Co−N Decorated Hierarchically Porous Graphene Aerogel for Efficient

Mar 3, 2016 - ABSTRACT: Nitrogen-functionalized graphene materials have been demonstrated as promising electrocatalyst for the oxygen reduction reacti...
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Co−N Decorated Hierarchically Porous Graphene Aerogel for Efficient Oxygen Reduction Reaction in Acid Xiaogang Fu, Ja-Yeon Choi, Pouyan Zamani, Gaopeng Jiang, Md. Ariful Hoque, Fathy Mohamed Hassan, and Zhongwei Chen* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute of Sustainable Energy, University of Waterloo, 200 University Ave. W, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: Nitrogen-functionalized graphene materials have been demonstrated as promising electrocatalyst for the oxygen reduction reaction (ORR), owning to their respectable activity and excellent stability in alkaline electrolyte. However, they exhibit unacceptable catalytic activity in acid medium. Here, a hierarchically porous Co−N functionalized graphene aerogel is prepared as an efficient catalyst for the ORR in acid electrolyte. In the preparation process, polyaniline (PANI) is introduced as a pore-forming agent to aid in the self-assembly of graphene species into a porous aerogel networks, and a nitrogen precursor to induce in situ nitrogen doping. Therefore, a Co−N decorated graphene aerogel framework with a large surface area (485 m2 g−1) and an abundance of meso/macropores is effectively formed after heat treatment. Such highly desired structures can not only expose sufficient active sites for the ORR but also guarantee the fast mass transfer in the catalytic process, which provides significant catalytic activity with positive onset and half wave potentials, low hydrogen peroxide yield, high resistance to methanol crossover, and remarkable stability that is comparable to commercial Pt/C in acid medium. KEYWORDS: graphene aerogel, hierarchically porous structures, cobalt−nitrogen moieties, oxygen reduction reaction, fuel cell

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INTRODUCTION Currently, the proton exchange membrane (PEM) fuel cell, owing to its high fuel efficiency and zero emissions at the point of operation, represents one of the most promising energy conversion technologies.1 For widespread commercialization of PEM fuel cell, one crucial factor is reducing the high cost caused by the exclusive use of expensive platinum-based catalysts at the cathode, due to the slow kinetics of the oxygen reduction reaction (ORR).2−4 Therefore, searching for efficient and less expensive nonplatinum catalysts (NPC) for ORR is extremely desirable to commercialize the clean operating, efficient electrochemical devices.5−9 Recent intense research efforts toward replacing platinumbased catalysts have shown that nitrogen-doped carbon nanomaterials could act as effective ORR catalysts due to their low cost, excellent electrocatalytic activity, long durability, and environmental friendliness.10−14 Among them, graphenebased carbon materials have attracted great interest for exploring alternative ORR catalyst due to its exceptional chemical and physical properties such as theoretically ultrahigh surface areas, superior conductivity and excellent mechanical/ chemical stability.15−20 However, most of the nitrogen-doped graphene materials which show promising ORR performance in alkaline electrolyte suffer from relatively low activity in acid medium, making them less competitive than commercial Pt/C catalyst. The introduction of transition metals (e.g., Fe, Co) to the above-mentioned graphene matrix results in metal− © XXXX American Chemical Society

nitrogen−carbon (M−N−C)-based NPC further enhances their ORR activity in acid media.21,22 Although the nature of the catalytically active sites in these NPC remains elusive,23−25 quantum mechanical calculations and experimental investigations have both suggested that M−N moieties, in which metal cations coordinated to pyridinic nitrogen atoms, play a vital role in catalyzing the ORR.26−29 In addition, according to previous report that one of the limitations for enhancing the catalytic activity of these catalysts is due to low density and utilization of M−N active sites as well as poor mass transport properties.30 Thus, turning to large surface area and high porosity graphene structures to afford both abundant accessible M−N catalytic sites and suitable channels for mass transport is a potential solution to produce advanced ORR catalysts.31 It is well-known that due to the inherent property of twodimensional (2D) materials, graphene sheets easily tend toward irreversibly planar stacking. This severe agglomeration thereby leads to a drastic loss of the high intrinsic specific surface areas, which results in inaccessibility of electro-active sites for catalysis and inferior mass transfer. To improve the utilization of active sites and facilitate mass transfer through the volumetric entirety of graphene-based catalysts, various strategies have been explored to prevent the stacking between individual graphene Received: December 28, 2015 Accepted: February 25, 2016

