PULSE ELECTRODEPOSITION OF Pt Co CATALYST ONTO GLASSY CARBON FOR OXYGEN REDUCTION REACTION TO USE IN PEMFC

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1 PULSE ELECTRODEPOSITION OF Pt Co CATALYST ONTO GLASSY CARBON FOR OXYGEN REDUCTION REACTION TO USE IN PEMFC Jittima Sriwannaboot a,b, Nisit Tantavichet a,b,* a) Center of Excellence on Petrochemical and Materials Technology b) Department of Chemical Technology, Faculty of Science, Chulalongkorn University Keywords: PEM fuel cell, Electrodeposition, Pulse current, Pulse reverse current, Pt Co alloys ABSTRACT Proton exchange membrane fuel cells (PEMFC) are energy conversion devices that convert chemical energy to electrical energy directly using hydrogen as a fuel. Since hydrogen can be obtained from renewable energy sources and is environmental friendly, PEMFC have a potential to solve the energy and environmental crisis. Practically, the performance of the PEMFC is limited by slow oxygen oxidation reaction (ORR). To overcome this, effective electrocatalysts are needed. Pt Co alloy is usually used as the electrocatalyst for the ORR due to its higher activity for the ORR than Pt. In this study, we prepared Pt Co alloys by pulse current (PC) electrodeposition. The Pt Co electrodeposits are then used to study the electrocatalytic activities. The effect of pulse parameters (peak current density, times of applied current and non-applied current) on the chemical composition of the Pt Co electrodeposited had been investigated. The pulse parameters were used to fine-tune the Pt Co deposit composition to have a wide Pt:Co range, between Pt 16 Co 84 and Pt 89 Co 11. It was found that the Pt Co alloys had higher ORR activities than Pt and the ORR activity was depended on the chemical composition of the Pt Co alloys, where Pt 78 Co 22 yielded the highest ORR activity. * Nisit.T@chula.ac.th INTRODUCTION The PEMFC are energy conversion devices that convert the chemical energy to electrical energy directly. In PEMFC, the oxygen reduction reaction (ORR) at cathode has a very slow kinetic rate. Normally, Pt is used as a catalyst on both anode and cathode. However, Pt catalyst is rare and very expensive. Thus, researches have be studied to improve the ORR activity and the utilization of Pt and to reduce the cost of catalyst. Generally, the Pt based alloy catalyst has been reported to improve the activity towards oxygen reduction with respect to Pt catalyst. Nowadays, Pt Co alloy is the most promising Pt-based alloy catalyst for the ORR. The Pt Co electrodes for PEMFC have been prepared by various methods such as the impregnation, polyol, microemulsion and electrodeposition methods [1]. The electrodeposition has the advantages of low energy consumption and easy control of the loading mass of Pt. For the galvanostatic electrodeposition, different current waveforms, including direct current (DC) and pulse current (PC) electrodepositions can be applied. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1

2 Pulse electrodeposition (PC) is widely accepted to be superior to DC electrodeposition since the metal coatings prepared by PC electrodeposition consist of smaller and finer grained structures, which lead to a higher surface area. The PC electrodeposition has various operating parameters, namely the cathodic current density (i C ), on time (T on ) and off time (T off ) to control the electrodeposition process to produce deposits with desired properties. [2] In this present work, we investigated the effect of electrical variables used to control the pulse electrodeposition in a NaCl electrolyte. The morphology and composition of Pt Co alloys electrodeposited were studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. By controlling the pulse deposition parameters, we can optimize the electrochemical properties of the Pt Co alloy catalysts for the ORR. EXPERIMENTAL A. Electrode preparation The sub layer solution was prepared from a mixture of carbon black (Valcan XC 72), DI water, isopropanol (QReC), Nafion 117 ionomer (Aldrich) with a carbon black to Nafion 117 volume ratio of 30:70 and then sonicated for 4 h. The glassy carbon GC ( = 2 mm.) working electrode was polished with emery paper and alumina slurries and finally cleaned with acetone and DI water. The 0.05 l sub layer solution was dropped onto the GC surface and dried at the room temperature. B. Electrodeposition of Pt Co on glassy carbon The electrodeposition method was conducted in a two compartment electrochemical cell, separated by a Nafion 115 membrane, using a standard three electrode system. The GC electrode was placed in a compartment containing a solution of M H 2 PtCl 6 6H 2 O (Aldrich), 0.01 M CoCl 2 6H 2 O (Fluka) and 0.5 M NaCl (Labachemie). The titanium gauze used as the counter electrode was placed in the other compartment containing 0.5 M NaCl. The Ag/AgCl reference electrode (3 M KCl, Metrohm) was placed in the same compartment as the working electrode to reduce the voltage drop when monitoring the electrode potential during the electrolysis. During the electrodeposition the solution was stirred by a magnetic stirrer at 600 rpm. All electrodeposition experiments were carried out using an Autolab PGSTAT 10 Potentiostat (Eco Chemie) equipped with voltage multiplier to widen the monitored potential range. The electrode responses recorded during the electrodeposition are reported in the Ag/AgCl scale. The cathodic densities and the off time duration were controlled in the range of ma cm 2 and s, respectively. After the electrodeposition, the Pt Co electrodeposited GC was removed from the solution and washed by DI water and then dried at the room temperature. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2

