Nature Chemistry 9, 457–465 (2017); published online 9 January 2017; addendum published after print 23 June 2017; corrected after print 21 December 2017. In the version of this Article originally published, a typographical error meant that the unit on the y-axis label of Fig. 3b incorrectly read 'mAcm−2disk'; it should have read 'mA cm−2oxide'.