Ultrasound-Assisted Nitrogen and Boron Codoping of Graphene

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Ultrasound-Assisted Nitrogen and Boron Co-doping of Graphene Oxide for Efficient Oxygen Reduction Reaction Mingli Zhang, Hengcong Tao, Yongchao Liu, Chao Yan, Song Hong, Justus Masa, Alex W. Robertson, Shizhen Liu, Jieshan Qiu, and Zhenyu Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05654 • Publication Date (Web): 01 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Sustainable Chemistry & Engineering

Ultrasound-Assisted Nitrogen and Boron Codoping of Graphene Oxide for Efficient Oxygen Reduction Reaction Mingli Zhang,†,a Hengcong Tao,†,a Yongchao Liu,b Chao Yan,b Song Hong,a Justus Masa,*,c Alex W. Robertson,d Shizhen Liu,a Jieshan Qiua, and Zhenyu Sun*,a a

State Key Laboratory of Organic–Inorganic Composites, College of Chemical Engineering,

Beijing University of Chemical Technology, Beijing 100029, P.R. China. b

School of Material Science & Engineering, Jiangsu University of Science and Technology,

Zhenjiang 212003, P.R. China c

Department of Chemistry, Kyambogo University. P. O. Box 1, Kyambogo, Uganda

d

Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom

†

These authors contributed equally to this work.

ABSTRACT: Development of naturally abundant, low cost, and energy-efficient electrocatalysts for the oxygen reduction reaction (ORR) is essential for commercialization of fuel cells. In this work, we report simple ultrasonication assisted synthesis of nitrogen and boron dual-doped graphene oxide (NB/GO) and demonstrate its application as an effective ORR catalyst realizing predominantly 4e− reduction of O2 to OH− in 0.1 M KOH. Enhanced ORR electrocatalysis of the dual B and N co-doped GO as opposed to GO singly doped with B or N arises from the synergistic interaction of the boron and nitrogen species. The content and configuration of both N and B dopants can be readily tailored by controlling the ultrasonic conditions, thereby permitting tuning of the ORR activity. Furthermore, the developed NB/GO metal-free catalyst exhibited very promising long-term durability and resistance to methanol poisoning compared to the state-of the art Pt/C catalyst.

KEYWORDS: Graphene, Doping, Oxygen reduction reaction, Electrocatalysis INTRODUCTION As the worldwide demand for energy supplies continues to soar, electrochemical energy systems including fuels cells and rechargeable metal-air batteries in which the oxygen reduction reaction (ORR) plays a crucial role continue to attract ever increasing attention.1, 2 The ORR is however inherently kinetically slow owing to the need to transfer up to four electrons and four protons via at least three reaction intermediates, which has frustrated progress towards the development of effective low-cost ORR catalysts for practical fuel cells. To date, pure platinum (Pt) and platinum alloys remain the widest used and most effective catalysts for the ORR,3 its scarcity and poor durability notwithstanding. The large-scale commercial implementation of fuel cells solely reliant on Pt and Pt-based materials as catalysts is however untenable, owing to scarcity and affordability concerns. Thus, reducing the loading of Pt or even completely replacing it with abundant and low-cost catalysts would be beneficial for mass production of fuel cells.4 Both theoretical and experimental studies have reported that doping graphene with

