymers with strikingly different hydrogen evolution activity, including some of the highest performing photocatalysts reported to date in this class of materials. A comparison to structurally related polymers with significantly lower activity allows us to identify the key determinants of hydrogen evolution activity in this series. To this end, we use transient absorption spectroscopy to monitor photogenerated reaction intermediates on time sales of femtoseconds to seconds after light absorption and correlate the type and yield of observed intermediates with the hydrogen evolution activity of the respective polymer. Computational simulations and calculations build on this transient data and extend the observations to the role of the solvent environment in the photoinduced reaction sequence. The presented results can provide design strategies for new materials and thus have implications for the development of more efficient organic photocatalysts.

References

1. Sprick, R. S. et al. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 137, 3265–3270 (2015).

2. Zhang, G., Lan, Z.-A. & Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chemie Int. Ed. 55, 15712–15727 (2016).

3. Vyas, V. S., Lau, V. W. & Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Production. Chem. Mater. 28, 5191–5204 (2016).

4. Sprick, R. S. et al. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chemie Int. Ed. 55, 1792–1796 (2016).

The concept of dye-sensitized  photoelectrosynthesis cells (DS-PECs) has recently been proposed as an alternative water splitting cell. The DS-PEC architecture involves an oxide semiconductor sensitized by an organic/inorganic chromophore and a water oxidation catalyst (WOC) either connected to the chromophore (a dyad assembly) or directly to the semiconductor. In this assembly, chromophore produces an electron-hole pair upon excitation with visible light followed by the injection of electrons to the semiconductor and then to the cathode compartment while holes are transferred to the catalytic site to activate the catalyst for water oxidation.

Despite the recent promising studies, achieving high stability for both the catalyst and the chromophore under DS-PEC operating conditions, extending the light absorption to visible and near IR region, and developing entirely earth abundant assemblies still remain as key challenges. 

Our recent research efforts have recently been devoted to developing dye-sensitized photoanodes involving cyanide-based assemblies. For this purpose, pentacyanoferrate building block was used not only as donor system for the construction of a new donor-acceptor chromophore but also as a cyanide precursor to produce a heterogeneous cobalt-iron Prussian blue electrocatalyst film. The photoelectrochemical performance and superior stability of the Prussian blue film coupled with TiO₂ will be presented in detail.

Integrated wireless monolithic solar water splitting devices, i.e. monoliths submerged in the electrolyte, is a promising approach for low-cost photoelectrochemical solar water splitting [1]. However, such a device design poses a significant limitation: the ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This turns out to be a bottleneck for reaching high efficiencies and maintain optimum performance when it comes to up-scaling.

In this work, we present a novel approach to tackle the aforementioned limitation. Our device design is based on the concept of porosity as an ionic shortcut, implemented in silicon-based monoliths for low-cost and scalable solar water splitting [2, 3]. Simulation and experimental results towards enabling the proof of concept device consisting of porous multi-junction thin-film silicon solar cells on perforated substrates are presented. Based on simulations, we highlight how porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. As a result, the overpotentials to drive the water splitting reaction can be reduced by more than 400 mV. Experimentally, the impact of porosity (square array of holes with a period of 100 μm and a diameter of 20 μm) on single-junction and multi-junction amorphous and microcrystalline thin-film silicon solar cells is explained. A minimal decrease in V_OC is seen, with porous triple-junction thin-film silicon solar cells reaching a value of 1.98 V. Additionally, we discuss the implementation of surface coatings by atomic layer deposition to i) alleviate the material degradation that occurs during silicon etching (electrical passivation) and ii) enable a chemically stable operation (protection against material corrosion). Finally, to demonstrate the beneficial effect of porosity on the hydrogen production we focus on two systems: i) a well-known simplified system based on platinum nanoparticle decorated porous silicon photocathodes (V_on ≈ 475 mV) immersed in a sulfite scavenger (cell potential ΔΕ⁰ = 170 mV) in absence of electrical bias and ii) the envisage device of porous multi-junction silicon solar cell monoliths in unbiased solar water splitting conditions. Overall, keeping a device oriented point of view, we discuss the results and challenges in our approach as well as some design guidelines.

References:

[1] S. Y. Reece et al., Science 334, 645-648 (2011).

[2] T. Bosserez et al., J. Phys. Chem. C 120 (38), 21242 – 21247 (2016)

[3] C. Trompoukis et al., Sol. Energy Mater. Sol. Cells 182, 196–203 (2018).

Photocatalytic conversion of CO2 and H2O is an interesting route to produce fuels and chemicals [1]; this process is also known as Artificial Photosynthesis (AP). In last years, extensive efforts have been made to develop efficient catalytic systems capable of harvesting light absorption and reducing CO2 especially when using water as the electron donor. Several modification pathways of inorganic semiconductors have been performed to improve the reaction efficiencies, including tailoring of the band structure, doping with metals and non-metallic elements and deposition of metal nanoparticles, etc [1-2].

Herein, we report different strategies and modifications of photocatalysts to increase process performance. Among them, an interesting approach to improve charge separation in photocatalytic systems is the use of heterojunctions. In this line, the combination of different semiconductors with noble metal nanoparticles or organic semiconducting polymers leads to a separation of the photogenerated charge carriers and thus to increasing their life time, facilitating charge transfer to adsorbed molecules.

The main products, using bare TiO2, were CO and H2, with low concentrations of CH4. The deposition of surface plasmon nanoparticles (SP-NPs) leads to changes in the selectivity to higher electron-demanding products, such as CH4. TAS measurements confirm that this behaviour is due to the electron scavenging ability of SP-NPs.

