The present disclosure relates generally to reactor systems that include (a) a housing having an interior surface that may be at least partially reflective, (b) at least one reactor cell disposed within an interior of the housing, the at least one reactor cell including an enclosure and a plasmonic photocatalyst on a catalyst support disposed within the at least one enclosure, where the enclosure is optically transparent and includes at least one inlet for a reactant to enter the at least one cell and at least one outlet for a reformate to exit the at least one cell and (c) at least one light source disposed within the interior of the housing and/or external to the housing. At least one light-management feature and/or at least one thermal-management feature is applied to the reactor cell, reactor system, or a reformer system comprising many reactor systems, in order to improve efficiency.