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44 CHAPTER3. EXPERIMENTALMETHODS 3.2 3.2.1 Electrochemical measurements Cyclic voltammetry In the study of Hummelgård et al. [15] cyclic voltammetry (CV) was used both for the quantification of the electrochemically active surface area (q∗) and to gain further information about the processes ongoing on the electrode at different po- tentials. For metal oxide electrodes, the voltammogram is characterized by the presence of broad diffuse peaks and thus cannot give the same understanding of the surface reactions as is possible e.g. for metallic Pt[197]. However, the determi- nation of q∗ based on the CV is a standard measurement in electrochemical studies of metal oxide electrodes. In the present work, the q∗ is determined by measuring the CV between two end potentials, and then integrating the current over time to obtain a positive and a negative charge. The average absolute value of these two charges is used as q∗. In turn, this charge is related to the real surface area of the electrode[97]. However, the connection is nontrivial for mixed oxide electrodes, and q∗ is mainly utilized to compare the surface areas between different electrodes, rather than to obtain an absolute value of the surface area in e.g. m2/g. 3.2.2 Galvanostatic polarization curve measurements In the study of Hummelgård et al. [15], we measured polarization curves (electrode potential plotted versus the logarithm of the applied current density, j) for mixed oxide electrodes. The slope of this curve is the Tafel slope. For industrial synthesis, it is important that the Tafel slope is minimized, since high production rates at high current densities then become achievable at a low total electrode voltage and total applied power. While polarization curves at low applied current densities can be measured with a relatively simple three-electrode setup, the high current densities relevant for in- dustrial electrosynthesis (i.e. several kiloamperes per square meter) require control of primarily two factors: the control of mass transport of reactants and products and accurate correction for the effect of the resistance of the electrolyte on the measured electrode potential. In the present work, the first factor is controlled by using rotating discs as elec- trodes. These discs can be rotated at high rotation rates (several thousand rotations per minute), which allows for a controlled rate of mass transfer to the surface of the electrode.[197] Generally, to reduce the impact of the electrolyte resistance, a Luggin capillary is employed to position the reference electrode close to the surface of the working electrode. However, at high current densities, the potential gradient between the opening of the Luggin capillary and the electrode surface becomes too high, pre-PDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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