Production of solar fuels by photoelectrochemical conversion of carbon dioxide

Ibrahim Erdem Irtem

Morante i Lleonart, Joan Ramon Andreu Arbella, Teresa

Growing global emission of carbon dioxide gas (CO2) reflects the world’s energy dependence on fossil fuels. The conversion of CO2 emission into value-added products, like fuels completes a circular CO2 economy which requires a renewable energy conversion and storage system. Amongst a few, photo/electrochemistry has been particularly appealing thanks to its energy efficiency and enormous potential for industrial applications. Formic acid (HCOOH) production from CO2 reduction appears as an alternative energy storage option based on the commercialization of this process. Herein, stable and selective catalysts working at low overpotential are needed to reduce CO2. Likewise, cell design is critical to have improved CO2 mass transport for obtaining high conversion efficiencies and to achieve feasible production yields. The initial work was conducted on the design and understanding of operational parameters of an electrochemical flow cell (ECf-cell) such as flow rates and electrode potentials. For CO2 reduction at the cathode site, two different gas diffusion electrodes were produced by electrodeposition: Sn-GDE and Cu-GDE. An optimum potential range was established to control HCOOH selectivity. The complementing reaction at the anode site, oxygen evolution reaction (OER), was studied using Mn-Co oxide nanoparticles to replace expensive DSA: Ir-Ta oxide catalyst. Subsequent efforts were devoted on the assembly of a photoelectrochemical flow cell (PECf-cell) which enabled coupling of Sn-GDE as cathode vs. TiO2 nanorods as photoanode. This led to nearly 1/3 reduction in overall cell voltage reaching an energy efficiency up to 70 %. The solar-to-fuel (STF) conversion efficiency was 0.25% which was one of the highest efficiencies reported amongst the data obtained from a cell in device level. The results proved that optimized system efficiency could be achieved with a large bandgap photoanode having superior stability and a GDE cathode with improved CO2 mass transfer. The deployment of renewable energy sources will require new technologies to emerge. The photoelectrochemical flow cell developed in this work can store energy from intermittent electricity sources (i.e. wind and solar) in a sustainable manner. This may pave the way for commercialization of this process and moving towards a circular CO2 economy.

The typical efficiency of the plants is around 0.1 – 0.2 %. The main interest of this work is to demonstrate how a photoelectrochemical device can continuously convert CO2, obtaining a higher yield than the capacity of biological photosynthesis: proof of concept of artificial photosynthesis applicable to industry. The reactions studied are the reduction of CO2 to HCOO– (formate with a price metric ton is currently around 1000 € with 2 % annual increase) and its complementary reaction, oxidation of H2O to O2. Since both reactions are endothermic processes, the energy input must be minimized and supplied by renewable energy sources to increase profitability, as shown in this work. Companies seeking to reduce their carbon footprint from plastics and fuels without sacrificing the economics of conventionally-made products could be the group of interest. We can create these low-carbon products from any CO2 source, but existing petrochemical and cement industry (ethylene, ethylene oxide and ammonia processes) represent an attractive initial source of CO2 due the purity of their emission. In Europe alone, they emit 100 Mt of pure CO2 gas per year, leaving $40Bn of potential value on the table and expose themselves to future regulatory risks. In addition, they already have supply chains in place for the products we can produce, i.e. formate, a stable liquid and CO/H2 mix, a syngas for synthetic fuel.

CO2 Emission; Fossil Fuels; Value-Added Products; Fuels; Circular CO2 Economy; Renewable Energy Conversion; Energy Storage System; Photo/Electrochemistry; Energy Efficiency; Industrial Applications; **Formic Acid (HCOOH)** Production; CO2 Reduction; Alternative Energy Storage; Commercialization; Stable Catalysts; Selective Catalysts; Low Overpotential; Cell Design; CO2 Mass Transport; High Conversion Efficiencies; Feasible Production Yields; Electrochemical Flow Cell (ECf-cell); Operational Parameters; Flow Rates; Electrode Potentials; Cathode; Gas Diffusion Electrodes (GDE); Electrodeposition; Sn-GDE; Cu-GDE; Optimum Potential Range; HCOOH Selectivity; Anode; Oxygen Evolution Reaction (OER); Mn-Co Oxide Nanoparticles; DSA; Ir-Ta Oxide Catalyst; Photoelectrochemical Flow Cell (PECf-cell); TiO2 Nanorods; Photoanode; Overall Cell Voltage Reduction; Energy Efficiency; Solar-to-Fuel (STF) Conversion Efficiency; Optimized System Efficiency; Large Bandgap Photoanode; Superior Stability; GDE Cathode; Improved CO2 Mass Transfer; Renewable Energy Sources; Intermittent Electricity Sources; Wind Energy; Solar Energy; Sustainable Energy Storage; Commercialization Path.