Amir Yadegariab,
Leila Samieec,
Saeedeh Tasharrofic,
Sanaz Tajikd,
Alimorad Rashidic,
Fatemeh Shoghie,
Morteza Rasoulianboroujenib,
Mohammadreza Tahriri*b,
Samuel J. Rowley-Nealefg and
Craig E. Banks*fg
aSchool of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran, Iran
bDepartment of Developmental Sciences, Marquette University, Milwaukee, WI 53233, USA. E-mail: mohammadreza.tahriri@marquette.edu
cResearch Institute of Petroleum Industry (RIPI), West Entrance Blvd., Olympic Village, P.O. Box: 14857-33111, Tehran, Iran
dFaculty of Chemical Engineering, Amirkabir University of Technology, Hafez Ave, P.O. Box 15875-4413, Tehran, Iran
eDepartment of Chemistry, Université de Montréal, 2900 Boulevard Edouard-Montpetit, Montréal, QC H3T 1J4, Canada
fFaculty of Science and Engineering, Manchester Metropolitan University, Manchester, M1 5GD, UK. E-mail: c.banks@mmu.ac.uk; Web: http://www.craigbanksresearch.com Fax: +44 (0)1612476831; Tel: +44 (0)1612471196
gManchester Fuel Cell Innovation Centre, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK
First published on 7th December 2017
The oxygen reduction reaction (ORR) is an important half reaction, which occurs at the cathode within a fuel cell and limits their range of applications due to slow electrochemical kinetics. To overcome this issue, electrocatalysts are sought, which need to be an alternative to expensive and unsustainable metallic catalysts. Herein we report the synthesis of nitrogen doped nanoporous graphene (NPG), which is a competitive alternative to currently employed metallic catalysts. The NPG is synthesised through a chemical vapour deposition methodology followed by a chemical functionalization step introducing oxygen functional groups (carbonyl and hydroxyl groups), which is then doped with nitrogen via ortho-phenylenediamine (OPDA). The NPG is physiochemically and electrochemically characterised. The NPG demonstrates outstanding electrocatalytic activity towards the ORR in alkaline media proceeding via a favourable 4-electron pathway and is comparable to commercially available platinum–carbon (20%). We demonstrate that the electrochemical activity of the NPG is tailorable such that through increased nitrogen doping the ORR transforms from a 2-electron process to that of the favourable 4-electron process via increasing the proportion of pyridinic nitrogen while the content of graphitic nitrogen remains almost constant. The NPG exhibits excellent electrochemical performance towards the ORR in alkaline media, long-term stability and appropriate methanol crossover as benchmarked to commercialised Pt/C electrodes; this outstanding electrocatalytic activity is related to the high proportion of defects, high porosity and (pyridinic) doping.
Acidic media:
O2 + 4H+ + 4e− → 2H2O, 4 electron pathway |
O2 + 2H+ + 2e− → H2O2, 2 electron pathway |
H2O2 + 2H+ + 2e− → 2H2O |
Alkaline media:
O2 + 2H2O + 4e− → 4OH−, 4 electron pathway |
O2 + H2O + 2e− → HO2− + OH−, 2 electron pathway |
HO2− + H2O + 2e− → 3OH− |
Attempts to increase the performance of PEMFCs have therefore focused on developing efficient, economical and sustainable new materials to reduce the over-potentials, increase the achievable current density and allow for a 4-electron pathway associated with the ORR. Platinum (Pt), and compounds containing it, are considered the most effective catalysts for ORR allowing the reaction mechanism to occur via a 4-electron pathway.6,7 This is a consequence of its low (near negligible) binding energy of adsorbates like O2 and H+. However, the high cost and poor global distribution of platinum limit the attractiveness of its application.8
