Enabling Direct Photoelectrochemical H₂ Production using Alternative Oxidation Reactions on WO₃

The efficient and inexpensive conversion of solar energy into chemical bonds, such as in H2 via the photoelectrochemical splitting of H2O, is a promising route to produce green industrial feedstocks and renewable fuels, which is a key goal of the NCCR Catalysis. However, the oxidation product of the water splitting reaction, O2, has little economic or industrial value. Thus, upgrading key chemical species using alternative oxidation reactions is an emerging trend. WO3 has been identified as a unique photoanode material for this purpose since it performs poorly in the oxygen evolution reaction in H2O. Herein we highlight a collaboration in the NCCR Catalysis that has gained insights at the atomic level of the WO3 surface with ab initio computational methods that help to explain its unique catalytic activity. These computational efforts give new context to experimental results employing WO3 photoanodes for the direct photoelectrochemical oxidation of biomass-derived 5-(hydroxymethyl) furfural. While yield for the desired product, 2,5-furandicarboxylic acid is low, insights into the reaction rate constants using kinetic modelling and an electrochemical technique called derivative voltammetry, give indications on how to improve the system

The role of excitons and free charges in the excited-state dynamics of solution-processed few-layer MoS₂ nanoflakes

Solution-processed semiconducting transition metal dichalcogenides are emerging as promising two-dimensional materials for photovoltaic and optoelectronic applications. Here, we have used transient absorption spectroscopy to provide unambiguous evidence and distinct signatures of photogenerated excitons and charges in solution- processed few-layer MoS₂ nanoflakes (10–20 layers). We find that photoexcitation above the direct energy gap results in the ultrafast generation of a mixture of free charges in direct band states and of excitons. While the excitons are rapidly trapped, the free charges are long-lived with nanosecond recombination times. The different signatures observed for these species enable the experimental extraction of the exciton binding energy, which we find to be ∼80 meV in the nanoflakes, in agreement with reported values in the bulk material. Carrier-density-dependent measurements bring new insights about the many-body interactions between free charges resulting in band gap renormalization effects in the few-layer MoS₂ nanoflakes

Effects of surface wettability on (001)-WO and (100)-WSe: A spin-polarized DFT-MD study

An extensive understanding of WO and WSe bulk crystalline structures and explicit solvent effects on (001)-WO and (100)-WSe facets are essential for design of efficient (photo) electrocatalysts. The atomistic level understanding of both WO and WSe bulk solids and how water solvation processes occur on WO and WSe facets are nowadays characterized by a noticeable lack of knowledge. Herein, forefront Density Functional Theory-based molecular dynamics have been conducted for assessing the role of an explicit water environment in the characterization of solid surfaces. Water at the interface and H-bonds environment, as well as WO and WSe surface activity, will be described in terms of surface wettability and interfacial water dynamics, revealing the relevance of treating explicitly liquid water and its dynamics in assessing catalytic features. We provide pieces of evidence of the hydrophobic character shown by (001)-WO and (100)-WSe facets. A preferential in-plane hydration structure of the first water layer has been detected at both (001)-WO and (100)-WSe water interface, in which the electric dipole moment of water molecules is re-oriented in a sort of 2-dimensional H-bond network. Bulk property calculations of WO and WSe are also provided

Examining architectures of photoanode-photovoltaic tandem cells for solar water splitting

Given the limitations of the materials available for photoelectrochemical water splitting, a multiphoton (tandem) approach is required to convert solar energy into hydrogen efficiently and durably. Here we investigate a promising system consisting of a hematite photoanode in combination with dye-sensitized solar cells with newly developed organic dyes, such as the squaraine dye, which permit new configurations of this tandem system. Three configurations were investigated: two side-by-side dye cells behind a semitransparent hematite photoanode, two semitransparent dye sensitized solar cells (DSCs) in front of the hematite, and a trilevel hematite/DSC/DSC architecture. Based on the current-voltage curves of state-of-the-art devices made in our laboratories, we found the trilevel tandem architecture (hematite/SQ1 dye/N749 dye) produces the highest operating current density and thus the highest expected solar-to-hydrogen efficiency (1.36% compared with 1.16% with the standard back DSC case and 0.76% for the front DSC case). Further investigation into the wavelength-dependent quantum efficiency of each component revealed that in each case photons lost as a result of scattering and reflection reduce the performance from the expected 3.3% based on the nanostructured hematite photoanodes. We further suggest avenues for the improvement of each configuration from both the DSC and the photoanode part

