Controlled synthesis of high metal-loading, Pt-based electrocatalysts with enhanced activity and durability toward oxygen reduction reaction

Abstract

High metal-loading Pt/C electrocatalysts are much sought after for the fabrication of thin-layered membrane electrode assemblies (MEA) suitable for mass transport and balance control of water and heat. However, such electrocatalysts are still limited to commercial Pt/C, comprised of carbon-supported, 2–5 nm Pt particles without size and size uniformity control, causing poor activity and durability toward oxygen reduction reaction (ORR). Herein, we report the controlled synthesis of 59.9 wt% Pt/C by using metallic ion-containing reversed micelles adsorbed on carbon. Combined with a simple surfactant removal method of washing with water, this unique confining approach enables easy control over the size and size uniformity of the resultant Pt/C, leading to significantly improved activity and durability for ORR compared with commercial 60 wt% Pt/C. The ORR activity enhancement arises from a smaller average size of Pt nanoparticles in combination with a narrower size distribution, with more Pt nanoparticles falling in an optimal size range of 2–4 nm with the highest ORR activity. The narrower size distribution of our Pt/C also makes the Pt nanoparticles resistant to the dissolution–re-deposition Ostwald ripening process, in which small Pt nanoparticles gradually shrink until they disappear, while large ones become bigger and bigger.

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Introduction

Worldwide intensive studies on proton exchange membrane fuel cells (PEMFCs) during the past two decades have brought this clean and efficient energy conversion technology close to the pre-commercialization stage for automotive applications.¹ A crucial component of PEMFCs is the Pt/C electrocatalyst for ORR at the cathode. However, the limited ORR activity and durability of Pt/C has become the main bottleneck, preventing PEMFC from widespread application.²

In order to improve the ORR activity and durability of Pt/C, two strategies have been investigated. One promising, yet long-term, solution is to replace noble metals with cheap and abundant counterparts while retaining or surpassing the overall performance of Pt/C. In this regard, significant progress in non-noble metal electrocatalysts has been made, especially for those applied in alkaline media.³ For example, Chung and Zelenay reported a novel nitrogen-doped carbon nanotube/nanoparticle composite ORR electrocatalyst obtained from pyrolysis of iron acetate as an iron precursor and cyanamide as a nitrogen and carbon nanotube precursor. When used at a sufficiently high loading, the composite outperforms commercial 20 wt% Pt/C at 60 μg_Pt cm⁻² in an alkaline solution.^3a Dai and his co-workers designed a class of 2D covalent organic polymers with precisely controlled locations of nitrogen heteroatoms and pore sizes, followed by complexation with metal and carbonization of the metal-incorporated materials, which exhibited a similar onset potential as that of the Pt/C, both with a loading of 0.2 mg cm⁻².^3f The other near-term and more realistic resolution is to develop novel platinum-based electrocatalysts,⁴ such as PtCo/C,⁵ PtNi/C,⁶ PtAu/C,⁷ and so on.⁸ Recently, Yang and Stamenkovic created an intriguing Pt₃Ni nanoframe electrocatalyst by leaching out nickel from platinum–nickel bimetallic nanocrystals. Compared with commercial Pt/C, this nanoframe electrocatalyst remarkably achieved an enhancement factor of 36 in mass activity and 22 in specific activity toward ORR.^4a Wang and Abruña reported ordered intermetallic platinum–cobalt core–shell nanoparticles with over 200% increase in ORR mass activity and over 300% increase in ORR specific activity relative to commercial Pt/C.⁵ Lou and his coworkers described a novel 3D Pt nanoassembly consisting of 1D single crystalline Pt nanowires with an excellent resistance to dissolution, migration, Ostwald ripening, and aggregation because of the 1D structural characteristics.^8a In particular, it is worth pointing out that high metal-loading Pt/C electrocatalysts (around or over 50 wt%) allow the fabrication of thin-layered MEA, which is highly desired for the purpose of improved Pt utilization efficiency and the balance control of water and heat.⁹ However, such electrocatalysts are still limited to commercial Pt/C comprised of carbon-supported Pt particles without size and size uniformity control, causing poor activity and durability toward ORR.¹⁰

