Impact of sp2 carbon material species on Pt nanoparticle-based electrocatalysts produced by one-pot pyrolysis methods with ionic liquids

Pt-nanoparticle-supported graphene nanoplatelets (Pt/GNPs) and multiwalled carbon nanotube composite (Pt/MWCNTs) electrocatalysts for the oxygen reduction reaction (ORR) can be prepared using a one-pot method through the pyrolytic decomposition of the platinum precursor, platinum(ii) bis(acetylacetonate) (Pt(acac)2) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C4mim][Tf2N]) or N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide ([N1,1,1,3][Tf2N]) ionic liquids (ILs) with the target sp2 carbon support. In this one-pot pyrolysis method, which does not require any reagents to reduce Pt metal precursors or stabilize Pt nanoparticles, Pt nanoparticles are readily immobilized onto the sp2 surface by a thin IL layer formed at the interface, which can work as a binder. We used three types of sp2 carbon materials with different geometric shapes (graphene nanoplatelets with <3 (GNPs-3) and 18–24 layers (GNPs-20) and multiwalled carbon nanotubes (MWCNTs)) to investigate Pt nanoparticle formation and anchoring. All the electrocatalysts, especially Pt/MWCNTs, showed higher durability than the commercial catalyst owing to the combined effect of the IL binder and sp2 carbon materials. Our findings strongly suggest that the original carbon surface properties are also an important factor for creating high-performance ORR electrocatalysts.


Introduction
Rationally designed Pt-nanoparticle-supported carbon (Pt/C) electrocatalysts have been used to expedite the sluggish oxygen reduction reaction (ORR) on the cathode for polymer membrane electrolyte fuel cells (PEMFCs), which are clean and high-power density solutions for powering vehicles. 1,2 However, such Pt/C catalysts are required to further reduce the overpotential for the ORR and increase the durability. The latter is regarded as an unavoidable issue owing to the dissolution 3 and Ostwald ripening 4 of Pt nanoparticles under harsh operating conditions. In addition, catalyst degradation is induced by carbon support corrosion. The most commonly used carbon support is carbon black (e.g., Vulcan® X72), which has a high specic surface area and many functional groups that can easily anchor Pt nanoparticles. Unfortunately, carbon black is gradually oxidized and deformed during PEMFC operation, especially during start-up and shut-down. [5][6][7][8] This aggravates Pt detachment and aggregation. sp 2 -hybridized carbon materials composed of two-dimensional hexagonal carbon planes such as carbon nanotubes (CNTs) and graphene analogs, e.g., graphene nanoplatelets (GNPs), have emerged as promising candidates to replace the widely used carbon black in PEMFC catalysts. These materials have attracted considerable attention in recent years owing to their unique physicochemical properties, including excellent electrical and thermal conductivities, high corrosion resistance, and large specic surface area. 9,10 The application of corrosion-resistant carbon materials substantially reduces Pt nanoparticle-related carbon oxidation and degradation. 5,[11][12][13] However, anchoring Pt nanoparticles onto the smooth and chemically inert surface of sp 2 carbon materials is difficult. In most cases, the honeycomb sp 2 structure is destroyed by the addition of functional groups when immobilizing Pt nanoparticles onto carbon materials 14,15 and heteroatoms are embedded into the sp 2 structure. 16,17 These approaches are accompanied by a high risk of lowering the expected stability of CNTs and GNPs. Wrapping a carbon surface with a polymer, which is categorized as a physical conjunction method, is another approach to xing Pt nanoparticles onto the surface. 18,19 We have previously reported a similar concept, but using an ionic liquid (IL) as the binder. [20][21][22] In this manner, highly durable Pt/C electrocatalysts for the ORR are easily produced by heating and/or agitating the Pt-nanoparticle-dispersed IL with the carbon support. [23][24][25] In electrocatalysts, small quantities of the IL, which can remain even aer electrochemical cleaning, impede direct contact between Pt nanoparticles and the carbon surface, thereby inhibiting carbon support corrosion.
Inspired by this approach, we recently established a simple and mass-producible one-pot pyrolysis method with IL (IL onepot pyrolysis method) for preparing composite Pt and PtNi alloy-nanoparticle-supported multiwalled carbon nanotube composite (MWCNT) electrocatalysts for the ORR. 26 Unlike commonly used colloid methods, wherein the size and shape of nanoparticles are directly controlled by capping, stabilizing, and reducing agents, [27][28][29] no additional agents are needed for the IL one-pot pyrolysis method. In this study, to investigate the impact of sp 2 carbon material species on the ORR performance of electrocatalysts, we applied this IL one-pot pyrolysis method to graphene-based carbon materials, graphene nanoplatelets with <3 (GNPs-3) and 18-24 layers (GNPs-20) and MWCNTs. Using two types of ILs, 1-butyl-3-methylimidazolium bis(tri-uoromethylsulfonyl)amide ([C 4 mim][Tf 2 N]) and N,N,N-trimethyl-N-propylammonium bis(triuoromethanesulfonyl) amide ([N 1,1,1,3 ][Tf 2 N]), we revealed the key factors responsible for controlling and maintaining the Pt nanoparticle size, which are directly related to the electrocatalytic performance.