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DOI: 10.1021/acsami.5b12746 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustration of the Synthetic Route for Co−N−GA Catalyst

Figure 1. (a, b) SEM and (c, d) TEM images of as-obtained Co−N−GA, (e) N2 sorption isotherms of Co−N−GS and Co−N−GA catalysts, and (f) pore size distribution from the BJH method of corresponding samples.

sheets. Spacer blocks such as carbon nanotubes,32 metal nanoparticles and templates were inserted between graphene sheets to prevent their stacking.33,34 Different physical and chemical treatments have also been employed to acquire porous frameworks that suppress their agglomeration.35−38 However, the use of hazardous reagents or the tedious synthesis procedures result in low production yield and high cost. Besides, the electrochemical performance still needs to be improved. Based on the above considerations, high ORR

performance materials are expected for example by designing and constructing interesting graphene framework enveloping a high surface area and suitable porosity, as well as more active M−N moieties. It will be certainly great if such a novel graphene structures could be fabricated through a simple but efficient way. Here, we report a Co−N moieties decorated graphene aerogel (Co−N−GA), which has unique hierarchical pores, large surface area and an abundant of potential Co−N active B

DOI: 10.1021/acsami.5b12746 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

smaller and compact GO hydrogel was formed due to the aggregation of GO sheets (Figure S1d). Second, it acts as an efficient nitrogen doping agent, owing to its high N/C atomic ratio (0.17) and unique aromatic structure which can facilitate the incorporation of nitrogen-containing active sites into the graphene matrix.42 Meanwhile, along with the decomposition of PANI during the pyrolysis process, a large amount of nitrogen containing gas was released, which could further expand the graphene sheets to have fewer-layer and increased porosity. The morphology and microstructures of Co−N−GA were first investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM imaging reveals large amount of open-pore structures that continue through the graphene framework (Figure 1a). Meanwhile, the magnified SEM image indicates the existence of numerous smaller pores contiguous with large pores (Figure 1b). These SEM results definitely certify that the Co−N−GA with an interconnected porous network was successfully prepared. This morphology is beneficial for a catalyst. Such a porous graphene skeleton could maximize the exposure of active sites to participate in the ORR process. Meanwhile, during the ORR process, these robust interconnected pores could function as arteries that shorten the diffusion length of reactant and product.43 To investigate the utilization of the pore structures, graphene-sheet-based Co− N−GS catalyst was also prepared. In contrast to Co−N−GA with loose porous networks, Co−N−GS exhibits a relatively agglomerated structure, which is disadvantageous for the contact of oxygen, electrolyte, and active sites (Figure S2). TEM characterization of Co−N−GA reveals almost transparent wrinkled surface characteristic of graphene, while some composite carbon structures which result from the carbonization of PANI and cobalt are also observed (Figure 1c). The high-resolution TEM (HRTEM) image of Co−N−GA further indicates the presence of graphitic carbon shells which are commonly observed from acid leached M−N−C catalysts (Figure 1d).44,45 The graphitic carbon shell structures are likely to result in improved ORR performance due to the increased exposure of sites responsible for oxygen adsorption and fast electron transport.33 The Brunauer−Emmmett−Teller (BET) analysis of nitrogen adsorption−desorption isotherms reveal that both the plots of Co−N−GS and Co−N−GA show close to a type IV patterns with a H3 hysteresis loop according to IUPAC classification (Figure 1e). The absence of saturation at a relatively high pressure indicates the presence of macro-pores and the rapid rise at low pressure region also indicates the presence of micropores. Meanwhile, the hysteresis in the middle pressure range suggests the existence of slit-type meso-pores structure typically formed by aggregates of lamellar graphene sheets. The detailed pore structure parameters of Co−N−GS and Co−N− GA are summarized in Table 1. Remarkably, the specific surface area of Co−N−GA is approximately 485 m2 g−1, which is about 2 times higher than that of Co−N−GS (222 m2 g−1). Such a high-surface-area graphene structure could achieve high volumetric surface area and therefor maximize the active site density to improve ORR performance. The pore size distribution (calculated by Barrett−Joiner−Halenda method) clearly shows the presence of multiple porosities ranging from a few nanometers to the near micrometer scale (Figure 1f). It is worth noting that the Co−N−GA shows predominantly macropores (53%), while in Co−N−GS, the macro-pores account for only 29% of the overall pore volume (Table 1). It is reported that the macro-pores could shorten the diffusion length of

