The Effect of Carbonate and pH on Hydrogen Oxidation and Oxygen Reduction on Pt-Based Electrocatalysts in Alkaline Media

Understanding the impact of carbonate on electrocatalysis has important practical implications for alkaline electrochemical cells operating on ambient air.  In addition to decreasing electrolyte conductivity¹carbonate formation can alter the pH at the electrode surface, possibly impacting reaction kinetics.

            In contrast with the acidic case, the hydrogen oxidation reaction (HOR) on Pt is kinetically limited at high pH. Our group² and others³ have addressed this problem by synthesizing various alloy catalysts with enhanced HOR kinetics. Competing theoretical frameworks have been proposed in the literature for understanding the rate-determining step (rds) on Pt alloys for alkaline HOR and pathways to more active catalysts. Such frameworks either hypothesize that hydrogen dissociative adsorption (Tafel step) is rate limiting⁴ or that electron transfer (Volmer/Heyrovsky step) is rate limiting⁵. A hydrogen dissociative adsorption rds is not sensitive to pH, while a concerted electron/proton transfer rds is⁶. Measuring activity changes in the presence of carbonate at different pH values will allow us to assess the sensitivity of the HOR towards carbonate, as well as to draw definitive conclusions with respect to the HOR rds on state-of-the-art Ru_xPt_yalloy nanoparticle catalysts. 

            To investigate these effects, Pt and Ru_xPt_y alloy nanoparticles were synthesized from metal-organic precursors on Vulcan carbon supports. Conventional electrochemistry was conducted in a three-electrode cell with a working electrode deposited onto a glassy carbon rotating disk, a Pt wire counter electrode, and a double-junction Ag/AgCl reference electrode. Activity data were collected under rotation at 1 atm H₂ or O₂ in alkaline electrolyte at 10 mV/s. Changes in HOR and ORR half-wave potential and Tafel slope were investigated with respect to pH, catalyst composition, as well as carbonate and phosphate concentration. A standard Tafel analysis was used to determine slopes from mass-transport corrected kinetic data. The in situ area and composition of the nanoparticle surfaces were determined using Cu-stripping and considered with ex situspectroscopy and microscopy.

            The HOR half-wave potential depended strongly on pH for the monometallic Pt catalysts (22 mV/pH), and it depended weakly on pH for the Ru-containing electrocatalysts (3.7, 2.5, and 4.7 mV/pH on Ru_0.2Pt_0.8, Ru_0.4Pt_0.6, and Ru_0.8Pt_0.2, respectively). The variation in HOR half-wave potential vs. Pt content at several pH values is illustrated in Fig. 1. Tafel slope analysis of the kinetic currents revealed an average slope for the monometallic Pt catalyst of 127 ± 18 mV/dec., while the average Tafel slopes for the Ru-containing catalysts were 31 ± 3, 30 ± 7, and 30 ± 4 mV/dec. for the Ru_0.2Pt_0.8, Ru_0.4Pt_0.6, and Ru_0.8Pt_0.2 electrocatalysts, respectively. These results indicate a change in rds from electron transfer on monometallic Pt to dissociative hydrogen adsorption on Ru_xPt_ycatalysts. There is no difference in the performance at comparable pH values in carbonate vs. phosphate on Pt indicating that water/hydroxide is the primary proton acceptor for alkaline HOR. Finally, we observe no pH or carbonate dependence for the ORR on monometallic Pt nanoparticles.

References

(1) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; et al. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci. 2014, 7, 3135–3191.

(2) St. John, S.; Atkinson III, R. W.; Unocic, R. R.; Papandrew, A. B.; Zawodzinski Jr., T. A. Submitted. J. Phys. Chem. C 2015.

(3) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Pt-Ru Catalyzed Hydrogen Oxidation in Alkaline Media: Oxophilic Effect or Electronic Effect? Energy Environ. Sci. 2014.

(4) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der Vliet, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Improving the Hydrogen Oxidation Reaction Rate by Promotion of Hydroxyl Adsorption. Nat. Chem. 2013, 5, 300–306.

(5) Durst, J.; Siebel, A.; Simon, C.; Hasche, F.; Herranz, J.; Gasteiger, H. A. New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7, 2255–2260.

(6) Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating Hydrogen Oxidation and Evolution Activity on Platinum at Different pH with Measured Hydrogen Binding Energy. Nat Commun 2015, 6.

Fig. 1: HOR half-wave potential vs. Pt content in the nanoparticle alloys indicate that the Ru-containing alloys are less sensitive to changes in pH than monometallic Pt catalysts. A minimum in the half-wave shift is observed for Pt content ~60 – 80%. Data for carbonate-free electrolyte (o) have been included for additional catalyst compositions.