Characterization
We observed the morphologies of specimens 1-6 using a Hitachi H-7650 transmission electron microscope (TEM). We analyzed the amount of Pt nanoparticles loaded on the sp 2 carbon materials using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Shimadzu ICPS-7510). We evaluated the Brunauer-Emmett-Teller (BET) surface area using nitrogen adsorption-desorption analysis (Quantachrome Instruments). We characterized the original carbon materials and specimens 1-6 using a Nanophoton RAMANview Raman spectrometer.

Electrochemical measurements
We examined the electrocatalytic activities of specimens 1-6 using a computer-controlled Hokuto Denko HZ-7000 potentiostat/galvanostat. We obtained electrochemical measurements using a three-electrode cell with a 0.1 M HClO 4 aqueous solution. We used a glassy carbon rotating disk electrode (GC-RDE, surface area ¼ 0.196 cm 2 ), Pt mesh electrode, and reversible hydrogen electrode (RHE) as the working, counter, and reference electrodes, respectively. We ionically connected the RHE to the electrolyte in the main cell compartment via a Luggin capillary tip positioned close to the working electrode (WE). Each sample prepared in this study was dispersed in an i-propanol solution with a weight concentration of 1.48 g L À1 . We added a 10 mL solution onto the GC-RDE (5 mm diameter) and allowed it to dry naturally. Finally, we covered the GC disk with 10 mL Naon® i-propanolic solution (0.1 wt%) to x the samples using a thin Naon® layer. We conducted voltammetric experiments at 10 mV s À1 between 0.050 and 1.20 V (vs. RHE) at $298 K. Prior to the experiments, we electrochemically cleaned the working electrode with 40-100 potential sweeps between 0.050 and 1.20 V at a scan rate of 50 mV s À1 under a N 2 atmosphere until the cyclic voltammograms (CVs) stabilized. Depending on the experimental purpose, we deaerated the electrolyte by bubbling with N 2 or O 2 for at least 30 min. We determined the electrochemical surface area (ECSA) of the Pt nanoparticles by hydrogen adsorption or desorption coulombic charge in the cyclic voltammogram under N 2 atmosphere aer subtracting the double-layer charge current. We calculated the ECSA (m 2 g À1 ) using the following equation: where Q H (C) is the charge of the hydrogen adsorption/ desorption. An average value for the charge associated with a hydrogen adsorption/desorption monolayer formed on smooth polycrystalline Pt is 2.1 C m À2 . 30 M Pt (g) is the Pt mass on the GC working electrode. We investigated the ORR performance of the samples using rotating disk electrode linear sweep voltammetry (RDE-LSV) in the anodic direction at a sweep rate of 10 mV s À1 under ve different RDE revolution speeds (200, 400, 800, 1200, and 1600 rpm) in an O 2 atmosphere. The resultant voltammograms were examined using the Koutecký-Levich equation.
(2) Fig. 1 TEM images and Pt nanoparticle size distribution of specimens (a-f) 1-6, respectively, prepared under the experimental conditions described in Table 1.
where I is the experimentally measured current at 0.85 V, I k is the kinetically limited current, u is the angular frequency of rotation (s À1 ), n is the electron transfer number, F is the Faraday constant, A is the electrode surface area, C* is the O 2 concentration in the electrolyte, D is the diffusion coefficient of O 2 in the electrolyte, and v is the kinematic viscosity of the electrolyte. We estimated I k from the intercept of the Koutecký-Levich plot (I À1 vs. u À1/2 ). For each catalyst, we normalized I k to the Pt loading to obtain the mass activities. We conducted the durability tests for the samples using the standard evaluation method recommended by the Fuel Cell Commercialization Conference of Japan. 