Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Platinum Nanoparticle-Supported Electrocatalysts Functionalized by Carbonization of Protic Ionic Liquid and Organic Salts Reiko Izumi,† Yu Yao,† Tetsuya Tsuda,*,† Tsukasa Torimoto,‡ and Susumu Kuwabata*,† †
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Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ‡ Department of Materials Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan S Supporting Information *
ABSTRACT: A novel approach involving a carbonization process of a protic ionic liquid (PIL) and organic salt (POS) is used to functionalize Pt-nanoparticle-supported carbon electrocatalysts for oxygen reduction reaction. The catalyst prepared using PIL N,N-diethyl-N-methylammonium hydrogen sulfate ([DEMA][HSO4]) and POS diphenylammonium hydrogen sulfate ([DPA][HSO4]) has a high long-term stability of catalytic performance because the Pt nanoparticles on the catalyst are well encapsulated with the carbonized PIL/POS mixture. Although the carbonization process enlarges the Pt nanoparticles and consequently reduces mass activity, surprisingly it realizes a high specific activity more than four times as much as a commercially available Pt nanoparticle electrocatalyst. KEYWORDS: ORR catalyst, ionic liquids, durability, platinum, catalytic activity
P
methylammonium hydrogen sulfate ([DEMA][HSO4]), and an electropolymerizable protic organic salt (POS), diphenylammonium hydrogen sulfate ([DPA][HSO4]).28 The conductive polymer layer electrochemically formed between the Pt nanoparticles and the carbon support improves the charge transfer capability (Figure 1c). What is interesting to note is that some PILs and POSs are easily carbonized under an Ar atmosphere.29 To take advantage of this unique feature, herein we attempt further functionalization of the aforementioned ILbased electrocatalysts by carbonizing the [DEMA][HSO4] and [DPA][HSO4] (Figure 1d) and investigate their electrocatalyst characteristics. Photographs and SEM images of the [DEMA][HSO4] PIL/ [DPA][HSO4] POS mixture and neat [DEMA][HSO4] PIL carbonized at 1273 K are shown in Supporting Information Figure S1. For the PIL/POS mixture, a self-standing carbon foam is obtained, and the carbonaceous residue yield was ca. 42.2 wt %. In the case of the [DEMA][HSO4], only a small amount of agglomerated small particles remains (carbonaceous residue yield, ca. 6.9 wt %). The carbonaceous residue yields are close to those reported by Watanabe et al.29 These results suggest that the carbon layer structure and thickness depend on the PIL and POS species. In order to confirm the carbonization reaction on the Pt-nanoparticle-supported carbon electrocatalysts prepared using the [DEMA][HSO4] PIL/[DPA][HSO4] POS mixture, catalyst 1, or the [DEMA][HSO4] PIL, catalyst 2, we compared the catalysts before and
olymer membrane electrolyte fuel cells (PEMFCs) have been rapidly developed for automobiles and household applications due to their low operating temperature, high power density, and compactness.1,2 One issue with PEMFCs is that the oxygen reduction reaction (ORR) at the cathode is slower than the hydrogen oxidation reaction at the anode. Development of high-performance ORR electrocatalysts has been pursued.3−9 In addition to this, improving the catalytic durability is an urgent task in terms of anticipated future market expansion and reduction in Pt usage.10−12 As is wellknown, Pt nanoparticle-supported carbon electrocatalysts show a favorable catalytic activity toward ORR, but the Pt nanoparticle also corrodes the carbon supports.13−16 To date, proposed methods to enhance the durability of ORR electrocatalysts include the utilization of sp2 carbon materials (carbon nanotubes17,18 and graphene19,20), noncarbon support materials,21,22 and SiO2 coating to carbon supports.23 These approaches are effective in reducing the carbon corrosion. However, novel ones are required because of their costly and time-consuming preparation procedures. We have found that highly durable Pt-nanoparticlesupported carbon electrocatalysts for ORR can be easily produced by heating and agitating the Pt-nanoparticledispersed ionic liquid (IL) mixture with carbon support (Figure 1a).24−28 In the electrocatalysts, the IL layer, which remains after the electrochemical cleaning, acts as a binder between the Pt nanoparticles and the carbon support and impedes their direct contact leading to carbon support corrosion13−16 (Figure 1b). Recently, we have succeeded in the modification of Pt-nanoparticle-supported carbon electrocatalysts using a protic ionic liquid (PIL), N,N-diethyl-N© XXXX American Chemical Society
Received: April 4, 2018 Accepted: June 19, 2018 Published: June 19, 2018 A
DOI: 10.1021/acsaem.8b00539 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 1. Schematic illustration of the Pt-nanoparticle-supported carbon electrocatalyst prepared in IL (a) before and (b−d) after the modification processes: (b) electrochemical cleaning, (c) electropolymerization, and (d) carbonization.
