关键词: 电化学测试, 欧姆电压降补偿, 电势阶跃法, 电化学阻抗法, 灵敏度

Abstract:

The solution resistance (R_u) between the working electrode (WE) and the reference electrode (RE) may lead to significant Ohmic drop (iR_u) and deviation of actually applied potential at the WE to the desired ones in electrochemical measurement. In the case of high current or large R_u, iR_u compensation is imperative. Errors associated with insufficient compensation of theiR_u drop may significantly affect the accuracy of data measured by conventional electrochemical methods, which may consequently result in wrong judgment and conclusions. In this article, we discuss important factors which may affect the accuracy of iR_u compensation as well as ways how to eliminate such errors through examples. Since R_u will be changed sensitively with the distance between the tip of the Luggin capillary of the RE and the WE, it should always be kept at a fixed distance (usually as close to the surface of the WE as possible but without affecting the behavior of electrode processes) during a set of measurements.
In addition to the structure of the cell (relative positions of WE and RE), the methods of measuring R_u, current sensitivity for recording data, percentage of R_u compensation, and instruments could cause the result to be deviated from the actual value, which are demonstrated by taken hydrogen evolution reaction on Pt electrode as model reactions measured using both CHI and Autolab PGSTAT 302N potentiostats. Comparing the AC impedance method with the potential step method in the automatic compensation function of CHI potentiostat, theR_u measured by the potential step method was smaller than the R_u measured by the AC impedance method and the actual value. It is suggested to use the AC impedance method to measure R_u, input the resistance value manually and complete the compensation by the instrument. Current sensitivity may limit the maximum R_u to be compensated, hence, one should select the right current sensitivity before recording data. In particular, when compensating by manual input using some types of potentiostat, one should be aware that the software may show the completely compensated resistance, in reality, however, the part of the resistance that exceeded the upper limit will not be compensated successfully. Furthermore, the percentage ofR_u compensation should be carefully optimized before carrying out the actual measurements, through comparing curves recorded with different percentages of compensation. Moreover, the same compensation level but conducted with different instruments may lead to significantly different results, using the same instrument in a series study is recommended to mitigate related errors. A set of strict compensation standards applied to all instruments and electrochemical systems is difficult to be established. Hence, we suggest carrying out some control experiments to optimize the iR_u compensation before the actual measurements, and write in detail their compensation methods and parameters in the published results, especially the factors mentioned in this paper.

Key words: measurement, iR_u compensation, STEP, EIS, sensitivity