Rare-earth elements have various applications in areas such as magnetism and hydrogen storage. The electrochemistry of rare-earth elements is important for their film formation and protection. Since the potentials of rare-earth metals are much lower than the electrochemical window of water, non-aqueous solutions have been conventionally used for their study. Our previous studies showed that the liquids confined in nanopores behave differently than in bulk solutions. Therefore, electrochemical processes that do not occur on flat electrodes may effectively occur in confined nanopores. In this study, we clarified the electrochemistry of aqueous solutions containing both Co(II) and Tb(III) using nanoporous electrodes. Although experiments on flat electrodes suggest that terbium oxide-embedded Co is deposited at an intermediate overpotential, the potential region is very narrow. However, the potential region becomes much wider when using nanoporous electrodes. In addition, it is likely that terbium oxide-embedded Co has catalytic activity for hydrogen evolution. This study shows that nanoporous electrodes are effective in producing deposits. (C) 2019 Elsevier Ltd. All rights reserved.