(Invited) Intercalation of Copper into Tungsten Oxide Nanostructures: Development of Highly Active Electrocatalytic Systems for CO2-Reduction
Authors: Rutkowska Iwona A. , Chmielnicka Anna , Kulesza Pawel 
Published: December 2024

There has been growing interest in the electrochemical reduction of carbon dioxide (CO2), a potent greenhouse gas and a contributor to global climate change, and its conversion into useful carbon-based fuels or chemicals. Numerous homogeneous and heterogeneous catalytic systems have been proposed to induce the CO2 reduction and, depending on the reaction conditions various products that include carbon monoxide, oxalate, formate, carboxylic acids, formaldehyde, acetone or methanol, in addition to various hydrocarbons at different ratios. Given the fact that the CO2 molecule is very stable, its electroreduction processes are characterized by large overpotentials. To produce highly efficient and selective electrocatalysts, the transition-metal-based molecular materials are often considered. It is believed that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate.
Because reduction of CO2 can effectively occur by hydrogenation, to optimize the conventional copper-based electrocatalytic approach, we explore such a model catalytic matrix as nanostructured tungsten oxide nanowires which are capable to undergo partial reduction to hydrogen-rich nonstoichiometric tungsten oxide bronzes. We demonstrate here that following the ntercalation of copper into tungsten(VI)-oxide-nanowires, the resulting electrocatalytic systems exhibits synergistic properties toward electroreduction of carbon dioxide, in addition to other inert inorganic reactants, such as oxygen, nitrates(V), nitrates(III) or bromates(V). In particular, evidence has been provided that hierarchically deposited films (on glassy carbon) of copper(I) oxide decorated with tungsten(VI) oxide nanowires and, subsequently, subjected to electroreduction and voltammetric conditioning to generate hybrid Cu/WO3 catalyst can be successfully utilized to drive reduction of carbon dioxide (saturated solution, concentration, ca. 0.033 mol dm-3)in a fairly strong acid medium of 0.5 mol dm-3 H2SO4. Here formation of the partially reduced tungsten oxides (HxWO3 and WO3-y) is accompanied by consumption of protons and sorption of hydrogen, and it tends to inhibit hydrogen evolution by shifting the proton discharge toward more negative potentials. Our observations are consistent with the view that copper is irreversibly trapped (i.e., it cannot be reoxidized) within the network of WO3 nanowires. The dispersed metallic copper sites seem to facilitate electron transfers and charge distribution in the catalytic layers. Among important issues are the capacity of copper-containing partially reduced tungsten oxides to induce reductions of nitrates, bromates and oxygen in acid medium. On mechanistic grounds, the existence of hydrogen-rich partially-reduced tungsten oxides, HxWO3, which contain large population of delocalized electrons and monoatomic H, or coexisting protons and electrons, H++ e-, is likely to induce hydrogenation of carbon oxo species, followed by protonation, in the vicinity of Cu to form oxo-hydrocarbon-type products. The hybrid Cu/WO3 system, is also characterized by improved durability, relative to pristine Cu2O-dervied copper, as evident from electroreductions under chronoamperometric conditions. Our research aiming at optimization the material’s activity and selectivity have also concentrated on application of mixed oxides, e.g. WO3-ZrO2. Finally, the present study demonstrates the usefulness of certain diagnostic electroanalytical approaches, such as ultramicroelectrode-based sensing,chronocoulometric probing of the diffusional-type charge propagation dynamics, or voltammetric stripping and monitoring of small organic or inorganic molecules as electroreduction products.