A facile synthesis of high activity cube-like Pt/carbon composites for fuel cell application

Abstract

High activity catalyst with simple low-cost synthesis is essential for fuel cell commercialization. In this study, a facile procedure for the synthesis of cube-like Pt nanoparticle (Pt_Cube) composites with high surface area carbon supports is developed by mixing precursor of Pt with carbon supports in organic batches, hence, one pot synthesis. The Pt_Cube grow with Vulcan XC-72 or Ketjen black, respectively, and then treated for 5.5 h at 185ºC (i.e., Pt_Cube5.5/V and Pt_Cube5.5/K). The resulting particle sizes and shapes are similar; however, Pt_Cube5.5/K has a larger electrochemical active surface area (EASA) and a remarkably better formic acid (FA) oxidation performance. Optimization of the Pt_Cube/K composites leads to Pt_Cube10/K that has been treated for 10 h at 185ºC. With a larger EASA, Pt_Cube10/K is also more active in FA oxidation than the other Pt_Cube/K composites. Impedance spectroscopy analysis of the temperature treated and as-prepared (i.e., untreated) Pt_Cube/K composites indicates that Pt_Cube10/K is less resistive, and has the highest limiting capacitance among the Pt_Cube/K electrodes. Consistently, the voltammetric EASA is the largest for Pt_Cube10/K. Furthermore, Pt_Cube10/K is compared with two commercial Pt/C catalysts, Tanaka Kikinzoku Kogyo (TKK), and Johnson Matthey (JM)Pt/C catalysts. The TKK Pt/C has a higher EASA than Pt_Cube10/K, as expected from their relative particles sizes (3–4 nm vs. 6–7 nm for Pt_Cube10/K), however, Pt_Cube10/K has a significantly better FA oxidation activity.

Keywords

synthesis / cube-like Pt / Pt/C composite / catalyst / impedance

Introduction

High activity catalyst with simple low-cost synthesis is considered mandatory for cost reduction that is required for fuel cell commercialization [1–10]. It has been shown that platinum (Pt) based catalysts are essential for low temperature fuel cells, both for performance and durability; but this has implication on the cost of fuel cells due to the high price of the precious metal Pt. From material point of view, Pt loading can be reduced through either improving catalytic activity or electrochemical active surface area (EASA). The physicochemical properties of the support material can greatly influence dispersion, particle size, crystallinity, and morphology of the forming support/metal composites [2,5,8,11–15], and these parameters, regardless of their synthesis procedure, can greatly affect the electrochemical responses of the produced catalysts. Comparing with traditional spherical Pt particles, non-spherical shape would provide larger surface area for a given amount of mass. Further, the cost associated with catalyst can be reduced if complex multi-step fabrication of the Pt based catalysts can be simplified. Therefore, the present study focuses on one-step synthesis of cube-like Pt composite catalysts with different supports. In addition, it has been reported that not only the synthesis procedure, but also the post synthesis treatments are highly influential in the properties and performance of the final catalyst materials [4,13,16,17].

Organic batch synthesis of Pt nanoparticles with various sizes and shapes for the electrocatalysis of fuel cell reactions have been the subject of numerous studies [9]. Such organic phase procedures consist of two main steps: nucleation and growth [1,9]. Over the course of nucleation and growth, parameters including metal precursor type, temperature, and its combination with reaction time, as well as the type of the surfactant have been shown to affect the size and shape of the resulting particles, which can generate quite different electrochemical activities [4,9]. Wang et al. showed that depending on the temperature regime, a certain combination of oleylamine, oleic acid, and octadecene led to cubic, truncated cubic, or octahedral Pt nanoparticles [4], whereas nanowires were obtained in mixtures of oleylamine and octadecene with controlled length [3]. Lee et al. also provided a useful example for the role of surfactant on the shape and size, where mono-dispersed cubic Pt was prepared in the presence of poly(vinyl pyrrolidone), while in the absence of which a spectrum of particles with various sizes and shapes were obtained [6].

