Evidence of local pH changes during ethanol oxidation at Pt electrodes in alkaline media**

Local changes of the interfacial pH can significantly affect the rate and mechanism in the course of an electrodic reaction. For instance, different pH values will have significant effect on the equilibrium properties of both solution and surface species altering the reactions kinetics. Ethanol oxidation at platinum electrodes, in alkaline media involves a fast consumption of OH species that will change the local pH at the electrode surface decreasing the reaction rate. In this study, the local pH change during ethanol oxidation in alkaline media is accomplished by using rotating ring disc electrode (RRDE) experiments. The current at the ring when polarised at the onset of hydrogen evolution, serves as a measure of the local pH in the vicinity of the electrode. The results show that the current at the ring at 0.1 V (vs RHE) becomes more negative during ethanol oxidation due to a change in the equilibrium potential of the hydrogen evolution reaction due to the change in the local pH. Recent advances in the development of anion exchange membranes (AEM) [1-4] have increased the interest in the study of alcohols oxidation for direct alcohol fuel cells (DAFC). The use of high pH electrolytes in DAFC presents many advantages: the oxidation of organic fuels (such as methanol or ethanol) is faster and takes place at lower overpotentials in alkaline media [5, 6]; alkaline conditions are also more favourable for the cathodic side (oxygen reduction reaction) [7] and the range of electrode materials that are stable is much wider allowing the use of cheaper and more abundant less noble metals [8]. Though AEM-DAFC are considered as a promising alternative to conventional polymer exchange membrane (PEM) DAFC, there are still many challenges to face in this area. The slow development of AEM-DAFCs seems, at least in part, to be caused by the lack of ideal and robust alkaline membranes. Besides, the pH changes in the anodic region due to proton release and carbonate formation will thermodynamically cause a voltage loss to the fuel cell [9]. This change of the pH in the vicinity of the electrode will also affect significantly the catalytic activity of the metal toward the alcohol oxidation reaction. During reactions involving H or OH as reactants or products, the local pH at the electrode/electrolyte interphase can differ from that in the bulk electrolyte with the progress of the reaction, especially when the electrode processes are much faster than the mass transport [10, 11]. In turn, the change of the pH will certainly affect the equilibrium potential and the kinetic behavior for such reactions. This phenomenon is particularly important when working under low volume confined environments or thin layer configurations (as in porous electrodes with thicknesses in the range of tens of μm used in fuel cells or during some spectroelectrochemical measurements). The overall oxidation reaction of ethanol in alkaline media is: CH3CH2OH + 12 OH  2 CO2 + 9 H2O +12 e (Eq. 1) This reaction involves the consumption of a great amount of OH species at the electrode and, therefore, it is very likely to produce a change in the local pH of the solution in contact with the surface of the electrode. This pH change will be significant if the OH concentration at the interphase is not replenished by mass transport. Under these conditions, as ethanol oxidation proceeds, the reaction will take place in progressively more acidic solution. There are numerous studies that mention local pH changes at the electrode surface during ethanol oxidation [12, 13], especially when spectroelectrochemical measurements are reported, in which CO2, and not carbonate, bands can be observed in alkaline media, signaling that local pH is sufficiently low to preserve the stability of the produced CO2. These changes of pH will have, as explained before, drastic consequences in fuel cells systems when ethanol is used as fuel. Scheme 1. Rotating Ring Disc electrode set-up used for this study. In this paper, by using rotating ring disc measurements, we reveal the existence of local pH changes in the vicinity of a Pt electrode during ethanol oxidation in alkaline media. This was achieved with a ring-disk system (Scheme 1), by measuring the increase of cathodic currents at the ring at a constant potential of 0.1 V (caused by the shift of the equilibrium potential of the hydrogen evolution reaction) while the disc potential was cycling in the ethanol oxidation potential range (0.1 to 1.25 V). The potential of 0.1 V for the ring was chosen for two reasons: first, no contributions from the oxidation of ethanol or any other intermediates are expected at 0.1 V and second, at pH close to 13 (0.1 M NaOH), the surface will be covered by hydrogen at this potential but no hydrogen evolution will occur. However, if local pH decreases, the onset potential for hydrogen evolution will increase, and this reaction will take place accordingly. [∗] Dr. M. C. Figueiredo, Dr. Tanja Kallio Chemistry Department, Research group of Fuel Cells Aalto University P.O. Box 16100, 00076 Aalto, Finland Fax: +358947022580 Current adress: Leiden Institute of Chemistry, Leiden University PO Box 9502, 2300 RA Leiden, Netherlands E-mail: [email protected] R. Arán-Aís, Dr. Víctor Climent, Prof. J. M. Feliu Instituto de Electroquímica Universidad de Alicante Apartado 99, 03080 Alicante, Spain