The imaging ammeter

A method for measuring local current density, not requiring segmentation of the electrode or spatial scanning, is presented. The motion of colloidal particles in response to local current density, characterized by the intensity of the light they scatter, is the fundamental phenomenon of the technique. The scattering was produced and measured with the electrochemical total internal reflection microscope, a tool that places an electrochemical cell within a total internal reflection apparatus. The electrolysis of water and the oxidation of ferrocene monocarboxylic acid were used as test reactions. Light scattered by a probe particle produced an "image" of current density; scattered light was converted to local current density by a function derived herein. Numerical simulations supplemented experimental evidence that local current density controlled the probe particle's vertical motion. The spatial resolution of the method was approximately the length scale of the probe particle, in this case 5.7 μm. The resolution of current density was better than 100 nA cm(-2). The method might find use in high throughput screening of electrocatalysts.