Electrochemical Behavior of a Pd Thin Film Electrode in Concentrated Alkaline Media

Electrocatalysis (2017) 8:295–300 DOI 10.1007/s12678-017-0379-5 ORIGINAL RESEARCH Electrochemical Behavior of a Pd Thin Film Electrode in Concentrated Alkaline Media K. Hubkowska 1 & M. Soszko 2 & M. Symonowicz 1 & M. Łukaszewski 1 & A. Czerwiński 1,2,3 Published online: 22 April 2017 # The Author(s) 2017. This article is an open access publication Abstract Pd limited volume electrodes (LVE) were obtained electrochemically from the PdCl2 aqueous solution. Hydrogen absorption was performed using cyclic voltammetry and chronoamperometry in concentrated (6 M) alkaline solutions (KOH, NaOH, the mixture of 2M LiOH + 4M KOH) and results were compared with 0.5 M H2SO4. It was found that hydrogen pretreatment procedure is crucial for further examination of hydrogen absorption in Pd electrodes. After many cycles of hydrogen absorption/desorption in Pd, these processes become electrochemically more reversible—faster than in a freshly prepared electrode. Results obtained for Pd electrodes in different concentrated alkaline media subjected to hydrogen pretreatment procedure are comparable with those for an acidic solution (e.g., α→β phase transition potential equals to ca. 0.050 V vs. RHE). The continuous Pd electrode cycling does not affect significantly the amount of absorbed hydrogen (ca. 0.72–0.74). The surface roughness factor in basic solutions decreases after multiple hydrogen sorption as in case of an acidic solution. Keywords Pd limited volume electrode . Hydrogen sorption . α↔β phase transition potential . Hydrogen pretreatment procedure . Alkaline solutions * K. Hubkowska 1 Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland 2 Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland 3 Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland Introduction Since Graham published his findings in 1866 [1], palladium properties have been thoroughly examined mainly due to its ability to hydrogen absorption. Nevertheless, Pd behavior in many aspects requires further research and explanation. In literature, there are many reports concerning Pd properties in acidic solutions [2–9] but there are relatively very few articles referring to Pd properties in basic solutions, especially in concentrated media. Martin and Lasia studied hydrogen absorption and adsorption in/on Pd monolayers in 0.1 M NaOH solutions. They pointed that the addition of benzotriazole (BTA) promotes kinetically absorption and inhibits adsorption of hydrogen [10]. The same research group stated that hydrogen sorption kinetics in Pd (several dozen nm thick) is inhibited in basic solutions in Pd as compared with acidic media and therefore hydrogen sorption isotherms are shifted into lower potential values [11]. Palladium 30 nm thick films obtained by pulsed layer deposition (PLD) were examined in the aspect of hydrogen sorption from 1 M KOH solution by Paillier and Roué [12]. The authors indicate that the kinetics of hydrogen sorption was facilitated with the decrease in Pd thickness; however, the amount of absorbed hydrogen does not exceed the values reported for coarse-grained films. In case of films obtained by PLD, there was no flat plateau of the α↔β phase transition, which is ascribed by Pallier and Roué to the wide distribution of energy levels in Pd thin films, being the main reason of good cyclic stability. Impedance analysis of hydrogen absorption in a Pd foil in 0.1 M NaOH was presented by Yang and Pyun [13]. The authors suggest that the phase boundary between the α- and β-phase hinders the hydrogen diffusion at palladium during charging with hydrogen. It was also stated that the β-phase of hydrogen formed near the electrode surface behaves as a barrier for hydrogen diffusion in the 296 potentials below 0.08 V (vs RHE), where direct hydrogen absorption reaction (har) is postulated. Rubes and Bilyková [14] showed the connection between catalytic activity of Pd and the amount of absorbed hydrogen in 6 M KOH. 0.3% at. hydrogen in Pd appeared to be optimal in case of HCOO− oxidation, while increasing hydrogen content in Pd resulted in a drop in its catalytic activity. Hu and Wen [15] from the experiment of hydrogen absorption in 1 M NaOH in oxidederived Pd in the presence of p-nitroaniline noticed that this organic compound reacts only with adsorbed hydrogen, which indicates that the β-phase of hydrogen is not formed from the α-phase. Thorough examination of oxide-derived Pd electrodes in acidic and basic media in comparison with Pd-bulk electrodes gives the ability to ascertain that the surface roughness of electrodes polarized in basic media is higher than in an acid [16]. The authors also postulate that the utilization of a Pd-oxide derived electrode enables to analyze separately signals originating from hydrogen adsorption and absorption, in contrast to a Pd bulk electrode. The electrochemistry of a palladized Pd electrode was examined by Burke and Casey mainly with the emphasis on the premonolayer oxidation [17]. It was found that on disordered surfaces there are a lot of sites where hydrous oxide mediators can be generated. The same group also stated that hydrogen absorption in activated Pd causes the activation of surface and results in the formation of recalcitrant hydrous oxides [18]. Czerwiński’s group studied the problem of alkaline metal insertion into Pd from 0.1 M basic solutions [19–21]. They noticed that alkaline metals influence the α → β phase transition potential, which is connected with alkaline metals incorporation into the bulk of Pd at lower potentials during hydrogen electrolysis. This effect was not observed in neutral and acid solutions with alkali cations [21]. Studying Pd properties in concentrated alkaline solutions is important not only in the aspect of basic research, but also in the application aspect, since Pd is still considered as a small addition [22] to commercially used alloys in hydride batteries [23–26]. Pd addition results in lower charge/discharge overpotentials, lower charge transfer resistance and improved cycle life [25]. Modification of the alloys with the palladium also accelerates the activation process and enhances the ratecapability [24, 26]. In this short paper, Pd behavior in concentrated (6 M) basic solutions of KOH, NaOH and the mixture of LiOH and KOH is presented (in comparison with 0.5 M H2SO4 solution). We describe the influence of the process of hydrogen pretreatment procedure on the hydrogen absorption properties of Pd thin films, namely the value of potential of hydrogen oxidation, the amount of absorbed hydrogen and the α → β phase transition potentials with respect to our previous works focused on electrochemical behavior of Pd thin film electrodes in acidic electrolytes. Electrocatalysis (2017) 8:295–300 Experimental Pd limited volume electrodes (0.5 μm thick) were deposited potentiostatically (deposition potent (...truncated)