31 This test overloads the cathode for a proton-exchange membrane (PEM) fuel-cell system by potential sweeps between 1.00 and 1.50 V (vs. RHE) at 500 mV s À1 , which easily causes the carbon support materials to corrode. Therefore, this electrochemical approach enables a quick durability evaluation of the cathode catalyst. To obtain further insight into the deterioration behavior, we estimated the catalytic activity retention rates for the ECSA and mass activities using the following equations: Catalytic activity retention rate for ECSA ð%Þ Catalytic activity retention rate for mass activity ð%Þ ¼ mass activity estimated at each cycle ðA g Pt À1 Þ initial mass acitvity ðA g Pt À1 Þ Â 100 Fig. 1 shows the TEM images and Pt nanoparticle size distribution of specimens 1-6 that we prepared using the IL one-pot pyrolysis method under the conditions listed in According to the product information, the specic surface area of GNPs-20 is 120-150 m 2 g À1 . 32 A similar, at, sheet-like, and ultrathin carbon material, GNPs-3, has a much larger specic surface area of 1243 m 2 g À1 , which is calculated from the typical H3-type nitrogen adsorption-desorption isotherm curve of GNPs-3 ( Fig. S2a (ESI †)). This higher specic surface area is extremely likely to result in smaller Pt nanoparticles, because GNPs-3 can provide more anchor points for Pt nucleation in the Pt(II) reduction process. However, the impact of specic surface area on Pt particle size seems to be limited, because the size variation is less than 0.5 nm even though the surface area is nearly 10 times larger. A schematic illustration of this plausible Pt nanoparticle formation mechanism is depicted in Fig. 2a and b. In addition, the geometric shape can affect the size of the nanoparticles. MWCNTs have completely different shapes but are sp 2 carbon materials, as are GNPs-3 and GNPs-20. Although the MWCNTs we used in this study had a higher surface area, i.e., 210 m 2 g À1 (Fig. S2b (ESI †)), than GNPs-20, the Pt nanoparticles on the MWCNTs were slightly larger ( The difference in the IL species had a more noticeable effect on the Pt particle size in specimens 1-6. As summarized in Table 1, the nanoparticles deposited became larger by changing the IL from [C 4 mim][Tf 2 N] (specimens 1-3) to [N 1,1,1,3 ][Tf 2 N] (specimens 4-6). Nanoparticle growth in [C 4 mim][Tf 2 N] was hampered. Similar behavior was also conrmed by Pt nanoparticle preparation using the same IL one-pot pyrolysis method but without carbon supports (Fig. S3 (ESI †)). The mean particle sizes of the Pt nanoparticles we obtained for [C 4 mim][Tf 2 N] and [N 1,1,1,3 ][Tf 2 N] were 3.5 and 6.9 nm, respectively. Because the surface of the Pt nanoparticles prepared in ILs is positively charged, the Pt nanoparticles are considered to be stabilized by anionic supramolecular aggregates, for example, [(organic cation) x (Tf 2 N) 1+x ] À . 33 In the present case, the accessibility of Pt metal precursors and/or atoms to the nanoparticle surface was directly controlled by the cationic steric effect. 34 Thus, a larger volume cation, [C 4 mim] + , could effectively block further Pt deposition on the nanoparticles. As a result, Pt nucleation was stimulated at other anchor points on the carbon supports, leading to less nanoparticle growth. A plausible process of Pt nanoparticle growth during the IL one-pot pyrolysis method is schematically summarized in Fig. 2c.

