Theoretical insight into effect of cation–anion pairs on CO2 reduction on bismuth electrocatalysts

This study presents theoretical insight into the mechanism of the CO2 reduction reaction (CO2RR) to formic acid (HCOOH) on Bi (012) surfaces in the presence of alkali metal cations (M+: Cs+, K+, and Li+) and/or halide anions (X−: Cl−, Br−, and I−) using density functional theory (DFT). The adsorption energy (Eads) and work function (Wf) of the anions increases with decreasing anion size (i.e., Cl− > Br− > I−). On the other hand, the larger the cation size is, the higher is the Eads (i.e., Li+ < K+ < Cs+) but the lower is the Wf (i.e., Cs+ < K+ < Li+). In the presence of the cation–anion pairs (M+/X−), Eads of the pairs on hydrated Bi (Bi-2H) becomes more negative than that in the cases of anions or cations alone, particularly when the ionic radius of the paired cation and anion do not differ significantly. Such a synergistic effect of the mixed ions is also observed for the work function values. In the case of anions alone, CO2 molecules prefer to coordinate directly with hydrated Bi atoms via the oxygen bidentate mode; in the case of cations alone, CO2 molecules directly bind to the cations via the oxygen monodentate mode, rather than the hydrated Bi atoms. Between two possible CO2RR pathways involving *OCHO and *COOH intermediates on Bi-2H pre-adsorbed with M+/X−, the former pathway requires less energy for all M+/X− pairs. In addition, cascaded reaction profiles from CO2* to HCOOH are obtained with Cs+/Cl− and K+/Cl− pairs in the former. This indicates that once CO2 is adsorbed, the following reactions proceed spontaneously on Bi-2H with Cs+/Cl− or K+/Cl− pairs. This study thus shed light on the positive effects of supporting electrolytes (e.g., CsCl and KCl) on catalytic CO2RR.- Qatar National Research Fund (QNRF) - grant #NPRP 10-1210-160019. - National Research Foundation of Korea (NRF) - grant #2018R1A6A1A03024962, 2019M1A2A2065616, 2019R1A2C2002602

Ion-Enhanced Conversion of CO2 into Formate on Porous Dendritic Bismuth Electrodes with High Efficiency and Durability

Facile synthesis of efficient electrocatalysts that can selectively convert CO2 to value-added chemicals remains a challenge. Herein, the electrochemical synthesis of porous Bi dendrite electrodes and details of their activity toward CO2 conversion to formate in aqueous solutions of bicarbonate are presented. The as-synthesized multilayered, porous, dendritic Bi electrodes exhibit a faradaic efficiency (FE) of approximately 100 % for formate production. Added halides and cations significantly influence the steady-state partial current density for formate production JFM (Cl?&gt;Br??I?; Cs+&gt;K+&gt;Li+). DFT calculations revealed that the reaction pathway involving the species *OCOH occurs predominantly and the presence of both Cs+ and Cl? makes the overall reaction more spontaneous. Photovoltaic-cell-assisted electrocatalysis produced formate with an FE of approximately 95 % (JFM?10 mA cm?2) at an overall solar conversion efficiency of approximately 8.5 %. The Bi electrodes maintain their activity for 360 h without a change in the surface states. - 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimScopu

Porous dendritic BiSn electrocatalysts for hydrogenation of 5-hydroxymethylfurfural

