Paper EH-TuE9
Electrochemical Reduction of CO2 as a Way to Store Energy from Intermittent Sources

Tuesday, December 9, 2014, 8:20 pm, Room Lehua

The desire to increase the utilization of sustainable energy sources such as solar and wind is hampered by their intermittent nature. Large scale energy storage capacity is needed to maximize utilization of these sources, specifically to avoid large amounts of renewable energy being wasted when their supply exceeds demand.

Over the last years we have studied the electrochemical reduction of CO₂ to various value-added chemicals such as carbon monoxide (CO), formic acid, and methane. When coupled to renewable energy sources such as wind and solar, this process can produce carbon-neutral fuels or commodity chemicals, possibly providing a method for storage of otherwise wasted excess energy from intermittent renewable sources [1].

For this process to become economically feasible, more active and stable catalysts as well as better electrodes are necessary such that CO₂ electrolyzers can be operated at sufficient conversion (current density >250 mA/cm²), reasonable energetic efficiency (>60%), and sufficient product selectivity (Faradaic efficiency >90%). For CO production, a key reactant in the Fischer-Tropsch process, the best performance reported to date is current densities on the order of 90 mA/cm² and energy efficiencies up to 45%, when operating at ambient conditions [2]. This presentation will focus on new catalysts systems for efficient conversion of CO₂ to CO: (i) Ag nanoparticles supported on TiO₂ [3]; (ii) Au nanoparticles supported on multiwall nanotubes; and (iii) metal-free N-doped carbons. These catalysts have been characterized in a 3-electrode cell and in an electrolyzer. Current densities of between 100 and 250 mA/cm² as well as energy efficiencies of up to 70% were obtained. The electrodes in all these cases are prepared using automated airbrushing [2], which reduced catalyst loadings to 0.75 mg/cm² for Ag and 0.17 mg/cm² for Au. These performance levels, together with the lower cost due to low precious metal loading (due to the use of catalyst supports), or even the elimination of precious metals altogether (N-doped carbons), brings electrochemical reduction of CO₂ to CO closer to economic feasibility.

We also performed an economic / life-cycle analysis of this process, to determine whether this technology can become, economically viable for large scale application in the storage of energy from renewable sources, and/or in the reduction of greenhouse gas emissions.

References

[1] H.R. Jhong, S. Ma, P.J.A. Kenis, Current Opinion in Chemical Engineering 2 (2013) 191.

[2] H.R. Jhong, F.R. Brushett, P.J.A. Kenis, Advanced Energy Materials 3 (2013) 589.

[3] S. Ma, Y. Lan, G.M.J. Perez, S. Moniri, P.J.A. Kenis, ChemSusChem 7 (2014) 866.