Herein, we report the controlled synthesis of 59.9 wt% Pt/C by using metallic ion-containing reversed micelles adsorbed on carbon.¹¹ The key to the controlled synthesis is that reduction of the platinum complex is confined in the separate micellar interior localized on the carbon, preventing individual platinum nanoparticles from growing into large ones and agglomerating. In addition, we developed a simple purification procedure to effectively remove the micelles by washing multiple times with hot water while keeping platinum nanoparticles intact. This unique synthetic approach enables easy control over the size and size uniformity of the resultant electrocatalysts at a high metal loading, leading to significantly improved activity and durability for ORR compared with commercial 60 wt% Pt/C. The improved ORR activity arises from a smaller average size of Pt nanoparticles supported on carbon in combination with a narrower size distribution. In this scenario, there are many more Pt nanoparticles falling in an optimal size range of 2–4 nm, which is well known to possess the highest ORR activity. The narrower size distribution of our Pt/C also makes the Pt nanoparticles resistant to the dissolution–re-deposition Ostwald ripening process, in which small Pt nanoparticles gradually shrink until they disappear, while large ones get bigger and bigger.¹² With a narrower size distribution, the size difference among individual Pt nanoparticles is relatively small, and thus the dissolution and re-deposition process of Pt occurs evenly upon each Pt nanoparticle, accordingly slowing down the Ostwald ripening process.

Experimental

Chemicals

Potassium tetrachloroplatinate (K₂PtCl₄, 99 wt%), potassium chloropalladate (K₂PdCl₄, 99 wt%), sodium tetrachloroaurate (NaAuCl₄, 99 wt%), potassium pentachlororuthenate (K₂RuCl₅, 99 wt%), and sodium hexachloroiridate (Na₂IrCl₆, 99 wt%) were purchased from Fengchuan Chemical Reagent Co. (Tianjin, China) and used as received. Cetyltrimethylammonium bromide (CTAB, 99%) (Sigma-Aldrich), chloroform (Sinopharm Chemical Reagent), and sodium borohydride (Acros organics, 99%) were used as received. All aqueous solutions were prepared with ultrapure water (18.2 MΩ cm at 25 °C) produced from a Millipore water system (Synergy® UV, France). The stock solution of platinum(II) complexes (20 mM) was prepared by dissolving K₂PtCl₄ in water and allowing it to age for at least 24 h before use. Aging the stock solution disproportionates the complex into an equilibrium mixture of 42% Pt(H₂O)₂Cl₂, 53% Pt(H₂O)Cl₃⁻, and 5% PtCl₄²⁻.

Synthesis of Pt/C with different metal loading

A specific amount of VXC-72 or EC-600 carbon black was added to 10 mL of chloroform containing 40 mM CTAB with the aid of sonication. The suspension turned black, and no black precipitates were observed, which suggested good dispersion of carbon black in chloroform. Then, 10 mL of 20 mM aged K₂PtCl₄ aqueous solution was mixed with the carbon-containing chloroform. The synthetic system was left stirring under ambient conditions for 1 h to guarantee the complete transfer of the metallic complexes to the micelle-containing chloroform phase. The mixture was transferred to a 250 mL glass bottle, and 90 mL of nanopure water and 10 mL of 300 mM sodium borohydride were added under stirring at a speed of 1600 rpm. Bubbles were observed during the reaction process. Finally, we purified the electrocatalyst simply by washing with a copious amount of water at 90–100 °C.

K₂PtCl₄ can be replaced by K₂PdCl₄, NaAuCl₄, K₃FeCl₆, and K₂CoCl₄ to thus synthesize a variety of electrocatalysts, including PtPd/C, PtFe/C, PtCo/C, and Pt₃PdAu/C.

