Generalized Synthesis to Produce Transparent Thin Films of Ternary Metal Oxide Photoelectrodes.

Developing facile approaches to prepare non-light-scattering ternary oxide thin film photoelectrodes is an important goal for solar water splitting tandem cells. Herein, a novel synthesis route is reported that employs ethylenediaminetetraacetic acid (EDTA) to enable compatible water solubility of diverse metal cations, which affords transparent films by solution processing. By using BiVO4 as a model material, a remarkable improvement in transparency is demonstrated, quantified by the direct transmittance at 600 nm of >80 % versus the <10 % observed with state-of-the-art electrodeposited thin films while maintaining reasonable solar-driven oxidation photocurrents (1.75 mA cm-2 in the presence of a sulfite hole scavenger). Furthermore, it is demonstrated that the synthesis technique can be applied in a general fashion towards the synthesis of diverse n- and p-type metal oxide materials, such as ZnFe2 O4 and CuFeO2 .

Direct photoelectrochemical oxidation of hydroxymethylfurfural on tungsten trioxide photoanodes.

An important target reaction for solar-powered biomass valorization is the conversion of 2,5-hydroxymethylfurfural (HMF) into key monomers for polyester production. Herein, photoanodes of WO3 are demonstrated to directly photo-oxidize HMF in aqueous electrolyte (pH 4) under simulated solar illumination. The addition of 5 mM HMF increases the saturation photocurrent by 26% and suppresses the water oxidation reaction, as determined by rotating ring-disk electrode experiments. Prolonged photoelectrochemical oxidation (64 h) illustrates system robustness and confirms the production of furandicarboxaldehyde (DFF), furandicarboxylic acid (FDCA), and related intermediates. Quantification of the reaction rate constants via a kinetic model gives insight into the modest DFF and FDCA yields (up to 4% and 1%, respectively)-which is due to the formation of by-products-and suggests routes for improvement.

Fully Conjugated Donor-Acceptor Block Copolymers for Organic Photovoltaics via Heck-Mizoroki Coupling.

The development of facile routes to prepare fully conjugated block copolymers (BCPs) from diverse monomers is an important goal for advancing robust bulk-heterojunction (BHJ) organic photovoltaics (OPVs). Herein we introduce a synthetic strategy for step-growth BCPs employing 1,2-bis(trialkylstannyl)ethene as one monomer, which, in addition to offering improved backbone planarity, directly yields a vinylene-terminated macromonomer suitable for Heck-Mizoroki coupling. The benefits of our strategy, which facilitates the preparation of functionalized macromonomers suitable for BCP synthesis, are demonstrated with a representative BCP based on a diketopyrrolopyrrole (DPP) copolymer coded pBDTTDPP as the donor block and a perylenediimide (PDI) copolymer coded as pPDIV as the acceptor block. Feed ratio optimization affords control over the macromonomer chain-end functionalities and allows for the selective formation of a tri-BCP consisting of pPDIV-b-pBDTTDPP-b-pPDIV, which is employed in a single-component BHJ OPV. Devices achieved a power conversion efficiency of 1.51% after thermal stress at 150 °C compared to 0.02% for a control device consisting of a comparable blend of pBDTTDPP and pPDIV. The difference in performance is ascribed to the morphological stability of the BHJ when using the BCP.

Advancing the Chemistry of CuWO4 for Photoelectrochemical Water Oxidation.

Photoelectrochemical (PEC) cells are an ongoing area of exploration that provide a means of converting solar energy into a storable chemical form (molecular bonds). In particular, using PEC cells to drive the water splitting reaction to obtain H2 could provide a clean and sustainable route to convert solar energy into chemical fuels. Since the discovery of catalytic water splitting on TiO2 photoelectrodes by Fujishima and Honda, significant efforts have been directed toward developing high efficiency metal oxides to use as photocatalysts for this reaction. Improving the efficiency of PEC cells requires developing chemically stable, and highly catalytic anodes for the oxygen-evolution reaction (OER). This water oxidation half reaction requires four protons and four electrons coupling in two bond making steps to form O2, which limits the rate. Our group has accelerated efforts in CuWO4 as a candidate for PEC OER chemistry. Its small band gap of 2.3 eV allows for using visible light to drive OER, and the reaction proceeds with a high degree of chemoselectivity, even in the presence of more kinetically accessible anions such as chloride, which is common to seawater. Furthermore, CuWO4 is a chemically robust material when subjected to the highly oxidizing conditions of PEC OER. The next steps for accelerating research using this (and other), ternary phase oxides, is to move beyond reporting the basic PEC measurements to understanding fundamental chemical reaction mechanisms operative during OER on semiconductor surfaces. In this Account, we outline the process for PEC OER on CuWO4 thin films with emphasis on the chemistry of this reaction, the reaction rate and selectivity (determined by controlled-potential coulometry and oxygen-detection experiments). We discuss key challenges with CuWO4 such as slow kinetics and the presence of an OER-mediating mid-gap state, probed by electrochemical impedance spectroscopy. We propose that this mid-gap state imparts the observed chemoselectivity of OER on CuWO4. We introduce insights into the chemical mechanism of PEC OER on CuWO4 using Tafel analysis of electrochemical polarization. We measure Tafel slopes of ∼161 mV/dec, showing that PEC OER proceeds at a slower rate on CuWO4 than on common electrocatalysts for this reaction. Moreover, the observed photocurrent is independent of the borate buffer concentration, signaling that the buffer plays no role in the rate-determining elementary step of the reaction. Finally, we explore some recent developments in doping this material with Co (a known electrocatalytically active metal) and in coupling it with a transparent manganese phosphate (MnPO) electrocatalyst. We find that introducing Co into the wolframite structure leads to detrimental recombination of photogenerated charge carriers. However, coupling CuWO4 with MnPO increases the photocurrent density. Despite some of these challenges, CuWO4 proves to be a robust, visible light absorbing photoanode that can oxidize water with a high degree of selectivity and is therefore worthy of further exploration. Even if new compositions emerge that show better reactivity, this material serves as an excellent proving ground for the common challenges in developing ternary-phase oxides and other compositionally complex materials.