The H2 evolution rates of organo-inorganic hybrid materials, is increased with the polymer content reaching the optimum with IEP-1@T-10 which improves the activity of TiO2 by a factor of 40. These hybrid materials also show a dramatic reactivity improvement in CO2 photoreduction. Hybrid materials also show a great change in the selectivity, enhancing the relative production of methane vs. carbon monoxide, and largely promoting selectivity towards hydrogen. Meanwhile, bare TiO2 gives rise to the formation of CO, a small amount of CH4 and H2 and traces of CH3OH and higher hydrocarbons (C2 mainly). To explain this behaviour a combination of in-situ NAP-XPS, FTIR, TAS spectroscopies and theoretical tools has been used, showing a more efficient light absorption and charge transfer in the hybrid photocatalyst compared with bare materials.

References

[1] V. A. de la Peña O’Shea, D. P. Serrano, J. M. Coronado, “Current challenges of CO2 photocatalytic reduction over semiconductors using sunlight”, in Molecules to Materials—Pathway to Artificial Photosynthesis, Ed. E. Rozhkova, K. Ariga (Eds.), Springer, London, 2015.

[2] L. Collado, A. Reynal, J.M. Coronado, D.P. Serrano, J.R. Durrant, V.A. de la Peña O'Shea, Appl. Catal. B: Environ. 178, 177, 2015

There is a need to rationalise the catalytic mechanisms occurring during solar fuels production if the design rules for improved materials are to be identified. With both direct solar to fuels materials (e.g. photoelectrodes) and indirect systems (e.g. PV driven electrolysis) there is a common challenge – how to characterise short-lived intermediates that are present only at the electrode surface in the presence of a high concentration of bulk solvent/reagent.

IR-Vis Sum-Frequency Generation (SFG) spectroscopy specifically probes molecules at interfaces offering a sensitive probe of only the catalytically relevant species at the electrode surface. SFG has been widely used to study model electrochemical systems but rarely to explore the mechanisms of carbon dioxide reduction and water oxidation. Here I will describe both early experiments to examine photoelectrode surfaces and our recent reports on the mechanisms of carbon dioxide reduction by group 6 and 7 transition metal electrocatalysts which allow for the identification of new catalytic intermediates and the first in-situ observation of the “protonation-first” pathway by [Mn(bpy)(CO)₃Br]. We will also discuss how the potential and light dependence of the non-resonant response recorded during the SFG experiment can provide important insights into the changing nature of the double layer structure during the study of solar fuels materials.

Storing the energy of sunlight into feedstock chemicals or energy-rich compounds, such hydrogen, appears as an enticing option to broaden the utilization of solar energy far beyond classical photovoltaics. Among different emerging technologies, photoelectrochemical (PEC) water splitting stands out with the promise of competitive solar-to-hydrogen conversion efficiencies (predicted to be above 20 %) and a conveniently simple design.¹ These devices, comprising two photoactive electrodes wired-stacked together, to leverage their complementary light absorption, and directly immersed in an aqueous electrolyte, generate under illumination a voltage greater than 1.23 V, driving the photoelectrosynthetic reactions of H₂ and O₂ separately at the different electrode-electrolyte interfaces. Unfortunately, current conversion efficiencies are far below the expectations. In recent years, there has been an encouraging progress on the refinement of the bulk properties of the semiconducting electrodes (viz. nanostructuring, doping, band gap engineering, etc.) as well as on the design of more active electrocatalyst, both contributing to an improved performance. However, another key component of these devices, the semiconductor-liquid junction (SCLJ), where the complex reactions occur and therefore, whose electrochemical and catalytic properties (namely, surface potential, overpotential, kinetics of charge transfer and recombination) control the conversion efficiency, remains hazy posing a major bottleneck for enhancing the conversion efficiency. A better understanding on the electrocatalytic properties of the SCLJ could not only nail down the processes limiting the performance of these devices but also provide specific routes to patch them.

Over the last few years, a wide variety of in-operando electrochemical based techniques have burst in the field of electrochemical-based solar fuel production offering new insights on the nature of intermediate species, the functioning of electrocatalysts, the carrier dynamics within the electrodes, etc.² Here, we introduce a new technique where a network-type electrical-contact at the electrode-electrolyte interface afford direct probing of the surface carrier dynamics when combined with a transient photocurrent/photovoltage technique. This technique applied to model semiconducting materials incorporating state-of-the-art electrocatalyst demonstrates to provide unprecedented access to steady-state (interfacial energetics) and transient (interfacial kinetics) electrochemical information of the SCLJ in-operando. We believe that these new tools will help to forge a better understanding on the SCLJ and to establish the designing principles for a next generation of more efficient electrodes.

[1] M. S. Prévot, K. Sivula. J. Phys. Chem. C, 2013, 117, 17879-17893

[2] W. A. Smith, I. D. Sharp, N. C. Strandwitz, J. Bisquert. Energy Environ. Sci. 2015, 8, 2851-2862

Due to their potential long term stability, ease of synthesis, and low production cost, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical water splitting devices. To date, most research has focused on binary semiconducting oxide materials. Since no binary oxide material has currently met all the criteria listed above, researchers have expanded the materials search database to include more complex multinary oxides. Among the multinary oxides investigated, bismuth vanadate, BiVO₄, is the highest performing material. However, charge carrier recombination at the BiVO₄/electrolyte interface remains a limitation. Reactions at the BiVO₄/electrolyte interface may give rise to surface states that can act as relay sites for charge injection into the electrolyte, but also as electron and hole traps that can enhance recombination rates. A detailed understanding of the chemical composition at the BiVO₄/electrolyte interface and its dependence on specific conditions (applied potential and illumination) would provide valuable input for strategies to suppress surface recombination and to further optimize BiVO₄-based photoanode materials.