Pt-based catalysts with enhanced electrochemical performances have been synthesized on various support materials, including carbon, conducting polymers (CPs), or metal oxide (MO) [7]. Nitrogen-rich CPs such as polyanilne, polypyrrole, polythiophene, and polycarbazole have been widely studied, which have shown to exert improvements in the activity of Pt towards fuel cell reactions [18,19]. Ruthenium oxide, known as a strong CO oxidation promoter, has been made in various forms to accelerate fuel cell reactions that involve oxidative removal of the adsorbed CO [20,21]. Further, other MOs such titanium oxide or nickel oxide have also speeded up fuel cell reactions at Pt [22–24]. Nevertheless, due to the ease of use, simplicity of preparation, and versatility, carbon is still a prevailing support material in fuel cell industry [2,12,14,15,25,26]. Even though CP and MO additive and support materials have shown interesting results in many studies [18–19], their use in actual fuel cells is still under fundamental investigation. On this basis, research on synthesis of better carbon materials is a high profile ongoing subject, where various carbon supports are being increasingly adopted to tailor single/bi-metallic Pt based fuel cell catalysts [2,5,7,12,14,27].

Post-synthesis treatment of Pt particles prepared in organic batches have repeatedly shown to influence the electrochemical properties of the resulting composites [4,16,28,29]. While organic capping agents are necessary in controlling the shape and size of the Pt particles, but they can physically (i.e., blocking the surface) and chemically (e.g., adversely reacting with reaction parties) decrease the electrochemical performance [29]. Luo et al. reported use of NaBH₄/tert-butylamine for removing polyvinylpyrrolidone adsorbate from Pt alloys [28], while Li et al. tested acetic acid to eliminate oleylamine-like surfactants from cubic Pt particles [16]. However, Li et al. also showed that temperature annealed (185ºC) Pt nanoparticles had greater electrochemical activities than those that had been treated with acetic acid or UV irradiation to remove organic surfactants.

Therefore, the objective of the present study is to develop a simple one-pot synthesis of high electrochemical performance composites of cube-like Pt catalyst particles with high area carbon supports, and optimize the condition for the composite catalyst preparation, including post synthesis treatment. Two high surface area carbon supports (Vulcan and Ketjen black) will be used for the assessment of the support impact on the catalyst performance; and post-synthesis temperature treatment, as a critical step in the preparation of composite catalysts, will be investigated. The resulting composite catalysts will be characterized by electron microscopy, voltammetry, electrochemical impedance spectroscopy, and potentiostatic polarization. The developed composite catalysts will be compared with commercial Pt/C catalysts to demonstrate their superior activity and performance.

Experimental

Chemical

Sulfuric acid (Sigma Aldrich, 95%-98%), formic acid (GR ACS-Finland, 98%), octadecene (Sigma Aldrich, 90% Technical grade), oleylamine (Sigma Aldrich, 70% Technical grade), oleic acid (Sigma Aldrich), hexanes (OmniSolv, HPLC grade), ethanol (Sigma Aldrich, HPLC grade), iso-propanol (Sigma Aldrich), and Fe(CO)₅(Aldrich, 99.99%), Nafion (Ion Power, 5% alcoholic), Pt(acac)₂ (Sigma Aldrich, 97%), Tanaka Kikinzoku KogyoPt/C (TKK46.6%, Japan), and Johnson Matthey Pt/C (JM HiSPEC60%), Ketjen Black (Akzonobel), and Vulcan XC-72 (Johnson Matthey) were used as received.

Preparation of Pt cube/carbon composites (50% Pt/C)

In a three-neck round bottom flask, ~1.04 g Pt(acac)₂ (97%) was stirred (at 450 r/min) under N₂ atmosphere with 80 mL octadecene, 10 mL oleylamine, and 10 mL oleic acid, while the temperature was increased to 65ºC to dissolve the precursor (dissolution taking typically ~ 60–70 min). Then, 0.5 g Ketjen black or Vulcan XC-72 (previously grinded) was added to the mixture, during which N₂ flow was momentarily disconnected to avoid splashing the carbon powder. Thereafter, temperature was raised to 160ºC, at which ~1 mL of a 1:10 freshly made solution of Fe(CO)₅: hexanes (Ar-purged) was injected with a syringe. Temperature was then increased to and kept at 200ºC for 1 h, before turning off the hotplate and pulling up the flask off the heating mantle. After the flask cooled down to room temperature, the component was transferred to a 1 L volumetric flask, to which 400 mL iso-propanol was also added. The precipitate was separated by centrifugation (3400 r/min for 1 h), and subsequently was mixed with a solution of 25% hexanes and 75% ethanol, and centrifuged. This process was repeated three times. Temperature treatment at 185ºC was performed following overnight drying at room temperature. The temperature of 185ºC was chosen following the previous study by Li et al.[16]. Carbon/Pt cube composites made with Vulcan and Ketjen after 5.5 h at 185ºC are denoted Pt_Cube5.5/K and Pt_Cube5.5/V, respectively. Two other Pt_Cube/K composites were also made similarly, with 10 (Pt_Cube10/K) and 24 h (Pt_Cube24/K) at 185ºC. Mass of the final material was ~1 g, demonstrating that the conversion of Pt(II) to Pt had been nearly complete. This was further confirmed by thermal gravimetric analysis that yielded ~50% Pt content. A similar procedure was performed (without adding carbon) to prepare pure cube-like Pt particles (Pt_Cube).