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heteroatoms, such as N, B, S, or P leads to modification of its electronic properties and chemical reactivity, particularly, its effectiveness as a substitute for Pt-based catalysts in catalyzing the ORR in fuel cells and metal-air batteries. Doping of graphene with B, N, and P can be achieved either by in situ routes or through post-treatment schemes.5 N-doped graphene has attracted the most research attention and interest since Dai and co-workers first reported the use of nitrogen doped graphene as a promising catalyst for the ORR in 2010.6 Doping graphene with boron facilitates chemisorption of oxygen (O=O) and subsequent cleavage of the O-O bond. Moreover, density functional theory (DFT) calculations indicate that doping graphene with B results in superior catalytic ORR performance compared to Pdoped and S-doped graphene systems owing to a relatively smaller ORR activation barrier.7 Co-doping graphene with both B and N has been investigated for further enhancement of the ORR activity of graphene and other carbon-based materials.8-12 Dual-atom or multiple atom doping can yield carbon materials with unique electronic structures, synergistic effects, and new catalytic properties. In particular, the electronegativity of B (χ = 2.04) and N (χ = 3.04), one with lower and the other with higher electronegativity than C (χ = 2.55),13 inducing additional electron-acceptor and electron-donor states, can tune the energy bandgap on the one hand and charge density on the other hand, through synergistic electron transfer interactions between the dopants and the surrounding carbon atoms, thereby providing potentially more active sites for the ORR.14 For instance, Qiao’s group reported a B, N codoped graphene catalyst for the ORR synthesized via a two-step method, which showed nearly 100% selective reduction of the ORR to OH- via the four-electron ORR pathway, much higher than that observed for B- or N- single atom doped graphene. Schiros et al. synthesized a N/B co-doped graphene catalyst by a CVD growth method under a B2H6 atmosphere. However, it is evident that the CVD method is unlikely a cost-effective technique for mass production of dual-doped graphene.15 Moreover, for N/B co-doped graphene oxide synthesized by easily scalable and economic ways, such as solid phase annealing, details of the origin for enhanced catalysis of the ORR, as well as the details of the pathways of the doping reactions, are not yet well understood. Here, we synthesized N and B co-doped graphene oxide simply by ultrasonic treatment of GO in aqueous ammonia and subsequently in boric acid solution. We showed for the first time that doping of GO with N and B dual atoms can be achieved at a temperature as low as 5 ºC with the aid of ultrasonication. This method (based on sonochemistry) has benefits of simplicity, straightforwardness, solution processability, and flexibility, which may also be readily extended to other (single or multiple) heteroatom-doping into graphitic networks (such as phosphorus, sulfur, halogen, and even transition metal species). Interestingly, the N and B doping levels can be easily tuned by controlling the ultrasonication conditions and calcination temperature. The resulting N and B co-doped GO showed very promising performance in catalyzing the oxygen reduction reaction in alkaline media, affording a 4ereduction of O2 to water. The content and configuration of the various nitrogen and boron


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species before and after annealing were probed by means of XPS in order to correlate the observed electrocatalytic behavior with the structural properties of the catalyst.

RESULTS AND DISCUSSION The typical FTIR spectra of GO and of the nitrogen and boron dual-doped graphene oxide (NB/GO) samples synthesized at various ultrasonication temperatures prior to annealing are displayed in Figure S1. For the GO sample, the characteristic O-H, C=O, C=C and C-O absorption bands located around 3400, 1731, 1597, and 1055 cm-1, respectively, are clearly discernible.16 After heteroatom doping, new stretching vibrational bands ascribed to N-C at 1250 cm-1 and to B-C at 1125 cm-1 are present.17

(a)

BN/GO_55

N 1s

Pyrrolic Oxidic Graphitic

(b)

Oxidic

Intensity/a.u.

Intensity/a.u.

NB/GO_55

NB/GO_75

(c)

8 6

403

N 1s

Graphitic

N-B

NB/GO_5_700

NB/GO_35

404

BN/GO_55_700 Pyrrolic Pyridinic

Pyridinic N-B

NB/GO_5

N Species Content/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


402

NB/GO_35_700

NB/GO_55_700

NB/GO_75_700

401

400

399

398

Binding Energy/eV NB/GO_75_700

Pyridinic N

Pyrrolic N

Graphitic N

Oxidic N

397

404

403

402

401

400

399

398

Binding Energy/eV

397

N-B

4 2 0

5 5 5 5 _5 00 00 00 00 00 _5 _7 _3 _5 _7 _7 _7 _7 _7 /GO GO GO GO 55 55 75 35 _5 /GO / / / B _ _ _ _ O N B B B N B N N N /G /GO /GO /GO /GO NB BN NB NB NB

Figure 1. High-resolution N 1s XPS spectra of the N and B co-doped GO samples prepared at different ultrasonic temperature (a) before and (b) after annealing at 700 °C. (c) Relative compositions of the pyridinic N, pyrrolic N, graphitic N, oxidic N, and N-B species (atomic fractions) in the samples.



(a)

BN/GO_55 BC2O BCO2

B 1s B-N

(b)

Intensity/a.u.

Intensity/a.u.

NB/GO_35

NB/GO_55

(c)4

194

NB/GO_35_700

NB/GO_55_700

NB/GO_75_700

NB/GO_75

195

BC2O B 1s B-N BC3

BN/GO_55_700 BCO2

NB/GO_5_700

NB/GO_5

B Species Content/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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193

192

191

190

Binding Energy/eV

189

195

194

193

192

191

190

189

Binding Energy/eV

3

BCO2

BC2O

BC3

B-N

2 1 0

5 5 5 5 _5 00 00 00 00 00 _7 _3 _5 _5 _7 _7 _7 _7 _7 /GO GO GO GO 55 55 75 35 _5 / / /GO / B _ _ _ _ O B B N B N N N B N /G /GO /GO /GO /GO NB BN NB NB NB

Figure 2. High-resolution B 1s XPS spectra of the N and B co-doped GO samples prepared at different ultrasonic temperature (a) before and (b) after annealing at 700 °C. (c) Atomic composition of various B species including BC3, BCO2, BC2O, and B-N in the samples.

