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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25179−25185
Enhanced CO Oxidation and Cyclic Activities in Three-Dimensional Platinum/Indium Tin Oxide/Carbon Black Electrocatalysts Processed by Cathodic Arc Deposition Hyunwoong Na,†,‡,§ Hanshin Choi,‡,§ Ji-Won Oh,‡ Ye Seul Jung,† and Yong Soo Cho*,† †
Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea Advanced Materials and Processing R&D Group, Korea Institute of Industrial Technology, Incheon 21999, Korea
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S Supporting Information *
ABSTRACT: There have been extensive efforts to develop competitive electrocatalysts using carbon black (CB) supports for high-performance proton-exchange membrane fuel cells with less usage of Pt. Herein, we propose a very promising electrocatalyst architecture based on the three-dimensional Pt/indium tin oxide (ITO)/CB support structure which was enabled by a nonconventional deposition process ensuring very uniform impregnation of Pt and ITO nanoparticles into the CB network. The unusual scales of the Pt (∼1.9 nm) and ITO (∼5.6 nm) nanoparticles were directly related to unexpectedly better performance of the electrocatalytic activities. As a highlight, the electrochemical surface area of the electrocatalyst was maintained very well after the 3000 cycle-accelerated durability evaluation by demonstrating an excellent retention of ∼74.9%. Particularly, the CO tolerance exhibited a low value of ∼0.68 V as the absorption current peak, compared to ∼0.79 V for a commercial Pt/CB catalyst containing twice more Pt. KEYWORDS: Pt catalysts, electrocatalyst, CO tolerance, cathodic arc deposition, ITO
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INTRODUCTION Proton-exchange membrane fuel cells (PEMFCs) are receiving considerable attention as a competitive alternative to conventional power sources such as fossil fuel power plants and internal combustion engines.1,2 Electrocatalysts have been regarded as one of the most critical materials for successful fuel cell operations owing to their ability to mediate the charge transfer process between reactants and electrodes.3 Most stateof-the-art electrocatalysts are based on platinum group-based noble metals.4−6 However, because of their high prices and limited supply, such Pt group-based metals pose difficulties for market activation and mass production. Therefore, searching for and designing efficient electrocatalysts with less use of the noble metals are essential for future power options as long as they sustain the effectiveness of the cathodic oxygen reduction reaction (ORR).7−10 In spite of easy poisoning and moderate stability, Pt-based electrocatalysts on carbon supports are still commonly considered, primarily because of their excellent ORR activity.11,12 Recently, oxide supports have been introduced as an alternative to carbon-based supports because they can partially overcome the issues of the corrosivity of carbon and the degradation of electrochemically active surface area (ECSA) during electrocatalytic operations.13,14 In addition, the oxide support is known to positively affect the electrocatalytic © 2019 American Chemical Society
activity based on strong metal−support interactions, which is presumably due to the modification of the electronic state of platinum toward nonpreference for the formation of Pt−OH groups.15,16 As a promising example, recently, SnO2 has been reported to affect the redox reaction of Pt catalysts positively by suppressing the formation of oxide species on the surface of Pt.9,17,18 The suppression of oxide species increases the ORR activity by keeping Pt surfaces clean and thus improves durability. Oxide species on Pt surfaces, if present, act negatively as they are associated with an intermediary core product during the dissolution process of Pt.19,20 Even with the positive aspects of SnO2, there are still some limitations to the contribution as a part of the cathode support. SnO2 impedes the redox process at potentials relevant to the ORR and becomes unstable with the progress of cell operation. A relatively high electrical resistivity (∼106 Ω·cm) of SnO2 also acts negatively.21,22 In this regard, highly conducting tin-doped indium oxide (ITO) has been suggested for better catalytic activities with the evidence of substantial improvements in the ORR activity and long-term stability.23−25 However, extra heat treatment at high temperatures for the synthesis of ITO Received: April 9, 2019 Accepted: June 19, 2019 Published: June 19, 2019 25179
DOI: 10.1021/acsami.9b06159 ACS Appl. Mater. Interfaces 2019, 11, 25179−25185
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic procedure of preparing the three-dimensional Pt/ITO/CB structure through CADs of ITO nanoparticles on the CB support and Pt nanoparticles on the ITO/CB, (b) size distribution of ITO nanoparticles (with an average of ∼5.6 nm) for the case of 200 000 pulses, (c) TEM images of ITO nanoparticle-dispersed CB with lattice fringes of the embedded ITO in the high-resolution image, (d) TEM images of the complete catalyst with Pt nanoparticles (with an average of ∼1.9 nm) deposited with 275 000 pulses, and (e) EDXS elemental mapping images of In Lα1 and Pt Lα1, indicating relatively uniform distributions of the elements over the 3D catalyst structure.
behavior is another valuable result, indicating the superiority of the three-dimensional system.
nanoparticles induces the insufficient surface with the partial crystallization of ITO.23,24,26 Furthermore, the reported system using the ITO-involved support was not effective in producing competitive hydrogen oxidation reactions and hydrogen evolution reactions. Unlike carbon, ITO is thermodynamically unstable at low potentials (below 0.6 V) and severely degraded due to the intermediate formation of secondary species, for example, In, SnO, and Sn.27 Herein, we introduce a nonconventional approach of using the merits of ITO to form a three-dimensional electrocatalyst network based on the Pt−carbon black (CB) support. This system was designed to ensure the positive effects of ITO incorporation during the high-potential cycling at 1.0−1.5 V and to overcome the low-potential limitation at
