59th Annual Report on Research 2014

Electrocatalytic oxygen reduction reaction (ORR) is central to alternative energy systems and sensors.  An important practical issue in these systems is the loss of active surface area of the catalyst.¹  During the first year of the grant period, we investigated unexpected dissolution of Pt at moderate negative potentials during ORR in water and organic media.  The combination of nanoelectrochemistry and atomic force microscopy (AFM) imaging allowed us to show that dissolution of Pt at moderate negative potentials during the ORR is unrelated to the Pt oxide formation.²  The Pt dissolution during ORR was most efficient in a pulse regime; however, we also observed the dissolution of Pt in voltammetric experiments after sweeping the nanoelectrode potential to sufficiently negative values (≤ –1 V vs. Ag/AgCl).  This process is relatively slow and difficult to detect at macroscopic electrodes; thus, it was not discovered in numerous previous studies of ORR at Pt.  A much higher current density at a nanoelectrode and the AFM capacity for detecting nanoscale topographic changes on its surface enabled observation of cathodic dissolution phenomena in our experiments.

The possibility of the connection between this phenomenon and the previously observed formation of hydroxyl radicals has been explored.   The loss of Pt in our experiments always occurred under the conditions at witch HO^• was recently observed.^3,4  The above findings can have important implications for the cathode stability in fuel cells and other devices in which Pt electrodes are used to generate current pulses (e.g., implantable pacemakers or stimulating electrodes in the artificial retina).

We also fabricated nanometer-sized platinized carbon electrodes and investigated their amperometric and potentiometric responses.⁵  Such electrodes can be used to detect reactive oxygen species (ROS) that play important roles in oxygen electrocatalysis, including the aforementioned dissolution of Pt.  To attain essentially flat geometry of these probes, Pt black was deposited into a nanocavity inside the carbon shaft.  The methodology was developed for slowly etching carbon surface in the oven at ~400°C to make it recess into quartz insulator.  The platinized electrodes obtained in this way are sufficiently well shaped to be used as amperometric SECM tips.  In a related study,⁶ we developed a new technique for sampling small volumes (attoliter to picoliter) of solution and rapid electrochemical analysis of sampled redox species.  The electrochemical nanosampler can be useful when direct electrochemical measurements are difficult, e.g., in biological vesicles and other subcellular compartments, in catalysis nanopores, and inside working batteries or fuel cells.

1.  Borup, B.; Meyers, J. P.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zeleney, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.;  Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Chem. Rev. 2007, 107, 3904.

2.  Noël, J.-M.; Yu, Y.; Mirkin, M. V. Langmuir 2013, 29, 1346.

3.  Yue, Q.;  Zhang, K.; Chen, X.; Wang, L.; Zhao, J.; Liu, J.; Jia, J. Chem. Commun. 2010, 46, 3369.

4.  Noël, J.-M.; Latus, A.; Lagrost, C.; Volanschi, E.; Hapiot, P. J. Am. Chem.Soc. 2012, 134, 2835.

5.  Hu, K.; Gao, Y.; Wang, Y.; Yu, Y.; Zhao, X.; Rotenberg, S. A.; Gökmeşe, E.; Mirkin, M. V.; Friedman, G.; Gogotsi, Y. J. Solid State Electrochem. 2013, 17, 2971.

6.  Yu, Y.; Noël, J.-M.; Mirkin, M. V.; Gao, Y.; Mashtalir, O.; Friedman, G.; Gogotsi, Y. Anal. Chem. 2014, 86, 3365.