X-ray photoelectron spectroscopy (XPS) measurements were performed to examine the chemical nature and valence states of C, O, B, and N in the samples prior to, and after annealing. The wide-scan spectra for the non-annealed NB/GO samples obtained at different ultrasonication temperatures are shown in Figures S2 and S3, respectively, clearly revealing the existence of N and B, as well as C and O in the samples. This unambiguously suggests successful doping of GO with both N and B atoms after ultrasonication even at a low temperature of 5 °C. The dopant precursors could attack GO’s vacancies with carbonyl, carboxylic, and hydroxyl groups to form C-NH2, C-NH, or C-B(OH)x (x = 1 or 2) bonds18, 19 facilitated by ultrasonication, which unlikely occurs under stirring. Subsequently, N-H or BOH dissociation and water formation took place to generate C-N or C-B moieties under ultrasonication. When N, with a smaller atomic radius than that of C, is doped into GO’s lattice, it may aid B doping by balancing the N induced lattice pressure stress.9 The binding energies of all species in the presented XPS spectra were fitted with respect to the peak in C


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1s at 284.8 eV, corresponding to sp2 graphite-like carbon (Figure S4).20 Figure 1a and Figure 1b showed the deconvoluted N 1s spectra of the NB/GO_X and NB/GO_X_700 samples (X = 5, 35, 55, and 75), respectively. Five prominent peaks assigned to pyridinic N (398.7 eV), pyrrolic N (399.65 eV), graphitic N (400.6 eV), oxidic N (402.4 eV), and N-B (397.6 eV) were obtained. The relative contributions of the various N configurations estimated from the N 1s spectra are depicted in Figure 1c. Note that the N content in all NB/GO_X samples is above 2.7%, reaching as high as 3.22% (for the NB/GO_35) close to that (3.2%) reported for N-doped graphene obtained through thermal annealing of GO in NH3 at 300 °C.18 The sum of total N groups and respective content of pyridinic N, graphitic N, and pyrrolic N configurations were higher in the samples annealed at 700 °C compared to their non-annealed NB/GO_X counterparts, which is possibly due to removal of unstable oxygen containing species and enhanced degree of graphitization after annealing. As opposed to graphitic N and oxidic N in NB/GO_X, the fractions of which decreased monotonically with the ultrasonic bath temperature (Tsonic), pyrrolic and pyridinic N groups tend to form with increase in concentrations at elevated temperatures (Figure 1c), similar to previous observations.21 When GO was first doped with B followed by N to yield BN/GO under similar ultrasonication conditions, the resultant percentage of oxidic N increased whereas the content of pyrrolic N and N-B groups decreased compared to its NB/GO counterpart. Deconvolution of the B 1s region revealed four chemical states of boron, namely: boron bound to three carbon atoms (BC3) at 191 eV, boron bound to two carbon and one oxygen (BC2O) at 192.3 eV, boron bound to one carbon and two oxygen atoms (BCO2) at 193 eV, boron singly bound to nitrogen (B-N) at 191.6 eV (Figures 2a and b).10, 22

No obvious peak of BC3, a moiety associated with the graphitic boron sites, was observed

for the non-annealed NB/GO_X samples (Figure 2a). The content of both BCO2 and BC2O dropped with the increase of Tsonic prior to annealing (Figure 2c). Such occurrence is likely due to the instability of BCO2 and BC2O species which were detached from the surface of GO when raising Tsonic. In contrast, the intensity of the B-N bond increased with Tsonic. This can be correlated with the increase of pyridinic N concentration at high Tsonic since pyrindinic-N with unpaired electrons tends to bond with B to form B-N structure.9 As expected, the percentage of B dopant in BN/GO is higher than that in NB/GO, possibly owing to more vacancy sites available after B doping in the former. Analogous to the case of N doping, the sum of B atoms in the annealed NB/GO samples is greater than for their respective nonannealed counterparts. Further, BC3 appeared attributable to the conversion of BCO2 or BC2O species. This matches theoretical calculations that BC3 was the most stable moiety.23 Such graphitic B-doping leads to improved conductivity, in favor of electrocatalysis. Interestingly, the N and B dual-doped catalyst contains a higher level of B functional groups compared to B singly doped GO samples (Figure S5). This provides benefits in enhancing electrocatalysis of the ORR, as will be shown later in the electrochemical results.