Instrumentation

TEM (JEOL JEM-2100, 200 keV) was performed on the obtained platinum nanomaterials. The samples for electron microscopy analysis were prepared by adding drops of colloidal suspensions onto a standard copper grid and absorbing the excess liquid with a filter paper. The grids were air-dried for at least 2 h before imaging. X-ray diffraction (XRD) spectra (PANalytical PW3040/60 X′Pert PRO) were recorded using a Cu Kα radiation source (λ = 0.15432 nm) in θ–2θ scan mode at a scanning rate of 5° min⁻¹. Thermogravimetry (TG) analysis was performed (SDT Q600) with dried air as the processing atmosphere and a heating rate of 10 °C per minute. UV-visible spectra were obtained with a spectrophotometer (Analytic Jena, Specord S600) and a 0.2 cm path length quartz cell.

Electrochemical measurements

An Autolab potentiostat/galvanostat (Echo Chemie BV Model PGSTAT-302N, Netherlands) was used for the electrochemical measurements in a conventional three-electrode electrochemical cell installed with platinum mesh as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, connected to the cell by a salt bridge (agar gel containing saturated KNO₃). All potentials in this study refer to the reversible hydrogen electrode (RHE). All electrochemical measurements were performed at 25 °C.

Specific amounts of water, ethanol, and Nafion solution (5 wt% Dupont) (1 : 9 : 0.06) were mixed with an electrocatalyst and sonicated in a water bath for 20 min to obtain catalyst ink (1 mg mL⁻¹). Glassy carbon rotating disk electrodes (RDE, 0.196 cm², Pine Instruments, USA) were polished with 0.05 μm Bio-Analytical Systems (BAS) alumina paste and purged with N₂ prior to deposition of catalyst ink. The catalyst ink (10 μL) was transferred onto the glassy carbon RDE, and then the solvent was evaporated in air. The metal loading is 30.6 and 30.0 μg_Pt cm⁻² for commercial 60 wt% Pt/C and 59.9 wt% Pt/C, respectively. Potential cycling (100 mV s⁻¹) between 0 and 1.2 V was applied to the electrodes to obtain stable and repeatable cyclic voltammetry (CV) curves in N₂-purged aqueous HClO₄ (0.1 M). The electrochemically active surface area (ECSA) of the catalysts was determined by integrating the hydrogen desorption area between 50 and 460 mV of CV curves obtained at a scan rate of 50 mV s⁻¹ (assuming 210 μC cm_Pt⁻² for the desorption of a monolayer of hydrogen).

The ORR polarization curve of the catalysts was recorded using catalyst-coated RDE in O₂-saturated HClO₄ (0.1 M) aqueous solution with a positive sweep rate of 10 mV s⁻¹ at 1600 rpm. Current densities in the ORR polarization curves were normalized to the geometric area of the glassy carbon RDE. Mass activity (MA) and specific activity (SA) were determined based on ORR polarization curves.

For the accelerated durability test (ADT), the same electrocatalyst-coated glassy carbon RDEs were used as the working electrode. The electrodes were cycled between 0.6 and 1.2 V for a total of 2500 cycles in O₂-saturated HClO₄ (0.1 M). The scan rate was kept at 100 mV s⁻¹ during the process of potential cycling. Meanwhile, CV curves and ORR polarization curves were collected in aqueous HClO₄ solution (0.1 M) at particular cycles to track the degradation of electrocatalysts.