We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to gain a molecular-level understanding of the BiVO₄/aqueous electrolyte interface. With soft X-rays water adsorption from the gas phase at pressures up to a few Torr can be studied providing information about the early stages of solid/electrolyte interface formation. The tender X-ray form (AP-HAXPES) can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Our AP-XPS measurements on the BiVO₄(010) single crystal surface indicate that the surface is significantly hydroxylated by ~0.5 Torr. Surface hydroxylation is accompanied by reduced vanadium at the surface which leads to occupied states above the valence band maximum. These states may act as recombination centers on BiVO₄-based photoanodes. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO₄/potassium phosphate electrolyte interface when illuminated with a solar simulator. Upon illumination we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and significant restructuring of the electrolyte near the interface. Bismuth phosphate formation under illumination may quench surface states that have been observed in capacitance versus voltage scans. Surface state suppression by bismuth phosphate formation is a also potential explanation for the increase in performance of BiVO₄ photoanodes that have undergone light soaking. In general, these results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces in water splitting devices.

The development of efficient photoelectrochemical (PEC) cells for solar hydrogen production is necessary for a prospective renewable energy supply of mankind. Semiconducting films in PEC cells are used as absorber layers in a triple system of electrolyte-catalyst-semiconductor for generating hydrogen which can be stored and afterwards reburned to water in gas turbines, fuel cells or for the production of chemicals [1]. Due to the low efficiency and poor stability of the absorber layers in the electrolyte their performance must be considerably improved.

We were successful in the preparation of highly (001)-textured polycrystalline WSe_2+x thin films with a Pd-promoter on TiN:O back contact substrates [2]. In this process, in a first step X-ray amorphous Se-rich WSe_x films have been deposited at room temperature by a reactive magnetron sputtering onto a thin metal-promoter Pd film which were afterwards annealed in an H₂S(e)/Ar atmosphere. During the crystallization process a liquid promoter PdSe_x forms and migrates to the surface of the growing WSe₂ film. It acts as a solvent agent and accumulates at the non van-der-Waals-planes finally initiating the lateral growth of WSe₂ platelets and their coalescence. A maximum hole mobility of 70 cm² V^-1s^-1 was reached for our polycrystalline films, which is close to the value for a single crystalline WSe₂ (100 cm² V^-1s^-1).

Recently, we improved the photoelectrochemical performance using Pt [3], Rh and ammonium thiomolybdate (ATM: (NH₄)₂Mo₃S₁₃) layers on top of the WSe₂ photocathode for hydrogen evolution reaction in an acidic solution. The surface states of WS₂ and WSe₂ crystallites can be passivated by photodeposition of the metal catalysts such as Pt, Rh and Ag. We show that the ATM can be used instead of expensive conventional metal catalysts on the WSe₂ photocathode. In addition, the ATM layer forms a heterojunction with the WSe₂ film, which improves the charge carrier separation, and allows lowering the prize for preparation of catalyst layers on top of different types of semiconductors.

1. Fichtner, M. J. Alloys Comp. 509S, S529-S534 (2012).

2. Bozheyev, F., Friedrich, D., Nie, M., Rengachari, M. & Ellmer, K. Phys. Stat. Sol. A 211, 2013-2019 (2014).

3. Bozheyev, F., Harbauer, Zahn, C., Friedrich D., & Ellmer, K. Sci. Rep. 7, 16003 (2017).

The synthesis of solar fuels and chemicals through artificial photosynthesis does not only require the coupling of solar light absorption and charge separation, but also the direct pairing with chemical redox processes. This approach is a one-step and versatile alternative to the more indirect coupling of a photovoltaic cell with electrolysis and enables potentially the synthesis of a wide range of fuels and feedstock chemicals. A common drawback in most artificial photosynthetic systems and organic photocatalysis is their reliance on expensive materials and device architectures, which challenges the development of ultimately scalable devices. Another limitation in many approaches is their inefficiency and reliance on sacrificial redox reagents, which may be system damaging and often prevent truly energy-storing chemistry to proceed. This presentation will give an overview about our recent progress in developing semiconductor hybrid materials to perform efficient full redox cycle solar fuel catalysis with inexpensive components, and our first steps in extending this approach for sustainable biomass photoreforming and fine chemical synthesis.

Representative recent references

(1) “Solar Hydrogen Generation from Lignocellulose”

Kuehnel, Reisner, Angew. Chem. Int. Ed., 2018, 57, 3290.

(1) “Photocatalytic CO₂ Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals”

Kuehnel, Orchard, Dalle, Reisner, J. Am. Chem. Soc., 2017, 139, 7217.

(2) “Solar-driven reforming of lignocellulose to H₂ with a CdS/CdO_x photocatalyst”

Wakerley, Kuehnel, Orchard, Ly, Rosser, Reisner, Nature Energy, 2017, 2, 17021.

(3) “Enhancing Light Absorption and Charge Transfer in Carbon Dots through Graphitization and Core N-doping”

Martindale, Hutton, Caputo, Prantl, Godin, Durrant, Reisner, Angew. Chem. Int. Ed., 2017, 56, 6459.

(4) “Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes”

Hutton, Reuillard, Martindale, Caputo, Lockwood, Butt, Reisner, J. Am. Chem. Soc., 2016, 138, 16722.