Hybrid heterojunctions of solution-processed semiconducting 2d transition metal dichalcogenides

Exfoliated transition metal dichalcogenides (2D-TMDs) are attractive light-harvesting materials for large-area and inexpensive solar energy conversion given their ability to form highly tolerant heterojunctions. However, the preparation of large-area heterojunctions with these materials remains a challenge toward practical devices, and the details of photogenerated charge carrier harvesting are not well established. In this work, we use all solution-based methods to prepare large-area hybrid heterojunction films consisting of exfoliated semiconducting 2H-MoS2 flakes and a perylene-diimide (PDI) derivative. Hybrid photoelectrodes exhibited a 6-fold improvement in photocurrent compared to that of bare MoS2 or PDI films. Kelvin probe force microscopy, X-ray photoelectron spectroscopy, and transient absorption measurements of the hybrid films indicate the formation of an interfacial dipole at the MoS2/organic interface and suggest that the photogenerated holes transfer from MoS2 to the PDI. Moreover, performing the same analysis on MoSe2-based hybrid devices confirms the importance of proper valence band alignment for efficient charge transfer and photogenerated carrier collection in TMD/organic semiconductor hybrid heterojunctions

Enabling Direct Photoelectrochemical H₂ Production using Alternative Oxidation Reactions on WO₃

The efficient and inexpensive conversion of solar energy into chemical bonds, such as in H2 via the photoelectrochemical splitting of H2O, is a promising route to produce green industrial feedstocks and renewable fuels, which is a key goal of the NCCR Catalysis. However, the oxidation product of the water splitting reaction, O2, has little economic or industrial value. Thus, upgrading key chemical species using alternative oxidation reactions is an emerging trend. WO3 has been identified as a unique photoanode material for this purpose since it performs poorly in the oxygen evolution reaction in H2O. Herein we highlight a collaboration in the NCCR Catalysis that has gained insights at the atomic level of the WO3 surface with ab initio computational methods that help to explain its unique catalytic activity. These computational efforts give new context to experimental results employing WO3 photoanodes for the direct photoelectrochemical oxidation of biomass-derived 5-(hydroxymethyl) furfural. While yield for the desired product, 2,5-furandicarboxylic acid is low, insights into the reaction rate constants using kinetic modelling and an electrochemical technique called derivative voltammetry, give indications on how to improve the system

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches (PTS)

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length via photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light the azobenzene undergoes trans -to- cis isomerization mechanically inducing the oligothiophene to assume a planar conformations (extended π-conjugation). Exposure to 254 nm wavelenght promotes azobenzene cis -to- trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis -to- trans thermal relaxation with rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized using steady-state UV-vis, X-ray crystallography and quantum-chemical modelling. Finally, we tested the proposed light-responsive oligothiophenes into field-effect transistors to probe the photo-induced tuning of their electronic properties