Results and discussion

In a typical synthesis, CTAB was dissolved in chloroform, forming reversed micelles via a molecular self-assembly process that we investigated previously.¹³ Initially, carbon black powder (VXC-72 or EC-600) was added with the aid of mild sonication in a water bath to disperse carbon thoroughly and to ensure the adsorption of reversed micelles (Fig. 1). Next, K₂PtCl₄ aqueous solution was placed in the mixture. The electrostatic interaction between positively charged CTA⁺ and negatively charged metallic PtCl₄²⁻ complexes drives metallic complexes to automatically transfer from the aqueous phase to the interior of reversed micelles in chloroform. The UV-visible spectrum of the supernatant obtained after phase transfer (Fig. S1†) shows no characteristic peaks of Pt(II) complexes, verifying that the transfer of PtCl₄²⁻ is complete. NaBH₄ aq. was then added to reduce metallic complexes confined in the micellar interior. Lastly, the obtained electrocatalyst was simply purified by washing with a copious amount of water at 90–100 °C.

Fig. 1 Schematic illustration of the synthesis and purification of electrocatalysts using metallic ion-containing reversed micelles adsorbed on carbon.

To demonstrate the efficacy of this approach and the significance of the high metal-loading electrocatalyst, we synthesized 60 wt% Pt/C electrocatalyst. After purification, the metal loading of the electrocatalyst was determined to be 59.9 wt% according to thermogravimetry analysis (TGA) as shown in Fig. S2.† Transmission electron microscopy (TEM) images reveal that all platinum nanostructures are well dispersed on carbon, without free-standing particles (Fig. 2a), suggesting the effective adsorption of reversed micelles and subsequent nucleation and growth of platinum on carbon. Platinum nanoparticles are evenly distributed on carbon without large agglomerations that apparently occurred in commercial 60 wt% Pt/C (Fig. 2d). The average diameter of the platinum nanoparticles of our 59.9 wt% Pt/C is 3.6 ± 0.7 nm, with a size distribution of 20.4%. In contrast, commercial 60 wt% Pt/C has a relatively large average diameter of 4.4 ± 1.3 nm with a much wider size distribution of 29.9% (Fig. 2d). It is worth pointing out that the high surface area of carbon (EC-600, 1270 m² g⁻¹) is a prerequisite for the synthesis of Pt/C with high metal loading. If the surface area of carbon is low, there would not be enough nucleation sites available for platinum, causing unsupported platinum nanoparticles.

Fig. 2 TEM images of Pt/C electrocatalysts synthesized by using metallic ion-containing reversed micelles at different metal loadings: (a) 59.9 wt%, (b) 38.1 wt%, (c) 18.6 wt%; commercial Pt/C at similar metal loadings: (d) 60 wt%, (e) 40 wt%, and (f) 20 wt%. (Insets: average diameters were measured for over 300 nanostructures, and their frequencies are plotted in the inset graphs. The average size and the standard deviations are given in the plots, along with the percentage ratio of the standard deviation to average size.)

By varying the ratio between platinum precursor and carbon, we can easily manipulate the metal loading of Pt/C electrocatalysts roughly from 20 to 60 wt% (Fig. S2†). As the metal loading decreases to 38.1 and 18.6 wt%, the average size of our Pt/C slightly changes to 3.1 ± 0.6 and 2.6 ± 0.5 nm, respectively. Meanwhile, the size distribution remains almost unchanged (18.9% and 18.6%) (Fig. 2b and c), further verifying the successful control over size distribution. In contrast, the average size of commercial 40 and 20 wt% Pt/C also decreases a little to 3.9 ± 1.3 and 3.1 ± 0.8 nm, respectively. However, the corresponding average size and size distribution are much larger than that of our Pt/C at a similar metal loading (Fig. 2e and f). This clearly shows the successful control of our approach over the electrocatalyst size and size uniformity in a wide range of metal loadings. This control originates from the confining effect of reversed micelles.

