The oxygen reduction reaction (ORR) is of great significance in energy science because it is the cathodic reaction for some important chemical power sources, such as fuel cells and metalair batteries. Platinum is up to now the most active catalyst towards the ORR in both acid and alkaline media; [1][2][3][4] however, the scarcity of platinum on earth makes it impossible to be used in large amounts. [5,6] Recently, it was reported that spinels and perovskites, especially when mixed with carbons, are surprisingly active toward the ORR in alkaline media. [7][8][9][10][11][12][13] The actual role played by carbon, except for improving conductivity, is less discussed and still undiscovered. For such a carbonsupported electrocatalyst, the presence of carbon complicates the ORR process as carbon itself is also reported to be able to catalyze the ORR, proceeding via a 2 e À process to produce HO 2 À in alkaline media (O 2 + H 2 O + 2 e À !HO 2 À + OH À ). Furthermore, the produced HO 2 À , when trapped in a porous carbon layer, has a large possibility to be chemically disproportionated (2 HO 2 À !O 2 + 2 OH À , HODR). This "trapped" effect in acidic solutions has been discussed widely; [14][15][16] however, both the catalytic and "trapped" effects from carbon in alkaline solutions are rarely referred to. In this communication, we prepare for the first time cobalt oxide nanoparticles (NPs) with a highly uniform particle size of around 2 nm and investigate the effect of carbon on the ORR process. A coupling effect between cobalt oxides and carbon is found, which influences the ORR process significantly. The results are expected to provide guidance for designing practical carbon-supported metal oxide-type ORR electrocatalysts. Cobalt oxides, prepared by a simple colloid method, were deposited on Vulcan XC-72 carbon (denoted as CoO x /C). The bright-field and dark-field high-resolution (HR)-TEM images of CoO x /C are presented in Figure 1 a and b, respectively. The black dots in Figure 1 a and the bright dots in Figure 1 b, corresponding to an average particle size of 2 nm, are cobalt oxide NPs. The large gray spheres of around 30 nm in diameter are carbon supports. Both images show clearly that the cobalt oxide NPs are distributed uniformly on the carbon support. However, no significant diffraction peak for cobalt is found in the XRD pattern of CoO x /C ( Figure S1 in the Supporting Information), indicating that the cobalt oxide sample is amorphous.The ORR polarization curve and the ring currents over CoO x / C measured by using a rotation ring-disk electrode (RRDE) in an O 2 -saturated 0.1 m NaOH solution is plotted in Figure 2. The calculated HO 2 À yield (h) and the electron-transfer number (n)are also plotted as a function of potential. To separate the current contributions from CoO x and carbon, the ORR curves over CoO x , carbon, and the electrode substrate (glassy carbon, GC) are also included. For CoO x /C, the ORR initiates at a potential of around 0.86 V, followed by a continuous increase in current as the potent...