(5) “Solar-driven Reduction of Protons Coupled to Alcohol Oxidation with a Carbon Nitride-Catalyst System”

Kasap, Caputo, Martindale, Godin, Lau, Lotsch, Durrant, Reisner, J. Am. Chem. Soc., 2016, 138, 9183.

(6) “Clean Donor Oxidation Enhances H₂ Evolution Activity of a Carbon Dot-Catalyst Photosystem”

Martindale, Joliat, Bachmann, Alberto, Reisner, Angew. Chem. Int. Ed., 2016, 55, 9402.

(7) “Electrocatalytic and Solar-driven CO₂ Reduction with a Mn Catalyst Immobilized on Mesoporous TiO₂”

Rosser, Windle, Reisner, Angew. Chem. Int. Ed., 2016, 55, 7388.

Semiconductor nanocrystals (quantum dots) emerged in the last years as an appealing alternative to molecular photosensitizers, owing to their superior stability, intense light absorption and bright luminescence. In particular, non-cadmium quantum dots seem to be a particularly interesting choice as they display good optoelectronic properties while containing no toxic elements with respect to CdSe and CdTe.[1] In the field of artificial photosynthesis, very efficient ”hybrid” photocatalytic systems for hydrogen production in water were obtained by associating Cd-based quantum dots as photosensitizers with molecular H₂-evolving catalysts in presence of a sacrificial reductant. [2,3]

In this communication, we describe new hybrid systems associating environmentally friendly (Cd-free) quantum dots with molecular catalysts based on earth-abundant metals, in order to perform photocatalytic H₂ production in purely aqueous environment.

Core-shell CuInS₂/ZnS nanocrystals capped with glutathione were synthesized in the aqueous phase, and their structural and optical properties were fully characterized. The nanocrystals exhibit a broad absorption throughout the visible range with orange luminescence in aqueous solution. For the photocatalysis process the colloidal solution was mixed with a cobalt macrocyclic catalyst and a sacrificial reductant, and the H₂ production under irradiation was quantified by gas chromatography. This hybrid system exhibited remarkable activity for hydrogen production under visible light irradiation at pH 5.0 with up to 7700 and 1010 turnover numbers versus catalyst and QDs, respectively. The CIS/ZnS nanoparticles were also compared to widely studied CdSe nanocrystals, using the same catalyst, and the former give remarkably better performances in terms of TON. [4]

1. M. Sandroni et al. ACS Energy Lett., 2017, 2, 1076–1088.

2. C. Gimbert-Suriñach et al., J. Am. Chem. Soc. 2014, 136, 7655-7661.

3. Z.J. Han et al., Science 2012, 338, 1321-1324

4. M. Sandroni et al. Energy Environ. Sci., 2018, DOI: 10.1039/c8ee00120k.

Artificial photosynthesis is a very promising method to turn solar energy into fuels.¹ In a photoelectrochemical (PEC) cell, the generation of H₂ by water splitting takes place on two different electrodes, which facilitates the separation of the products by the addition of a proton exchange membrane.  PEC cells have led to some of the highest solar-to-hydrogen efficiencies achieved to date.² The setup of a successful PEC cell requires the preparation of efficient and stable photoelectrodes.

Subestequiometric copper telluride, Cu_2-xTe, is a very interesting material that, as far as we know, has not been used in a PEC cell yet. It has already been used in photovoltaic cells due to its small band gap of around 1.5 eV and its high conductivity.³ In this work, we used Cu_2-xTe nanocrystals synthesized by a colloidal method⁴ to prepare thin films and use them as photoelectrodes. After their organic capping was successfully removed by a thermal treatment in Ar, two different crystallographic phases were obtained depending on the heating temperature. Then, the photoelectrochemical and optical properties of both phases were measured. Electrochemical impedance spectroscopy was used to build the Mott-Schottky diagrams and determine the flat band potential, charrier density and conduction type, which is p-type for both phases. This conductivity type was ratified by photopotential measurements. Both electrochemical impedance spectroscopy and UV-Vis-nIR spectroscopy allowed us to build the bands energy diagram, which confirms the ability of this material to reduce water to hydrogen. This means that Cu_2-xTe thin films are good candidates to be used as photocathodes, which was also confirmed by the measurement of their photocurrents. As they show some stability problems, two strategies were proved to solve this: the addition of an amorphous TiO₂ layer and the change of the electrolyte pH. Moreover, H₂ generation by water splitting in a two-electrode PEC cell was observed and quantified.

References

1.           Sivula, K. & van de Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 1, 1–16 (2016).

2.           Young, J. L. et al. Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures. Nat. Energy 17028, 1–8 (2017).

3.           Rungtaweechai, N. & Tubtimtae, A. Cu_2-xTe/MnTe co-sensitized near-infrared absorbing liquid-junction solar cells. Mater. Lett. 158, 70–74 (2015).

4.           Arciniegas, M. P. et al. Self-Assembled Dense Colloidal Cu₂Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

All-Inorganic Halide Perovskite Quantum Dots (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different disciplines like photovoltaics, lasing and emission.[1] In the present talk, we will discuss about the application of these materials for solar-driven environmental remediation. To that aim, it is essential to elucidate the energy levels of CsPbxSn1-xX3 QDs by different techniques and to identify candidate target molecules, which can be oxidized with photogenerated holes in these QDs. As a proof of concept, we demonstrate the efficient photocatalytic and photoelectrochemical oxidation of 2-Mercaptobenzothiazol with CsPbBr3 QDs.

