Electroreduction of CO₂ to CO: Analyzing the Effect of Electrolytes on Ag, Au Electrodes and Defining Technoeconomic Benchmarks

Developing cost competitive technologies that can stabilize and eventually reduce the rising levels of anthropogenic carbon dioxide (CO2) emissions remains one of the most important challenges of the 21st century.1 A variety of approaches such as switching from fossil fuel to renewable energy sources, increasing the energy efficiency of vehicles and buildings, as well as the capture and sequestration of CO2 from point sources (such as coal fired power plants and cement plants) need to be implemented to enable the transition of modern society towards a carbon neutral future.2 Electroreduction of CO2 to value added chemicals and fuels (such as carbon monoxide (CO), formic acid (HCOOH), methanol, ethanol, and ethylene) offers another interesting way of utilizing excess CO2 emissions.3 When the electroreduction process is driven by intermittent solar or wind energy, the process can be used to store otherwise wasted renewable electricity as well. This presentation will focus on the effect of different electrolytes (such as KOH, KCl, and KHCO3) on the electroreduction of CO2 to CO when using two different cathode catalysts: (i) < 100 nm silver (Ag) nanoparticles, and (ii) 4 nm gold nanoparticles on polybenzimidazole wrapped multi walled carbon nanotubes (Au/PyPBI/MWCNT) in an electrochemical flow cell.4 The concentration of the electrolyte (high concentration improves partial current density for CO (jCO )) and the nature of the anion (shifts the onset potential for CO in the order OH- < HCO3 - < Cl-) were found to play an important role in the process for both the cathode catalysts studied here.5 Onset cathode potentials as low as -0.05 V and -0.13 V vs. RHE for CO production were observed when using Ag and Au/PyPBI/MWCNT as the cathode catalyst and KOH as the electrolyte. Furthermore, combining these cathode catalysts with an IrO2 anode6 (electrolyte = 2.0 M KOH) resulted in jCO as high as 104 and 158 mA cm-2 at an overall system energy efficiency of 60% (cell potential = -2.25 V) for the case of Ag and Au/PyPBI/MWCNT cathode catalyst respectively (Figure 1). The Au/PyPBI/MWCNT catalyst exhibited remarkably high mass activity of nearly 1100 A g-1 Aufor CO production at a cathode overpotential of -0.4 V as well. The second part of the talk will focus on a gross margin model to evaluate the technoeconomic feasibility of CO2 electroreduction to different C1-C2 chemicals. The performance benchmarks required for an economically viable process of CO2 electroreduction (in terms of minimum operating current density, maximum operating cell potential (Vmax ), Faradaic efficiency, catalyst durability, and catalyst cost) were derived using the model. A comparison of Vmax with the E0 cell (thermodynamic cell potential) required for the electrochemical production of a particular chemical revealed that CO and HCOOH are the most economically viable products. The economic viability of other process designs such as co-producing different chemicals and combining CO2capture with conversion were also evaluated. References [1] S. Chu, A. Majumdar, Nature 2012, 488, 294-303. [2] S. Pacala, R. Socolow, Science 2004, 305, 968-972. [3] H. R. M. Jhong, S. Ma, P. J. A. Kenis, Curr. Op. Chem. Eng. 2013, 2.2, 191-199. [4] D. T. Whipple, E. C. Finke, P. J. A. Kenis, Electrochem. Solid-State Lett. 2010, 13, B109-B111. [5] S. Verma, X. Lu, R. I. Masel, P. J. A. Kenis, Phys. Chem. Chem. Phys. 2016, 18, 7075-7084. [6] S. Ma, R. Luo, S. Moniri, Y. Lan, P. J. A. Kenis, J. Electrochem. Soc. 2014, 161 .10, F1124-F1131. Figure 1