ID/IG

(a)

GO

(b)

NB/GO_5_700

(c)

1.20

NB/GO_35_700

1.16 1.12 1.08 1.04 1.00 0.96 0.92 0.88

2μm

(d)

NB/GO_55_700

2μm

(e)

2μm 1.1

NB/GO_75_700

0.84 0.80

(f)

1.01

1.00

0.98 0.95

1.0

ID/IG

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NB /G NB O_ /G 75 O_ _7 NB 55 00 NB /G _7 O_ 00 /G 35 O _5 _7 _7 00 00

0.86

0.9

GO

0.8

2μm

2μm

0

100

200

300

400

500

600

Data Point Number

Figure 3. (a-e) Two-dimensional Raman maps and (f) the average D-to-G peak intensity ratios of the NB/GO samples synthesized at various ultrasonication temperatures and subsequently annealed at 700 °C.

Raman spectra of the NB/GO catalysts are given in Figure S6. Two clearly distinct peak at 1346 and 1595 cm-1 are identified, attributed to the D and G bands, respectively. The D band is associated with the breathing mode of aromatic rings or defect sites, whereas the G band corresponds to the tangential vibration mode of sp2 carbon atoms.24 The D-band position of graphene upshifted to higher frequencies by up to 20 cm-1 after doping with nitrogen and/or boron, which is ascribed to heteroatom substitution in the plane. The dependence of the ratio of the intensity of the D band (ID) to that of the G band (IG) on Tsonic is shown in Figure 3. The ID/IG intensity ratio maps were recorded over an area of 10 × 10 μm2 for all the studied NB/GO samples, as displayed in Figures 3a-e. The (ID/IG) ratio provides insight on the degree of defects, disordered structures, and level of graphitization. The ID/IG ratio of NB/GO_55_700 was slightly higher than for the other samples (Figure 3f), indicating that the defects generally increased with Tsonic in the range 5-55 °C. This trend is consistent with the increase of the overall N content. However, the ID/IG ratio at 75 °C showed a decreasing tendency, which can be attributed to the loss of some defects (such as oxygen-containing groups).


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Figure 4. (a) SEM and (b) TEM images of NB/GO_55_700. (c) STEM image of NB/GO_55_700 with corresponding EDX maps of C (d), N (e), and B (f) over the region shown in c. (g) Enlarged TEM images of few-layer NB/GO_55_700 showing 7 layers, 3 layers, and 2 layers, respectively (from left to right). (h) HRTEM and (i) AFM images NB/GO_55_700.

SEM and TEM were used to probe the morphology and microstructure of the as-prepared NB/GO catalysts. Similar results were obtained in all the cases. As a representative example, NB/GO_55_700 clearly exhibits shape of flakes that are slightly wrinkled and folded (Figures 4a, 4b, S7a, and S7e). The crumpled structure exposes a high density of active sites, which is beneficial to realizing high utilization of the active sites for the oxygen reduction reaction. The EDX elemental mapping in both SEM and STEM analyses disclosed a homogeneous distribution of boron, nitrogen, and carbon (Figures 4c-f and Figures S7a-d), and further confirmed the doping of N and B on the surface of GO. The TEM image of NB/GO_55_700 in Figure 4b shows a nanosheet morphology with thin flakes typical of two-dimensional materials such as graphene.25 High-resolution TEM imaging revealed that the majority of nanoflakes were comprised of only a few layers (2-7 layers) (Figure 4g). Figure 4h illustrates a moire pattern originating from an interference effect due to irregular stacking of the doped GO layers. However, we did not observe significant deviation from lattice spacing by analyzing the FFT of the sheets.26 Fig. 4i shows an AFM image of the NB/GO_55_700 sample, further confirming the few-layer nature of the resulting NB/GO flakes.


0 -1 -2 O2 N2

-3 0.4

0.6

0.8

1.0

(b)

400 rpm

GO

-1 Graphene 5% Pt/C

-2 NB/GO_55_700

-3

1.2

20% Pt/C

0.2

0.4

Onset Potential/mV (vs. RHE)

Current Density/mA cm

-2

1.2

(d)

