A Copper Based Catalyst for the CO₂ Reduction Electrochemically Synthesized on a Gas Diffusion Layer

The gaseous CO₂ concentration in the environment has reached alarming levels¹. The uncontrolled exploitation of fossil fuels is one of the main causes for the huge increase of this greenhouse gas and nowadays the living beings have to face the biggest threat to our planet: the Global Warming. In this scenario, a great effort has been demanded to all fields of human sciences in order to guarantee a sustainable future for our society. Among different solutions, the concept of "artificial photosynthesis"² has gained momentum in view of a smart use of the atmospheric CO₂. Inspired by the leaf photosynthesis, the carbon dioxide is converted back to chemicals and fuels by exploiting an electrochemical process³.

This contribution describes an easily synthesized catalyst made of metallic copper that can be used for the heterogenous CO₂ electroreduction (CO₂RR) in liquid phase in order to produce small organic molecules such as formic acid and acetic acid. The metallic copper-based catalyst was electrochemically deposited on a gas diffusion layer (GDL) support made of carbon fibers with a total surface area of 4 cm². Electrosynthesis allows the one step formation and deposition of the Cu film directly on the carbon paper substrate, whose surface properties were optimised by a pre-treatment in 1M H₂SO₄ followed by soaking in pure EtOH⁴. The pre-treatment times were thoroughly evaluated and the ratio between crystalline and amorphous carbon phases was investigated by Raman analysis on the single carbon fibers. Afterwards, as shown in Figure 1, an homogenous electrodeposited Cu film on the carbon fibers support was achieved, starting from an aqueous solution containing Cu(NO₃)₂ at room temperature and atmospheric pressure. In addition, the reproducibility of the catalyst formation process was tested. The optimized catalyst was employed to carry out the CO₂RR in a H-type cell, in which a cation exchange membrane (Nafion-115) divided the cathode side, containing the working electrode, from the anode side where a platinum gauze and Ag/AgCl electrode were used as counter and reference electrodes, respectively. The reaction was performed for one hour in NaHCO₃ aqueous solution. At the end of the reaction, all the liquid products were analysed by quantitative H¹-NMR, while the total amount of gas products were collected in a gas sampling flask and analysed by Gas Chromatography. As first preliminary tests, several reactions were carried out both in continuous and discontinuous CO₂ flux at -1.1 V and -0.7 V vs RHE in which a CO faradaic efficiency (FE) up to 4.36% was obtained with a FE for H₂ evolution of 63.80% for the gaseous product. As for the liquid products, formic acid (up to FE = 4.46%) and acetic acid (up to FE = 6.39%) were obtained.

References

1. https://www.esrl.noaa.gov/gmd/ccgg/trends/

2. Bensaid S. et al. ChemSusChem 5 (2012) 500 – 521

3. Genovese C. et al. J. Energy Chem. 22 (2013) 202–213

4. Xuli Ma et al., Int. J. Hydrog. Energy 41(2016) 14553-14561

Figure 1