Transmission electron microscopy

A Zeiss Libra 200 MC high resolution transmission electron microscope (HR-TEM) was used to study the morphology of the catalysts. The TEM images were captured at 200 kV by a TRS Sharpeye camera at 2048 × 2048 pixels. Samples were prepared by sonicating (30 min) proper amounts of catalyst in hexane. After that, ~10 µL of the homogenous suspension was dropped on a copper grid and dried at room temperature.

Electrochemical measurements

A 760E CH Instrument workstation through CHI software was used to perform the electrochemical measurements in a conventional three-electrode glass cell. A glass three column cell was used for the measurements, where the reference electrode column was connected to the working electrode column via a lug in capillary, to minimize contamination while maintaining a good flow of liquid through. Electrochemical impedance spectroscopy was carried out over the frequency range of 100 kHz–0.1 Hz using an AC amplitude of 10 mV. Glassy carbon disc electrodes (GC; CH Instruments; 0.071 cm²), saturated calomel electrode (SCE; Fisher), and a graphite rod formed the working (WE), reference (RE), and counter electrodes (CE), respectively. Catalyst inks were made with sonicating carbon/Pt with 1.6 g H₂O and 0.4 g iso-propanol containing 20 µL 5% Nafion for 30 min. The catalyst inks were drop coated on the GC electrodes using a micropipette to give ~7.7 µg(Pt)/cm² loadings. All electrochemical measurements were conducted following 25 consecutive scans between-0.25 and+1.05 V_SCE in 0.5 mol/L H₂SO₄ for activation until a stable voltammetric pattern was obtained. Forformic acid (FA) oxidation, the WE was activated as described, then FA was injected into the cell at open circuit while the WE was maintained inside the solution. All electrochemical experiments were performed at room temperature under a nitrogen atmosphere following purging for 15 min.

Results and discussion

Figure 1 shows transmission electron micrographs (TEM) of Pt_Cube, Pt_Cube/V and Pt_Cube/K composites and that for a TKK Pt/C as well. The Pt_Cube (A) displays cube-like particles of ~ 6–7 nm. A mixture of complete and truncated cubes is observed for the Pt_Cube5.5/V (B, C) and Pt_Cube5.5/K (D, E) composites. Comparing histograms for the Pt_Cube5.5/V (C; inset) and Pt_Cube5.5/K (E; inset) show somewhat smaller particles for the Pt_Cube5.5/K composite (~5 nm, versus 6 nm for Pt_Cube5.5/V), while both composites have significantly larger particles than the TKK Pt/C (1–3 nm, I, inset). The TEM images of Pt_Cube10/K (F, G), and Pt_Cube24/K (H) are quite different. First, a broader spectrum of particles, with more diverse shapes and sizes can be seen on both. Interestingly, the Pt_Cube24/K particles (H) are rather parallelogram-like and not cubic. Inspection of the particles on the micrographs of the Pt_Cube10/K (F, G) reveals that the parallelogram-like structures seen for Pt_Cube24/K (H) may have been formed via merging of the neighboring cubic particles over the prolonged exposure to the high temperature. In fact, the TEM images of Pt_Cube10/K(F, G) support the assumption, as it appears that adjacent particles appear to be merging.

Potentio dynamic profiles (50 mV/s) of the Pt_Cube5.5/V and Pt_Cube5.5/K composites in 0.5 mol/L H₂SO₄ are shown in Fig. 2. The H signals at Pt dominate the voltammograms below ~0.1 V_SCE over both catalysts, with H adsorption (H_Ads) and desorption (H_Des) on the cathodic and anodic sweeps, respectively. The oxide formation/stripping region of the Pt_Cube/Kelectrode seems to locate at less positive potentials than Pt_Cube/V. Further, the Pt_Cube5.5/K electrode shows stronger signals on both H and oxide regions. Another notable difference can be observed within 0.1 to 0.45V_SCE, where Pt_Cube/K composite shows a larger charging current. By integrating the area under the 0.1 to 0.45V_SCE range (i.e., charging current), charges equal to ca. 0.18 and 0.70 mC were estimated for the Pt_Cube5.5/V and Pt_Cube5.5/K composites, further approximating voltammetric capacitance values of ca.0.05 and 0.20 mF, respectively.