Identifying Reactive Sites and Surface Traps in Chalcopyrite Photocathodes

Gathering information on the atomic nature of reactive sites and trap states is key to fine tuning catalysis and suppressing deleterious surface voltage losses in photoelectrochemical technologies. Here, spectroelectrochemical and computational methods were combined to investigate a model photocathode from the promising chalcopyrite family: CuIn0.3Ga0.7S2. We found that voltage losses are linked to traps induced by surface Ga and In vacancies, whereas operando Raman spectroscopy revealed that catalysis occurred at Ga, In, and S sites. This study allows establishing a bridge between the chalcopyrite's performance and its surface's chemistry, where avoiding formation of Ga and In vacancies is crucial for achieving high activity.This work was supported by the Swiss National Science Foundation (SNSF) under the Ambizione Energy grant (PZENP2_166871) and by the Gaznat-EPFL Research Program. M.B. and U.A. were supported by the Swiss National Science Foundation Professorship Grants PP00P2_157615 and PP00P2_187185. Calculations were performed on UBELIX, the HPC cluster at the University of Bern. M.X. is grateful for the support from the China Scholarship Council (No. CSC201806160172) and the Strategic Japanese–Swiss Science and Technology program (514259). Open access funding provided by Ecole Polytechnique Federale de Lausanne

Injectable, Self-opening, and Freestanding Retinal Prosthesis for Fighting Blindness

Purpose:In the past decade, retinal prostheses emerged as promising technology to restore a primitive, although clinically useful, form of vision. However, fighting blindness with retinal prostheses require challenges not yet achieved. From a clinical perspective, sight restoration requires to reach two main goals: enlarging the visual field of the patient and improving its visual acuity. From the engineering point of view, these needs demand the overcoming of two major issues: implanting a prosthesis (i) large enough to cover the retinal surface and (ii) embedding a high number of highly dense stimulatory elements. Our goal is the development of an injectable, self-opening, and freestanding retinal prosthesis restoring at least 40° of visual field, therefore covering at least a retinal surface of 12 mm in diameter. Moreover, the prosthesis must have a hemispherical shape in order to minimize the distance from the targeted cells over its entire surface, it should operate according to a photovoltaic stimulation principle and it must be injected trough a minimal scleral incision. Methods:Using solution processes and micro-fabrication techniques, we designed a retinal prosthesis based on polydimethylsiloxane (PDMS) as shell material, embedding photovoltaic pixels made of conjugated polymers. The prosthesis is shaped with a molding technique. Results:The prosthesis consists in a photovoltaic PDMS-interface, embedding 2345 organic stimulating pixels (100 µm and 150 µm in diameter, density 54.34 px/mm2) with a biomimetic distribution in an active area of 13 mm (44° of visual field). Our results indicate that those photovoltaic pixels can deliver up to 54.22±10.55 mA/cm2 and generate an electrode potential of 182.22±6.72 mV when illuminated with a pulse light of 10 ms, 32.47 µW/mm2, at 530 nm. Sample tested n = 20. Accelerated aging tests and experiments with explanted retinas are currently under evaluation. Conclusions:These preliminary results show the potential of organic photovoltaic technology in the fabrication of a retinal prosthesis with large surface area and high stimulation efficiency. The biocompatibility and mechanical compliance of the materials represent an additional step forward in building advanced photovoltaic retinal prostheses

An Organic Semiconductor Photoelectrochemical Tandem Cell for Solar Water Splitting

Photoelectrochemical cells employing organic semiconductors (OS) are promising for solar-to-fuel conversion via water splitting. However, despite encouraging advances with the half reactions, complete overall water splitting remains a challenge. Herein, a robust organic photocathode operating in near-neutral pH electrolyte by careful selections of a semiconducting polymer bulk heterojunction (BHJ) blend and organic charge-selective layer is realized. The optimized photocathode produces a photocurrent density of >4 mA cm???2 at 0 V vs the reversible hydrogen electrode (VRHE) for solar water reduction with noticeable operational stability (retaining ???90% of the initial performance over 6 h) at pH 9. Combining the optimized BHJ photocathode with a benchmark BHJ photoanode leads to the demonstration of a large-area (2.4 cm2) organic photoelectrochemical tandem cell for complete solar water splitting, with a predicted solar-to-hydrogen (STH) conversion efficiency of 0.8%. Under unassisted two-electrode operation (1 Sun illumination) a stabilized photocurrent of 0.6 mA and an STH of 0.3% are observed together with near unity Faradaic efficiency of H2 and O2 production