Kinetic‐Modulated Crystal Phase of Ru for Hydrogen Oxidation

Crystal‐phase‐engineering provides a powerful strategy for regulating the catalytic performance yet remains great challenge. Herein, the kinetic‐modulated crystal‐phase‐control of Ru nanosheet assemblies (Ru NAs) is demonstrated by simply altering the concentration of citric acid (CA). Detailed experimental results reveal that high concentration of CA retards the growth kinetics and thus leads to the formation of metastable face‐centered cubic (fcc) Ru NAs, while low concentration of CA results in the fast growth kinetics and the preferential formation of Ru NAs with stable hexagonal close packed (hcp) phase. Moreover, Ru NAs with different phases are used as catalyst for hydrogen oxidation reaction (HOR) to evaluate the effects of crystal phase on catalytic performance. Impressively, Ru NAs with fcc phase display a mass activity of 2.75 A mgRu−1 at 50 mV, which is much higher than those of Ru NAs with fcc/hcp (1.02 A mgRu−1) and hcp (0.74 A mgRu−1) phases. Theoretical calculations show that fcc Ru NAs display weaker adsorption toward *H and lower energy barrier toward the rate‐determining step (RDS) during HOR. This work provides a facile strategy for regulating the crystal phase of Ru nanocrystals, which may attract rapid interests of researchers in materials, chemistry, and catalysis. Here, it is found that the high concentration of CA retards the growth kinetics and thus leads to the formation of metastable face‐centered cubic (fcc) Ru NAs, while low concentration of CA results in the fast growth kinetics and the preferential formation of Ru NAs with stable hexagonal close packed (hcp) phase. Meanwhile, the fcc Ru exhibits higher HOR performance than that of hcp Ru.