Attempts have been made to develop non-precious metal catalysts as cost effective alternatives to Pt. Recently, carbon nanostructures, such as carbon nanotubes,9,10 graphene11–13 carbon quantum dots,14 and mesoporous carbon,15 which have been doped with heteroatoms (N, S, P, B, I, or F), have shown promising metal free ORR catalysis.16 Table 1 presents a thorough overview of the surrounding literature. The introduction of heteroatoms increases the electron density near the Fermi level, which induces an increase in the electropositive charge on the adjacent carbon atom, via a polarization effect, thus facilitating the binding of the electronegative O atoms within the electrolyte and making the electropositive carbon atoms the sites responsible for the ORR electrocatalytic activity.17,18 Of particular interest within this field has been the utilisation graphene as a carbon material for doping, due to it processing unique and beneficial electrochemical properties compared to other carbon-based materials.19–21 whilst, pristine (defect free) graphene possesses poor ORR activity it has been shown that by synthesising mesoporous graphene one can greatly increase graphene's ORR activity. There are several reported methods within the literature for the fabrication of mesoporous graphene, such as; chemical etching,22,23 photocatalytic reaction,24 lithography,25 electron beam26 and chemical activation.27 An elegant study by Lin et al.28 prepared mesoporous nitrogen doped graphene, through pyrolysis of graphene oxide and polyaniline at 1000 °C, and observed it to have beneficial ORR electrocatalytic activity and lowered the electronegativity of the ORR onset allowing the mechanism to occur via the desirable 4 electron pathway, whilst also increasing the stability and methanol tolerance, compared to undoped graphene. This study clearly highlights the potential for heteroatom doped graphene to be a cost effect alternative to Pt based electrocatalysts, however by it utilising chemically exfoliated graphene oxide as the carbon precursor and implementing a chemical activation method, the resulting graphene based material is prone to restacking and crumpling of its sheets, which limits the stability and electrocatalytic potential of the material. Lin et al.28 is not alone in their use of graphene oxide as a precursor, the vast majority of the literature utilises a similar fabrication methodology and thusly encounters the same detrimental effects.
Method/Precursor | Electrocatalyst | Mass loading (ca. μg cm−2) | Electrolyte | Number of electrons in the ORR mechanism | Onset potential (V) at limiting current vs. Pt/Ca | Fuel tolerance | Reference |
---|---|---|---|---|---|---|---|
a The commercial Pt/C catalysts are commonly used as references: 20 wt% Pt on Vulcan X72R from ElectroChem, Inc., 20 wt% Pt on carbon black from Alfa Aesar, PANI; polyaniline, GO; graphene oxide, rGO; reduced graphene oxide, —; value unobtainable, 3-MBP-dca; 3-methyl-1-butylpyridine dicyanamide, CMIM-Cl; 3-(3-cyanopropyl)-1-methyl-1H-imidazol-3-iumchloride, GQD; graphene quantum nanodots, SWCNTs; single walled carbon nanotubes, CVD; chemical vapour deposition, DNA; deoxyribose nucleic acid. | |||||||
Template method/PANi | N,O-doped mesoporous carbon | 100 | 0.1 M KOH | 4.0 | Comparable@larger | — | 44 |
Carbonization/nucleobases and ionic liquid | N-Doped mesoporous carbon | 800 | 0.1 M KOH | 4.1 | Comparable | CH3OH | 45 |
Spray pyrolysis/xylene and ethylenediamine | N-Doped carbon spheres | 286 | 0.1 M KOH | 3.9 | Comparable@smaller | CH3OH | 46 |
Template method/3-MBP-dca or CMIM-Cl | N-Doped hollow carbon hemisphere | 459 | 0.1 M KOH | 3.8 | Comparable@lower | CH3OH | 47 |
Template method/triphenylphosphine + phenol | P-Doped mesoporous carbon | — | 0.1 M KOH | 3.9 | Comparable | CH3OH | 48 |
Electrochemical approach/graphene sheets | N-Doped GQD | 286 | 0.1 M KOH | 3.6–4.4 | Smaller | CH3OH | 49 |
Solution chemistry approach | N-Doped GQD | 77 | 0.1 M KOH | 3.9 | Better | — | 50 |
Pyrolysis/carbon black + NH4F | F-Doped carbon black | 400 | 0.1 M KOH | 3.9 | Better | CH3OH and CO | 51 |
Pyrolysis/carbon black + melamine (C3H6N6) and ammonium fluoride (NH4F) | N,F-co-doped carbon | 510 | 0.1 M KOH | 4.2 | Comparable | CH3OH and CO | 52 |