The efficient production of solar fuels requires catalysts capable of accelerating complex multi-electron reactions at electrified interfaces. These reactions can be carried out at the metallic surface sites of heterogeneous electrocatalysts or via redox mediation at molecular electrocatalysts. Molecular catalysts yield readily to synthetic alteration of their redox properties and secondary coordination sphere, permitting systematic tuning of their activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts because they typically exhibit a distribution of active site geometries and local electronic structures, which are recalcitrant to molecular-level synthetic modification. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states which distribute the redox burden of a multi-electron transformation, enabling more efficient catalysis. We have developed a simple synthetic strategy for conjugating well-defined molecular catalyst active sites with the extended states of graphitic solids. Electrochemical and spectroscopic data indicate that these graphite-conjugated catalysts do not behave like their molecular analogues, but rather as metallic active sites with molecular definition, providing a unique bridge between the traditionally disparate fields of molecular and heterogeneous electrocatalysis. Our efforts to deploy these new hybrid materials for solar fuels production will be discussed.

Dimensionally stable electrodes that are able to sustain the harsh chemical environment involved during electrochemical reactions is a requirement for materials to reach industrial applications. Dimensionally stable anodes (DSA) have been obtained previously for the electrochemical generation of chlorine gas and for water splitting applications.¹ So far the most successful approach has been the generation of a thin layer of a stable electrocatalytic and conductive coating deposited on a base metal, usually Ti. Unfortunately, this approach renders materials with a small contact surface area between catalyst and electrolyte which could limit the overall activity of the reaction. The current presentation will focus on how DSA can be prepared from powder metallurgy processes. The composition can be controlled to modify the porosity of the electrode. This type of electrodes can be used in combination with photovoltaic materials by using different photoelectrochemical configurations for generation of solar fuels.^2-4

References

1.         Mousty, C.; Fóti, G.; Comninellis, C.; Reid, V., Electrochemical behaviour of DSA type electrodes prepared by induction heating. Electrochim. Acta 1999, 45, 451-456.

2.         Guerrero, A.; Bisquert, J., Perovskite semiconductors for photoelectrochemical water splitting applications. Current Opinion in Electrochemistry 2017, 2, 144-147.

3.         Guerrero, A.; Haro, M.; Bellani, S.; Antognazza, M. R.; Meda, L.; Gimenez, S.; Bisquert, J., Organic photoelectrochemical cells with quantitative photocarrier conversion. Energy Environ. Sci. 2014, 7, 3666-3673.

4.         Haro, M.; Solis, C.; Blas‐Ferrando Vicente, M.; Margeat, O.; Dhkil Sadok, B.; Videlot‐Ackermann, C.; Ackermann, J.; Di Fonzo, F.; Guerrero, A.; Gimenez, S., Direct Hydrogen Evolution from Saline Water Reduction at Neutral pH using Organic Photocathodes. ChemSusChem 2016, 9, 3062-3066.

The morphology of copper electrodes significantly affects the performance of electrochemical CO₂ reduction devices. Porous morphologies are important to increase the active reaction sites and to tune the selectivity towards desired products, but it can also limit the mass transport through the pore space. A better understanding of morphology-induced performance limitations or enhancements in porous electrodes is needed.

We used a coupled experimental-numerical approach to quantitatively characterize morphologically-complex porous electrodes (micrometer thick macroporous films with mesoporous structural details). We utilized a 3D-microscopy method, FIB-SEM tomography [1] with a high resolution of 4x4x4 nm³, to obtain a grey value array representing the photoelectrode morphology. The digital structure was segmented based on trainable machine-learning algorithms to subsequently quantify performance-related morphological parameters.

We applied this method to a hierarchically structured indium−tin oxide (ITO) electrode, covered by electrodeposited copper. The ITO scaffold has a macroporous inverse opal (IO) architecture and a mesoporous skeleton to increase the effective surface area [2]. Various samples with IO film thickness of 10 – 30 μm and pore diameters of 0.5 – 2 μm were scanned.

The digitalized electrode morphologies were then used in direct pore-level simulations to understand the mass transport within the macro- and mesopores. The diffusive transport of reactants and products in the electrolyte were investigated with a finite volume solver on a meshed representation of the exact geometries. Local current densities at the solid-liquid interface and pH distributions in the pore space were determined for the different film thickness and pore diameters.

The FIB-SEM tomography, with its nanometer-scale resolution, and the advanced pore-level simulations allowed for direct linking of the multi-physical transport to the morphological parameters of the porous ITO/Cu films. This study lays the ground for the optimization of the CO₂ reduction efficiency by tuning the morphological parameters on digitally modified electrode representations (e.g. modified pore diameters, pore distribution and film thickness).

References

[1]         M. Cantoni and L. Holzer, “Advances in 3D focused ion beam tomography,” MRS Bull., vol. 39, no. 4, pp. 354–360, 2014.

[2]         D. Mersch et al., “Wiring of Photosystem II to Hydrogenase for Photoelectrochemical Water Splitting,” 2015.

The electrocatalytic reduction of CO2 has attracted high attention recently, not only to reduce the carbon footprint, but for the possibility to produce value-added chemicals, such as carbon monoxide, formic acid, methanol or more complex organic molecules like ethanol or ethylene, under ambient temperature and pressure conditions. However, the real application of these electrochemical techniques are still a challenge, and efforts have to be made in decrease the overpotential, improve the product selectivity and stability of the electrocatalyst. Among all the CO2 reduction products that can be obtained, we have focused our research in formic acid, as it has been receiving great industrial attention for its several applications in fuel cells, textile or chemical industries.

