Project 12

Recently, we have tested the electrocatalytic performance of di-tungsten carbide (W2C) nanoflakes, for eCO2RR. Our results –published in Nature Communications1 – indicated that W2C nanoflakes –having the best CO2RR activity compared to its counterparts (i.e., Mo2C, Nb2C, and V2C)– are selective for production of hydrocarbons at relatively low overpotentials (excess input energy).1 In this project we propose to perform in-situ x-ray absorption fine structure (XAFS) to gain more understanding about CO2RR mechanism and identify structure­–performance relationships for this catalytic system. More specifically, in-situ XAFS study will help us to characterize (i) the chemical composition of the interfacial region and (ii) the electronic structure of local edge atoms of di-tungsten carbide (W2C) nanoflakes in contact with the electrolyte during the CO2RR. Online observation of changes in the XAFS spectrum of oxygen (O) K-edge will elucidate the type and composition of intermediates formed at different overpotentials that are most likely present at the electrical double layer (EDL). This information will be utilized to explain the selectivity and activity parameters and how they change with respect to the applied potential. Observation of these atoms at the W K-edge can also show us how the W-W coordination number changes with time, applied potential, as well as in the plane structure and for the edge atoms. This information leads to additional insights describing the strong link between the edge type and reaction rates.

In-situ XAFS experiment methods and materials - Single layer W2C nanoflakes will be studied using in-situ XAFS to determine the type and the composition of chemical species formed in EDL as well as the local electronic structure and oxidation state of the surface transition metal atoms as a function of potential. To do so, synthesized nanoflakes will be deposited on a gas diffusion layer. Then, the prepared electrode with a copper contact will be placed and sealed between the electrolyte chamber and a PEEK sample holder using sealing O-rings (Figure). In this configuration, the surface area of the working electrode (electrode under study) can be also changed using different sizes of O-rings (WE, limiting O-ring) as shown in Figure. A platinum (Pt) wire and an Ag/AgCl electrode will be used as the counter and reference electrodes in this cell, respectively. The quartz window shown in this figure allows for in-situ X-ray

measurements. Electrochemical characterization during the in­-situ XAS data acquisition will be performed using a Bio-Logic potentiostat. In detail, cyclic voltammetry (CV) experiments will be measured for W2C nanoflakes in 1M KOH electrolyte. The electrolyte is saturated with CO2 prior to the experiment. The in­-situ XAFS spectra will be acquired at the Sector 10-ID beamline of the Advanced Photon Source at ANL. A cell potential will be held constant for each acquisition for a sufficient time to collect a certain number of continuous scanning XAFS spectra at the W and O K-edge. The in-situ XAFS will be recorded for different cell potentials such as -1.0, -1.1, -1.2, -1.3, -1.4, -1.5, -1.6 and -1.7 V vs Ag/AgCl on the anodic sweep. The collected XAFS data will be analyzed, and the information will be used as inputs for DFT-calculations to obtain a fundamental understanding of the structure-performance relationships of this catalytic system for eCO2RR.