Template method/sucrose + thiourea | S,N-co-doped carbon foam | 143 | 0.1 M KOH | 4.0 | Comparable | CH3OH | 53 |
Pyrolysis/ethylenediamine + GO | g-C3N4/RGO nanosheet | 70.7 | 0.1 M KOH | 4.0 | — | CH3OH | 54 |
Incorporation/g-C3N4 + mesoporous carbon | g-C3N4@carbon | 84.8 | 0.1 M KOH | 4.0 | Lower | CH3OH | 3 |
Sputtering and annealing | N-Doped carbon film | — | 0.1 M KOH | 4.0 | Comparable | CH3OH | 55 |
Pyrolysis/gram flour | N-Doped porous carbon | 255 | 0.1 M KOH | 4.0 | Comparable@smaller | CH3OH | 56 |
Pyrolysis/sugar and urea | N-Doped graphene | 283 | 0.1 M KOH | 4.0 | Comparable | CH3OH | 57 |
Thermal treatment/polypyridine + carbon black | N-Doped carbon | 135 | 0.1 M KOH | 3.8 | Comparable | — | 58 |
Carbonization/sucrose + 4-hydro xyphenylboronic acid | B-Doped mesoporous carbon | — | 0.1 M KOH | 4.0 | Lower@larger | CH3OH and CO | 59 |
Pyrolysis/carbohydrate-based derivatives and phenolic compound | N-Doped carbon aerogels | 287 | 0.1 M KOH | 3.7 | Lower | CH3OH | 60 |
Pyrolysis/DNA + CaCO3 | N-Doped mesoporous carbon | 200 | 0.1 M KOH | 4.0 | Lower@larger | — | 61 |
Template method/(1-methyl-1H-pyrrole-2-yl) methanol | N-Doped porous carbon | 200 | 0.5 M H2SO4 | 2.0 | — | — | 62 |
Pyrolysis/pyromellitic acid dianhydride + 4,4′-oxidianilin | Fe-free N doped carbon | 40–300 | 0.5 M H2SO4 | 2.0 | Lower@lower | — | 63 |
C-ORNL-1 and NH3 | N-OMCs | 102 | 0.5 M H2SO4 | 3.7 | Comparable | CO | 64 |
Metal-free CVD of CH4 and H2 | N-Doped SWCNTs | — | 0.5 M H2SO4 | >3.5 | — | — | 65 |
Calcination/Methyl orange, banana peel and waste water | N,S-doped porous carbon | — | 0.1 M KOH | 3.9 | Comparable | CH3OH | 66 |
Hummers method, carbon from glucose | N-Doped graphene/carbon | 200 | 0.1 M KOH | 3.8 | Lower | CH3OH | 67 |
CVD and pyrolysis of ortho-phenylenediamine | N-Doped nanoporous graphene | 350 | 0.1 M KOH | 4.0 | Comparable | CH3OH | This work |
In order to address the problem of restacking and crumpling graphene sheets, which is endemic within the literature, this paper presents a facile two-step methodology for the fabrication of nitrogen-doped nanoporous graphene. This involves, producing the nanoporous graphene via a chemical vapour deposition (CVD) method followed by introducing nitrogen groups via pyrolysis using ortho-phenylenediamine as a precursor. Table 1 summarises the current literature devoted to developing heteroatom-doped catalysts, which demonstrates that our approach is a novel methodology to realise nitrogen doped nanoporous graphene with outstanding electrochemical properties. The nitrogen doped nanoporous graphene electrocatalyst does not suffer from the restacking and crumpling problems associated with other heteroatom doped graphene based ORR electrocatalysts. This study therefore significantly increases the attractiveness of graphene-based electrocatalysts as potential cost-effective alternatives for Pt in PEMFCs.
Fig. 2A and S1† show the SEM images and corresponding elemental mapping analysis of the PG and NPG 1–5. The 3D morphological structure can be observed to be one of high porosity for the both PG and NPG 1–5. The TEM images of the PG and NPG 1–5 were also taken to further investigate the crumpled, porous, and corrugated structure of PG and NPG 1–5 (see Fig. 2B and C). As presented in the TEM images, the more transparent and apparently fewer layers of the NPG 1–5, compared to PG, might be attributed to the nitrogen doping effect in agreement with the XRD and Raman results; this indicates that the NPG does not suffer from restacking and crumpling problems which is associated with other non-doped and other heteroatom doped graphenes.
The interconnected framework and ultrathin nanosheets of the NPG 1–5 maintain lots of slit-shaped porous structures as the desirable sites for ORR.31 Simultaneously, the presence of amorphous carbon and highly graphitized structure of the NPG 1–5 can be obviously seen in the TEM images.