Figure 3 shows voltammograms at 50 mV/s (after the CV activation) for the Pt_Cube/K composites treated at 185ºC for 5.5, 10, and 24 h. For comparison, voltammetry of an as-prepared Pt_Cube/K (i.e., untreated) is also included. The overall shapes of the voltammograms are similar, whereas outstanding differences are observed. The charging current region for Pt_Cube10/K is ~2 µA larger than that of Pt_Cube5.5/K, while it diminished dramatically (~6–7 µA) over continuing the treatment to 24 h, Pt_Cube24/K. The H and oxide signals have changed similarly over the temperature treatment, with the Pt_Cube10/K showing the most pronounced signals. The untreated Pt_Cube/K catalyst displays a quite poor electrochemistry.

Table 1 summarizes the electrochemical active surface area (EASA) values for the Pt_Cube/V and Pt_Cube/K composites obtained from their H adsorption (cathodic sweep) and desorption (anodic sweep) twin signals, located typically below 0.1V_SCE. The data were corrected for the background current, and the EASA values were estimated assuming 0.210 mC/cm² for one electron transfer over adsorption/desorption of a single H atom at Pt [30,31]. As shown in Table 1, the largest EASA values were obtained for the Pt_Cube10/K electrode, which was also significantly higher than those for the Pt_Cube/V composite (also see Fig. 2).

Electrochemical impedance spectroscopy (EIS) was conducted to further explore the electrochemical characteristics of the Pt_Cube/K composites. Here only data at 0.15 V_SCE are considered, because Faradaic events at Pt over the double layer window (i.e., 0.1 to 0.2 V) are insignificant, hence such potential would be suitable to obtain insights into the intrinsic electron/ion transport properties of the catalysts [32]. Figure 4 shows Nyquist plots (i.e., real vs. imaginary impedance; main panel) for the Pt_Cube/K composites at 0.15 V_SCE in 0.5 mol/L H₂SO₄. The EIS experiments were performed following 30 s conditioning at the DC offset potential (0.15 V_SCE), subsequent to the voltammetric activation at 50 mV/s (see section 2.4). Further, the Nyquist data are corrected for the uncompensated resistance of each set (i.e.,~5–6 Ω).

An incomplete semicircle at high frequencies (100 to ~10 kHz) is observed for all the Pt_Cube/K composites, followed by small Warburg-like features in mid frequencies (~ down to 1 kHz). A non-ideal capacitive response dominates the lower frequency impedance, where a slightly curved profile is noticeable. In fact, an ideal capacitive impedance at low frequencies would be a vertical line with no drift to larger real Z values. This can more pronouncedly be observed at the series capacitance plots (see Fig. 4, inset). Such drift has been attributed to the inhomogeneous current distribution on porous electrodes, also known as leak current effect or DC back ground resistance [33,34]. Nevertheless, to obtain numerical insights, the impedance data were modelled using a commercial software based on a non-linear least square algorithm (Zview 2- Scribner Inc.). The equivalent circuit shown in Fig. 4 (inset) was used for fitting the impedance data, whereR_u represents the uncompensated resistance (mostly solution resistance, also known as equivalent series resistance ESR), while impedance of the composites at high frequency was modelled with a constant phase element (CPE_DL) in parallel with a charge transfer resistance (R_CT). CPE_DL and R_CT represent double layer capacitance and the resistance associated with interfacial redox processes, respectively. An open Warburg element (W_O; semi infinite diffusion) was required to properly model the mid frequency impedance. Even though the Warburg-like features were minor, best fits were obtained only when such element was included in the circuit. The CPE_F and R_DC represent the catalytic film pseudocapacitance and DC background resistance, respectively. Modelling the low frequencies (<1 Hz) was problematic and required using very highR_DC values to obtain acceptably good fits. Due to this problem, and that the low frequency DC background resistance has a limited importance in this context, the modelling results are discussed only over the high to mid frequencies.

