This work was supported by the Research Council of Norway (grant 256264) and the Spanish Government (SEV-2016-0683 grant). NORTEM is acknowledged for access to transmission electron microscopes.

Author contributions

H.M.-F., D.C., R.Z. and C.K. performed the experiments. H.M.-F., J.M.S., R.H., D.C., P.K.V., T.N. and C.K. designed the experiments. D.B. fabricated the tubular membrane electrode assembly. H.M.-F., C.K., R.H., T.N. and J.M.S. analysed electrochemical data. H.M.-F., J.M.S., S.H.M., R.Z. and C.K. analysed the catalytic data. I.Y.-T. and D.C.-M. designed and performed modelling studies. D.C. collected scanning and transmission electron microscope data. P.K.V. and C.K. initiated the project. H.M.-F., D.C., I.Y.-T., D.C.-M., D.B., S.H.M., P.K.V., T.N., R.H., J.M.S. and C.K. wrote the manuscript, while all authors discussed the results and commented on the manuscript.