During his period at IFPEN as a scientific visitor, Edward Brightman, an electrochemist at the National Physical Laboratory (NPL) in the United Kingdom, worked on a new method for measuring the acidity of ion-conducting polymer membranes. The method marks a significant breakthrough for the electrochemical conversion of CO2 and may be extended to other applications, in the field of energy storage.
The electro-reduction of CO2 to useful products or chemical intermediates - for example formic acid - is an interesting avenue since it could pave the way for transforming this greenhouse gas into non-fossil-sourced exploitable compounds. However, while the phenomena at play in this process are well known, significant research is still required to develop it further, particularly in terms of methodologies and equipment.
In this case, an experimental challenge to be overcome (see box) related to the capacity to measure the proton activity of a polymer electrolyte membrane (PEM).
In order to address this difficulty, the starting point for this research involved employing a reference electrode, initially designed for fuel cells, that makes it possible to measure a membrane's surface potential. The measuring principle hinges around the evaluation of the differential potential between the membrane and a pH-sensitive working electrode.
A miniature apparatus was developed (see figure 1) using electrodes and a membrane with a small surface area (Ø 4 mm). The equipment was then used to test two types of pH-sensitive electrodes: (i) A dynamic hydrogen electrode on Pt, and (ii) a hydrated iridium oxide electrode, IrOx.
Once the working electrodes had been calibrated on the basis of standard solutions of known pH, the method was validated using Nafion® membranes modified via the neutralization of protons with various protic bases (such as imidazole, butylimidazole, diethylamine and triethylamine).
The results obtained with different membranes confirmed lower acidity for the ones that had been modified.
Polymer membranes with suitable acidity were selected for subsequent validation in a pilot unit for the electro-reduction of CO2.
In addition to this miniature assembly, another set of apparatus was designed to characterize membranes with a larger surface area (Ø 50 mm). It is currently being used to validate these results in more representative conditions. Ionic conductivity, the other major parameter affecting the performance of these PEMs, can also be measured.
Developed to overcome a major technological challenge in the context of the electrochemical conversion of CO2, this new measurement method may be advantageously extended to a broad spectrum of key applications in the field of energy storage, such as the development of membranes for redox flow batteries, methanol fuel cells and high-temperature fuel cells, or for the study of corrosion between the bipolar plates of fuel cells.
A joint NPL/IFPEN publication concerning this methodological development and the results obtained from the research conducted by Edward Brightman during his period as a scientific visitor is currently being written.
Concretely, the implementation of the electrochemical reduction of CO2, which takes place in an electrochemical cell of standard configuration, requires low proton activity in order to avoid the release of hydrogen as a side reaction. Nafion®, the reference polymer material for the polymer electrolyte membrane (PEM) which separates the anode and cathode compartments of the cell, contains "sulfonic acid" functions that are undesirable for this application since they give it highly acidic properties.
Moreover, it is difficult - both practically and theoretically - to measure proton activity in a solid polymer electrolyte membrane and there is very little literature on the subject. However, given that it is a major intrinsic property of PEM-type ionic conducting materials, such a measurement is required for the development of membranes with controlled proton activity.
Acidity (pH) can be determined by measuring the electrochemical potential of a proton-sensitive half-cell, using electrochemical theory, and more specifically the Nernst equation, which relates these two values. The fact remains, however, that the experimental implementation of this theory for PEMs is hampered by the absence of a reliable reference electrode, in the required geometry (a thin membrane).