Table 2 lists R_CT, CPE_DL, and W_R values estimated from modelling of the EIS at Pt_Cube/K composites (5.5, 10, and 24 h). An R_CT value of ~6.5 Ω was estimated for Pt_Cube10/K, which is lower than that for both the Pt_Cube5.5/K and Pt_Cube24/K electrodes. The high frequency capacitance calculated from CPE_DL gave similar numbers for all the three composites, while that for the Pt_Cube10/K electrode was slightly larger. Similarly, while quite small W_R values were obtained for all the three Pt_Cube/K composites, the W_R for Pt_Cube10/K was slightly smaller (0.09 vs. 0.11 Ω for the other two Pt_Cube/K composites). Nevertheless, except for the R_CT that showed a convincing difference between the Pt_Cube10/K (giving the lowest) and the other two Pt_Cube/K composites, modelling the high-mid frequencies gave rise to values that were only slightly different, which could be partially originated from experimental and/or fitting inaccuracies.

While we postpone further discussion of the low frequency impedance to future, one distinctive difference between the EIS responses of the Pt_Cube/K composites can actually be found in this frequency range. Figure 4 (inset) shows the series capacitance plots (C_Series) of the Pt_Cube/K composites, overlaid with that of the Pt_Cube5.5/V electrode for comparison. Inversely proportional to the imaginary impedance (i.e.,C_Series = 1/(2pfZ_Image), where f is the frequency), such C_Series plots provide two useful information ① visual comparison of the W_R, which can be envisaged from the slope of the rising portion (i.e., mid frequencies); ② the low frequency plateau that gives the overall capacitance of the films. Here, theW_R region seems quite vertical for all, indicating insignificant ionic/electronic resistances of the composites. This observation is consistent with the modelling results in Table 2, where very smallW_R values were estimated for all the Pt_Cube/K composites. Nevertheless, the limiting capacitance for Pt_Cube10/K is ~0.36 mF, which is twice that of Pt_Cube24/K (0.18 mF), and remarkably larger than that for the Pt_Cube5.5/K electrode (0.22 mA). For comparison, the maximum capacitance (i.e., at 0.1 Hz) for an as-prepared Pt_Cube/K was calculated to be ~14.3 µF (EIS data not shown), which is over 10 times smaller than that for Pt_Cube24/K.

Figure 5 shows cyclic voltammograms (50 mV/s) of the TKK Pt/C (46.6%), JM Pt/C, and a Pt_Cube10/K in 0.5 mol/L H₂SO₄. The H and oxide formation/stripping electrochemistry of the TKK Pt/C are considerably more pronounced than the other two electrodes. The TKK Pt/C and Pt_Cube10/K catalysts display similar background currents, both larger than that for the JMPt/C. The EASA (based on the H_Ads and H_Des signals) for the TKK catalyst are 120.8 and 122.7 m²/g, respectively, fairly higher than an earlier report (83 m²/g) [35]. Assuming complete spheres of 4 nm and 30% empty spaces, a theoretical EASA of ~492.7 m²/g (Pt loading ~7.7×10⁻⁶ g/cm²) would be estimated for the TKK, which gives an average catalytic utilization of ~25%. Using the data in Table 1, the catalytic utilization for the Pt_Cube10/K electrode can be estimated as ~35%. Even though the EASA of the TKK is higher (obviously the larger H signals) than both the JM and Pt_Cube10/K, the catalytic utilization of Pt_Cube10/K showed the largest catalytic utilization.

Figure 6 (a) (raw data) and (b)(EASA normalized) show the linear sweep voltammograms for formic acid (FA) oxidation at Pt_Cube/V and Pt_Cube/K composites compared with the TKK (46.6%) and JM (60%) Pt/C catalysts in 0.5 mol/L H₂SO₄ containing 0.5 mol/L FA. The potentiodynamic FA oxidation at Pt is known to produce two peaks at low and high potentials [36], which here are centered at ~0.2–0.3 and ~0.6 V_SCE, respectively. The earlier peak is assigned to the direct pathway of FA oxidation (HCO₂H → H₂ + CO₂), whereas the second peak is attributed to the oxidative removal of the adsorbates (mainly CO) formed over the dehydration of FA (HCO₂H → H₂O+ -CO_Ads; -CO_Ads+ -OH_Ads → CO₂ + H⁺) [36–38]. The Pt_Cube10/K electrode is significantly superior to the TKK Pt/C and all other electrodes at both potential ranges. As well, consistent with the voltammetric profiles in Fig. 2, Pt_Cube5.5/K is more active than Pt_Cube5.5/V, indicating that the same procedure on different carbon supports would lead to quite different EASA values, and differently active FA oxidation catalysts. That the Pt_Cube10/K is remarkably superior to Pt_Cube5.5/K indicates that the temperature treatment had greatly influenced the electrochemical properties of the resulting composites. Such increase in EASA has been attributed to temperature assisted removal of the organic surfactants, which would increase the number of clean reaction sites [16]. It is also notable that the Pt_Cube10/K with a larger particle size and lower EASA is more active than the TKKPt/C. In fact, the catalytic utilization of PtCube10/K is the only electrochemical evidence for its superiority to the TKK Pt/C.