We have studied two different metals and the combination of both as electrocatalysts for CO2 reduction to formic acid: tin (Sn)1 and palladium (Pd)2. Electrodeposited Sn has shown a potential around -0,8 VRHE at 10 mA/cm2 and a faradaic efficiency to formate around 70% using KHCO3 as electrolyte. On the other hand, under the same experimental conditions, the electrode containing Pd nanoparticles supported on carbon black shows a potential around -0.25 VRHE at 10 mV/cm2 and more than 80% of faradaic efficiency. The cathode potential obtained with Pd electrodes in comparison with Sn is 500 mV lower however, the catalyst working at such high current density is rapidly poisoned with CO (subproduct of the CO2 reduction) and the faradaic efficiency to formic is decreased dramatically. In this work, we present how the combination of both catalysts can be used to modulate the overpotential of the CO2 electroreduction reaction, the selectivity and the electrode stability under continuous operation. Also, a novel technique of co-electrodeposition of both metals is achieved, showing an alternative method to produce easily tunable electrodes, which facilitates and enhance the possible coupling to a photo-absorbing element in a complete photoelectrochemical cell.

References:

1. J. Mater. Chem. A,2016, 4, 13582–13588

2. ACS Energy Lett. 2016, 1, 764−770

The transformation of solar energy into chemical energy stored in the form of hydrogen, through photoelectrochemical water splitting is a promising method that has the important advantages of being environment friendly and free from carbon dioxide emission. Metal oxides are promising candidates for photoanode but the strong electron-hole recombination may explain their low efficiency. It has been recently proposed to use the spontaneous electric field of a ferroelectric compound to separate photogenerated charges in photoanode. In our laboratory we have been developing model oxide thin films for solar water splitting prepared by oxygen assisted molecular beam epitaxy for several years, in order to understand the relevant parameters to improve photoanodes performance. In this context, in order to unravel the role of ferroelectricity we have been investigating epitaxial TiO₂ films and TiO₂/BaTiO₃ heterojonctions deposited on Pt(001) where BaTiO₃ is the prototypical ferroelectric material. We have studied the growth, the electronic structure, the ferroelectric properties and the photoelectrochemical performance (photocurrent and impedance spectroscopy) as a function of the position of the ferroelectric layer in the film (above, in the middle and below) and of the orientation of ferroelectric polarization. Thereby, we show that the TiO₂ films adopt a TiO₂-B (001) structure for thickness lower than 3 nm and an anatase (001) structure for higher thicknesses, while the BaTiO₃ films adopt a tetragonal (001) structure with a natural electrical polarization perpendicular to the surface. We show that the performance of the TiO₂ photoanode can be improved by a polarized layer of BaTiO₃, the best improvement is when the ferroelectric film is below the TiO₂ film and when the electrical polarization is downward polarized. We explain this finding by the presence of an internal electric field which favors the separation of photo-generated charges, and explain how the electronic structure is modified by the electrical polarization in each case.

In this contribution, we report on strategies for high yield syngas production from solar CO₂ recycling. To make CO₂ re-use available at industry level, versatile and cost-effective processes at large-scale are in dire need. Therefore, we focused our study on efficient processes, which are scalable to large areas and compatible with state-of-the-art photovoltaics and electrocatalysts. Efficient processes include low overpotentials for the CO₂ reduction reaction (CO₂RR) and the oxygen evolution reaction (OER), respectively, along with excellent selectivity for the desired products (in particular for the case of CO₂RR). We demonstrate that the coating of three-dimensional metallic foam electrodes with adapted nanosized catalysts resulted in high yield CO₂ conversion to syngas. Furthermore, we provide evidence that photovoltaic structures based on cheap and mature silicon technology can be adapted to provide the required photovoltage to break the CO₂ molecule into the targeted product.

In particular, we have investigated the application of silicon heterojunction photovoltaic cells as photoanodes, which requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient OER. We show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm² can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we explored the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles as OER catalyst. We demonstrate OER overpotentials below 300 mV (for 10 mA/cm²).

The CO₂RR was performed by using large-scale metallic foam electrodes as highly conductive catalyst scaffolds. In this context, we developed a deposition process, which enables tunable coating of Cu and Ni foams, respectively, with highly active nanosized metal catalysts, such as Zn or Ag, for selective syngas production. The performance of the as-produced electrodes has been evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. The developed electrodes exhibit overpotentials below 400 mV for CO₂RR. Furthermore, stable and tunable H₂:CO ratios between 5 and 1 along with high CO Faradaic efficiencies of up to 96% and CO current densities of 40 mA/cm² were measured. Finally, we demonstrate a bias-free operation of the complete  device (2-electrode configuration) providing a photocurrent density of 5.0 mA/cm² measured under 100 mW/cm² illumination. This corresponds to a solar-to-syngas conversion efficiency of 4.3%.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

During this presentation we will introduce the A-LEAF project: "An artificial leaf: a photo-electro-catalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels" (FET-PROACT-732840). This project, funded by the European Union, is a collaborative effort by thirteen partners from eight European countries, coordinated by the Institute of Chemical Research of Catalonia (ICIQ), in Spain.

Our A-LEAF initiative aims to design and develop a fully functional scheme to transform sunligh, water and carbon dioxide into useful, sustainable and environmentally neutral fuels and/or fine chemicals. We have gathered a truly multidisciplinary consortium to cover all scientific expertise needed to overcome the major challenges: physicists to achieve sunlight capture; surface scientists to use this energy to oxidize water, extracting protons and electrons; electrochemists to use these equivalents to reduce carbon dioxide into useful stock; and system engineers to implement and fine-tune all components into a viable and cost-effective process. Because, in addition to the general scientific challenge, we have committed to use, exclusively, scalable processes, earth abundant non-critical raw materials and inexpensive platforms. Only with these added values, we may dream of bringing artificial photosynthesis, and solar fuels, to the future Energy pull, where societal impact is the ultimate target.

