Gas-Phase Electrochemical Conversion of Methane on Boron-Doped Diamond Gas Diffusion Anodes
Authors: Vass Adam , Ghanem Hanadi , Rosiwal Stefan M. , Franken Tanja , Palkovits Regina , Mul Guido , Tsampas Michail N. , Katsoukis Georgios , Altomare Marco 
Published: February 2024

Methane as the primary component of natural gas is used as fuel for energy supply and serves as a major feedstock for the chemical industry to produce hydrogen. In addition, renewable methane is becoming available via thermochemical, photo- or electrochemical, and biogenic routes. It remains a challenge, however, to efficiently activate the C-H bond of methane while keeping the reaction kinetics under control to form value added products (hydrocarbons or alcohols) and avoid complete oxidation to CO2 [1]. A promising way to selectively activate the C-H bond of methane is to utilize reactive oxygen species (ROS) [2]. ROS play a major role in the partial electrochemical oxidation of water to produce hydrogen peroxide with boron-doped diamond (BDD) anodes [3]. However, little is known about the role of ROS towards CH4 activation under electrochemical conditions. The goal of our work is to achieve control over product selectivity in the gas-phase electrochemical conversion of methane towards methanol or C2+ products [4, 5]. We also aim to explore the role of water in the methane conversion mechanism using in-situ electrochemical FT-IR spectroscopy.
Experimentally, we use a BDD coated mesh as gas-diffusion anode electrode in a zero-gap gas-phase electrolyzer. The anode side of the cell is fed with humidified or dry CH4 (or He for control experiments) and humidified He or H2O are fed to the cathode side (to keep the Nafion 117 membrane humidified). The electro-oxidation of water is paired with hydrogen evolution.. The scheme of the membrane electrode assembly is shown on Fig. 1. A.
In this talk, we discuss the effects of water content, gas flow rate and methane pressure on product selectivity. We observe the formation of CO2 and CO along with O2 when humidified CH4 is fed to the anode side of the electrolyzer (and H2O is fed to the cathode). CO2 and CO originate from the oxidation of CH4, as confirmed by control experiment with only He fed to the anode (Fig. 1. B). This suggests that ROS, generated at the BDD electrode by water partial oxidation, can oxidize methane. However, O2 is still the major product even in the presence of CH4 and both conversion and selectivity are low. The missing Faradaic efficiency (Fig. 1. C) can be related to non-detected CH4 oxidation products – for this, we are currently deploying additional analytical techniques besides gas-chromatography, e.g., IR, NMR, and HPLC, to detect minor products in both gas and liquid phases.
When running experiments with humidified CH4 gas feed on the anode and humidified He gas feed on the cathode, we observe that the CO to CO2 ratio increases. This indicates that the selectivity changes significantly while the cell (and the membrane) dries out. Hence, we propose that the product selectivity can be controlled by tuning the humidity level.
Finally, in experiments with dry CH4 gas fed to the anode and H2O recirculated to the cathode, we observe that both the increase of the CH4 gas flow rate (from 10 to 20 cm3 min-1) and pressure (atmospheric, +1 and +2 bar) lead to a consistent increase of the CO2 and CO formation yield. Hence, controlling CH4 gas flow rate and pressure provides further opportunity to control the conversion and selectivity.
The results achieved so far motivate us to further investigate the joint effect of the above parameters by using a membrane capable to operate under dry conditions, in a system with improved H2O management.

References

[1] R. Horn et al. Cat. Lett., 145, 23–39 (2015).
[2] R. F. B de Souza et al. Catalysts, 12, 217 (2022).
[3] K. Wenderich et al. ACS Sustainable Chem. Eng., 9, 23, 7803–7812 (2021).
[4] J. A. Arminio-Ravelo et al. Curr. Op. in Green and Sustainable Chemistry, 30, 100489 (2021).
[5] A. S. Ramos et al. ChemCatChem, 12, 18, 4517-4521 (2020)

Figure 1. Schematic figure of the membrane electrode assembly (A). Chronovoltammetric curve (top) and product formation rates in the anode compartment (bottom, O2, CO2 and CO respectively) recorded during continuous electrolysis applying humidified He or CH4 anodic gas feeds (B). Faradaic efficiencies for anodic processes of electrolysis with humidified He or CH4 anodic gas feeds (C). Electrolysis conditions: Tcell = 25 °C, j = 20 mA cm–2, 10 cm3 min-1 gas feed rate, atmospheric pressure, H2O recirculated in the cathode compartment.

Figure 1