Oxygen Reduction Kinetics in Deuterated Phosphoric Acid

Abstract

tion of hydrogen. Electrolytic procedures, however, may turn out to be useful for washing and concentrating oil shale, and the hydrogen gas produced at the cathode would be a useful byproduct. Further work is in progress along these lines in our laboratory.Despite the substantial research on the reduction of O~ on platinum in phosphoric acid, the mechanistic aspects still remain very uncertain. The situation has become even more in question with the recent observation that the Tafel slope is almost independent of the temperature over the range 298--500 K, indicating that the apparent transfer coefficient is proportional to the temperature (1). In an attempt to gain further insight into the mechanistic aspects, polarization measurements for O~ reduction on Pt have been carried out in deuterated and undeuterated 85% H3PO4 at 25~ This article reports the preliminary results. ExperimentalThe most critical aspect of such isotope studies is that of reproducibility of surface and electrolyte conditions between the deuterated and undeuterated acid solutions. A major effort has been made to reproduce conditions and to eliminate impurity effects.The O2 reduction measurements have been made with the rotating disk electrode technique. The measurements were carried out in two identical Teflon cells, each consisting of a main working electrode compartment and two separate compartments for the reference and counterelectrodes (2). The working electrode was a disk electrode (Pine Instruments Company) with an area of 0.46 cm 2. The reference electrodes were reversible H2/H + and DJD § using a platinized-platinum surface. The counterelectrode was bright Pt.The working electrode surface was first polished with alumina (0.05 ~m) to a mirror finish, submerged in distilled water for 15 rain with ultrasonic vibrations, and then rinsed in distilled water, followed by purified H3PO4. The distilled water used in this work was prepared by reverse osmosis of tap water followed by distillation with a counterflow of N~ to purge volatiles such as COs and organics. The reverse osmosis-distillation system was assembled by B. D. Cahan at Case Western Reserve University and is based on a design developed by Gilmont and Silvis (3).A Pine Instruments RDE-3 bipotentiostat with input impedance of 10'2~ in the reference circuit and built-in triangular sweep generator was used for all measurements. The current-potential curves were recorded on a Yokagawa Electric Works Type 3086 X-Y recorder. Prior to electrochemical measurements, the Pt disk electrode was cycled several times between potentials of 1.0 and 0.3V vs. a reversible hydrogen or deuterium electrode (p = 1 atm) at a sweep rate of 10 mV/s. For preparation of the D3PO4 and H:~PO4 electrolytes, Mallinckrodt 85% HsPO4 was first purified with H~O~ ac-*Electrochemical Society Active Member. 'Permanent address: Tanta University, Tanta, Egypt.cording to the procedure of Ferrier et al. (4). Two samples of this purified H3PO4, each 120 ml, were then placed in Teflon containers and concentrated by boiling...

Kinetic Isotope Effect as a Tool To Investigate the Oxygen Reduction Reaction on Pt‐based Electrocatalysts – Part I: High‐loading Pt/C and Pt Extended Surface

“…As the ORR rate‐determining step is known to be, on Pt surfaces and nanostructures, the O 2 +H + +e − →OOH ads step, it indicates that the limiting elementary step should either be the oxygen adsorption (O 2 →O 2,ads ) or an electron transfer without a coupled proton transfer (O 2,ads +e − →O 2,ads − ). The result is in agreement with previous studies of ORR kinetic isotope effect on platinum, such as Yeager's group original work in phosphoric acid and Gewirth's group more recent work . While the kinetic isotope effect under these specific conditions is defined, future work will investigate low platinum loading electrocatalysts, where inter‐particle spacing, and support interactions can be relevant.…”

“…In the first study of a KIE for the ORR on platinum catalysts, Ghoneim and Yeager carried out the reaction in a phosphoric acid electrolyte (phosphoric acid being known to poison Pt‐based surfaces). They compared the reduction currents at a kinetically limiting potential regime and calculated a KIE sufficiently close to unity to conclude that the rate determining step did not include a proton transfer . Since then, a growing interest in non‐precious metal ORR catalysts to lower the costs of the electrocatalyst has led to recent KIE studies of the ORR on different catalyst surfaces.…”

Observation of kinetic isotope effect in electrocatalysis with fully deuterated ultrapure electrolytes

“…[12][13][14][15][16][17][18][19][20][21][22][23] From this point, kinetic isotope effect (KIE) can be a powerful tool to clarify the basic reaction mechanism of electrocatalysis. [24][25][26] A use of heavy water in order to substitute proton in the system to deuteron (H-D exchange) is a classical topic in electrochemistry, [27][28][29][30][31][32][33][34] and this method was applied to analyze complicated electrode processes, such as electrocatalytic reactions on carbon-based electrocatalysts. [35][36][37][38][39] These non-noble metal based catalysts show a promising properties, [40][41][42][43][44][45] therefore is expected that KIE can be used for a part of approaches to find out the alternatives to present Pt-based electrocatalysts.…”

Mechanistic Study of the Electrochemical Oxygen Reduction Reaction on Pt(111) Using Density Functional Theory

“…An alternate view, proposed by Yeager et al and based on kinetic isotope experiments, is that oxygen adsorbs dissociatively, but corroborating evidence is lacking. 5,12 Additionally, Yeager speculated that O 2 adsorption could possibly be the rate-determining step. 5 Much work has been previously performed to study behavior of oxygen on Pt(111) [13][14][15][16][17][18][19][20][21] and its reaction with hydrogen in a vacuum.…”