(Invited) Elucidating Hydrogen Oxidation/Evolution Kinetics in Base and Acid By Enhanced Activities at the Optimized Pt-Shell Thickness on the Ru Core
Authors: Ma Zhong , Elbert Katherine , Hu Jue , Zhang Yu , Chen Guangyu , An Wei , Liu Ping , Isaacs Hugh , Adzic Radoslav R , Wang Jia X. 
Published: February 2020

Hydrogen oxidation and evolution reactions are slower by two orders of magnitude in base than in acid on Pt and other metal catalysts.1It is important to understand the cause and to develop catalysts that are more active than Pt in base for advancing the performance of anion-exchange-membrane fuel cells and water electrolyzers.
Here, we detail a four-fold enhancement in Pt mass activity that we achieved using single crystalline Ru@Pt core-shell nanoparticles with two-monolayer-thick Pt shells,2 which doubles the activity on Pt-Ru alloy nanocatalysts. For Pt specific activity, the 2-ML and 1-ML thick Pt shells, respectively, exhibited an enhancement factor of 3.1 and 2.3 compared to the Pt nanocatalysts in base, larger than the values of 1 and 0.4 in acid.3

To explain such behavior and the huge difference in acid and base, we performed kinetic analyses of polarization curves over a wide range of potential from -250 to 250 mV, which were measured in 1 M KOH and 1 M HClO4 using the gas diffusion electrode method.4

The results of fitting polarization curves in Fig. 2 indicate (1) H adsorption free energy, DGad, increases from bulk Pt to 2 ML and to 1 ML, consistent with the weakening trend found by DFT-calculated hydrogen binding energy (H-BE); (2) the Tafel-Volmer is still the dominant pathway as the activation barriers are lower for the Tafel than the Heyrovsky reaction (DG+T < DG+H) on all three catalysts.
More importantly, the activation free energies increase the most for the Volmer reaction, e.g., from 177 to 352 meV (Fig. 3a), resulting in a switch of the rate-determining step from the Tafel- to the Volmer-reaction, and a shift to a weaker optimal H-BE. The shift makes bulk Pt less active than 1-ML Pt in base (red in Fig. 3b), while the 2-ML Pt with an intermediate H-BE remains near the top of volcano.
For both acid and base, the H-BE is a descriptor of catalysts’ activities, however, the huge difference between acid and base (~100 times in scale on the left than the right y-axis in Fig. 3b) has to be caused by a factor not directly associated with the H-BE because it is a general phenomenon for various catalysts.1 The much higher activation barrier for the Volmer reaction in base than in acid indicates that the key is the proton interaction with water. Theoretical studies showed that the main contribution to the barrier for the Volmer reaction comes from initial proton transfer from water to the electrode,6 which is superfast along hydrogen bond network in acids.7 In base, OH- transport is slow, and the HO-H bond breaking/formation likely has a considerable activation barrier, differing from the virtually activation-less hydrogen bond breaking/formation for hydrated proton (H2O-H+) in acid.

Acknowledgements

This research was supported by the US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division under contract DE-SC0012704.

References

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