Nevertheless, the formic acid (FA) oxidation activities of the Pt_Cube/K composites closely track their relative EASA and catalytic utilization values. The voltammetric and impedance studies demonstrated that the temperature treatment had a large effect on the electrochemical responses of the Pt_Cube/K composites. Wang et al. obtained similar effects for pure Pt nanoparticles [4,16]. Here, the EASA increased with heat treatment initially (up to 10 h; Pt_Cube10/K), whereas further residence at 185ºC produced the Pt_Cube24/K composite that had an EASA smaller than Pt_Cube5.5/K yet larger than the untreated Pt_Cube/K. The increased EASA should likely be due to temperature-assisted removal of organic surfactants [16]. This can only elucidate the changes from the untreated Pt_Cube/K to the Pt_Cube10/K, whereas the impedance data showed a greatly reduced capacitance for the Pt_Cube24/K. Such decrease can also be understood comparing the double layer window of the CVs in Fig. 3, which cannot be due to any loss of the active materials for two reasons: first, 185ºC is not adequately high to burn out carbon effectively, considering that the limiting capacitance decreased from 10 to 24 h; second, monitoring the mass over the 24 h treatment showed that no observable weight loss occurred after the initial 4–5 h. Therefore, the change in performance could only be addressed by structural and/or morphological changes.

Figure 7 shows moderate term EASA-normalized FA oxidation at 0.2 V_SCE (t = 1000 s) over the Pt_Cube10/K and TKK Pt/C catalysts to obtain preliminary evaluation of their catalytic durability. The 0.2V_SCE was chosen as it nearly coincides the first voltammetric FA oxidation peak in Fig. 5. After the initial stages where current rapidly decreased on both catalysts, quite stable patterns were recorded afterwards, as expected for FA oxidation at Pt [37,38]. The rapidly decreasing current over the initial 100 s should be due to the formation of the blocking adsorbates (mainly-CO), over the incomplete oxidation (i.e., dehydration) of formic acid [38,39]. Nevertheless, consistent with the voltammetric results in Fig. 6, it is noteworthy that the Pt_Cube10/Kelectrode showed a greater potentiostatic FA oxidation activity, while the potentiostatic patterns of the both catalysts were similarly stable.

Conclusions

A one-pot facile method is described for the direct synthesis of carbon/cube-like Pt (Pt_Cube) composites with Vulcan XC-72 (i.e., Pt_Cube/V) and Ketjen black (i.e., Pt_Cube/K), respectively. The Pt_Cube/K treated at 185ºC for 5.5 h (Pt_Cube5.5/K) had a remarkably larger electrochemical active surface area (EASA) and better formic acid (FA) oxidation activity than Pt_Cube5.5/V. The Pt_Cube/K composite was also prepared with treatment at 185ºC for 10 and 24 h, where Pt_Cube10/K appeared to yield the best performance, i.e., superior H and oxide electrochemistry, and higher FA oxidation activity compared to the untreated Pt_Cube/K, Pt_Cube5.5/K, and Pt_Cube24/K composites. Impedance spectroscopy was invoked to further investigate the voltammetric differences, where lower resistances and a larger capacitance were recorded for Pt_Cube10/K. Further, the Pt_Cube10/K electrode produced a remarkably better FA oxidation activity than the commercial TKK and JMPt/C catalysts. The EASA of Pt_Cube10/K was similar to JM Pt/C and lower than TKKPt/C, however, the catalytic utilization (EASA over theoretical active area) of Pt_Cube10/K was higher (35%) than both TKKPt/C (25%) and JMPt/C (14%). Interestingly, the FA oxidation activities of the catalysts explored in this work rightly followed their relative catalytic utilization values, but not their EASA. It appears that the catalytic utilization may be a more accurate measure to predict FA oxidation activities of catalysts.

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