sites. A combined hydrothermal self-assemble, freeze-drying, and pyrolysis process was employed to efficiently prepare this porous graphene framework. Specifically, polyaniline (PANI) is carefully selected as pore-forming agent to aid in the selfassembly of graphene oxide (GO) species into a highly porous hydrogel structure, while also being an effective nitrogen precursor due to its unique chemical structure.39−41 The asfabricated graphene aerogel was used as a catalyst to achieve several merits, including (1) to maximize the Co−N active sites density; (2) to maximize the utilization of active sites; (3) to facilitate good transfer of reactant and product. Benefiting from these excellent structural properties, the Co−N−GA exhibits impressively electrochemical performance in acid medium.

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EXPERIMENTAL SECTION

Synthesis of Co−N−GA. In a typical experiment, a 30 mL of GO (2 mg mL−1) aqueous solution containing Co(NO3)2·6H2O (15 mg) and PANI (80 mg) was sonicated for 1 h to form a stable complex solution. Subsequently, the stable suspension was sealed in a Teflonlined autoclave and hydrothermally treated at 180 °C for 12 h. After that, a columnar hydrogel was lyophilized to prevent the agglomeration of graphene sheets during the drying process. The maintained monolithic architecture was then heated at 900 °C for 1 h under Ar. The heat-treated sample was then preleached in 2 M H2SO4 at 80 °C for 24 h to remove unstable and inactive species from the catalyst, and thoroughly washed in deionized water. Finally, the catalyst was heat-treated again in Ar at 900 °C for 3 h referred to as “Co−N−GA” catalysts (Figure S1a−c). For comparison, the Co−Nmodified graphene sheets (Co−N−GS) catalyst was prepared with a slightly modified approach. In the case of Co−N−GS, a 120 mL of diluted GO (0.5 mg mL−1,) aqueous dispersion with cobalt salt (15 mg) and PANI (80 mg) was prepared, and then followed the same synthetic procedures. Due to the relative low concentration of GO solution, an amorphous precipitation instead of a hydrogel formed during the hydrothermal treatment (Figure S1e). N−GA was prepared with the same procedures as Co−N−GA but without cobalt salt. GA was also prepared with the same procedures as Co−N−GA but without PANI and cobalt precursors. Electrochemical Measurements. Rotating ring-disk electrode (RRDE) testing was conducted to evaluate ORR activity. First 10 mg of catalyst was dispersed in 1 mL of a 1-Propanol ink, and 12 μL of the ink was then deposited on a 0.19635 cm2 electrode. Electrochemical testing was conducted in 0.5 M H2SO4 and 0.1 M KOH at room temperature by sweeping the electrode potential from 1.0 to 0.0 V vs the reversible hydrogen electrode (RHE) under oxygen saturation. Accelerated durability testing (ADT) was conducted by cycling the electrode potential 5000 times between 0.6 and 1.0 V vs RHE under nitrogen saturation.

