Dopamine-induced Pt and N-doped carbon@silica hybrids as high-performance anode catalysts for polymer electrolyte membrane fuel cells

Abstract

We report a simple and bio-friendly method to synthesize platinum (Pt) and nitrogen (N)-doped carbon@silica using polydopamine (PDA). This silica-based composite permits greater humidifying capacity to sufficiently hydrate membrane electrode assemblies (MEAs), thus improving electrochemical properties for polymer electrolyte membrane fuel cells (PEMFC).

---

Organic and inorganic hybrid catalysts have been extensively studied for development of polymer electrolyte membrane fuel cells (PEMFCs) because of their durability and interaction qualities.¹ Recently, the combination of conductivity enhancement and water management became a popular approach to improve PEMFC performance without an external humidifying system. Current requirements for prospective PEMFC materials are (1) high proton conductivity, (2) humidifying ability for membrane electrode assemblies (MEAs), and (3) a non-toxic fabrication method.²

To meet such criteria, platinum (Pt) and silica composites have been considered the most suitable materials.³ In fuel cells, humidity control is essential in MEA proton conductivity. The hygroscopic characteristics and self-humidifying capability of silica permits maintenance of the MEA humidity level to sustain fuel cell output under little or no external humidification.⁴ Furthermore, silica improves mechanical and thermal strength. However, the enhancement of Pt loading and silica conductivity are challenging. Recent reports indicate that N-doped carbon electrodes can address these issues via chemical vapor deposition (CVD) and plasma using an N precursor such as pyridines, nitriles and ammonia.⁵ However, these methods still require expensive equipment and complex reaction processes, thereby limiting large-scale production.

To overcome these challenges, polydopamine (PDA), a polymer that is commonly found it from mussel adhesive proteins, could be an alternative solution by providing such advantages as (1) multifunctional groups (i.e., amino and catechol groups), (2) versatile adhesive properties for inorganic and organic substrates and nanostructure, (3) hydrophilic and non-toxic properties, (4) self-reducing ability, and (5) in situ N-doping through carbonization for enhancing catalytic performance.⁶

Herein, we propose a simple and green chemical synthetic approach for increasing Pt loading onto silica by using PDA as bio-glue and making N-doped thin carbon films. Silica was employed as a core template and PDA was applied as the adhesive layer for Pt coating and as the chemical source for formation of N-doped carbon. Moreover, the catalytic capability of Pt and N-doped carbon@silica was investigated in PEMFCs under non-humidified conditions.

The overall fabrication processes of Pt and N-doped carbon@silica is depicted in Scheme 1 and ESI.† The synthetic strategy is a two-part process as follows: (a) formation of monodispersed silica and secondary PDA coating and (b) Pt deposition on the PDA-coated silica (Pt–PDA–silica) and further carbonization of PDA. Silica was prepared following the Stöber method, using 10.8 mL of tetraethylorthosilicate and 17.0 mL of ammonium hydroxide as the reducing agent.⁷ After ultrasonically dispersing silica (200 mg) in Tris buffer solution (pH 8.5), dopamine solution (2 mg mL⁻¹) was then introduced for polymerization to occur to form the thin PDA film. Particles of silica coated with PDA (PDA–silica, 40 mg) were then re-dispersed in buffer solution (40 mL, pH 10.0). Pt precursors were slowly injected into the mixture and stirred for 12 h at 90 °C. Experimental procedure details are described in ESI.†

Scheme 1 Overall synthetic procedure for preparation of Pt and N-doped carbon@silica.

Typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of pristine silica and PDA-decorated silica are shown in Fig. 1(a–d). The images indicated highly dispersed silica with an average diameter of 304 ± 12.6 nm, as calculated by measuring the diameters of the nanoparticles in SEM images. Thin PDA film with a thickness of 11.5 nm on the silica surface was also observed from the TEM image (Fig. 1(d)). This indicated a uniform thin coating of PDA film over the silica. The overall polymerization pathway of PDA onto silica is illustrated in Fig. S1 in ESI.† The weight ratio of PDA and silica for the PDA–silica was confirmed by thermal gravimetric analysis (TGA) (Fig. S2 in ESI†).

Fig. 1 SEM and TEM images of (a and b) silica, (c and d) PDA–silica, and (e and f) Pt and N-doped carbon@silica. Scale bars are 100 nm.