Our  thirteen partner institutions: ICIQ, ETH Zürich, Universitet Leiden, IMDEA Nanociencia, Technische Universität Wien, Universitat Jaume I, Imperial College, Technische Universität Darmstadt, Forschungzentrum Jülich, Université de Montpellier, INSTM and COVESTRO; started the collaborative work in Januay 2017, and the major results and current state of the project (successful achievements and unexpected problems) will be highlighted and discussed.

Efforts in the solar water splitting field for the past decade have established BiVO₄ as the highest performing metal oxide photoanode. Within this period, the AM1.5 photocurrents of BiVO₄ have been increased from hundreds of mA/cm² to ~7 mA/cm².^[1] However, high photocurrents (> 5 mA/cm²) are only achieved with nanostructuring, which poses additional complexities (e.g., light scattering) for the design and scalability of a tandem device for water splitting. Non-nanostructured BiVO₄ is limited by its poor carrier transport properties. In addition, with reported photocurrents already within 90% of the theoretical maximum, the solar-to-hydrogen (STH) efficiency of BiVO₄ is limited to < 9% due to its relatively wide bandgap of 2.4-2.5 eV.

Here, we demonstrate that these limiting factors can be alleviated by controlled anion and cation substitution in BiVO₄. To overcome the bandgap limitation, we incorporated sulfur into BiVO₄ by post-annealing in sulfur-rich atmosphere. As a result, the bandgap is reduced by up to ~0.3 eV, which increases the theoretical maximum STH efficiency to ~12%. We confirmed that sulfur substitutes oxygen in the lattice of BiVO₄ by a series of structural and chemical characterization (e.g., XRD, Raman, XPS). Moreover, hard X-ray photoelectron spectroscopy (HAXPES) reveals that the bandgap decrease is a result of an upward shift of the valence band maximum. Simultaneously, time-resolved microwave conductivity (TRMC) measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases by a factor of ~5. Wavelength-dependent TRMC measurements confirm the photoactivity of the sulfur-incorporated BiVO₄ up to 560 nm, well beyond the bandgap of typical BiVO₄ films. In addition, we successfully introduced calcium into BiVO₄ thin films;^[2] calcium substitutes bismuth and acts as an acceptor-type dopant. Although it seems counterintuitive to introduce an acceptor dopant into n-type BiVO₄, this would allow the fabrication of p-type, and eventually p-n homojunctions based on BiVO₄. HAXPES measurements reveal that calcium out-diffuses towards the surface of the film, thereby creating a spontaneous p-n homojunction within the film. As a result of the internal electric field, the carrier separation efficiency was enhanced by a factor of ~2. Overall, this work underlines the importance of controlled ionic substitution in complex metal oxides, which may not only improve the performance but also enable new device architectures.

[1] Abdi and Berglund, J. Phys. D. Appl. Phys. (2017)

[2] Abdi et al. ChemPlusChem (2018)

Recently, Ham et al. reported that aluminum incorporation into SrTiO₃ microparticles followed by modification with Rh_2-yCryO₃ produces a superior overall water splitting catalyst with 30% apparent quantum efficiency at 350 nm. Based on transient IR spectroscopy, the improved activity was attributed to the removal of Ti³⁺ recombination sites. Here we use results from photoelectron spectroscopy (XPS) and density functional theory to show that Al³⁺ incorporation into the perovskite lattice not only reduces the number of Ti³⁺ deep recombination sites and but also promotes the formation of oxygen vacancies in the material. The oxygen vacancies form a mini-band 0.2 eV above the valence band whose energy position strongly depends on the spatial separation between Al³⁺ sites and oxygen vacancies. The vacancy band is believed to aid photochemical charge separation in this metal oxide. Particle suspensions of the material inside of a baggie reactor promote overall water splitting under direct sunlight illumination with 0.1% solar to hydrogen efficiency.

References

Ham, Y., T. Hisatomi, Y. Goto, Y. Moriya, Y. Sakata, A. Yamakata, J. Kubota, and K. Domen, Flux-Mediated Doping of SrTiO₃ Photocatalysts for Efficient Overall Water Splitting. Journal of Materials Chemistry A, 2016. 4(8): p. 3027-3033. http://dx.doi.org/10.1039/C5TA04843E

Cu(I)-based delafossites (CuMO2) have garnered significant attention for their small optical bandgaps, strong light absorption characteristics, and native p-type conductivities, which make them suitable photocathode candidates for solar fuel production. While resolving the inherently complex electronic structures of these ternary oxides is often challenging, doing so is imperative for understanding optical excitations and carrier transport mechanisms, as well as for furthering material design and discovery. Combining density functional theory (DFT) calculations and X-ray spectroscopy techniques, this research gives a detailed portrait of the band structure of rhombohedral 3R-CuFeO2, prepared by reactive co-sputtering. In particular, element-specific contributions from 3d orbitals of Fe and Cu, as well as 2p orbitals of O, in the valence and conduction bands are revealed by resonant inelastic X-ray scattering (RIXS) measurement. By combining this knowledge of electronic structure, with measurements of optical properties, carrier relaxation dynamics, and photoelectrochemical performance characteristics, this research provides insights into current limitations of this novel photocathode material and informs strategies to overcome them.