Figure 2. TEM images of the Pt-nanoparticle-supported carbon electrocatalysts. Electrocatalysts are (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6. (Insets) High-magnification TEM images of 1−6.
after the carbonization process by means of TEM. As a reference, a commercially available electrocatalyst, 3, was also employed. Catalysts 4, 5, and 6 were prepared by heat treatment of catalysts 1, 2, and 3, respectively, at 1273 K for 2 h. Figure 2 shows TEM images of catalysts 1−6. After the heat treatment, the mean particle size of the Pt nanoparticles supported on the carbon black (Vulcan XC-72) clearly increases (Table S1), but a homogeneous dispersion of the nanoparticles is maintained. The mean particle sizes for 4 and
5 were 6.1 and 6.3 nm, respectively. As for 6, the size is larger than these two catalysts (10.3 nm), although their original sizes, ca. 2.5−2.8 nm, are quite similar. The moderate Pt nanoparticle aggregation observed on the 4 and 5 may be due to the thin IL layer on the catalysts. Indeed, high-magnification TEM images of 1 and 2 show that a number of Pt nanoparticles are covered with the IL layer (Figure 2a,b (inset)). After the carbonization process, the nanoparticles were encapsulated by the carbonized PIL and POS (Figure B
DOI: 10.1021/acsaem.8b00539 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
S4). Their mass activities and specific activities were calculated from the current density at 0.85 V and the Pt loading. These data are summarized in Figure 4a and Table S1. The low initial
2d,e). This process yielded both spherical and polyhedral shaped Pt nanoparticles. Similar morphology change was recognized, when commercially available catalyst 3 was heated under the same condition (Figure 2c,f). It means that the PIL and POS are not involved in the morphology change. As depicted in Figure S2, the XRD patterns indicate smaller halfvalue widths for the Pt metal on the 4−6 than original catalysts 1−3, since the Pt crystallite size increased via the heat treatment at 1273 K. On the other hand, no diffraction pattern derived from graphitic structure is observed in ones for the catalysts 4 and 5, suggesting that carbon materials in the catalysts have amorphous structure.29 Figure S3 indicates the cyclic voltammograms recorded at the electrodes with the electrocatalysts having relatively small Pt nanoparticles, 1−5, in a N2-saturated 0.1 M HClO4 aqueous solution before and after the durability test. All the voltammograms show commonly observed electrochemical behaviors. That is, the redox waves for hydrogen adsorption/ desorption and platinum oxidation/reduction appear at the potential ranges of ca. 0.10−0.35 V (vs RHE) and ca. 0.80− 1.20 V, respectively.3−9,17−23 A pair of peaks observed at ca. 0.60 V are attributed to the redox reaction of quinone and hydroquinone moieties on the carbon materials.30 Figure 3
Figure 4. Comparison of (a) the mass activities and (b) the specific activities recorded at 1−5 before and after the potential cycling tests.
