The catalytic activities of sputtered cobalt metal electrocatalysts for polymer electrolyte membrane fuel cells

Solid State Ionics 225 (2012) 395–397

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The catalytic activities of sputtered cobalt metal electrocatalysts for polymer electrolyte membrane fuel cells Ki-Seong Lee a, Changhyun Jang b, Dongil Kim c, Hyunchul Ju d, Tae-whan Hong e, Whangi Kim f, Dongmin Kim a,⁎ a

Department of Materials Science and Engineering, Hongik University, 300 Shinan, Jochiwon, Yeongi, Chungnam 339-701, South Korea Department of Chemistry, Gachon University, San 65, Bokjeong, Sujeong, Seongnam, Gyeonggi, 461-701, South Korea Department of Fuel Cells, Dongjin Semichem. Co., Ltd. 472-2 Gaja, Seoku, Incheon 404-250, South Korea d School of Mechanical Engineering, Inha University, 253 Yonghyun, Namgu, Incheon, 402-751, South Korea e Department of Materials Science and Engineering, Chungju National University, 50 Daehakro, Chungju, Chungbuk 380-702, South Korea f Department of Applied Chemistry, Konkuk University, 322 Danwol, Chungju, Chungbuk 380-701, South Korea b c

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Article history: Received 10 September 2011 Received in revised form 10 February 2012 Accepted 13 February 2012 Available online 20 March 2012 Keywords: Fuel cell Electrocatalyst PEMFC Non-precious metal Sputtering Cobalt

a b s t r a c t A pure Co metal target was sputtered onto carbon paper and heat treated in an NH3 environment to fabricate Co-based electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFC). A 100 nm-thick Co-based electrocatalyst sample showed the highest current density among the samples examined. It was determined that the optimal level of Co needed to yield content for maximum current density is directly related to the formation of ligands between Co and carbon paper when linked by nitrogen. Unfortunately, lower levels of Co in carbon paper are not able to form sufficient ligands while excessive amounts of Co will in fact hinder the formation of ligands. Although the catalytic activity observed is not comparable to that of Pt/C catalysts yet, sputtered Co-based electrocatalysts show potential as nonprecious metal electrocatalysts in fuel cell. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Fuel cells are electrochemical devices to generate electrical power directly from the chemical energy of their chemical components [1,2]. Among various types of fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) is regarded as one of the most promising devices for the forthcoming age of electric vehicles due to its high efficiency, high energy density, and simple construction [3]. In addition, PEMFCs generally operate at relatively low temperatures (~80°) and are silent due to the absence of moving parts. Platinum is widely used as the electrocatalyst for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) due to its high catalytic activity and excellent chemical stability in a fuel cell environment. However, the limited quantity and high costs of Pt are critical hurdles to the commercialization of this type of fuel cell. To resolve such issues in PEMFCs, researchers have begun investigating alternative fuel cell catalysts using non-precious metals. Several types

⁎ Corresponding author at: Department of Materials Science, Hongik University, 300 Shinan Jochiwon Yeongi, Chungnam, 339-701, South Korea. Tel.: + 82 41 860 2558; fax: + 82 41 862 2774. E-mail address: [email protected] (D. Kim). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.02.025

of non-precious metal catalysts have been explored in recent years such as transition metal alloys and chalcogenides [4–9]. In 1964, Jasinsky [10] discovered that Co porphyrin was able to reduce oxygen in an acidic medium. Since then, it was determined that catalysts could be obtained by adsorbing Fe–N4 or Co–N4 macrocycles on a carbon support before heat treating them in an inert atmosphere [11,12]. Since many researchers have thought the transition metal ion is at the heart of non-precious metal catalyst for ORR in an acidic medium [13], this research aims to address this critical concern. Although these non-precious catalysts show promising results, their catalytic activity is still far lower than that of Pt catalysts. Further research is therefore required to find and improve the catalytic activity of non-precious metal catalysts. The sputtering process has been investigated for more than a decade as a potentially more effective tool in producing fuel cell electrode. This method allows nano-scale metal layers to be fabricated [14] with more precise control of metal content and film thickness [15]. In this study, pure Co metal was sputtered onto carbon paper (CP) and the samples were heat treated in an ammonia environment. These fabricated electrocatalysts are termed herein as Co/N/CP. The catalytic activity of Co/N/CP is examined in relation to changes in thickness due to the amount of Co used in the carbon paper to stimulate ligand formation.

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treated at 750 °C. The furnace was a quartz tube 6 cm in diameter and 120 cm in length. The furnace was kept at a positive pressure with NH3 gas to form a Co/N/CP catalyst. The quartz boat containing the sample was then inserted into the heating zone of the quartz tube. The tube was first purged with NH3 for 60 min. The oven was then turned on to a temperature of 750 °C, increasing 10 °C/min. The NH3 gas stream remained constant during each temperature increment. 2.2. Electrochemical characterizations of the Co/N/CP catalysts

Fig. 1. SEM morphology of the Co-grown on carbon paper.