The electrocatalytic hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF) is an alternative to conventional heterogeneous catalysis with H2 at high temperatures and pressures. Although Ag is the most representative electrocatalyst, it works only under limited conditions. This study synthesizes highly porous dendritic Bi, Sn, and BiSn electrocatalysts using an in situ generated hydrogen bubble template. Density functional theory computations on the adsorption energy and elementary hydrogenation reaction steps of HMF predict the superiority of Bi to Sn and the intermediate behavior of BiSn between Bi and Sn. The dendritic BiSn catalyst generates a current density of ∼144 mA cm−2 at a faradaic efficiency (FE) of ∼100% for BHMF production at pH ∼ 7 (corresponding to the BHMF production rate of ∼2.7 mmol h−1 cm−2) in prolonged electrolysis. Considering the material cost (<5% of Ag price) and quick synthesis (<40 s), dendritic BiSn should be a promising candidate for HMF hydrogenation.- Qatar National Research Fund (QNRF) - grant no. NPRP 13S-0202-200228. - Ministry of Trade, Industry and Energy (MOTIE). - Korea Evaluation Institute of Industrial Technology (KEIT) - grant no. 20018904, NTIS-141518011. - National Research Foundation of Korea (NRF) - grant no. NRF-2018R1A6A1A03024962, NRF-2021K1A4A7A02102598, NRF-2021M3I3A1082880

Homogeneous photoconversion of seawater uranium using copper and iron mixed-oxide semiconductor electrodes

Sunlight-driven conversion of hexavalent uranium (U(VI)) in seawater is achieved with mixed p-type CuO and CuFeO2 (CuO/CuFeO2) photocatalyst film electrodes synthesized via electrodeposition (ED) of Cu(II) and Fe(III), followed by annealing in air. The mixed photocatalysts exhibit a double-layer configuration with crystalline structures of CuO and CuFeO2. On irradiation of the CuO/CuFeO2 electrodes (held at −0.5 V vs. SCE) with solar simulated light (air mass 1.5; 100 mW cm−2), the U(VI) concentration decreases with time, while the total amount of uranium in solution does not change. This indicates that virtually all conversion reactions of U(VI) occur in the bulk solution, while surface reactions are limited due to insignificant adsorption of U(VI). U(VI) conversion leads to the mixed production of lower oxidation states U4+, U14/3+, and U16/3+ at a ratio of 42:28:30, with an overall Faradaic efficiency of ∼98%. The kinetics and induction time for U(VI) conversion are significantly influenced by the conditions of photocatalyst synthesis (CuO/CuFeO2, CuO, and CuFeO2; ED times of 2–4 h), the applied potential value (−0.4, −0.5, and −0.6 V vs. SCE), and the seawater condition (air-equilibrated vs. N2-purged; pH 3–10.4). Based on the obtained results, O2 is proposed to play a key role in shuttling photogenerated electrons between the electrodes and U(VI). In addition, the existence of an induction time is discussed in terms of material and reaction pathway. © 2017 Elsevier B.V.

Direct In Situ Growth of Centimeter‐Scale Multi‐Heterojunction MoS2/WS2/WSe2 Thin‐Film Catalyst for Photo‐Electrochemical Hydrogen Evolution

To date, the in situ fabrication of the large-scale van der Waals multi-heterojunction transition metal dichalcogenides (multi-TMDs) is significantly challenging using conventional deposition methods. In this study, vertically stacked centimeter-scale multi-TMD (MoS2/WS2/WSe2 and MoS2/WSe2) thin films are successfully fabricated via sequential pulsed laser deposition (PLD), which is an in situ growth process. The fabricated MoS2/WS2/WSe2 thin film on p-type silicon (p-Si) substrate is designed to form multistaggered gaps (type-II band structure) with p-Si, and this film exhibits excellent spatial and thickness uniformity, which is verified by Raman spectroscopy. Among various application fields, MoS2/WS2/WSe2 is applied to the thin-film catalyst of a p-Si photocathode, to effectively transfer the photogenerated electrons from p-Si to the electrolyte in the photo-electrochemical (PEC) hydrogen evolution. From a comparison between the PEC performances of the homostructure TMDs (homo-TMDs)/p-Si and multi-TMDs/p-Si, it is demonstrated that the multistaggered gap of multi-TMDs/p-Si improves the PEC performance significantly more than the homo-TMDs/p-Si and bare p-Si by effective charge transfer. The new in situ growth process for the fabrication of multi-TMD thin films offers a novel and innovative method for the application of multi-TMD thin films to various fields