The XRD patterns of PG, NPG 1–0.5, NPG 1–1, NPG 1–10 and NPG 1–5 samples are shown within Fig. 2D. The XRD patterns reveal the (002) reflection of graphene structure at around 2θ ∼ 29° and 26° for non-doped and nitrogen doped samples, respectively, suggesting that nanoporous graphene exists in single or few layers. The scattered peaks at ca. 44–45° can be assigned to the (100) crystal plane of graphite, implying the presence of both crystalline and amorphous carbon structures.32
As observed, the strong peaks of nitrogen doped nanoporous graphene are broadened and shifted toward lower angles in comparison with non-doped nanoporous graphene, indicating that the pyrolysis treatment with OPDA moderately disorders the graphene framework, possibly due to the creation of more defects resulting from heteroatom doping effect.33 The number of graphene sheets can be estimated by calculating the interlayer spacing of the lattices and the mean crystallite size of the powder, which is obtained by the Scherrer equation:34 where Lhkl is the mean crystallite size of powder for the related Miller indices (hkl), λ is the wavelength of radiation (1.542 Å), β is the full widths at half maximum (FWHM) in radians, and θ is the scattering angle. The interlayer spacing of graphene sheets can be calculated by Bragg's law:34
where dhkl is the interlayer spacing of the lattices for the related Miller indices (hkl) and n represents an integer for the order of the diffraction peak. Accordingly, the number of graphene layers can be estimated trough the following equation:34
. The number of layers, FWHM, Lhkl, and dhkl are summarized in Table 2 (main paper) with respect to the XRD spectra.
Sample | 2θ (°) | FWHM (°) | Lhkl (nm) | dhkl (Å) | Number of layers ≈ |
---|---|---|---|---|---|
PG | 29.01 | 4.22 | 1.95 | 3.26 | 6–7 |
NPG 1–0.5 | 26.65 | 5.78 | 1.41 | 3.34 | 4–5 |
NPG 1–1 | 26.58 | 5.91 | 1.38 | 3.36 | 3–4 |
NPG 1–10 | 26.54 | 5.95 | 1.37 | 3.36 | 3–4 |
NPG 1–5 | 26.50 | 6.12 | 1.33 | 3.37 | 3–4 |
The number of graphene sheets for PG and NPG 1–0.5 was estimated by calculating the interlayer spacing of the lattices and the mean crystallite size of the powder and found to be 7 and 5, respectively. Whilst the number of graphene layers for NPG 1–1, NPG 1–5, and NPG 1–10 is equal to 4 (see Table 2). The decreasing number of graphene layers can be attributed to increasing the space layer for nitrogen doped nanoporous samples versus un-doped nanoporous graphene.
The augmentation of FWHM quantities of nitrogen doped graphene compared to the pristine nanoporous graphene further verifies the successful doping of N atoms into the carbon skeleton.
Furthermore, by increasing the OPDA concentration, the (002) peaks get somewhat broader from NPG 1–0.5 to NPG 1–5 (FWHM in Table 2), signifying that the higher concentration of OPDA leads to the creation of more disorders in the graphene network. However, further increasing the concentration of OPDA to NPG 1–10 reduces the FWHM, suggesting the NPG 1–5 as the optimized ratio for the preparation of nitrogen doped nanoporous graphene.