The catalytic activity for the oxygen reduction reaction (ORR) was studied using the rotating disk electrode (RDE) technique in 0.5 M H2SO4 solutions at room temperature. The heat treated sample was first cut to a round shape, then attached by carbon tape onto a glassy carbon disk 0.5 cm in diameter, the samples was then mounted to an interchangeable RDE holder (Pine Instruments, USA). A Pt counter electrode and a saturated Hg/HgSO4 reference electrode were used in a standard three compartment electrochemical cell. The Co catalyst loading ranged from 0.00445 to 0.178 mg/cm 2. The catalytic activity was determined using cyclic voltammetry in 0.5 M H2SO4 solutions at room temperature and at a sweep rate of 20 mV s − 1. ORR polarization curves were measured at 1600 rpm for positive-going potential sweeps between 0 and 1.3 V versus NHE.

2. Experiment 3. Results and discussion 2.1. Sputter deposition of cobalt on carbon paper A pure cobalt metal target was sputtered onto the carbon paper at room temperature using RF magnetron sputtering. The pure Co target had a purity of 99.95% and was purchased from Kurt J. Lesker Company. The size was 2 in. in diameter and 0.125 in. in thickness. The sputter deposition system consisted of a stainless-steel chamber evacuated to 7 × 10− 5 Torr using a turbo molecular pump backed up by a mechanical pump. The distance between the target and the sample attached to the substrate holder was 17 cm. The film thickness was controlled by the sputtering time. The sputtered Co on the carbon paper was then heat

Fig. 1 shows the SEM images of a Co-based electrocatalyst grown on carbon paper. The carbon paper was purchased from

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Fig. 2. (a) EDX of Co/N/CP. (b) Schematic of ligand formation between metal and carbon paper linked by nitrogen.

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Fig. 3. (a) ORR polarization curves for the Co/N/CP in an O2-saturated 0.5 M H2SO4 solutions. Sweep rate = 20 mV s− 1 at room temperature at 1600 rpm. (b) Comparison of current density for Co/N/CP with variations in film thickness. Current density at 0.8 V versus NHE.

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provide a basis for further exploring the possibility of adapting nonprecious metal electrodes since sputtered pure metal Co on carbon paper is capable of producing catalytic activity in an ORR. More intensive research in this area is therefore required.

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Potential / V (NHE) Fig. 4. Cyclic voltammograms of Co/N/CP in an O2-saturated 0.5 M H2SO4 solution. Sweep rate = 20 mV s− 1. Normalized in reference to the geometric area of a RDE (0.196 cm2).

E-tek and showed a highly porous morphology. The sputtered Co is positioned inside the pores of the carbon paper. The EDX data in Fig. 2(a) shows the existence of Co, nitrogen and carbon. These likely accounts for the formation of Co–N4 macrocycles in the carbon paper as shown in Fig. 2(b). The fabricated samples are stable after RDE measurement which measured at 0.5 M H2SO4 environment. There is no destruction of samples after RDE measurement at acid media. Fig. 3(a) shows the ORR polarization curves for Co/N/CP catalysts in O2-saturated 0.5 M H2SO4 solutions obtained using a rotating disk electrode (RDE) at 1600 rpm. The heat treatment temperature of Co/N/CP in an NH3 environment was 750 °C. This yielded better activity and mechanical stability. The performance of the electrocatalyst is shown for different levels of Co which can be regarded as film thickness in the sputtering system. It was determined that the catalytic activity of a 100 nm thick sample was much higher than that of other thickness samples when the applied voltage was 0.8 V. It is important to note that the optimal amount of Co to ensure maximum current density is directly related to the formation of additional ligands between the Co and carbon paper used when linked by nitrogen. This is consistent with EDX data as shown Fig. 2(a). Under optimal Co levels, ligands are not formed sufficiently. Furthermore, excessive amounts of Co will hinder ligand formation. Hence, it is necessary to optimize the amount of Co used when fabricating Co-electrocatalysts for PEMFCs. Fig. 4 shows the cyclic voltammetry (CV) curves of the catalysts examined. They were recorded in O2-saturated 0.5 M H2SO4 solutions at a sweep rate of 20 mV s − 1. The cyclic voltammetry of the Co-based electrocatalysts showed a similar trend in terms of current density to the thickness level found at 100 nm. Although the current density of a Co-based electrocatalyst is not as high as that of a Pt/C electrocatalyst yet, the results of this research

Co-based electrocatalysts were fabricated on carbon paper by sputter deposition and heat treatment in an NH3 environment. A 100 nm-thick Co-based electrocatalyst sample showed the highest current density among the samples examined. The optimal level of Co needed to produce maximum current density is directly related to the formation of ligands between Co and carbon paper when linked by nitrogen. Lower levels of Co in carbon paper are not able to form sufficient ligands while excessive amounts of Co will in fact hinder the formation of ligands. Hence, it is necessary to optimize the level of Co used when fabricating Co-electrocatalysts for PEMFCs. Although the current density of a Co-based electrocatalyst is not as high as that of a Pt/C electrocatalyst yet, the prospect of exploring and commercially adapting non-precious metal electrodes remains promising since sputtered pure metal Co on carbon paper can produce significant catalytic activity in PEMFC.

Acknowledgments This work was supported by the National Research Foundation of Korea grant funded by the Korean Government (MEST) (NRF-2009C1AAA001-2009-0093168). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0024237).

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