Nature Communications 5: Article number: 5787 (2014); Published: 19 December 2014; Updated: 23 February 2015

The affiliation details for Winson Chun Hsin Kuo are incorrect in this Article. The correct affiliation details for this author are given below:

Johnson Matthey Technology Centre, Blount's Court, Sonning Common, Reading RG4 9NH, UK.