3 C. Analysis of the Pt Co deposit Elemental Analysis Pt and Co atomic percentages and morphology of all the alloy catalysts were studied by scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM-EDS, JSM-6610LV and X-MaxN 50). Electrochemical Characterization Cyclic voltammograms (CVs) of the Pt Co alloys were recorded at room temperature between 0.05 V and 1.0 V vs RHE at a scan rate of 20 mv s 1 under N 2 saturated. The ORR kinetics of the deposited alloys were studied at the room temperature through hydrodynamic voltammetric measurements by employing a rotating disk electrode (RDE) in the potential range of 0.2 V to 1.0 V vs RHE in the positive direction with a scan rate of 10 mv s 1 under O 2 saturated. Both the CV and RDE experiments were conducted in 0.5 M H 2 SO 4 where a Ag/AgCl was used as the reference electrode and a Pt gauze was used as the counter electrode. All results present were converted into RHE scale. RESULTS AND DISCUSSION A. Effect of cathodic current density in PC electrodeposition The composition of the Pt Co alloys prepared by the PC electrodeposition using various cathodic current densities (i c ) between 10 and 30 ma cm 2 are shown in Table 1. The Pt deposit with a wide composition (between Pt 16 and Pt 89 ) where can be produced the on time (T on ) and off time (T off ) were fixed at 0.5 and 0.1 s, respectively The Pt Co alloy deposited at the lower applied cathodic current densities had a higher Pt contents than those deposited at the higher applied cathodic current densities. The higher applied cathodic current density increases the electrode potential to a more negative value so that more Co atom can react at electrode surface. From SEM images shown in Fig. 1, the Pt Co alloy catalysts produced at higher applied cathodic current densities are made of smaller grains. Fig. 2(a) shows the CVs of Pt Co alloys deposited from PC electrodeposition where the charge of hydrogen desorption, respectively the active surface area of the catalyst, is calculated and shown in Table 1. The total charge corresponding to the hydrogen desorption can be related to the integral of the curve for a certain interval of potentials where the atoms are being desorbed [3,4]. The results show that the Pt Co alloys produced by PC have a significantly higher a charge than that produced by DC electrodeposition. Moreover, the deposited Pt Co alloys produced from the lower applied cathodic current densities have higher charges due to the higher Pt contents. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3

4 (a) (b) (c) (d) Fig. 1. SEM images (x5000) of the Pt Co alloy deposits prepared by DC and PC electrodeposition at a cathodic current density range of (a) DC: i c 20 ma cm 2,PC: i c at (b) 10 ma cm 2, (c) 20 ma cm 2, (d) 30 ma cm 2. Table 1 Chemical composition, charge of hydrogen desorption of the Pt Co alloy prepared under DC and PC electrodeposition using different cathodic current densities. Mode i c T on T off Pt Co Q des (ma cm 2 ) (s) (s) (at.%) (at.%) (ma s ) DC PC (a) (b) Fig. 2. Cyclic voltammograms (CVs) of Pt Co deposits prepared by DC electrode-position at 20 ma cm 2, PC electrodeposition at (a) cathodic current densities of 10, 20, 30 ma cm 2 and (b) different off times of 0.1, 0.2, 0.3 and 0.5 s. B. Effect of off time Since the pulse off time (T off ) is an important parameter for mass transport enhancement, during PC electrodeposition we have investigated effect of T off. Normally, the crystallization process involves two steps which include the formation of new crystals or nucleation process and the building up of old crystals or growth process [5]. Fig. 2(b) shows the CVs of the Pt Co alloys deposited from PC electrodeposition with different off times between 0.1 and 0.5 s in N 2 saturated. The properties of the Pt Co alloys Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4

