62nd Annual Report on Research 2017

The research project is mainly focused on the materials chemistry of Cu-containing bimetallic and metal/oxide heterostructured nanomaterials as catalysts for electrochemical CO₂ reduction reactions, with the aims to develop electrocatalysts that are able to selectively and rapidly convert CO₂ to useful products as well as to understand the metal-metal or metal-oxide interactions and their influences on electrocatalytic reactivity.

We first studied a Cu-Ag bimetallic nanoparticle system. The nanoparticles were synthesized by a polyol method, in which a mixture of Cu²⁺ and Ag⁺ were reduced by ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP). Depending on the reaction time, the structure of the obtained Cu-Ag nanoparticles changes from Ag nanoparticles decorated with discontinuous Cu patches on the surface to Ag@Cu core@shell nanoparticles (Figure 1a). The interactions between the Ag component and the Cu component in the material structure generate interesting electrocatalytic properties toward CO₂ reduction. In the Ag-rich composition range, the intrinsic catalytic behavior of Ag is improved to give even higher selectivity toward production of CO (Figure 1b). In the Cu-rich range, Ag modifies the catalytic selectivity of Cu, achieving higher Faradaic efficiency (FE) for CO₂ reduction to ethylene (C₂H₄). This work has been published as a research article in J. Phys. Chem. C.

Figure 1. (a) Schematic illustration of the structural evolution of Ag-Cu bimetallic nanoparticles as the synthesis reaction time increases. (b) Composition-dependent product selectivity for the Ag-Cu materials as electrocatalysts for CO₂ reduction at -1.06 V vs RHE in 0.1 M aqueous KHCO₃ (pH 6.8) saturated with CO₂.

We also studied a Cu-SnO_x heterostructured nanoparticle system, using mildly oxidized carbon nanotubes (CNTs) as the catalyst support. By adjusting the Cu/Sn ratio in the catalyst material structure, we can tune the products of the CO₂ electrocatalytic reduction reaction from hydrocarbon-favorable to CO-selective to formic-acid-dominant (Figure 2). In the Cu-rich regime, SnO_x dramatically alters the catalytic behavior of Cu. The Cu/SnO_x-CNT catalyst containing 6.2% of SnO_x converts CO₂ to CO with a high FE of 89% and a j_CO of 11.3 mA·cm^-2 at -0.99 V vs RHE, in stark contrast to the Cu-CNT catalyst on which ethylene and methane are the main products for CO₂ reduction. In the Sn-richer regime, Cu modifies the catalytic properties of SnO_x. The Cu/SnO_x-CNT catalyst containing 30.2% of SnO_x reduces CO₂ to formic acid with a FE of 77% and a j_HCOOH of 4.0 mA·cm^-2 at -0.99 V, outperforming the SnO_x-CNT catalyst which only converts CO₂ to formic acid in a FE of 48%. This work, which has been published as a research article in ACS Appl. Mater. Interfaces, represents one of the first systematic study on composition-dependent influences of metal-oxide interactions on electrocatalytic CO₂ reduction.

Figure 2. Correlations between material structures and product distributions for composition-tunable Cu/SnO_x-CNT materials as electrocatalysts for CO₂ reduction.

These results have brought us new knowledge of utilizing metal-metal and metal-oxide interactions to improve electrocatalysis for the important CO₂ conversion reactions. We have developed Cu-containing nanoparticle electrocatalysts with tunable product selectivity for the CO₂ reduction reactions, and have gained preliminary insights into how Cu interacts electronically with another metal or a metal oxide and manifests different catalytic properties.

Our research in the following year will answer the remaining questions in this project. We would like to explore how metal-metal and metal-oxide interactions can be utilized to achieve more active and stable electrochemical CO₂ conversion, in addition to the product selectivity that we focused on investigating in the past year. Our current understanding has mainly been on electronic interactions between two components within a material structure, we would like to examine the possibilities of other cooperation modes, such as reaction intermediate co-stabilization and hydrogen spillover. We anticipate that at the end of this project we will have a toolbox for designing advanced electrocatalysts for CO₂ reduction reactions by coordinating metal-metal and metal-oxide interactions.