The Raman spectra of the samples are presented in Fig. 2E. The three expected characteristic peaks are observed at ca. 1350, 1580, and 2685 cm−1 for all samples correspond to D, G, and 2D bands, respectively (see Table 3). The three distinctive peaks at around 1350, 1580, and 2685 cm−1 for all samples correspond to D, G, and 2D bands, respectively (Table 3). The G band is considered as the first-order scattering of the E2g mode of sp2 carbon atoms in graphene matrix, whereas the D band is related to the breathing mode of A1g symmetry, indicating the presence of defects, microstructure disordering and amorphous carbon.35 Commonly, the ID/IG reveals the information about the relation of the sp3 hybridized carbon (disordered structures) to sp2 hybridized carbon (graphitic structures). It should be taken into account that the higher ratio of ID/IG signifies the higher in plane and edge defects emanating from doping, functionalization and exfoliation of graphene.36 Obviously, the intensity of D and G band of PG is approximately equal, suggesting the presence of crystallite and amorphous structures in nanoporous graphene at the same time. The ID/IG ratio of PG is about 0.93, while doping nitrogen increases the amount of ID/IG ratio to 1.1, 1.23, 1.42, and 1.54 for NPG 1–0.5, NPG 1–1, NPG 1–10, and NPG 1–5, respectively (Table 3). The augmentation of ID/IG can be attributed to the disruption of aromatic π–π electrons in the graphene matrix confirming successful nitrogen doping procedure. The ID/IG ratio increases by increasing the concentration of OPDA in the ratio of (1:
5), despite higher concentrations of OPDA (1
:
10) reducing the of ID/IG ratio. This indicates NPG 1–5 is the best proportion. Apart from that, the shape and intensity of the 2D bands are known as one of the most prominent features of Raman spectra for identifying the number of layers in two-dimension carbon nanostructures. As the 2D band gets broader and positively shifted toward higher wave number, the number of graphene layers increases.37 The 2D bands of nitrogen doped porous graphene are partially sharper than that of non-doped porous graphene (Fig. 2E), indicating that the number of graphene layers can be decreased via nitrogen doping. However, as the XRD results showed, there is no significant alteration between the number of graphene layers before and after nitrogen doping. Also, the intangible augmentation of the I2D by introducing nitrogen atoms (Table 3) leads to the increase in the I2D/IG ratio, which further verifies the diminishing the number of graphene sheets. It can be concluded that the nitrogen doped samples are composed of few-layered graphene as well as sufficient defects as the desirable sites for oxygen reduction reaction.
Sample | D band (cm−1) | G band (cm−1) | 2D band (cm−1) | ID | IG | ID/IG | I2D | I2D/IG |
---|---|---|---|---|---|---|---|---|
PG | 1351.8 | 1578.3 | 2673.6 | 327.9 | 350.7 | 0.93 | 189.7 | 0.55 |
NPG 1–0.5 | 1350.6 | 1580.97 | 2681.1 | 348.1 | 316.8 | 1.10 | 195.1 | 0.62 |
NPG 1–1 | 1349.4 | 1582.3 | 2687.4 | 394.8 | 320.9 | 1.23 | 202.4 | 0.63 |
NPG 1–10 | 1351.6 | 1582.5 | 2683.2 | 497.5 | 352.6 | 1.42 | 222.4 | 0.63 |
NPG 1–5 | 1353.4 | 1582.3 | 2685.2 | 544.6 | 354.1 | 1.54 | 225.1 | 0.64 |
XPS was performed to investigate the elemental composition of the electrocatalysts and verify the degree and type of nitrogen doping. The XPS surveys of porous graphene presented in Fig. S2 and S3† shows two distinctive peaks at 285 and 531 eV corresponding to C 1s and O 1s, respectively, without any other impurities. In the NPG spectra, the appearance of the characteristic peaks at about 400 eV confirms the successful doping of nitrogen atoms in graphene matrix. Last, BET surface area (m2 g−1), pore volume (cm3 g−1) and mean pore size (nm) for PG sample was determined to correspond to 721.3, 2.3 and 12.6, respectively (see ESI and Fig. S4†). Also, the BET surface area (m2 g−1), pore volume (cm3 g−1) and mean pore size (nm) for NPG 1–5 sample is 665.2, 1.8 and 13.7, respectively. In addition to BET surface area, the pore volume and pore size of the PG and NPG 1–5 remains approximately unchanged, indicating that nitrogen doping does not principally deteriorate the mesoporous structure of porous graphene, which is in good agreement with TEM images and Raman spectra. Thus, the PG and NPG variants utilised throughout this study have been fully characterised and revealed to be of high quality and can be accurately described as such.