After deposition of Pt nanoparticles (NPs), Pt NPs on PDA–silica exhibited uniform distribution with an average diameter of approximately 17.9 ± 6.2 nm, as shown in Fig. 1(e–f) and S3 in ESI.† Interplanar spacing of 0.22 nm from TEM image (inset image, Fig. 1(f)) corresponded to Pt (111). It also matched the X-ray diffraction (XRD) pattern, as shown in Fig. 2(a). The five significant XRD peaks are assigned to the Pt (111), (200), (220), (311) and (222) planes, which indicates the formation of face-centered cubic structures of Pt in agreement with the JCPDS database.⁸ In addition, energy-dispersive spectroscopy (EDS) and element mapping provided evidence for the existence of Pt over PDA–silica (Fig. S4 and S5 in ESI†).

Fig. 2 (a) XRD patterns of Pt and N-doped carbon@silica, Pt–PDA–silica, PDA–silica and pristine silica; (b) FT-IR of pristine silica, PDA–silica, Pt and N-doped carbon@silica and Pt and N-doped carbon@silica; high-resolution XPS spectra of C 1s for (c) PDA–silica and (d) Pt–PDA–silica.

The role of PDA for Pt coating on silica was investigated using different analytical tools. The results showed the enhancement of Pt loading over PDA–silica compared to pristine silica due to the strong adhesion capability of PDA (Fig. S6†). In addition, the catechol groups of PDA served as nucleation sites and self-reducing agents for reduction of Pt ions into Pt NPs.⁹ The self-reducing ability of PDA was investigated via Fourier transform infrared spectroscopy (FT-IR). The absorbance changes from FT-IR spectra in hydroxyl groups indicated conversion of catechol to quinone, as shown in Fig. 2b and S7.†¹⁰ This phenomenon was also confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The spectral intensity changes of the C=O (288.3 eV) and C–O peaks (286.3 eV) from C 1s were decreased after Pt deposition on PDA–silica, as shown in Fig. 2(c) and (d). The slight positive binding energy shifts of N 1s from PDA–silica to Pt–PDA–silica must be attributed to the chemical interaction between Pt and the amine group of PDA, as shown in Fig. S8.†¹¹ Therefore, the electrons from transforming the R–OH group to a C=O group reduce PtCl₆²⁻ ions into Pt NPs, while serving as self-reducing agents.

After carbonization of PDA, thin PDA layers converted into N-doped carbon film and incorporated N-source improved electrical conductivity of the composite.¹² Furthermore, conductivity enhancement was also confirmed by I–V curves of PDA–silica and N-doped carbon@silica, as shown in Fig. S9, ESI.† One of the most promising potential uses of Pt and N-doped carbon@silica composites is to apply them as electrocatalytic electrodes for PEMFCs. The presence of PDA and silica could help in operating PEMFC under non-humidified conditions. Galvanostatic polarization and power density curves as functions of current density for Pt and N-doped carbon@silica-based composites for a single PEMFC under non-humidified conditions are shown in Fig. 3. Pt supported on commercial carbon (Vulcan XC72) was also tested as a control sample.

Fig. 3 I–V polarization (left axis) and power density (right axis) curve for PEMFC single cell (H₂/O₂) with Pt and N-doped carbon@silica composites (red triangle) and commercial Pt/carbon (black circle) used as an anode catalyst under relative humidity at 0%.

The Pt and N-doped carbon@silica-based PMFC exhibited a maximum power density of 0.55 W cm⁻², which exceeds that of commercial Pt-based PEMFC (0.45 W cm⁻²) and other reported composites (Fig. S10†). This enhancement is attributed to the combined effects of silica humidity management and N-doped carbon coating (Fig. S11†).¹³ To be more precise, compared to the silica-free PEMFC, silica NPs were able to accommodate water molecules that resulted from back-diffusion, and thus they compensated for water loss at the anode. This high level of hydration at the anode can lead to high power output of PEMFCs.¹⁴ Moreover, N-doped carbon coating enhanced electrical conductivity for improvement of charge transfer at interfaces. Therefore, these superior properties of the Pt and N-doped carbon@silica composite could be responsible for improved performances under non-humidified conditions compared to the commercial Pt catalyst.

