As a result of active research, PtCo nanoparticle alloy catalysts were commercialized in the first and second generations of the Toyota MIRAI. First-principles (FP) calculations have greatly contributed to revealing the mechanism of enhanced activity. The theory and modeling of ORR began with the reproduction of the so-called volcano plot of ORR by Nørskov and co-workers. In series of these studies, ORR was assumed to proceed mainly through the following associative reaction mechanism according to the past experimental studies: 20