Electrochimica Acta 47 (2001) 501 – 508 www.elsevier.com/locate/electacta On the origin of oscillations in the electrocatalytic oxidation of HCOOH on a Pt electrode modified by Bi deposition Jaeyoung Lee *, Peter Strasser, Markus Eiswirth, Gerhard Ertl Department of Physical Chemistry, Fritz -Haber -Institut der Max -Planck -Gesellschaft, Faradayweg 4 -6, D-14195 Berlin, Germany Received 21 May 2001; received in revised form 10 July 2001 Abstract We report experimental observations on the temporal dynamics in the electrocatalytic oxidation of formic acid (HCOOH) on a polycrystalline Pt electrode modified by deposition of bismuth. Bismuth modification significantly enhanced the current density of HCOOH oxidation, since it suppressed the poisoning branch and increased the apparent direct oxidation rate. Impedance spectroscopy and the galvanostatic scan in HCOOH oxidation on Bi/Pt exhibited a hidden negative differential resistance and a Hopf bifurcation. The electrocatalytic oxidation of HCOOH on Bi/Pt spontaneously underwent transitions from homogeneous catalytic activity to spatiotemporally inhomogeneous distributions of the interfacial electrode potential, in the form of traveling pulses of the interfacial potential. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electrocatalytic oxidation; Formic acid; Bi/Pt electrode; Galvanostatic oscillations; Hidden negative impedance 1. Introduction The electro-oxidation of small organic molecules such as formic acid (FA), formaldehyde, or methanol, on noble metal electrodes is of great interest in anodic reactions of direct electrochemical energy conversion [1 – 5]. Dynamical instabilities in electrochemistry most frequently arise through the interplay of the outer electrical load line and a sequence of faradaic processes with non-monotonous current – potential characteristics [6,7]. However, more important in the present context are non-stationary operation points in which the mea- sured currents or potentials temporally exhibit au- tonomous electrochemical oscillations [7,8]. The origin and the existence conditions of these oscillations in electrocatalytic oxidation of fuels have recently been clarified [9–11]. The variation of the phase shift be- tween fluxes and forces through periodic perturbations can lead to superior non-stationary operating condi- tions (oscillatory), enhancing the efficiency compared to stationary states [12 – 14]. This idea has recently at- tracted renewed interest in the context of electrochemi- cal energy conversion devices [15,16]. Aside from temporal aspects, control of spatial coupling across electrified interfaces may provide interesting applica- tions. For example, electrochemical devices are conceiv- able where local activation of a poisoned electrocatalytic surface is sufficient to restore complete activity, if the interface supports propagating active fronts. This result can be advantageous for operating self-poisoning processes under such dynamic condi- tions. It has been shown that bismuth adatoms signifi- cantly enhanced the susceptibility of the catalytic interface towards self-activation process [17]. In this work, we try to elucidate the mechanistic origin of current oscillations in the electro-oxidation of formic acid on a Pt electrode modified by bismuth deposition. 2. Experimental A smooth polycrystalline Pt ring with inner diameter of 34.5 mm and outer diameter of 40.5 mm (thus exhibiting a geometric surface area of 7 cm 2 ) was used as the working electrode (WE). A concentric platinized Pt wire ring (thickness of 1 mm wire, 70 mm ring * Corresponding author. Fax: +49-30-8413-5106. E-mail address: jlee@fhi-berlin.mpg.de (J. Lee). 0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(01)00744-7