In conclusion, we developed a facile and green chemical method to synthesize Pt and N-doped carbon@silica hybrids. Silica can serve not only as the core template but can also provide hygroscopic capability to manage humidity. PDA acted as a self-reducing agent and was found to provide specific binding sites to anchor and grow Pt NPs on silica surfaces. Furthermore, N-doped carbon resulting from carbonization of PDAs significantly improved electrical conductivity of Pt and N-doped carbon@silica composites, which produced a dramatic performance improvement of the PEMFC. This promising synthetic method and combination of dopamine and silica will be a valuable material in developing the next generation of energy conversion systems.

Acknowledgements

This work was supported by the BioNano Health-Guard Research Center funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea, Global Frontier Project (Grant number H-GUARD_2013M3A6B2078945); Public Welfare & Safety Research Program, National Research Foundation of Korea (NRF), MSIP of Korea (NRF-2013M3A2A1073991); and National Fisheries Research & Development Institute (NFRDI; contribution number RP-2013-BT-XXX).

Notes and references

1. (a) L. Zhang, L. Wang, C. M. B. Holt, B. Zahiri, Z. Li, K. Malek, T. Navessin, M. H. Eikerling and D. Mitlin, Energy Environ. Sci., 2012, 5, 6156; (b) D. Long, W. Li, W. Qiao, J. Miyawaki, S.-H. Yoon, I. Mochida and L. Ling, Chem. Commun., 2011, 47, 9429.
2. (a) Handbook of Fuel Cells—Fundamental, Technology and Applications, Fuel Cell Technology and Applications, ed. W. Vielstich, A. Lamm and H. A. Gasteiger, John Wiley & Sons, Ltd., New York, NY, USA, 2003, vol. 3; (b) Z. Chen, D. Higgins, A. Yu, L. Zhang and J. Zhang, Energy Environ. Sci., 2011, 4, 3167.
3. M. L. Anderson, R. M. Stroud and D. R. Rolison, Nano Lett., 2002, 2, 235.
4. I. Choi, K. G. Lee, S. H. Ahn, D. H. Kim, O. J. Kwon and J. J. Kim, Catal. Commun., 2012, 21, 86.
5. (a) S. Maldonado and K. J. Stevenson, J. Phys. Chem. B, 2005, 109, 4707; (b) C. W. Miller, D. H. Karweik and T. Kuwana, Anal. Chem., 1981, 53, 2319; (c) H. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012, 2, 781.
6. (a) H. Li, L. Shen, K. Yin, J. Ji, J. Wang, X. Wang and X. Zhang, J. Mater. Chem. A, 2013, 1, 7270; (b) J. H. Waite, Nat. Mater., 2008, 7, 8.
7. W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62.
8. Powder Diffraction Data File 24-0734, Inorganic Phases, JCPDS International Centre for Diffraction Data, Swathmore, PA, 199.
9. (a) X. C. Liu, G. C. Wang, R. P. Liang, L. Shi and J. D. Qiu, J. Mater. Chem. A, 2013, 1, 3945; (b) S. Hong, J. S. Lee, J. Ryu, S. H. Lee, D. Y. Lee, D.-P. Kim, C. B. Park and H. Lee, Nanotechnology, 2011, 22, 494020.
10. R. Baron, M. Zayats and I. Willner, Anal. Chem., 2005, 77, 1566.
11. S.-F. Zheng, J.-S. Hu, L.-S. Zhong, L.-J. Wan and W.-G. Song, J. Phys. Chem. C, 2007, 111, 11174.
12. J. Kong, W. A. Yee, L. Yang, Y. Wei, S. L. Phua, H. G. Ong, J. M. Ang, X. Li and X. Lu, Chem. Commun., 2012, 48, 10316.
13. T. Matsuoka, H. Hatori, M. Kodama, J. Yamashita and N. Miyajima, Carbon, 2004, 42, 2346.
14. M. Ji and Z. Wei, Energies, 2009, 2, 1057.

---

Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedure, TEM, TGA, EDS, element mapping data, high-resolution XPS spectra, I–V curves, PEMFC performance comparison data, and schematic illustrations for both PDA formation, self-reducing pathway of PDA for Pt NPs and contact angle. See DOI: 10.1039/c4ra06819j

---