ECSAs for 4 and 5 relate to insufficient initial mass activities. Surprisingly the mass activities for 1 and 4 improve after the 15,000-cycle test. As previously reported, the former is due to the reduction of charge transfer resistance between Pt nanoparticles and carbon support by a conducting polymer, poly(diphenylamine), formed by the electropolymerization of [DPA][HSO4] (Figure 1c).28 The latter should be attributable to the increase in O2 diffusion routes derived from the aforementioned carbon corrosion of the carbonized PIL/POS during the durability test. The mass activity retention rate for 4 is much higher (245.9% at 15,000 cycles) than the other catalysts, and the mass activity remains virtually unchanged up to 45,000 cycles. Sufficient encapsulation of Pt nanoparticles by the carbonized [DEMA][HSO4]/[DPA][HSO4] plays a role in the unexpected result. However, the mass activity of 5 functionalized by the carbonization process similar to 4 slightly decreases after the cycle test, because, as described above, the insufficient carbon layer originating from [DEMA][HSO4] PIL is formed on the Pt nanoparticles. From the data given in Table S1, specific activity was also estimated (Figure 4b). The initial specific activity for 4 is comparable to commercially available catalyst 3. After the cycle test over more than 15,000 cycles, 4 exhibits the highest value among 1−5. It even exceeds 1, which has a conducting polymer layer produced between the Pt nanoparticles and carbon black, and the specific activity for 4 finally reaches 2.1 mA cm−2 after 45,000 cycles. These results reveal that the carbonized layer formed by the [DEMA][HSO4]/[DPA][HSO4] mixture greatly enhances both catalytic durability and specific activity. In summary, the carbonization of the [DEMA][HSO4] PIL/ [DPA][HSO4] POS mixture on the ORR electrocatalysts prepared using the Pt-nanoparticle-monodispersed IL enables a high specific activity up to 2.1 mA cm−2 after the 45,000 cycle test because of the improvement of the charge transfer ability between Pt nanoparticles and carbon support by the formation of a fine carbonized PIL/POS layer. Sufficient encapsulation of Pt nanoparticles on the carbon support is necessary to realize the electrocatalysts with a long-term stability. But, moderate carbon corrosion of the carbonized PIL/POS layer on the Pt nanoparticles is effective for increasing O2 diffusion routes, i.e., for obtaining a better catalytic property. Several findings reported in this article offer helpful suggestions to functionalize the IL-based electrocatalysts for ORR.
Figure 3. Variation in the ECSA as a function of cycle number. Electrocatalysts are (green filled circles) 1, (orange open circles) 2, (black triangles) 3, (blue filled squares) 4, and (orange open squares) 5.
plots ECSAs estimated from the hydrogen desorption peaks for 1−5 as functions of the cycle number. Catalysts 1−3 show relatively high initial ECSAs (40.5−57.8 m2 g−1, Table S1), and their ECSAs gradually decrease with increasing cycle number. As expected from the voltammograms in Figure S3, ECSAs for 4 and 5 are very low, 11.4 and 10.3 m2 g−1, respectively (Table S1), but their ECSAs are stable without significant change. The encapsulation of Pt nanoparticles by the carbonized PIL/POS would relate to the high stability. These two catalysts exhibited exceptionally high surface retention rates of 112.2 and 129.0%, respectively. Considering that the redox waves associated with the quinone-like structure, which come from carbon corrosion, became clearer after the durability test compared to the other three catalysts, probably the carbon corrosion of the carbonized PIL/POS layer on the Pt nanoparticles during the cycle test made it possible to expose Pt surface to the aqueous solution directly and resulted in the increase in electrochemical active area. The ORR performance of 1−5 was examined by rotating disk electrode−linear sweep voltammetry (RDE-LSV) in O2sataurated 0.1 M HClO4 before and after the cycle test (Figure C
DOI: 10.1021/acsaem.8b00539 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00539. Experimental methods, summary of electrocatalysts used, photographs and SEM images of mixtures and neat PIL, XRD patterns, and cyclic and hydrodynamic voltammograms (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(T. Tsuda) E-mail:
[email protected]. *(S.K.) E-mail:
[email protected]. ORCID
Tetsuya Tsuda: 0000-0001-9462-8066 Tsukasa Torimoto: 0000-0003-0069-1916 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially supported by JSPS KAKENHI Grant Nos. JP15H03591, JP15K13287, JP15H02202, JP16H06507, and JP16K14539.
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DOI: 10.1021/acsaem.8b00539 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsaem.8b00539 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