The nanostructured W-doped BiVO₄ photoanodes were prepared by electrospinning. The role of surface states (SS) during water oxidation for the as-prepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 V_RHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (N_SS) with respect to the charge donor density (N_d), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest N_d and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V⁵⁺/V⁴⁺ in semiconductor bulk to form water oxidation intermediates by electron trapping process. Otherwise, the irreversible surface reductive reaction of VO₂⁺ to VO²⁺ by electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of BiVO₄ electrode. It can tune the electron trapping process to obtain high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface.

One of the critical challenges for efficient solar water splitting is the identification of a stable photoelectrode material with a bandgap of 1.6 - 1.9 eV that can be used as a top absorber in an efficient D4 tandem device. Recently, n-type α-SnWO₄ was identified as a potential photoanode candidate due to the combination of its ideal bandgap (~1.9 eV) and a very negative photocurrent onset potential (~0 V vs. RHE) [1-3]. However, up to now, α-SnWO₄ photoelectrodes have shown very low photoconversion efficiencies. The reason for this is not fully understood, and many of the essential material parameters are still elusive.

In this study, we identify the major limiting parameters of pulsed laser-deposited α-SnWO₄ photoanodes: (I) the oxidation of Sn²⁺ to Sn⁴⁺ at the surface creating a hole blocking layer and pushing the photoconversion efficiency to almost zero, and (II) the relatively poor charge carrier dynamics resulting in a mismatch between the charge carrier diffusion length and the light penetration depth. To address these challenges, we explored several strategies such as the deposition of a hole-conducting NiO_x protection layer resulting in a new benchmark sulfite oxidation photocurrent of 0.75 mA cm^-2 at 1.23 V vs. RHE. We furthermore show that a high-temperature treatment enhances the charge carrier transport by improving the film crystallinity, resulting in a charge carrier diffusion length comparable to state-of-the-art BiVO₄ photoanodes. Finally, the remaining challenges for oxygen evolution using our α-SnWO₄ films will be discussed.

Our findings provide important insights into the limitations and key properties of α-SnWO₄ photoelectrodes and will help to further improve the performance of this promising photoanode material.

[1] Ziani et al., APL Materials, 3 (2015) 096101

[2] Zhu et al., ACS Appl. Mater. Interfaces, 9 (2017) 1459-1470

[3] Pyper et al., Chin. Chem. Lett., 26 (2015) 474-478

The development of technologies for H2 production or CO2 reduction strongly relies on an abundant supply of protons and electrons liberated by water oxidation. [1-2] Therefore, photoelectrochemical (PEC) water oxidation is an important anodic half-cell process in the development of a sustainable artificial solar fuel system. In the PEC devices design, coupling water oxidation catalysts with active photoanode materials has become the most promising methodology, since the attachment/integration of the catalyst on the semiconductor light absorbers could kinetically facilitate interfacial charge transfer reactions.

In this contribution, we have fabricated ITO/Fe2O3/Fe2TiO5/FeNiOOH multi-layers nanowire heterostructures via combination of sputtering, hydrothermal, ALD, photo-electrodepositon methods for photoelectrochemical (PEC) oxygen evolution application. Structural, spectroscopic and electrochemical investigations disclose that the origin of the superior catalytic performance is owing to the interfacial coupling effect of ITO underlayer (Sn doping and conductivity promoter), ultrathin Fe2TiO5 coating (Ti doping, energetics and surface state density modulation) and FeNiOOH eletrocatalyst (varying surface state energy level). [2]

Meanwhile, an alternative earth-abundant CoFe prussian blue analogues (CoFe PBA) is incorporated in Fe2O3/Fe2TiO5 core-shell type II heterojunction nanowires as photoanodes for PEC water oxidation. The observed photocurrent is improved from 0.12 mA cm-2 to 1.25 mA cm-2 at 1.23 V vs. RHE under illumination by involvement of ultrathin Fe2TiO5 layer and CoFe PBA WOCs coating. Further investigation of the PEC mechanisms via photoelectrochemical impedance spectroscopy unveils that the enhanced PEC performance is attributed to the enhanced charge transfer efficiency owing to the tuned energy level and density of surface state. [3-4]

References

[1] Félix Urbain, Pengyi Tang, Nina M. Carretero, Teresa Andreu, Luís G. Gerling, Cristóbal Voz, Jordi Arbiol, Joan R. Morante, Energy & Environmental Science, 10, 2256-2266 (2017).

[2] Pengyi Tang, HaiBing Xie, Carles Ros, LiJuan Han, Martí Biset-Peiró, Yongmin He, Wesley Kramer, Alejandro Perez-Rodriguez, Edgardo Saucedo, Jose Galan-Mascaros, Teresa Andreu, Joan R. Morante, Jordi Arbiol, Energy & Environmental Science, 10, 2124-2136 (2017).

[3] Lijuan Han, Pengyi Tang, Alvaro Reyes-Carmona, Barbara Rodriguez-Garcia, Mabel Torrens, Joan Ramon Morante, Jordi Arbiol, Jose Ramon Galan-Mascaros, Journal of the American Chemical Society, 138, 16037-16045 (2016).

[4] PengYi Tang, LiJuan Han, Paul Paciok, Marti Biset Peiro, Hong-Chu Du, Xian-Kui Wei, Lei Jin, Hai-Bing Xie, Qin Shi, Teresa Andreu, Joan Ramon Morante, Mónica Lira-Cantú, Marc Heggen, Rafal E. Dunin-Borkowski, José Ramón Galán-Mascarós, Jordi Arbiol, to be submitted.