Electrochemical CO oxidation catalyzed by Rh complexes of tetraaza [14]annulene was examined. Rh complexes on carbon black exhibit much higher CO oxidation activity than Rh porphyrins or conventional Pt alloy catalysts. The onset potential for CO oxidation is lower than 0 V vs. a reversible hydrogen electrode. As a result, a combination of electrochemical CO oxidation by Rh tetraaza [14]annulene and proton reduction by Pt catalysts generates slight electricity. The combined overall reaction is a water-gas shift reaction (CO + H 2 O!CO 2 + H 2 ). The co-presence of Rh tetraaza [14]annulene catalyst and Pt catalyst promotes the water-gas shift reaction.For several decades, CO electro-oxidation (Eq. (1)) has attracted considerable interest from researchers engaged in both basic science and studies on various applications.This reaction has been applied to the development of polymer electrolyte fuel cells (PEFCs). In stationary PEFC systems, a reformate gas of hydrocarbon is used as a fuel, and hence CO is inevitably contained in the anode gas. CO severely poisons the surface of a Pt anode and significantly decreases the performance of PEFCs. CO-tolerant anode catalysts are required to counteract the negative impact of CO on PEFCs. [1,2] CO electro-oxidation (Eq. (1)) can decrease the concentration of CO at the anode, and hence this reaction helps to improve COtolerant anode catalysts for PEFCs. This reaction also plays an important role in the design of an amperometric CO sensor. [3] Noble metal electrodes, exemplified by a Pt electrode, catalyze electrochemical CO oxidation. However, on Pt electrodes, this reaction needs extremely high overpotentials [4] , which leads to loss of performance and efficiency of PEFCs. While alloying Pt with a second metal such as Ru or Sn decreases these overpotentials, [1,2,5] overpotentials remain high. High overpotentials could also be a significant problem for amperometric CO sensors in terms of interference.We focused on complex-based electrocatalysts to achieve CO oxidation at low overpotentials. Veen et al. found that a Rh porphyrin catalyzes the electro-oxidation of CO at low overpotentials. [6] We found that the catalytic activity of Rh porphyrin electrocatalysts significantly depends on the ligand structure. [7] The activity increased with the use of an appropriate ligand and substituents. [7a,b] We demonstrated that a PEFC that uses a Rh porphyrin catalyst as an anode delivers high power when neat CO is supplied as a fuel. [8] A mechanistic analysis demonstrated that CO is adsorbed on a Rh atom in Rh porphyrin. [9] Rh porphyrin can be adsorbed on the PtRu catalyst, and the composite material acts as a CO-tolerant anode catalyst. [10] However, even when a state-of-the-art Rh porphyrin catalyst is used, some overpotential is left. Improvement of the porphyrin ligand alone might not be enough to decrease the CO oxidation potential. We tried to change macrocycle ligands to develop a catalyst that can oxidize CO at low overpotentials. We paid special attention to a dibenzo...