-1.2

-1.6

0.7 V 0.65 V 0.6 V

-2.0

0.4 V 0.3 V 0.2 V

-2.4 4.4

4.8

5.2

5.6

-1

0.8

-1.8

0.4



/S

1/2

rad

0.25 -1/2

4.4

0.30

0.35

4.8

5.2

5.6

6.0

Total N Content/%

0.84

0.8

-2.4 0.82

-1.7

-1.6

-1.6

0.81

0.81

0.79

0.78

0.78

-1 -2 -3

0.73

-4

0.69

4.0

-5

Gr NB GO NB BN N/G B/G NB NB ap /GO /GO /GO /GO /GO O_ O_ he 55 55 _3 _7 _5 _5 _5 ne _7 _7 5_ 5_ 5_ 5_ _7 00 00 70 70 70 70 00 0 0 0 0 0.5

(g)

3.8

3.2

NB/GO_55_700

0.20

4.0

-1.8

3.4

-1/2

0.76

-1.9

3.6

0.6

0.15

0.78

1.0

1.0

Electron Transfer Number/n

0.6V 0.5V 0.4V 0.3V 0.2V

0.10

0.80

-0.73

6.0

-1

2

(f)

0.2 0.05

0.82

-1.1

Total N Content/%

1.0

0.84

(e)

0.6

4.0

0.8

(c)

Potential/V (vs. RHE)

Potential/V (vs. RHE) -0.8

0.6

0.86

Overpotential/V (vs.RHE)

0.2

0

NB B/G N/G NB BN NB NB /G /G /G /G /G O_ O_ O_ O_ O_ O_ O_ 55 55 75 35 55 55 5_ _7 _7 _7 _7 _7 _7 70 00 00 00 00 00 00 0

(h)

0.4 -1

0.3 0.2

V dec

c V de 68 m -1 ene h p Gra dec mV 90 GO

32 20% Pt/C

0.1

mV dec

-1

V dec

3m 700 4 O_55_ NB/G

0.0 -4.0

-3.5

-3.0

-2.5

-2.0 -2

Log j/mA cm

Figure 5. (a) CVs of NB/GO_55_700 in O2- (solid line) and N2- (dashed line) saturated 0.1 M KOH at a scan rate of 100 mV s-1. (b) LSVs of GO, liquid-phase exfoliated graphene (in NMP), 5% Pt/C, 20% Pt/C and NB/GO_55_700 in O2-saturated 0.1 M KOH at a scan rate of 5 mV s-1. Correlation of (c) onset potential and (d) current densities at 0.7, 0.65, 0.6, 0.4, 0.3, and 0.2 V (vs. RHE) with N contents. (e) Comparison of the onset potential and current density at 0.4 V (vs. RHE) for GO, liquid-phase exfoliated graphene, B/GO_55_700, N/GO_55_700, NB/GO_5_700, NB/GO_35_700, NB/GO_55_700, NB/GO_75_700, and BN/GO_55_700. (f) K-L plots derived from LSVs recorded at different rotation speeds. (g) Number of electrons, n, transferred during the ORR. (h) Tafel plots for GO, liquid-phase exfoliated graphene, 5% Pt/C, 20% Pt/C, and NB/GO_55_700 samples extracted from the LSV results.

The electrocatalytic activity of the N and B co-doped graphene oxide samples for the ORR was examined in alkaline solutions. The ORR results were presented in Figure 5. Figure 5a shows cyclic voltammograms (CVs) of the NB/GO_55_700 catalyst obtained in O2- and N2saturated 0.1 M KOH in the potential range from 0.2 to 1.1 V (vs. RHE). NB/GO_55_700 exhibited a well-defined irreversible reduction wave at about 0.80 V (vs. RHE) under O2 saturated conditions, whereas no obvious faradaic ORR current was observed for the CV recorded under N2 in the same potential range. To gain deeper insight into the ORR performance of NB/GO_55_700 in relation to other catalysts, the LSV of NB/GO_55_700 recorded in O2-saturated 0.1 M KOH at a scan rate of 5 mV s-1 and with electrode rotation at


-1

C 45 m

5% Pt/

-1.5

-1

-2

1


-2

(a)



-2

2

J /cm mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Onset Potential/V (vs. RHE)