Results and discussion
We collected further information on the carbon structure using Raman spectroscopy for specimens 1-6, and the asprepared samples are shown in Fig. S4 (ESI †). Two characteristic peaks, the D band at 1350 cm À1 and G band at 1580 cm À1 , are associated with defects or disorders and sp 2 -bonded carbons, respectively. 35 We noted little difference between the Raman spectra before and aer the Pt nanoparticle modication, indicating that the IL one-pot pyrolysis method employed in this research had no effect on the surface structure of the sp 2 carbon materials.
As expected from the characterization of the specimens, their electrochemical performance differed signicantly. Fig. 3 shows the cyclic voltammograms before and aer 15 000 potential cycle durability tests recorded at the GC electrodes with specimens 1-6 prepared by the one-pot pyrolysis method in a N 2 -saturated 0.1 M HClO 4 aqueous solution. The several commonly observed redox waves concerning the hydrogen and oxygen electrochemical reactions are recognized at potentials ranging from 0.10 to 0.35 V and 0.60 to 1.00 V, respectively. The ECSAs for specimens 1-6 estimated from the voltammograms for hydrogen adsorption-desorption are shown in Table 2. These values are lower than those of commercial Pt-C catalysts. The H 2 adsorption was highly likely hampered by the IL remaining on the Pt nanoparticle surface. [36][37][38] Aer the durability test, we found that the ECSA retention rates for the graphene nanoplatelet-based samples, 1 (51.2%), 2 (47.6%), 4 (26.9%) and 5 (65.2%), were inferior to that of Pt-C (71.1%). Conversely, specimens 3 (75.1%) and 6 (92.8%), prepared using MWCNTs, showed higher retention rates than Pt-C. Considering the larger number of defects or disorders on the MWCNTs, corroborated by the Raman spectra derived from the G-band (Fig. S4 (ESI †)), we deduced that their presence triggered slower nanoparticle aggregation during the durability test, that is, a higher retention rate of ECSA by Pt/MWCNTs.
To evaluate the catalytic ability of specimens 1-6 toward the ORR, we measured the RDE-LSVs at the GC-RDEs with each specimen in an O 2 -saturated 0.1 M HClO 4 aqueous solution before and aer 15 000 potential cycle durability tests (Fig. S5  (ESI †)). Before the durability test, the increase in current ow for O 2 reduction initiated at ca. 1.05-1.10 V, as observed for commonly used electrocatalysts for the ORR. Koutecký-Levich plots at 0.85 V were constructed using the RDE-LSVs obtained at speeds of 200-1600 rpm. The mass activities of the specimens that we calculated from the Koutecký-Levich plots and the mean Pt particle sizes before the durability tests are summarized in Table 2 and Fig. 4. The mass activity decreased with increasing nanoparticle size. We obtained the highest mass activity, 329.9 A g À1 , for specimen 1, which had the smallest mean nanoparticle size of 2.1 nm, corresponding to the size that can obtain more Pt (111) facets. 39 As focusing on Pt/GNPs-3 and Pt/GNPs-20, which are at, sheet-like carbon supports, would provide an opportunity to discuss the effect of Pt nanoparticle density on catalytic ability, we prepared two extra GNPs-20based specimens by the same procedure using [N 1,1,1,3 ][Tf 2 N]  but with different Pt precursor concentrations of 1 and 2.5 mmol L À1 . TEM observation of the GNPs-20 prepared with 1, 2.5, and 5 mmol L À1 Pt(acac) 2 revealed that the mean particle sizes were 3.8 AE 0.6 nm, 3.6 AE 0.7 nm and 3.8 AE 0.5 nm, respectively (Fig. 5). In other words, size does not depend on the precursor concentration. However, the Pt loading varied with precursor concentration. From these ndings, we concluded that the number density of Pt nanoparticles on GNPs-20 increases with increasing precursor concentration. The mass activities of these three specimens are shown in Fig. 6a as a function of Pt loading. The mass activities increase with increasing Pt loading, but are nonlinear. Because their particle sizes are almost the same, the differences in their mass activities are not dependent on size. This nonlinear behavior should be linked to the number density of the Pt nanoparticles on the GNPs-20. Watanabe et al. reported that the active area territory on the Pt surface is an important factor in attaining higher mass activity. 40 As illustrated in Fig. 6b, oxygen accessibility reduced when the Pt nanoparticle number density was too high.
Based on the RDE-LSVs aer the 15 000-cycle tests ( Fig. S5 and S6 (ESI †)), the mass activity data estimated at 0.85 V for specimens 1-6 and Pt-C are provided in Table 2. A comparison of all the mass activity data obtained in this study, with the catalytic activity retention rates, is presented in Fig. 7. All the specimens prepared showed a higher mass activity retention rate than Pt-C (49.6%). In particular, specimens 3 and 6, resulting from the MWCNTs, showed a much higher durability. The results of Raman spectroscopy indicated that, as more functional groups exist on MWCNTs than on the other two sp 2 carbon materials, MWCNTs would suppress the displacement and aggregation of the Pt nanoparticles. Although the catalyst durability is improved by the IL binding between the Pt nanoparticles and the carbon material surface, the results here provide evidence that the original carbon surface properties also affect durability.

Conclusions
In this study, we successfully synthesized Pt-nanoparticle-modied sp 2 carbon materials by the IL one-pot pyrolysis method using three different sp 2 carbon supports, GNPs-3, GNPs-20, and MWCNTs, without any pretreatment. The higher surface area of the carbon supports led to the formation of smaller Pt nanoparticles. The cation size in the IL also signicantly affected the Pt nanoparticle size. We obtained the highest mass activity when the Pt nanoparticle size was $2 nm. All the resulting specimens showed higher durability in terms of mass activity than the commercial catalyst. The samples prepared from MWCNTs had a much higher retention rate. More defects on the MWCNTs resulted in a stronger ability to x the nanoparticles and prevent nanoparticle aggregation. This study provides useful information on the IL one-pot pyrolysis method for the synthesis of ORR electrocatalysts with high activity and durability.

Author contributions
T. T. conceived the idea of the one-pot process used in this research. S. K. gave the suggestions on experimental plan. Y. Y., Q. X. M. K. and H. Y. prepared and evaluated the specimens. Y. Y. and T. T. wrote the manuscript. All the authors participated in discussions of the results and in preparing the manuscript.

Conflicts of interest
The authors have no conicts to declare.