To confirm the size control over Pt/C at varied metal loadings, the electrocatalysts were subjected to XRD measurements (Fig. S3†). The broadened peaks at 39.6°, 46.2° and 67.8° originate from the (111), (200), (220) diffraction, respectively, of face-centred cubic (fcc) platinum. The Scherrer equation was used to calculate the average crystalline size based on full width at half maximum (FWHM) of the (220) peak for each sample. The as-determined crystalline size of our Pt/C increases from 2.3 to 2.7 to 2.8 nm with the increase in metal loading, analogous to the sizes determined by TEM. The overall size incremental is merely 0.5 nm, clearly exemplifying the successful size control due to micellar confining. In contrast, the size of commercial Pt/C changes from 2.5 to 3.3 to 3.8 nm as metal loading increases, and the total size variation is 1.3 nm (Fig. S4†). According to the XRD data, the average size of our electrocatalyst appears to be smaller at each metal loading than that of the commercial one, consistent with our TEM results, as shown in Fig. 2.

Furthermore, the phase-transferable metallic complexes have been extended to PdCl₄²⁻, AuCl₄⁻, FeCl₆³⁻, and CoCl₄²⁻, as shown in Fig. S5.† This makes our approach more general and allows us to easily manipulate the composition of electrocatalysts by combining the above-mentioned two or more metallic complexes at certain molar ratios. We synthesized a variety of binary and ternary electrocatalysts, such as PtPd/C and PtPdAu/C. Fig. S6† is the TEM image and EDX spectrum of PtPd/C; all PtPd nanostructures are well dispersed on carbon without free-standing particles.

For its applications, the surface of electrocatalysts is required to be clean and thus accessible for reactants, so CTAB molecules and other by-products need to be removed. We chose 59.9 wt% Pt/C to investigate the purification process and the electrochemical performance. The electrocatalyst was purified simply by washing with a copious amount of water at 90–100 °C, different from the surfactant burning method that frequently causes size increase and shape modification.¹⁴ CV was used to detect the surface cleanness of the purified electrocatalyst. The initial 10 consecutive CV curves of Pt/C were recorded at 25 °C in N₂-purged 0.1 M HClO₄ aq. with a scan rate of 100 mV s⁻¹ using a thin-film glassy carbon RDE. Fig. S7a† shows that the hydrogen adsorption and desorption peak of 59.9 wt% Pt/C appeared in the first cycle and remained unchanged during the following cycles. This suggests that the surface of the electrocatalyst is clean. Otherwise, the hydrogen adsorption/desorption peak would not appear initially, or the peak would evolve with the removal of species by electrochemical oxidation. For comparison, CV curves (Fig. S7b†) were also collected for commercial 60 wt% Pt/C. Similar to 59.9 wt% Pt/C; hydrogen adsorption/desorption peak was observed right at the beginning of the CV test, and it did not change during the whole process. This indicates that our purified 59.9 wt% Pt/C is as clean as commercial 60 wt% Pt/C.

Fig. 3a shows ORR polarization curves of 59.9 wt% Pt/C and commercial 60 wt% Pt/C obtained at 25 °C in O₂-saturated HClO₄ (0.1 M) using a thin-film glassy carbon RDE at 1600 rpm. The half-wave potential (E_1/2) of 59.9 wt% Pt/C is 8.4 mV more positive than that of commercial 60 wt% Pt/C. This suggests that our Pt/C is much more electrocatalytically active than commercial 60 wt% Pt/C. The mass activity (MA) of 59.9 wt% Pt/C is 232.7 mA mg_Pt⁻¹ at 0.9 V (vs. RHE), which is 1.31 times that of commercial 60 wt% Pt/C (178.1 mA mg_Pt⁻¹) (Fig. 3b), and the specific activity (SA) for 59.9 wt% Pt/C is 308.6 mA cm_Pt⁻², which is 51% higher than that of commercial 60 wt% Pt/C (204.9 mA cm_Pt⁻²) (Fig. 3c). The much enhanced MA and SA of our Pt/C can be attributed to its small average size of 3.6 nm and its relatively narrow size distribution of 20.4%. It is well known that 2–4 nm platinum nanoparticles possess the highest ORR activity when considering both ECSA and SA.¹⁵ Unlike commercial Pt/C (with an average size of 4.4 nm with a wider size distribution of 29.9%), there are more platinum nanoparticles for our Pt/C falling in the optimum size range, thus leading to the much-enhanced ORR activity.