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400 rpm was compared with that of commercial Pt/C (Pt/C (5%) and 20%)), the state of the art ORR catalyst, pristine liquid-exfoliated graphene, and GO as reference materials recorded under similar conditions (Figure 5b). NB/GO_55_700 showed a significantly more positive onset potential (0.82 V vs. RHE) compared to GO (0.69 V vs. RHE), graphene (0.73 V vs. RHE), and 5% Pt/C (0.78 V vs. RHE), but a bit lower than that of 20% commercial Pt/C (0.95 V vs. RHE). NB/GO_55_700 exhibited a comparably higher limiting current density -2.43 mA cm-2, outperforming all the other catalysts in this work except 20% Pt/C. The electrocatalytic ORR activity of NB/GO_55_700 is comparable to many previously reported N and B co-doped graphene materials (as shown in Table S1), and is better than that of the catalysts prepared under similar conditions but with only N or B doping of GO (Figures 5e and S8). A possible reason that may account for such enhancement is that the B and N in dualdoped samples are predominantly in separated state, in which the extra electron of N and the empty orbital of B conjugate into carbon system. The electron density near the Fermi level and atomic spin density of neighboring carbon atoms are thus enhanced, induced by the host-guest electronic interaction.9 Such synergic effect makes NB/GO superior to either singly doped GO in the ORR. For B and N dual-doped GO, the pyridinic N groups could be changed from “inactive” in N-graphene to “active” after B incorporation, which may be another probable reason for the better ORR activity of (NB/GO) samples compared to the singly doped B only (B/GO) and N only (N/GO) samples.27 The improved catalytic activity toward the ORR was also suggested to be due to reduction of the HOMO-LUMO (the lowest unoccupied molecular orbital) gap on the basis of cluster type quantum mechanical calculations.28, 29 A smaller HOMO-LUMO gap means that it is easier to add electrons to a high-lying LUMO or to remove them from a low-lying HOMO, benefiting the ORR. Due to the higher electronegativity of N compared to carbon, in N-doped carbon species, ORR enhancement arises from the possibility that nitrogen attracts electrons from the highest occupied molecular orbital (HOMO) of carbon, inducing a partial positive charge on adjacent carbons with high spin density (in ortho positions to N dopants) and subsequently decreasing the OOH or O2 adsorption energy.30 Boron doping enhances oxygen chemisorption due to accumulation of electrons in the vacant 2pz boron orbital from the π-conjugated carbon system, thereby facilitating subsequent electron transfer to the chemisorbed oxygen and weakening the O-O bond. The content of N species in N-doped GO and N, B co-doped GO is very close, while the content of B (especially the active BC3 species) is greatly improved upon N and B dual-doping (explaining the higher ORR activity of NB/GO than B/GO). These aforementioned aspects may contribute to the enhanced ORR performance of the N and B codoped GO catalysts compared to either singly doped GO samples. The performance of N- and B-co-doped carbon catalysts seems to depend on the synthetic approach, degree of doping and the resulting distribution of the B and N functional groups within the catalyst.31, 32 Gong et al. showed superior electrocatalysis of the ORR by graphene nanoribbons co-doped with both B and N compared to Pt/C, the state of the art ORR catalyst. 33 In these studies, the



optimal level of doping with both N and B was about 10 at%. Such a catalyst achieved onset potential for ORR as high as 1.07 V (vs. RHE) with the half-wave potential (0.97 V vs. RHE) being higher than that for Pt/C (0.87 V vs. RHE). Another crucial factor of the ORR performance of B and N co-doped carbon materials reported by Zhao et al.,32 pertains to whether B and N are chemically bonded, or exist as discrete species located separately, and the B/N ratio. Since B and N exert opposite electronic effects on the adjacent carbons, a neutralization effect occurs between the extra electrons from N and the vacant orbitals of B, in the case of chemical bonding between B and N thereby disfavoring the energetics of O 2 chemisorption and subsequent reduction. A study by Zheng et al. indeed demonstrated that the ORR activity of hexagonal boron nitride in which B and N are chemically bonded was considerably inferior to that of B and N co-doped graphene.13 As shown in Figures 5c and d, the onset potential and current density at different potentials increased with the overall N content. Meanwhile, the ORR performance also scaled with the contents of pyridinic N (Figures S9a and S9b), graphitic N (Figures S9c and S9d), BC3 (Figures S9e and S9f) as well as the total content of the three species (Figures S9g and S9h). KouteckyLevich (K-L) plots of NB/GO_55_700 at various potentials exhibited good linearity (Figure 5f), indicating that there are likely no parasitic reactions and the ORR was first order with respect to oxygen concentration. The number of electrons transferred during electrocatalysis of the ORR by NB/GO_55_700 calculated from the slope of K-L curves at the potential of 0.4 V (vs. RHE) was 3.98, which was well higher than that of GO singly doped with B (3.32) and N (3.62) (Figure 5g). For the B/GO catalyst, the number of electrons transferred during the ORR is in good agreement with literature reports.34 In the pioneering studies of doping carbon materials with boron, Yang et al. observed a two-electron reduction of oxygen to hydrogen peroxide. Increasing the boron content decreased the overpotential for ORR and led to enhancement of the electrocatalytic current but hydrogen peroxide remained the dominant product. Improved ORR activity by the CNTs through boron-doping was attributed to electron transfer from the anti-bonding orbitals of boron to the p orbitals of the conjugated carbons leading to electron deficiency on the boron, thereby enhancing oxygen chemisorption at the boron sites. This interaction weakens the O=O bond thus facilitating the ORR.35 Although some studies have reported the reduction of O 2 to OH− in alkaline electrons,36 questions concerning the suspicious promotional effect of metal impurities remain to be resolved.37, 38 Several studies of the ORR on nitrogen-doped materials without substantial metal impurities suggest that O2 reduction proceeds via the two-electron pathway with substantial formation of hydrogen peroxide.38, 39 In the present studies, the ORR on the dual B and N co-doped graphene oxide, NB/GO_55_700, proceeded predominantly via the direct 4e− transfer pathway forming OH− as the main product of O2 reduction. It can be concluded that the favorable electrocatalytic reduction of O2 by NB/GO_55_700 is due to a high density of active sites and synergistic interactions between the moieties of the dopant elements B and N in NB/GO_55_700. The results of our studies


