Direct Electrocatalytic CO2 Reduction

Another promising route of CO2 reduction is via electrochemical reduction using catalysis. This method mimics photosynthesis. Electrochemical CO2 reduction reactions (CO2RR) can be carried out in ambient conditions through the application of external bias and provides the opportunity to be coupled with electricity generated from renewable energy resources.

It is generally accepted that ideal electrochemical CO2 reduction reactions catalysts should possess the following characteristics:
• high product selectivity,
• long term stability,
• large current density and
• low costs.

A drawback to most electrocatalysts is high overpotentials requirements and typically this exhibit a trade-off between selectivity and current density. To circumvent such challenges, the Particles and Catalysis Research Group has developed three-dimensional and porous Ag Foam electrodes that converts CO2 to CO with a Faradaic Efficiency (FECO) of 94.7% with a current density of -10.8 mA cm-2 at a low applied potential (-0.99 V).

To minimise catalyst costs, the team has fabricated metal-free Graphitic carbon-based catalysts that achieved a moderate FECO of 60% at a low potential of -0.75 V. Lastly, through defect engineering, the team has prepared metal-free nitrogen removed mesoporous carbon (NRMC) catalysts which were able to convert CO2 to CO with a FECO of ~ 80% and a partial current density for CO (jCO) of -2.9 mA cm-2 at an applied overpotential of only 490 mV. This catalytic performance is among the highest of metal-free catalysts and is even on par with the benchmarked metallic catalysts.

The Particles and Catalysis Research Group has also developed simple, scalable and cost-effective catalysts for the conversion of CO2 in the gas phase to liquid products. For this the team are designing a gas diffusion electrode (as shown in Figure 3) for the conversion of CO2 to products such as formate/formic acid.

Modified Sn foil showed high selectivity towards the production of formate (HCOO-) at a moderate Faradaic efficiency (77%) and a low applied potential (-1.09 V). By designing Sn catalyst to be highly crystalline and mesoporous, the mass transport rate was improved without significantly affecting Faradaic efficiency. Lastly, the team has developed a unique three-dimensional heterostructured copper electrode (referred as Cu sandwich) that was obtained via a simple two-step treatment of commercially available copper foam (Cu-f). The designed catalyst achieved a FE toward alcohols of >50% at an applied potential as low as -0.3 V vs reversible hydrogen electrode (RHE). 

As the present production rate achieved with the catalysts are well-below the target that would make this technology financially feasible. In this regard, the team is now further developing the high throughput gas diffusion electrode system.