A, K, VIJH
506
Electrolytic Hydrogen Evolution Reaction on Aluminum, Oxide-Covered Electrodes by A. K. Vijh Research & Development Laboratories, Sprague Electric C o m p a n y , N o r t h A d a m s , Massachusetts (Received J u n e 3, 1 9 6 8 )
The electrolytic hydrogen evolution reaction (h.e.r.) on aluminum covered by thin “spontaneous” oxide has been examined iii aqueous, buffered acetate solutions of pH ~ 5 . 5 .Tafel parameters and their temperature dependence have been determined galvanostatically, with emphasis on the effect of time of polarizatioii 011 these parameters. Galvanostatic cathodic charging curves and open-circuit decay of the electrode potential have also been examined. Tafel plots obtained by procedures involving short polarizatioiis show slopes between 2.3 X 3RT/F and 2.3 X 4RT/P, depending on the temperature. On long polarizations, some cathodic “activation” is manifested, which also reduces the values of b to 2.3 X 2RT/F approximately. The initial discharge step is suggested as the probable rate-determining step (rds) iii the overall reaction. This conclusion is based on the values of Tafel slopes and on the lack of appreciable electrode coverage by adsorbed hydrogen as deduced from transient studies. The values of “apparent” exchange current densities (ca. A cm-2, before surface activation) and apparent heat of activation (11 kcal mol-‘) have been concluded to be consistent with this mechanism. Relation of present studies to the mechanisms of rectification is briefly pointed out.
I, Introduction
Preparation of the working electrode surface, however,
In a previous report,1 the iiicclianisni of hydrogen evolution reaction (h.e.r.) was exaniined on aluminum electrodes which wcre essentially free of “spontaneous” surface oxidrq2 In thc present investigation the iiiechanisni of 1i.e.r. has IWYI studied on nluniinuin electrodes covered hy a [‘natural” surfacc oxide in solutions (pH d T . 5 ) in wliirh the aluiiiiiiiiin oxidc is thermodynamically stsblc.2 The oxidr-covcred aliiniinuni has purposely not been called passivc aluminum here, since oxide filiii~on valve nietals3 are not passivc in thc sense of oxide films, e.g., on 5 4 4 which are noiiinsulating. ‘I’liere are fern previous iiivestigations on tlie niechaiiisni of he,r. 011 oxide-covered electrodes,j-* probably because the difficulties involved in the deteriiiinatioii of reliable kinetic parameters on these electrodes are considerablc,s owing to rather pronounced irreproducibility from one electrode to anothcr.8-”0 However, it is still possible to draw unanibiguous mechanistic conclusions froni the general niagnitude of the various kinetic parameters, without coiicediiig precise quantitative sigiiificancc to the data, as mill br attempted. An additional difficulty associated with the electrode processes on oxide-covcred electrodes is the interpretation of anornalous transfcr coefficient^.^-^^"
was modified in the present investigation. After the
11. Experimental Section The electrochemical measurements mere carried out in CHSCOOK in an aqueous solution which mas 1 and 0.22 M in CHaCOOH and had pH -5.5. Preelectrolysis was purposely not carried out since it mas observed in a previous study‘ that in acetate solutions, preelectrolysis tends to produce rather than remove impurities. All other experimental procedures were similar to those used in other modern work on electrode kinetics12-’6 and in our previous closely related study.’ The Journal of Physical Chemistry
chemipolishing, etching, washing, etc., sequence,’ the electrodes were either exposed to air for a fern dags or were lrft in distilled water (which was not deaerated) for sevcral days so that the clectrode surfacc acquired a “spontaneous” oxide. Tlir iiieasurcineiits to hc reported here were obtained in descending directioii of current density or temperature, unless stated otherwise, since it was observed that data in the descending direction were inore reproducible than those in the ascending direction. This is probably because the electrode becoinex rapidly “coiiditioiied” at higher current deiisities or temperatures by achieving either a steady-statc contaminatioii17 or a “steady-state” hydration which (1) A. K. Vijh, J. P h y s . Chem., 72, 114s (1968).
(2) 31. Pourbaix, “Atlas D’Equilibres Electrochimiques,” GauthierVillars and Co., Paris, 1963, p 16s. (3) L. Young, “Anodic Oxide Films,” Academic Press, New York, N . Y . , 1961. ( 4 ) I