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are consistent with previous reports where synergistic interaction between B and N resulted in enhanced electrocatalysis of the ORR on dual boron and nitrogen co-doped CNTs.40 For example, Ozaki et al. reported a catalytic ORR enhancement factor of 4-5 when carbon was singly doped with either B or N alone, whereas an enhancement factor of 20 was obtained for the dual N and B co-doped system.41 Furthermore, the Tafel slope of the ORR on NB/GO_55_700 was 43 mV decade-1, which was considerably lower than that on GO (90 mV decade-1) and graphene (68 mV decade-1), but similar to that of the ORR on 5% Pt/C (45 mV decade-1) and higher than on 20% Pt/C (32 mV decade-1) (Figure 5h). The results thus reveal superior kinetics of the ORR on NB/GO_55_700 compared to pure non-doped GO and pristine liquid-phase exfoliated graphene. 3 -2

(a)

1.0


1.5

NB/GO_55_700

j/j0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20% Pt/C

0.5

(b)

2 1 20% Pt/C

0 -1 -2

3 M CH3OH

-3

0.0 0

2

4

6

Time/h

8

10

0

500

1000

1500

2000

Time/s

Figure 6. (a) Durability evaluation of NB/GO_55_700 and 20% Pt/C during the ORR at a constant potential of 0.5 V (vs. RHE) in O2-saturated 0.1 M KOH and electrode rotation at 100 rpm for 10 h. (b) Chronoamperometric responses at 0.5 V (vs. RHE) in O2-saturated 0.1 M KOH on NB/GO_55_700 and commercial 20% Pt/C electrodes (100 rpm) followed by introducing CH3OH (3 M) at 300 s.

We evaluated durability of the ORR on NB/GO_55_700 by means of continuous chronoamperometric polarization at 0.5 V for 10 h. As can be seen (Figure 6a), NB/GO_55_700 maintained stable performance in the course of 10 h of continuous polarization while the ORR activity of Pt/C declined by more than one half. Meanwhile, we tested the ORR selectivity and poisoning resistance of NB/GO_55_700 and Pt/C in the presence of methanol (Figure 6b). NB/GO_55_700 displayed marginal loss in ORR performance after the addition of 3 M methanol, indicative of excellent immunity against poisoning from methanol crossover. On the other hand, the ORR current of the electrode modified with 20% Pt/C decayed sharply upon the addition of methanol to the electrolyte.

CONCLUSIONS In summary, we have successfully developed a simple and effective method for rapid and scalable synthesis of N and B co-doped graphene oxide by ultrasonic treatment. The content and configuration of both N and B can be readily modulated by manipulating ultrasonic and subsequent annealing temperatures. The obtained NB/GO samples showed better electrocatalytic ORR activity (onset potential of 0.82 V (vs. RHE) and limiting current density



of 2.43 mA cm-2) compared to the samples doped with only B or N, thus indicating a synergistic interaction between the B and N species. O2 reduction on the optimized sample, NB/GO_55_700, proceeded via a pathway dominated by a one-step 4e− transfer process forming OH− as the main product. XPS studies revealed pyridinic N, graphitic N, and BC3 as predominant species in the NB/GO catalysts and are thus the most likely active sites for the ORR. Our results therefore indicate that co-doping graphene oxide with nitrogen and boron can lead to the formation of very promising metal-free ORR catalysts for fuel cells.

