Schematic of the Dioxide Materials patented  3-compartment electrolyzer design configuration producing formic acid using only CO2, water , and electricity. Schematic adapted from Kaczur et. al .
The electrochemical conversion of carbon dioxide (CO2) into fuels and feedstocks has received increased attention during the past five years due to the global goal of reducing green house gas emissions (GHG). A wide variety of chemicals, such as carbon monoxide (CO), methane (CH4), ethylene (C2H4), formate/formic acid (HCOO– and HCOOH), as well as alcohols can be obtained through the electrochemical CO2 reduction reactions. Specific cathode catalysts have been found that help promote these reactions.
Among all the potential chemicals generated through CO2 reduction, formic acid has attracted great interest because the reduction reaction requires only two electrons per molecule. In addition, formic acid has a wide range of commercial applications in addition to having a commercial selling price in the $500 – $1200/metric ton range depending on global location.
The Dioxide Materials novel 3-compartment cell design is the only demonstrated technology that can directly produce a pure formic acid product from CO2 using only water and electricity [3,4]. Researchers at Dioxide Materials have recently published a new paper with data showing long term stable performance using a new catalyst .
3-Compartment Formic Acid Electrolyzer Chemistry
The electrochemical reduction of CO2 occurs in the presence of water at the cathode, forming formate (HCOO−) and hydroxide (OH−) ions:
CO2 + H2O + 2e− → HCOO− + OH− (1)
Most of the hydroxide ions are converted to bicarbonate/carbonate from reactions with the CO2 gas passing into the cathode compartment:
CO2 + OH− → HCO3− (2)
CO2 + 2OH− → CO3−2 + H2O (3)
Formate ions, any unreacted hydroxide ions, bicarbonate ions, and carbonate ions all migrate through the adjoining anion exchange membrane into the center flow compartment.
Simultaneously, the oxidation of water occurs at the anode, forming oxygen gas and hydrogen ions (H+).
2H2O → 4H+ + 4e− + O2 (4)
The H+ ions leave the anode compartment, passing through the adjoining cation exchange membrane into the center compartment.
In the center compartment, H+ ions react with formate ions as well as any OH– ions producing formic acid and water:
H+ + OH− → H2O (5)
H+ + HCOO− → HCOOH (6)
In the center compartment, H+ ions react also react with bicarbonate/carbonate ions forming CO2 as follows:
HCO3− + H+ → CO2 + H2O (7)
CO3−2 + 2H+ → CO2 + H2O (8)
The formic acid electrolyzer center compartment actually produces a combination stream of pure formic acid as well as a pure CO2 gas that can be recovered and recycled.
1000 Hr Electrolyzer Testing
In the recent published paper, Yang et. al , the 3-compartment formic acid electrolyzer design was operated in a 1000 hr performance and stability test using a nanoparticle bismuth oxide (Bi2O3)-based cathode catalyst, instead of the nanoparticle tin oxide (SnO2) catalyst that was utilized in the previous papers [2,3,4].
An example of the 1000 hr performance of the formic acid electrolyzer is shown in Fig 1. The formic acid Faradaic efficiency (FE) operated in the 70 – 80% range and showed stable voltage performance while producing a nominal 10 wt% (2.2 M) formic acid product at a current density of 200 mA/cm2.
Fig. 1. Performance of the 3-compartment formic acid cell in a 1000 hr performance run. The formic acid concentration was set at about 10 wt%. The cell operated in a formic acid FE range between 70% to 80% at a current density of 200 mA/cm2. The cell voltage was stable, operating in the range of 3.6 – 3.7 V over the entire run.
Cathode Catalyst Characterization after 1000 Hr Operation
Examination of the Bi2O3 catalyst before and after by X-Ray diffraction (XRD) and scanning electron microscopy (SEM) showed some interesting results. The SEM results generally showed that the rough cathode surface became relatively flattened and smooth but showing no other significant structure changes. XRD results indicated a partial conversion of the Bi2O3 to Bi metal and bismuth subcarbonate (Bi2O2CO3). These catalysts have been reported to have high catalyst activity in some recent papers. Additional work in the future will need to be done to characterize the bismuth-based cathode catalysts in this electrolyzer design.
- J. J. Kaczur, H. Yang, S. D. Sajjad, and R. I. Masel. Method and system for electrochemical production of formic acid from carbon dioxide, assigned to Dioxide Materials, issued Aug. 14, 2018. https://patents.google.com/patent/US10047446B2/en
- Kaczur J. J., Yang H, Liu Z, Sajjad S .D. and Masel R. I. Carbon dioxide and water electrolysis using new alkaline stable anion membranes. Front. Chem. 6:263 (2018). https://doi.org/10.3389/fchem.2018.00263.
- H. Yang, J.J. Kaczur, S.D. Sajjad, R.I. Masel, Electrochemical conversion of CO2 to formic acid utilizing Sustainion™ membranes, J. CO2 Util., 20 (2017), pp. 208-217, https://doi.org/10.1016/j.jcou.2017.04.011
- H. Yang, J.J. Kaczur, S.D. Sajjad, R.I. Masel, CO2 conversion to formic acid in a three compartment cell with Sustainion™ membranes, ECS Trans., 77 (2017), pp. 1425-1431, https://doi.org/10.1149/07711.1425ecst
- Hongzhou Yang, Jerry J. Kaczur, Syed Dawar Sajjad, Richard I. Masel. Performance and long-term stability of CO2 conversion to formic acid using a three-compartment electrolyzer design, J. CO2 Util., 42 (2020). https://doi.org/10.1016/j.jcou.2020.101349.