The CV profiles (in O2 saturated 0.1 M KOH) of the optimized nitrogen doped porous graphene (NPG 1–5), bare/unmodified PG and commercialised Pt/C 20% are shown in Fig. 3 for comparative purposes. As stated above the ORR onset potential for the NPG 1–5 is more electropositive than the bare/unmodified PG and comparable to that of Pt/C 20%. Upon inspection of this figure it is also evident that NPG 1–5 has a larger peak potential than either the bare/unmodified PG or Pt/C 20%. This supports the interference that N doping the PG increases its catalytic activity towards the ORR. In order to assess the limiting current and whether there is a change in the ORR mechanism associated with the observed increase in the ORR, we employed rotating disk electrochemistry as is common within the literature. This was performed by carrying out LSV in O2 saturated 0.1 M KOH at scan rate of 5 mV s−1, utilising various rotation rates. The LSV plots at the rotation rates from 250 to 3500 rpm show that by increasing the rotation rate, the limiting current increases as expected (see Fig. S6,† with a comparison of the LSVs at 1500 rpm being given in Fig. 4b). As presented, for all of the nitrogen doped samples, the ORR current density initiates at ca. 0.82–1.05 V and then reaches a limiting current density at ca. 0.7–0.8 V, indicating high ORR activity. Among all nitrogen doped nanoporous electrocatalysts, NPG 1–5 possesses the highest limiting current of ca. 7.8 mA cm−2. In order to deduce the ORR mechanism the number of electron transfers, n, participating in the electrochemical reaction need to be deduced. A commonly employed approach is to calculate this via rotating disc analysis via the Koutecky–Levich equation:10–12,38 where B = 0.62nFD2/3υ−1/6C0. In these equations, J is the experimental current density, JK is the kinetic-limiting current density, Jd is the diffusion-limiting current density, ω is the rotation speed (rpm), n is the electron transfer number in ORR, F is the Faraday constant (96
485 C mol−1), D is the diffusion coefficient of O2 in 0.1 M KOH solution (1.9 × 10−5 cm2 s−1), υ is the electrolyte velocity (0.01 cm2 s−1) and C0 is the bulk concentration of O2 (1.2 × 10−6 mol cm−3). The Koutecky–Levich (K–L) plots of J−1 versus ω−1 at various potentials (RHE) from RDE measurements exhibit straight and parallel lines, implying the first-order reaction (See Fig. S7†). Analysis of the K–L slope plots for the PG suggest that ORR mechanism occurs via a two electron pathway, whilst the nitrogen doped samples where observed to have an ORR mechanism that occurred by the desirable four-electron pathway, suggesting the almost complete O2 reduction to OH−. The exact number electrons involved in the PG and NPG variants reaction mechanism are displayed within Fig. 4a and Table 5. It can be inferred that the doping of PG with N increases the ORR activity by enabling the reaction mechanism to occur by the desirable 4 electron pathway.
![]() | ||
Fig. 3 (a) CVs of PG and the variants and (b) optimised NPG 1–5 compared against Pt/C 20% and PG. Solution composition: 0.1 M KOH (O2 saturated), scan rate 50 mV s−1 (vs. RHE). |
The obtained results suggest that the high electrocatalytic activity of the NPG can be attributed to the degree and type of nitrogen doping. Whilst the detailed role of different nitrogen containing phases is still unclear,39 with various studies within the literature offering alternative theories. For example Lai et al.40 reported that the graphitic nitrogen phase determines the limiting current density, while the pyridinic phase has an undeniable role in onset potential improvement.40 Contrary to this, it has been reported that the high electrocatalytic activity of nitrogen doped carbon nanostructures can be attributed to pyrrolic nitrogen content.39 It was therefore important to assess the surface groups present on the NPG samples to determine their involvement in observed ORR activity. It was initially theorised as the incorporation of OPDA increased the percentage of nitrogen groups present on the graphene's surface would increase, however, according to the XPS results, the coverage of graphitic nitrogen remained almost unchanged with increasing usage of OPDA. This suggests that the electocatalytic activity can be much more affected by pyridinic nitrogen as the favourable active sites for ORR rather than the graphitic nitrogen. The pyridinic percentage in NPG 1–5 is higher than other N-doped electrocatalysts, possibly explaining why it displayed the most beneficial ORR activity. The pyridinic nitrogen doping into graphene network can increase the 2pπ state of C–N, as well as, weaken the C–C electron density.41 Moreover, the increase in N/C ratio (Table 4) would alter the electronic density of state close to the Fermi level, which enhances the adsorption of O2 and facilitates the electron transfer between catalyst and oxygen molecules.17,41 Thus, the best electrocatalytic performance of NPG 1–5 among the other electrocatalysts can be attributed to it having the highest percentage of pyridinic content (33.5%).