EXPERIMENTAL SECTION Materials. All chemicals were purchased and used without further purification. Graphene oxide powder (TM-01PR-01) with a purity of > 98.5 wt% was purchased from Sinocarbon Graphene Marketing Center. Boric acid (CAS number: 10043-35-3) and aqueous ammonia (CAS number: 1336-21-6) were obtained from Aladdin. Ethanol was acquired from Beijing Chemical Works. Synthesis of N, B Co-Doped Graphene Oxide. The NB/GO catalysts were synthesized by dispersing 30 mg of N/GO in 10 mL of aqueous boron acid in sealed vials followed by ultrasonication in a bath sonicator for 3 h at 55 ºC. The ultrasonication temperature was varied from 5 ºC to 35 ºC, 55 ºC, and 75 ºC for a fixed duration of 3 h. Finally, the obtained NB/GO composite was annealed under a N2 atmosphere at 700 ºC for 1 h. The temperature ramp was 5 ºC min-1. The non-annealed product was designated as NB/GO_X (X is the ultrasonication temperature) and correspondingly the annealed product was designated as NB/GO_X_700. Characterization. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a K-Alpha spectrometer (Thermo Fisher Scientific Inc., Switzerland) equipped with a monochromatic Al Kα source operated at 150 W. All spectra were calibrated based on the C 1s binding energy at 284.8 eV. Scanning electron microscopy (SEM) of the samples were performed on a field emission microscope (FEI Quanta 600 FEG) operated at 20 kV and equipped with an energy-dispersive X-ray spectrometer (EDX). Transmission electron microscopy (TEM) was conducted using a JEOL 3000F microscope. TEM samples were prepared by depositing a droplet of the sample suspension onto a Cu grid coated with a lacey carbon film. Atomic force microscopy (AFM) images were obtained on a 5500 AFM/SPM system (Agilent Technologies Inc., USA) in the tapping mode at room temperature. Raman spectra of the samples deposited on 300 nm thick SiO2/Si substrates were collected using a Renishaw in Via Raman microscope with a He/Ne Laser excitation at 532 nm. Electrochemical Measurements. Electrochemical measurements, including cyclic voltammetry (CV) and linear-scan voltammetry (LSV) were performed in a three-electrode glass cell using Ag/AgCl as reference electrode, Pt wire as counter electrode, and glassy carbon as working electrode and a CHI 760E electrochemical workstation (Shanghai Chenhua Co., China). Rotating disk electrode (RDE) experiments were run on an AFMSRCE


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RDE control system (Pine Inc., USA). The details of preparation of catalyst inks are described in the Supporting Information. The electrochemical experiments were conducted in a 0.1 M aqueous solution of KOH saturated with O2 for oxygen reduction reaction. The onset potential in this work is determined by taking the potential at the intersection point of the tangent at maximum slope. The number of electrons, n, transferred per oxygen molecule during the ORR process was calculated by the Koutecky-Levich (K-L) equation [Eq. (1)]:

1 1 1 1 1 = + = + j jk jL jk B 

(1)

where B = 0.62nFAD2/3ν-1/6C*, j, jL, and jK are the measured current density, diffusion-limiting current density, and kinetic-limiting current density at a specific potential, respectively; n, the number of electrons transferred; D, the diffusion coefficient of the analyte, in this case O2 (1.9 × 10-5 cm2 s-1); F, the Faraday constant (96,485 C mol-1); A, the geometric area of the electrode (0.19625 cm2); ν, the kinematic viscosity of the electrolyte (0.01 cm2 s-1), and C* the solubility of oxygen in the electrolyte, which for the case of KOH (0.1 M) was taken to be 1.2 × 10-6 mole cm-3.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details; FTIR spectra; XPS spectra; Raman spectra; SEM image together with EDX maps; TEM image; LSV plots; correlation of onset potential and current density with N and B configurations and their contents; table of ORR activities of the NB/GO_55_700 catalyst and some reported carbon materials

AUTHOR INFORMATION Corresponding Authors *(Z.S.) E-mail: [email protected]. *(J.M.) E-mail: [email protected]. Author Contributions The paper was written by M.Z, Z.S., and J. M. through contributions of all authors. The project was supervised by Z. S. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the State Key Laboratory of Organic-Inorganic Composites (No. oic-201503005); Fundamental Research Funds for the Central Universities (No. buctrc201525);



Beijing National Laboratory for Molecular Sciences (BNLMS20160133); State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University, No. M2201704). AWR thanks the support of the Royal Society.

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For Table of Contents Use Only.

We report simple ultrasonication assisted synthesis of nitrogen and boron dual-doped graphene oxide with tunable doping configuration and content for ORR.


Ultrasound-Assisted Nitrogen and Boron Codoping of Graphene

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