Defining the onset potential

The onset potential is a widely reported and generally accepted measure of electrocatalytic activity; however, there is no single and agreed definition, nor does the scientific community have a standard measurement procedure. This review starts by summarising the main definitions employed in the literature. However, regardless of how the measurement is undertaken, there are fundamental problems with the use of the onset potential as a figure of merit. Even if a defined threshold current density is used, the reported onset potential is inherently sensitive to the surface catalyst loading and the specific surface area of the catalytic material. A simple theoretical model is used to exemplify this problem and to further evidence why the mass-activity or the specific-activity of an electrocatalyst is a superior method for measuring the activity of a material.