Fig. 3 (a) ORR polarization curves measured in O₂-saturated 0.1 M HClO₄ with a positive sweep rate of 10 mV s⁻¹ at 1600 rpm (25 °C), (b) MA at 0.9 V (vs. RHE), and (c) SA at 0.9 V (vs. RHE) for 59.9 wt% Pt/C (red line) and commercial 60 wt% Pt/C (black line), their corresponding normalized ECSA values (d), and their corresponding MA values (e) recorded during the process of potential cycling (0.6–1.2 V vs. RHE, 100 mV s⁻¹, 0.1 M O₂-purged HClO₄).

Durability is also of special importance for practical applications.¹⁶ ADT over 59.9 wt% Pt/C and commercial 60 wt% Pt/C was performed by potential cycling between 0.6 and 1.2 V (vs. RHE) at a sweep rate of 100 mV s⁻¹ in O₂-purged HClO₄ solution (0.1 M). As shown in Fig. S8,† the CV curves of our Pt/C and commercial Pt/C shrink with increased potential cycles. After the ADT test, the ECSA loss of Pt/C is 48.6%, which is lower than that of the commercial one (68.1%) (Fig. 3d). The ORR curve of both electrocatalysts shift negatively during the ADT test. Interestingly, the degradation rate of our Pt/C is much slower than that of the commercial one as shown in Fig. 3e. After the ADT test, the MA value of our Pt/C (127.6 mA mg_Pt⁻¹) is 2.27 times of that of commercial Pt/C (56.2 mA mg_Pt⁻¹).

In order to elucidate the origin of the improved durability, we investigated the morphological changes by TEM before and after ADT (Fig. 4a–d). The size of Pt nanoparticles in commercial Pt/C increased from 4–5 nm to 30–60 nm because of the Pt dissolution–redeposition Ostwald ripening.¹⁷ In contrast, the size alteration of our Pt/C is much smaller after ADT (3–4 nm to 10–15 nm); this is likely related to the narrower size distribution, which is inherently resistant to Ostwald ripening, which drives small nanoparticles gradually to become smaller and large ones to become larger via the platinum dissolution and redeposition process. Similarly, 18.6 and 38.1 wt% Pt/C also show an improved durability compared with 20 and 40 wt% commercial Pt/C (Fig. S9†), due to the narrow size distribution resistant to Ostwald ripening. The size changes of 18.6 and 38.1 wt% Pt/C nanoparticles during ADT are also smaller than that of their corresponding commercial counterparts (Fig. 4e–l).

Fig. 4 TEM images of 60 wt% commercial Pt/C, 59.9 wt% Pt/C, 40 wt% commercial Pt/C, 38.1 wt% Pt/C, 20 wt% commercial Pt/C and 18.6 wt% Pt/C before (a, c, e, g, i and k) and after (b, d, f, h, j and l) accelerated durability test (0.6–1.2 V vs. RHE, 100 mV s⁻¹, 0.1 M O₂-purged HClO₄).

Conclusion

In conclusion, we have demonstrated a controllable synthetic approach to the creation of platinum-based electrocatalysts, especially at a high metal loading, by using metallic ion-containing reversed micelles adsorbed on carbon. This unique approach enables concurrent easy control over the size, size uniformity, metal loading, and composition of electrocatalysts, resulting in much-improved ORR activity and durability. This controlled approach combined with a facile surfactant removal method offers a versatile and effective avenue for producing the next generation of high-performance electrocatalysts.

Acknowledgements

This work was supported by National Key Basic Research Program of China (973 project, no. 2012CB215500), National Natural Science Foundation of China (project no. 21306188, 21103163, 21373211, 21306187, and 21003114).

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c4ra16026f

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