Sample | Atomic percentages (%) | O | N | N/C | Nitrogen composition atomic percentages (%) | |||
---|---|---|---|---|---|---|---|---|
C | Pyridinic N | Pyrrolic N | Graphitic N | Oxidized N | ||||
PG | 95.6 ± 4.3 | 4.4 ± 0.2 | — | — | — | — | — | — |
NPG 1–0.5 | 92.1 ± 3.2 | 3.8 ± 0.1 | 4.1 ± 0.1 | 0.045 | 15.2 ± 0.6 | 24.7 ± 1.3 | 40.3 ± 2.2 | 19.8 ± 0.7 |
NPG 1–1 | 91.5 ± 3.9 | 2.2 ± 0.1 | 6.3 ± 0.2 | 0.069 | 25.4 ± 1.3 | 12.1 ± 0.4 | 45.2 ± 1.8 | 17.3 ± 1.1 |
NPG 1–10 | 90.4 ± 2.7 | 1.9 ± 0.1 | 7.7 ± 1.2 | 0.085 | 27.4 ± 3.6 | 10.0 ± 1.7 | 44.4 ± 2.7 | 18.2 ± 0.8 |
NPG 1–5 | 89.9 ± 3.1 | 1.7 ± 0.2 | 8.4 ± 0.4 | 0.093 | 33.5 ± 1.5 | 8.2 ± 0.6 | 45.6 ± 2.4 | 12.7 ± 0.6 |
Electrochemical impedance spectroscopy (EIS) was employed to further elucidate the electrocatalytic performance of the samples. Nyquist plots of the samples and Pt/C at different potential are presented in Fig. S10.† The Nyquist curves show an arc-like or semicircle profile with different diameter corresponding to the charge transfer resistance (Rct). For all of the nitrogen doped samples and Pt/C, the diameter of the semicircles shrinks by decreasing the applied potential to +0.87 V (vs. RHE) and slightly increases at +0.82 V, indicating higher electrical conductivity in the vicinity of +0.865 V (vs. RHE), which is close to the peak potential in cyclic voltammetry. A similar trend does is not observed for the non-doped porous graphene, further highlighting the role of nitrogen atoms and high specific surface area, which facilitates electron transfer in the conductive channels of NPG. The comparison of Nyquist plots at +0.87 V (vs. RHE) and the corresponding equivalent circuit are shown in Fig. S11.† Based on the equivalent circuit fitting, the Rct of PG, NPG 1–0.5, NPG 1–1, NPG 1–10, NPG 1–5, and Pt/C were calculated to be 2335, 283, 255, 212, 155, and 496 Ω, respectively. The semicircle diameter of NPG 1–5 is slightly smaller than other samples. The Rct of all nitrogen doped samples are noticeably lower than that of PG and Pt/C, suggesting improved electron and ion transfer between electrode and electrolyte. Prior literature has demonstrated that the nitrogen atoms, especially pyridinic groups, next to the carbon atoms provide beneficial electrochemical/electrocatalytic sites due to their localised density of states in the occupied region near the Fermi level.41 Moreover, density functional theory (DFT) indicates that carbon atoms in the vicinity of the pyridinic nitrogen can act as Lewis bases because of its tendency to donate the electron pair.42 Therefore, in accordance to the XPS results, the high concentration of pyridinic nitrogen's of the electrocatalysts (i.e. NPG 1–5 and NPG 1–10, Table 5) result in enhanced electron transport and reduced charge transfer resistance.
Samples | Number of electrons transferred (n) | Onset potential (V) | Steady state current density (mA cm−2) |
---|---|---|---|
PG | 2.2 | +0.82 | 4.52 |
NPG 1–0.5 | 3.8 | +1.00 | 5.79 |
NPG 1–1 | 3.9 | +1.01 | 7.31 |
NPG 1–5 | 4.0 | +1.03 | 7.42 |
NPG 1–10 | 4.0 | +0.99 | 7.45 |
Pt/C 20% | 4.0 | +1.05 | 7.61 |
In summary, this study has shown that by producing nitrogen doped nanoporous graphene, via a CVD method followed by introducing nitrogen groups via pyrolysis using ortho-phenylenediamine as a precursor, a stable and cost effective alternative to the Pt/C can be fabricated for use as a cathodic material in a PEMFC.
Footnote |
† Electronic supplementary information (ESI) available: BET isotherms of nanoporous and NPG, XPS spectra of wide survey of electrocatalysts and C1s and N1s high resolution spectra, SEM images and mapping analysis of nanoporous graphene and NPG, cyclic voltammetry in O2 or N2 saturated electrolyte and after addition of methanol, chronoamperometry in O2 saturated and after addition of methanol, linear sweep voltammetry at various rotation speeds, Koutecky–Levich plots, and electrochemical impedance spectroscopy of the electrocatalysts. See DOI: 10.1039/c7ra10626b |
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