On the Temperature Performance of Ethanol Oxidatio

Authors: Boguslaw Pierozynski Tomasz Mikolajczyk Marcin Turemko

Publish Date: 2014/10/01

Volume: 6, Issue: 2, Pages: 173-178

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Abstract

The present paper reports on ethanol oxidation reaction EOR investigated at catalytically modified nickel foam material The EOR was studied in 01 M NaOH supporting electrolyte on Pdactivated nickel foam catalyst material obtained by a spontaneous deposition method Catalytic modification of Ni foam resulted in a composite material having superior EOR kinetics as elucidated through corresponding values of ac impedancederived chargetransfer resistance parameter including temperaturedependence of the EOR over the temperature range 20–60 °C The presence of a catalytic additive was disclosed from SEM and XRD analysesElectrooxidation of ethanol attracts significant attention because of its potential application in direct ethanol fuel cells DEFCs 1 Ethanol is an important renewable fuel which could conveniently be produced in great quantities eg by fermentation of sugarcontaining biomass As compared to methanol its key competitor ethanol is not only characterized by significantly higher by ca 30  energy density but is also substantially nontoxic 2 3 The most desired anodic reaction for a DEFC device is complete oxidation of a C2H5OH molecule to form water and CO2 However ethanol oxidation reaction EOR typically leads to numerous intermediates and byproducts that could potentially become adsorbed on the catalyst surface In fact the main difficulty here especially at lowoperating temperatures is to achieve significant cleavage of the C–C bond 2As kinetics of ethanol oxidation process were found to become significantly facilitated in alkaline environments 4 5 6 7 8 the range of suitable EOR catalyst materials was extended to cover nonnoble but highly corrosionresistant metals such as nickel which also possesses reasonable electrical and thermal conductivities and good mechanical durability 9 10 Nickel foams have commercially been available for more than two decades Application of highly modifiable base material having large specific surface area is of superior importance for the development of low noble metal level highly electroactive catalyst composites In this respect nickel foam might potentially become a key player within such important electrochemical technologies as alkaline PEM fuel cells water electrolysers hydrogen storage and pollutant degradation systems 11 12 13In this study Pdmodified Ni foam sample electrodes were prepared by means of a spontaneous deposition method as described in ref 13 Such obtained catalyst materials were employed as electrocatalysts for the EOR over the temperature range 20–60 °C in 01 M NaOH supporting electrolyte Palladium was chosen as Pd was previously found to exhibit high catalytic activity for ethanol oxidation in alkaline media along with superior tolerance against CO poisoning effect 14 15 16 17 18 It should be stressed that pure Ni foam itself does not possess any electrocatalytic activity towards the EOR 19 However nickel foam material could be made catalytic for alcohol oxidation reaction through the surface formation of extensive oxide/hydroxide layer 20An electrochemical cell made of Pyrex glass was used during the course of this work The cell comprised three electrodes a Ni foambased working electrode WE in a central part a reversible Pd 05 mm diameter 999  purity Aldrich hydrogen electrode RHE as reference and a Pt 10 mm diameter 999998  purity Johnson Matthey Inc counter electrode CE both placed in separate compartments Nickel foam was provided by MTI Corporation purity 9999  Ni thickness 16 mm surface density 346 g m−2 porosity ≥95  where no information on the specific surface area of this foam was given However the electrochemically active surface area of the MTI foam has been estimated at 192 cm2 548 cm2 g−1 in recent work from this laboratory 21 compare with similar values recorded by Grden et al 22 and by van Drunen et al 23 based on the impedance and the cyclic voltammetryderived data correspondingly All studied working electrodes were 1 cm × 1 cm Spontaneous deposition of Pd on nickel foam samples 13 was carried out in several steps Freshly cut foam samples were subjected to acetone and CH2Cl2 wash 15 min + ultrasonication following air drying and acid etching in 2 M HCl 15 min at 60 °C then spontaneous deposition of Pd was realized by dipping such pretreated foam electrodes in 0005 M PdCl2 pH = 10 t dep = 30 s and T dep = 25 ± 1 °C to obtain Pdmodified Ni foam composite electrodes Cyclic voltammetry and electrochemical impedance spectroscopy techniques were employed in this work All measurements were registered over the temperature range 20–60 °C by means of Solatron 12608 W Full Electrochemical System Data analysis was performed with ZView 29 Corrview 29 software package where the impedance spectra were fitted using a complex nonlinear leastsquares immitance fitting program LEVM 6 written by JR Macdonald 24 All other experimental details including preparation of supporting electrolyte 025 M C2H5OH in 01 M NaOH pretreatments applied to electrochemical cell and electrodes and employed ac impedance protocol were as those given in refs 7 8 and 18a SEM micrograph picture of Pdmodified Ni foam surface ca 027 wt Pd taken at 15000× magnification b XRD pattern for spontaneously deposited Pd element on Ni foam substrate diffraction lines correspond to the following sequence of fcc indices 111 200 220 311 and 222 for both Ni and Pd elementsCyclic voltammograms for ethanol electrooxidation on Pdmodified Ni foam carried out in 01 M NaOH at a sweep rate of 50 mV s−1 and in the presence of 025 M C2H5OH at the stated temperature values notations 1 and 2 correspond to the sequence of sweepsAn increase of the reaction temperature from 20 to 60 °C caused significant amplification of the CVrecorded current densities Interestingly the corresponding centres for the high temperature peaks A and B became considerably shifted towards more positive potentials both by ca 200 mV see Fig 2 again It should also be noted that the temperature increase has a significant impact on the EOR onset potential which at 60 °C became substantially displaced towards the H2 reversible potential see inset to Fig 2

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