5 prepared by PC electrodeposition using different T off are summarized in Table 2. The results show that the short off time results in a decrease in Pt contents. Such results indicate the variation of electrochemical reaction rates during off time associated with the mass transport of the electroactive species from the bulk solution to the electrode surface during off time. When the cathodic current is applied (during on time), Pt ion which is reduced more easily than Co ion, is consumed and depleted on the electrode surface. During the off time where no reduction of metal ions is expected to take place, the Pt ion can diffuse from the bulk solution to the electrode surface to compensate the Pt ion reacted in the previous on time period. Thus, the larger off time allows more Pt ion to diffuse to the electrode surface so that more Pt ion is available to react when the next cathodic current is applied. The enhancement of the mass transfer of Pt ion in the longer off time then leads to more Pt content in the Pt Co alloys. Table 2 Chemical composition, charge of hydrogen desorption of the Pt Co alloy prepared under different off time in PC electrodeposition. i c T on T off Pt Co Q Mode des (ma cm 2 ) (s) (s) (at. %) (at. %) (ma s) PC The ORR voltammograms for the Pt Co catalysts with different alloy compositions in O 2 saturated are shown in Fig. 4(a). The onset potentails and the kinetic current densities (i k ) obtained from the ORR voltammograms curve shown in Fig. 4(b). The ORR voltammograms 3 regions: consist of (i) kinetic control, (ii) mixed kinetic mass transfer control and (iii) mass transfer control. The better catalysts for ORR have higher positive onset potential and higher i k comparing between Pt and Pt Co alloy catalysts, the results show that some compositions of Pt Co alloy (Pt 78 Co 22 ) have higher catalyst activities than that of Pt where the Pt 78 Co 22 presents the highest ORR activity with the highest onset potential, steepest slop and biggest limiting current. This result agrees with those reported in the literatures [6-9] which suggested that atomic ratio of Pt 3 Co yields the higher ORR activity and fuel cell performance. (a) (b) Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5

6 Fig. 4. Linear sweep voltammograms test for ORR on Pt Co catalyst with different alloy compositions in O 2 saturated 0.5 M H 2 SO 4. The scan rate was 10 mvs 1 and the rotating speed was 2000 rpm. (a) Cathodic polarization curve and (b) Onset potentials and kinetic current density at 0.83 V. CONCLUSIONS The Pt Co alloy catalyst electrodes with different Pt and Co contents have been prepared by pulse current electrodeposition to investigate the influence of pulse parameters on the composition of the Pt Co deposited alloys, the charge of hydrogen desorption and the catalyst activity of the ORR. The applied cathodic current density can be used to control composition of Pt Co alloys where the Pt Co compositions between Pt 16 Co 84 and Pt 89 Co 11 can be produced. The Pt rich contents were obtained from lower applied cathodic current densities. For pulse electrodeposition at different off times, the compositions between Pt 26 Co 74 and Pt 88 Co 12 at.% can be obtained where the higher Pt contents were obtained from the longer applied off times. Comparing to DC electrodeposition, the Pt Co alloys prepared by PC electrodeposition has smaller and finer grained structures than DC electrodeposition. The CV results indicated that Pt rich, Pt Co alloys show higher charges of hydrogen desorption which indicate higher active surface areas. The Pt 78 Co 22 alloy catalyst which was prepared by cathodic current density 20 ma cm 2 at off time 0.3 s showed the highest ORR activity. This study shows that the Pt based bimetallic alloys can be prepared and fine tune by PC electrodeposition, which indicates the enhancement for improving the ORR activity. ACKNOWLEDGEMENTS The author would like to express their gratitude to Center of Excellence on Petrochemical and Materials Technology for financial support. REFERENCES [1] S. Woo, I. Kim, J.K. Lee, S. Bong, J. Lee, H. Kim. Preparation of cost-effective Pt Co electrodes by pulse electrodeposition for PEMFC electrocatalysts. Electrochim. Acta 56 (2011) [2] N. Chaisubanan, N. Tantavichet. Pulse reverse electrodeposition of Pt Co alloys onto carbon cloth electrodes. J. Alloy Compd. 559 (2013) [3] Bio-Logic Application note #11. [4] K. R. Cooper. In situ pem fuel cell electrochemical surface area and catalyst utilization measurement. Fuel Cell Magazine. Jan-Feb (2009). [5] M.S. Chandrasekar, M. Pushpavanam. Pulse and pulse reverse plating Conceptual, advantage and applications. Electrochim. Acta 53 (2008) Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6

7 [6] S. Koh, M. F. Toney, P. Strasser. Activity stability relationships of ordered and disordered alloy phases of Pt 3 Co electrocatalysts for the oxygen reduction reaction (ORR). Electrochim. Acta 52 (2007) [7] A. Uzunoglu, A. S. Ahsen, F. Dundar, A. Ata, O. Ozturk. Structural, electronic, and electrochemical analyses of sputtercoated Pt and Pt Co/GCE electrodes with ultralow metal loadings for PEM fuel cell applications. J. Appl Electrochem 47 (2017) [8] S. Choi, S.U. Lee, W. Y. Kim, R. Choi, K. Hong, Composition-Controlled PtCo Alloy Nanocubes with Tuned Electrocatalytic Activity for Oxygen Reduction. ACS Appl. Mater. Interfaces 4 (2012) [9] D. Wang, H. L. Xin, R. Hovden, H. Wang, Y. Yul. (2012). Structurally ordered intermetallic platinum cobalt core shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nature Mater. Vol(12) Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 7