Fig. 5: Build-up of concentration gradients in the vertical electrode cell.

a Temperature map and b Co²⁺ concentration map obtained from the vertical electrode cell with the gel electrolyte. c One-dimensional Co²⁺ concentration profiles across the centre of the cell, extracted from images obtained at the times t and under the load resistances R indicated (the yellow plot in (c) was taken from the image in (b)). Average powers P_avg drawn from the cells during the acquisition of each image are also quoted. The full series of images are provided in Supplementary Fig. 16, along with plots of cell current and voltage versus time for these experiments (Supplementary Fig. 12).