A continuous hydrothermal flow synthesis method was used to produce copper(I) oxide nanoparticles, which were used as an electrocatalyst for the reduction of CO2. A rotating ring-disc electrode (RRDE) system was used to study the electroreduction processes, including a systematic study (including quantitative NMR analysis) to identify product species formed at the disc and detected at the ring. In 0.5 M KHCO3 electrolyte with a pH of 7.1, carbon dioxide was found to be exclusively reduced to formate. In the potential range -0.5 to -0.9 V vs the reversible hydrogen electrode (RHE), an active material/ glassy-carbon disc electrode was shown to produce formate, with a maximum Faradaic efficiency of 66% (at -0.8 V vs RHE). (C) 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).