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RESULTS AND DISCUSSION The fabrication process for Co−N−GA is demonstrated in Scheme 1. In the first step, a stable aqueous suspension containing graphene oxide (GO), Co(NO3)2·6H2O, and PANI (Figure S1a) was hydrothermally treated to synthesize a graphene-based hybrid hydrogel (Figure S1b). Subsequently, a freeze-drying and pyrolysis process was applied to obtain the aerogel with a monolithic architecture (Figure S1c). Finally, the as-prepared aerogel was preleached in 2 M H2SO4 to remove unstable and inactive species from the catalyst, and then pyrolyzed again to yield the Co−N−GA. Here, the PANI serves two important functions in preparing the Co−N−GA catalyst. First of all, it helps with the assembly of graphene to form a spongy hydrogel, owing to the hydrogen bonding and π−π interactions with GO during the hydrothermal treatment which can prevent the restacking of GO so as to form a hierarchical porous structure. Otherwise, in the absence of PANI, a much C

DOI: 10.1021/acsami.5b12746 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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its oxides can be observed. This shows that the majority of large inorganic cobalt particles were most likely removed during the acid leaching step, and any particles that may remain are too small or present in amounts too low to be detected. X-ray photoelectron spectroscopy (XPS) measurements were further carried out to probe the chemical compositions and contents of the materials. The measured survey spectra of Co− N−GA and Co−N−GS clearly reveal the presence of C, O, N, and Co peaks (Figure 2c, Figure S3). Table S1 outlines the atomic surface concentrations with only small differences observed between the two catalysts. The high-resolution N 1s spectra of the two Co−N modified catalysts were deconvoluted into five different peaks with binding energies of 398.6, 399.3, 400.9 401.5, and 403.2 eV, corresponding to pyridinic N, Co− N, pyrrolic N, graphitic N, and oxidated N, respectively (Figure 2d, Figure S3a, Table S2).48,49 Here, the assignment to the Co−N type of N species is based on previous investigations of pyrolyzed metal-containing macrocycles.50 Specifically, Co−N moieties are proposed as a complex in which a cobalt cation is coordinated to two or four pyridinic-type nitrogen atoms that are doped in carbon matrix.51,52 Moreover, in the Co 2p range (Figure S3b,c), a peak centered at 781.8 eV could be related to Co−N structures due to cobalt cation associated with N atoms,53 which goes well with the corresponding peak detected in the N 1s spectrum due to Co−N species. For comparison, the N 1s of nitrogen doped graphene aerogel (N−GA) was peak-fitted with four peaks, due to the absence of no

Table 1. Pore Structure Parameters of Co−N−GS and Co− N−GAa samples Co−N− GS Co−N− GA

SBET (m2 g−1)

Vtotal (cm3 g−1)

V1 (cm3 g−1)

V2 (cm3 g−1)

V3 (cm3 g−1)

222

0.41

0.11

0.18

0.12

485

0.71

0.05

0.29

0.37

a SBET, BET specific surface area; Vtotal, total pore volume; V1, volume of micropores ( 3.94) and low H2O2 yield ( 3.75), excellent electrochemical durability and high methanol tolerance in acidic solution. The outstanding electrochemical performance makes the Co−N−GA a promising NPC for PEM fuel cell.

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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/acsami.5b12746. Detailed information regarding certain reagents, GO and PANI synthesis, Co−N−GA fabrication process and RRDE measurement; SEM images, XPS N 1s and Co 2p of Co−N−GS; RRDE polarization curves of the N−GA, Co−N−GS, Co−N−GA, and Pt/C in 0.1 M KOH and the corresponding electron-transfer number and H2O2 yield; polarization curves of the Co−N−GA catalysts pyrolyzed at various temperature; and elemental composition and atomic concentrations (atom %) of heterocyclic N components of the samples obtained from XPS results. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the University of Waterloo, and the Waterloo Institute for Nanotechnology.

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DOI: 10.1021/acsami.5b12746 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b12746 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX