Copper Nitride Nanostructure for the Electrocatalytic Reduction of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Copper Nitride Nanostructure for the Electrocatalytic Reduction of Oxygen: Kinetics and Reaction Pathway Siniya Mondal, and C. Retna Raj J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03840 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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The Journal of Physical Chemistry

Copper Nitride Nanostructure for the Electrocatalytic Reduction of Oxygen: Kinetics and Reaction Pathway

Siniya Mondal and C. Retna Raj*

Functional Materials and Electrochemistry Lab, Department of Chemistry Indian Institute of Technology, Kharagpur Kharagpur 721302, India E-mail: [email protected]

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Abstract Synthesis of inexpensive electrocatalytically active non-precious metal-based catalyst for oxygen reduction reaction (ORR) is of significant importance for the development of energy conversion and storage technologies. Herein we describe a new single-step solvothermal method for the synthesis of nanostructured Cu3N and its electrocatalytic activity toward ORR. Our synthetic approach involves reduction of Cu(II) to Cu(I) and subsequent nitridation of Cu(I) by hexamethylenetetramine in argon atmosphere at 200 ºC. At elevated temperature, hexamethylenetetramine hydrolyses to formaldehyde and ammonia and the hydrolyzed products efficiently function as reducing and nitridating agents of the copper precursor. The crystalline Cu3N nanoparticles have quasi-spherical shape with an average size of 80 nm. The nanoparticles are supported on reduced graphene oxide (rGO) and nitrogen-doped rGO (N-rGO) catalyst support and the electrocatalytic activity towards ORR is evaluated in terms of onset potential, mass specific activity, Tafel slope, and kinetics and reaction pathway. The N-rGO-supported Cu3N (N-rGO/Cu3N) has superior ORR activity compared to the as-synthesized Cu3N and rGOsupported Cu3N. The rate constant for the reduction of O2 to H2O and the disproportionation of intermediate H2O2 are calculated. The kinetic analysis shows that N-rGO/Cu3N favors fourelectron reduction of oxygen to water and the disproportionation of trace amount of in situ generated HO2− (~6%) is negligible. N-rGO/Cu3N is durable and has good tolerance toward the anode fuel methanol. The synergistic effect of N-rGO and Cu3N plays important role in controlling the electrocatalytic activity.


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Introduction The transition metal nitrides, an important class of materials, have unique catalytic, optoelectronic and magnetic properties owing to the localization of electrons in anion lattice. Though the metal nitrides are known for more than a century, the unique properties have been exploited for various applications very recently.1-4 The electronic structure of metal nitrides affords noble metal-like catalytic properties and they are being used as potential candidate in the development of electrochemical energy conversion and storage devices such as fuel cell, batteries and supercpacitors.2-4 Their electrocatalytic properties toward various electrochemical reactions have been investigated. Among the transition metal nitrides, copper nitride has drawn considerable attention for catalytic, molecular spintronic, energy storage, sensing, etc. applications.5-6 The shape-controlled solution-based synthesis of nanoscale metal nitride is a challenging task and the synthetic method often requires high temperature and pressure and reactive nitrogen precursors. Traditionally, the vapor deposition, plasma laser, solid-state metathesis approaches have been employed for the synthesis of metal nitrides.1,5,7-21 Copper nitride has been synthesized by solvothermal, radio-frequency reactive, pulse magnetron sputtering, plasma enhanced atomic layer deposition and chemical vapor deposition methods.5,1221

Suitable nitridating reagent or the ammonia atmosphere is critically required for the

solvothermal methods.5,17-21 The development of solvothermal synthetic methods without involving tedious procedure is highly desirable. Growing energy requirements and depletion of fossil fuel demand for renewable energy sources. Fuel cell is one of the promising green energy sources. The function of polymer electrolyte membrane fuel cell involves the cathodic reduction of oxygen to water and anodic oxidation of fuel. The sluggish electron transfer kinetics associated with cathodic oxygen 3 ACS Paragon Plus Environment


reduction reaction (ORR) requires efficient electrocatalyst.22 Traditionally, noble Pt-based materials that favors the four-electron reduction of oxygen to water are used as cathode catalyst.23 The high cost, lack of durability and poor tolerance toward anode fuel are some of the serious concerns with the traditional Pt-based catalysts.24 In the past few years, the design and synthesis of inexpensive durable and efficient non-Pt group metals and metal-free carbon-based cathode electrocatalyst received significant attention.25-27 For instance, transition metals, metal macrocycles, chalcogenides, oxides, sulfides, carbides, ntirides, etc. and metal-free heteroatomdoped catalysts are emerging as potential cathode catalyst.24-27 It has been shown that some of these non-Pt and metal-free catalysts have Pt-like electrocatalytic activity toward ORR though the actual mechanism is not yet well established.25-28 Moreover, the electrocatalytic activity of these non-precious catalysts depends on the size, shape, surface and electronic structure and catalyst support. It is generally accepted that the physical and chemical nature of carbon support largely control the electrocatalytic activity. The ideal catalyst support should have large surface area, high stability in harsh condition, and they should have high electronic conductivity.29-31 Reduced graphene oxide (rGO), carbon nanotube (CNT), activated and heteroatom doped carbon, etc. have been proved to be an efficient catalyst support for metal based ORR catalysts.30-32 Although the ORR activity of titanium nitride-based catalyst is known since 1966,33 the ORR activity of transition metal nitrides received considerable interest in the recent years.2,5,27,34-38 The careful survey of literature reveals only a handful of papers report the ORR activity of Cu3N5,19,38 and to the best of our knowledge the kinetics and the reaction pathway for the reduction of oxygen on Cu3N based catalyst is not yet explored. Our group is interested in developing novel non-precious and metal-free electrocatalysts for ORR.30,39-40 In continuation of our efforts, herein we demonstrate a new single-step


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solvothermal

approach

for

the

synthesis

of

nanoscale

copper

nitride

using

hexamethylenetetramine (HMT) as a reducing and nitridating agent and its electrocatalytic activity toward ORR. We also demonstrate the kinetics, reaction pathway for ORR and the influence of the chemical nature of catalyst support on the electrocatalytic activity. Our nitrogendoped reduced graphene oxide supported Cu3N (N-rGO/Cu3N) has significantly high activity compared to the as-synthesized Cu3N and reduced graphene oxide supported Cu3N (rGO/Cu3N).

Experimental section Materials Copper nitrate (Cu(NO3)2•5H2O), urea, N,N-dimethylformamide (DMF) and n-hexanol were purchased from Merck. Nafion® (5% in lower aliphatic alcohol) and graphite powder were purchased from Sigma Aldrich. HMT was obtained from TCI chemicals. Other chemicals used in this experiment were of analytical grade. All the solutions used in this investigation were prepared with Millipore water (Milli-Q system). Instrumentation X-ray diffraction (XRD) analysis was carried out with a Bruker D8 advance unit using Cu−Kα (λ = 1.54 Ǻ) radiation. Transmission electron microscopic (TEM) images of the nanoparticles were acquired with JEOL JEM 2010 electron microscope operating at 200 kV. Xray photoelectron spectroscopic (XPS) analysis was performed with PHI 5000 Versa Probe II scanning XPS microprobe (ULVAC-PHI) using the energy source Al (Kα, hυ=1486.6 eV). Fourier transform infrared spectroscopic (FTIR) measurements were performed with a PerkinElmer FTIR spectrophotometer RX1. Electrochemical measurements were performed in a singlecompartment three-electrode cell using rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) of geometrical surface area 0.197 and 0.282 cm2, respectively as working, Pt 5 ACS Paragon Plus Environment


wire as auxiliary, and Hg/HgO (1 M NaOH) as reference electrodes. All electrochemical experiments were performed with Autolab potentiostat-galvanostat (302N), using computer controlled NOVA 1.11 software. Synthesis of rGO and N-rGO Graphene oxide (GO) was synthesized from natural graphite powder using modified Hummer’s method (supporting information).41 N-rGO was synthesized by thermal annealing of the physical mixture of GO and urea (mass ratio of 1:3) at 700 0C for 2 h in Ar atmosphere according to the earlier procedure.42 rGO was obtained by calcination of GO at an identical condition in absence of urea. Synthesis of Cu3N Cu(NO3)2•5H2O (0.05 M) was dispersed in 10 ml n-hexanol by ultrasonication and then HMT (0.05 M) was added. The reaction mixture was transferred to a quartz pressure tube and was degassed by purging with Ar gas for about 30 min and closed with pressure tube stopper to prevent the entry of atmospheric oxygen. The temperature was then slowly raised to 200 ºC in an oil bath on a magnetic stirrer. The color of the solution turned to brown during the reaction. The temperature was maintained at 200 ºC for an hour under constant stirring. The product was collected at room temperature by centrifugation and washed with ethanol. The as-synthesized Cu3N was supported on rGO or N-rGO catalyst support by mixing rGO or N-rGO (1 mg/ml) and Cu3N (2 mg/ml) in DMF (10 ml) and stirred for 24 h. The rGO and N-rGO supported Cu3N is referred as rGO/Cu3N and N-rGO/Cu3N composite material, respectively. The composite material was collected by centrifugation and washed extensively with copious amount of water and dried at ambient condition.


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Electrode Modification The working electrodes were polished well with alumina slurry (0.3 followed by 0.05 µm) on a polishing cloth and sonicated in Millipore water for 5-8 min. Finally the electrode was thoroughly rinsed with Millipore water and dried in Ar flow. The ink was prepared by sonicating 0.5 mg of as-prepared catalyst with the mixture of 20 µL of Nafion® and 180 µL of ethanolwater-DMF (3:1:2 v/v ratio) solution for 30 min. An aliquot (10-15 µL) of the as-prepared ink was drop casted on the surface of RDE/RRDE and dried at ambient condition.

Results and discussion Synthesis and characterization Cu3N was synthesized solvothermally in single-step at an elevated temperature of 200 ºC in inert atmosphere using HMT as reducing and nitridating agent (Figure 1). The synthetic approach involves (i) in situ generation of ammonia and formaldehyde, (ii) reduction of Cu(II) and (iii) nitridation of Cu(I) by in situ generated ammonia according to the following reactions. ∆

C6H12N4 + 6H2O → 6HCHO + 4NH3

(1)

2Cu2+ + HCHO + H2O → 2Cu+ + HCOOH + 2 H+

(2)

3Cu+ + NH3 Cu3N

(3)

It is well known that HMT undergoes hydrolysis at elevated temperature and yields HCHO and

Figure 1. Schematic illustration of the synthesis of N-rGO/Cu3N composite.



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NH3.43 It should be noted that the reaction was performed in n-hexanol and Cu(NO3)2•5H2O was the source of Cu(II) and water. The reduction of Cu(II) to Cu(I) by in situ generated HCHO and the subsequent nitridation of Cu(I) by NH3 produces Cu3N nanoparticles. Reaction performed in the absence of inert atmosphere failed to yield Cu3N presumably due to ariel oxidation of Cu(I). Wu et al and Xi et al synthesized Cu3N nanocubes at 110-250 ºC using long chain high boiling solvents and nitridating agents such as 1-octadecylamine, octadecene, oleylamine, etc.5,19 It is desirable to avoid the use of such high boiling solvents as they can passivate the catalyst surface and inhibit the activity. Recently, Nakamura and co-workers demonstrated the solution-based synthesis of coper nitride nanoparticle by ammonolysis in long chain alcohols such as 1-nonanol and 1-octonal at 150 oC.20 Nitridation of copper could not be achieved in short chain alcohols like 1-heptanol, 1-hexanol and 1-pentanol due to the formation of CuO. In our case, Cu3N was successfully obtained in 1-hexanol in inert atmosphere at the temperature of 200 ºC. Inert atmosphere is critically required to obtain Cu3N.

(100)

(110)

(111)

N-rGO/Cu3N (200) (210) (211) rGO/Cu3N

Cu3N JCPDS 74-0242

20

30

40

50

60

2θ

Figure 2. XRD profile of as-synthesized Cu3N, rGO/Cu3N and N-rGO/Cu3N. XRD profile of as-synthesized Cu3N nanoparticles shows characteristic signature (JCPDS no. 74-0242) corresponding to anti-ReO3 cubic crystal structure of Cu3N (Figure 2). The 8 ACS Paragon Plus Environment

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diffraction profile does not show any peaks for impurity or oxide species confirming that Cu3N is free from oxides and other impurities. Both N-rGO/Cu3N and rGO-Cu3N show similar signature for Cu3N along with a broad diffraction corresponding to the (002) plane of rGO/N-rGO at low angle. The chemical nature of N and Cu in Cu3N was analyzed by examining the core level N 1s and Cu 2p XPS profiles. The deconvoluted N 1s spectrum of Cu3N shows sharp peak at 397.4 eV which is the characteristic signature of N bonded to Cu in Cu3N (Figure 3A).5,38 The origin of the less intense peak at 398.6 eV is not very clear and it could be associated with the nitrogen in Cu4N phase.44,45 The chemical state of nitrogen in N-rGO was also examined and it has pyridinic (46%), pyrrolic (36%) and graphitic nitrogen (18%) (Figure S2). The N-rGO/Cu3N also shows

406 404 402 400 398 396 394 392 Binding energy (eV) raw fitted background 932.4 eV 942.0 eV 952.25 eV

970

pyridinic: 398.61eV pyrrolic: 399.51 eV graphitic: 400.5 eV

406

404

920

394 (D)

2p3/2 2p1/2

intensity

2p3/2

950 940 930 Binding energy (eV)

402 400 398 396 Binding energy (eV)

raw baseline 932.4 eV 952.24 942.2

(C)

2p1/2

960

(B)

raw fitted background Cu3N: 397.5 eV

(A)

intensity

intensity

raw fitted background 397.4 eV 398.6 eV

intensity

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


960

955

950 945 940 935 Binding energy (eV)

930

925

Figure 3. High resolution N 1s (A, B) and Cu 2p (C, D) XPS profiles of as-synthesized Cu3N (A, C) and N-rGO/Cu3N composite (B, D). 9 ACS Paragon Plus Environment


characteristic signature for Cu3N and N-rGO (Figure 3B, D). The deconvoluted N 1s spectrum of N-rGO/Cu3N shows additional one peak at 397.5 eV corresponding to the nitrogen in Cu3N along with other three nitrogens from N-rGO (Figure 3B). The high resolution spectrum of Cu 2p shows two peaks at the binding energy of 952.25 and 932.4 eV, respectively, corresponding to Cu 2p1/2 and Cu 2p3/2, respectively of Cu(I) in Cu3N (Figure 3C) and N-rGO/Cu3N (Figure 3D);5,38 the satellite peak is observed at 942 eV. Figure 4 is the TEM images of Cu3N and the N-rGO/Cu3N. The nanoparticles have quasispherical shape with an average size of 80 nm (Figure 4A). The high resolution TEM (HRTEM) image shows the fringe spacing of 0.26 nm corresponding to (100) plane of cubic Cu3N nanoparticles. The TEM image of N-rGO/Cu3N and rGO/Cu3N shows the presence of Cu3N (A)

(B) (100)

30 average particle size: 73 nm 20

Probability (%)

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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(C)

10 0

60 70 80 90 100 Particle size (nm)

0.26 nm

(D)

(E)

(F) (110) (100)

0.26 nm

Figure 4. TEM (A, C, D) and HRTEM (B, E, F) images of Cu3N (A, B) and N-rGO/Cu3N (C-F). The corresponding inverse fast Fourier transform patterns are shown in the inset of B and E. The SAED pattern of N-rGO/Cu3N (F). The histogram illustrating the particle size distribution is shown in the inset of C. 10 ACS Paragon Plus Environment

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nanoparticles and rGO/N-rGO sheets (Figure 4B and Figure S3). The rGO/N-rGO sheets prevent the self-aggregation of Cu3N nanoparticles. HRTEM images and the corresponding spotty selected area diffraction (SAED) pattern confirm the crystallinity of the Cu3N particles (Figure 4 inset). The band gap of as-synthesized cubic Cu3N was 1.6 eV which is in well accordance with the literature value.5 Electrocatalytic reduction of oxygen The electrochemically active surface area (ECSA) of Cu3N, N-rGO/Cu3N and rGO/Cu3N was obtained from specific capacitance measured at the potential of 0.3 V (Figure S4). The (B)

2 j (mA/cm2)

2

3

(A)

0

0

Cu3N rGO/Cu3N

-3

N-rGO/Cu3N

-6

20% Pt/C

0 -0.6 -0.3

0.0

j (mA/cm2)

4

-2

-2 200 500 800 1100 1400 1600

-4

1600 rpm Scan rate: 5 mV/s

Mass specific activity (mA/mg)

-0.9 180

-0.6 -0.3 E/V vs Hg/HgO

(C)

Scan rate: 5 mV/s

-6

-1.0

0.0 4.5

Mass specific activity

no. of electrons transferred 4.2 150

loading: 63 µg/cm2 120

3.9 3.6

90

3.3

60

3.0

30

0.4 E/V vs Hg/HgO

-4

no. of electrons transferred

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


-0.8

-0.6 -0.4 E/V vs Hg/HgO

-0.2

0.0

(D)

Cu3N (106 mV/dec) rGO/Cu3N (62 mV/dec) N-rGO/Cu3N (53 mV/dec)

0.2

20% Pt/C (62 mV/decade)

0.4

0.0

0.2

-0.2

-0.2

0.0

-3

0

3

-0.4

2.7

N-rGO/ rGO/ Cu3N Cu3N

N-rGO rGO

-0.6 -1.0 -0.5

Cu3N

0.0

0.5

1.0 1.5 log jk

2.0

2.5

3.0

Figure 5. Polarization curves illustrating the electrocatalytic activity of Cu3N, rGO/Cu3N, N-rGO/Cu3N and 20% Pt/C (A). Polarisation curves obtained with N-rGO/Cu3N at different rotation rate (B). Electrolyte: O2 saturated 0.1 M KOH. Plot illustrating the mass specific activity and number of electrons transferred (C). Tafel plot for ORR (D).



ECSA of Cu3N, N-rGO/Cu3N and rGO/Cu3N was calculated to be 14.12, 51.25 and 43.75 cm2, respectively. The ECSA of both N-rGO/Cu3N and rGO/Cu3N is significantly higher than the assynthesized Cu3N owing to the presence of rGO/N-rGO sheets. Such large surface area would favor facilitated electron transfer kinetics for ORR. The electrocatalytic activity of Cu3N-based composite toward ORR was evaluated by hydrodynamic voltammetry in O2 saturated alkaline electrolyte. As shown in Figure 5A N-rGO/Cu3N exhibits high electrocatalytic activity with respect to the as-synthesized Cu3N and rGO/Cu3N. We could achieve a limiting current density of 5.1 mA/cm2 with N-rGO/Cu3N which is comparable to that of 20% Pt/C, though the onset potential is slightly less positive (∼100 mV). The onset potential on N-rGO/Cu3N is 20-90 mV more positive than that of as-synthesized Cu3N, rGO/Cu3N catalysts (Figure 5A and Figure S5A). Such positive shift in the onset potential suggests the enhanced electrocatalytic performance of N-rGO/Cu3N. The limiting current density gradually increases with increasing the rotation speed, indicating the diffusion controlled process (Figure 5B). Further insight into the kinetics of ORR was obtained from Koutecky-Levich (K-L) analysis (equation 4 and 5) (Figure S6).46

=

+

. … … … … . . . . . 4

$ $ # . ! " % &

=

+

'

. … … … … … . . . 5

where, iK is kinetic current and iD is the diffusion limited current. ‘n’ is the number of electrons transferred, ‘F’ is Faraday’s constant, ‘A’ is the surface area of the electrode, ‘C0’ is the concentration of oxygen, ‘)*

2/3

’ is the diffusion coefficient of oxygen, ‘ν’ is the kinematic

viscosity, ‘ω’ is the angular velocity, and ‘k’ is the heterogeneous rate constant. The mass 12 ACS Paragon Plus Environment

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specific activity was obtained by normalizing the current with the mass of Cu3N loaded on the electrode surface. The mass activity of N-rGO/Cu3N is 1.2−7 times higher than the other catalysts (Figure 5C). The mass normalized current density as well as onset potential largely depends on the amount of catalyst loaded on the electrode surface. High mass specific activity was obtained at the loading of 63 µg/cm2 (Figure S7). The number of electron transferred (n) was calculated by K-L analysis and it was found that both N-rGO/Cu3N and rGO/Cu3N favor 4electron pathway for ORR (Table S1). The n value for ORR on N-rGO/Cu3N calculated by K-L analysis (n,- = 4.05) slightly exceeds the theoretical value of 4 and it could be associated with the limitation of K-L analysis for ORR as reported recently.47 The electrocatalytic performance was further evaluated by Tafel analysis using mass transport-corrected kinetic current density (Jk) (equation 6).23,40 Jk= (Jlim × Jdiff)/(Jlim - Jdiff)

.…………..... (6)

Tafel slope of 53 mV/dec was obtained with N-rGO/Cu3N and is very close to that of traditional Pt-based catalyst, suggesting N-rGO/Cu3N favor similar mechanistic pathway as Pt (Figure 5D). To get further insight into the electrocatalytic performance of the composite material, RRDE experiments were performed (Figure S8). The number of electrons (n) and peroxide yield (%HO2‾ relative to total products) was calculated from the ratio of ring and disk currents (IR/ID), using the equations 7 and 8,40,48 where, N is the current collection efficiency of RRDE (0.37) n = 4NID/(NID+IR)

.………….....(7)

% HO2‾ = 200IR/(NID+IR)

..………….....(8)

The RRDE analysis (Figure S9) yields the ‘n’ value (n../ ) of 3.87, which is less than that was obtained in the K-L analysis. Only ~6% of HO2‾ was generated in the limiting current region, further confirming the direct 4-electron pathway for reduction of oxygen (Figure S9). However,



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12−28% HO2− was obtained with the other catalysts examined in this investigation. It is interesting to note that the catalyst support (N-rGO or rGO) has significant role in controlling the activity of the catalyst. For instance, the onset potential, mass specific activity and number of electron transferred on N-rGO/Cu3N is higher than rGO/Cu3N. It should be highlighted here that the mass activity obtained for N-rGO/Cu3N is significantly higher than the other Cu3N based catalysts available in literature.5,38 The % of HO2‾ generated during ORR on N-rGO/Cu3N is 2 times lower (Table S1) than at on rGO/Cu3N, implying that the N-rGO support and possible synergistic effect of N-rGO and Cu3N favors the 4-electron pathway. It is well known that the chemical nature of nitrogen in N-doped graphene has significant contribution in the electrocatalytic reduction of oxygen.32,40 In our case, it is considered that the presence of large amount of pyridinic nitrogen promote the electron transfer kinetics of ORR. The RDE kinetic analysis shows the direct 4-electron reduction of O2 to H2O. However, the RRDE analysis indicates the generation of small amount of HO2‾ (~6%). In order to further understand the possible reaction pathways and the associated rate constants with the ORR kinetics was further analyzed according to Damjanovic49 (Scheme 1). Two possible pathways have been proposed for the reduction of oxygen. In the parallel pathway, simultaneous 2- and 4electron transfer occurs. In the series pathway, oxygen is reduced to H2O (k2, k3) through the k1

O2*

O2,b

k2

H2O2*

H2O2,b

k3

k4

H2O

H2O + O2

Scheme 1. Proposed mechanistic pathway for electrochemical ORR and chemical disproportionation of H2O2. * and b at the subscript denote the species absorbed on the electrode surface and in the bulk, respectively.


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formation of intermediate H2O2. The individual rate constants (k1, k2, k3) at different potentials were obtained using the equations (9, 10 and 11).50,51 k1 = Z1S2(I1N − 1)/(I1N + 1)

(9)

k2 = 2Z1S2/(I1N + 1)

(10)

k3 = Z2NS1/(I1N + 1)

(11)

where, Z1 = 0.62)*

2/3

0−1/6 and Z2 = 0.62)1

2/3

*

0−1/6. )* and )1

*

are the diffusion

coefficient of O2 and H2O2, respectively. I1 and S1 are the intercept and slope of the plot of −ID/IR vs ω-1/2, respectively. The slope of the plot of IDL/(IDL − ID) vs ω−1/2 provides S2 (IDL is the disklimiting current). Rate constants k1, k2 and k3 were calculated in the mixed kinetic-diffusion region (0.3-0.6 V) and the value depends on the potential. As shown in Figure 6A k1 is significantly higher than the k2 and k3, supporting the direct reduction of O2 to H2O. Moreover, the k1/k2 value is more than one for N-rGO/Cu3N (3.3−4.7) as well as rGO/Cu3N (2.7−3.6) (Figure 6B), confirming that oxygen reduction follows the 4-electron pathway rather than

(A)

1.4

(B)

1.0

Cu3N rGO/Cu3N

0.8

NrGO/Cu3N

5

1.2

0.09

0.6 -0.7 -0.6 -0.5 -0.4 -0.3 E/V vs Hg/HgO

k1 k2

0.06

4 k1/k2

0.12

rate constant (µ m/s)

peroxide pathway.52 The careful analysis of rate constants indicates that the value of k3 is very

rate constant (cm/s)

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


3 N-rGO/Cu3N

k3

0.03

2

-0.7

-0.6

-0.5 -0.4 E/V vs Hg/HgO

-0.3

-0.6

-0.5 -0.4 E/V vs Hg/HgO

-0.3

Figure 6. Plot illustrating the potential-dependent rate constants k1 and k2 (A). Plot of k1/k2 vs potential (B). Inset in (A) illustrates the potential dependency of k3.



small as compared to k1 and k2. The very small k3 value could be either due to the generation of negligible amount of HO2‾ (series pathway)51 or due to the facile disproportionation reaction of in situ generated HO2‾, if any. In order to verify whether the in situ generated HO2‾ electrochemically reduces to H2O or it disproportionates to O2 and H2O, the rate constant for peroxide disproportionation (k4) has been evaluated.53 In a typical procedure, the Cu3N and NrGO/Cu3N catalysts were dispersed in O2 saturated supporting electrolyte and linear sweep voltammograms were recorded at 1600 rpm using 20% commercial Pt/C modified RDE. Timedependent linear sweep voltammograms were obtained immediately after the addition of H2O2. The diffusion-limited current for H2O2 reduction 23456,1

*

was obtained from the difference

between diffusion-limited current after addition of H2O2 at time t (23456 ,8 ) and in the absence of H2O2 (23456 ,* ), i.e., (23456 ,1

*

= 23456 ,8 ─23456 ,* ). The slope of the plot of log 23456 ,1

*

vs time

provides the rate constant k4 (s-1) for the disproportionation reaction (Figure 7). The rate constants k1, k2 and k3 were normalized with the thickness (l) of catalyst on the electrode surface (32±5 µm) to compare with k4. The 9 ⁄: , 9< ⁄: and 9 ⁄: were calculated to be 33, 7 and 0.04 s-1,

4 3 log (idlim,H2O2)

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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2

N-rGO/Cu3N

1

Cu3N

0

-1 -2 -3 0

300

600 time (s)

900

1200

Figure 7. Variation of the H2O2 diffusion-limited current at 0.6 V with time after introducing 10 mM H2O2 in O2-saturated 0.1 M KOH solution containing 2 mg of each catalysts.


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respectively, for N-rGO/Cu3N and 8.8, 2.9 and 0.4 s-1 for Cu3N. It is worth noting here that the apparent disproportionation rate constant k4 depends on the amount of catalyst present in the solution.53 The first order homogeneous rate constant (k4*) for H2O2 disproportionation reaction was obtained by mass correction of k4 according to the earlier procedure (equation 12).51 k4* = k4 x (=⁄>)

...................(12)

Here, w is the weight of 20% Pt/C catalyst loaded on the RDE surface and m is the mass of catalyst dispersed in the electrolyte. The k4* value for N-rGO/Cu3N and Cu3N was calculated to be 1.14x10-4 and 1.12x10-4 s-1 which is much lower than 9 ⁄: , suggesting that the catalytic disproportionation of H2O2 with Cu3N-based material is negligible. It further implies that the in situ generated small amount of peroxide is reduced electrochemically to water (Scheme 1). The charge-transfer at the composite-modified electrode-solution interface was studied using the electrochemical impedance spectroscopy (EIS). The Nyquist plots were fitted with an equivalent circuit (inset of Figure S10) which consists of solution resistance (Rs), charge-transfer resistance (Rct), constant phase element (CPE) and Warburg impedance (Zw). The N-rGO/Cu3N shows lowest Rct (2.16 kΩ) compared to the as-synthesized Cu3N (85.4 kΩ) and N-rGO (15 kΩ), suggesting the enhanced charge transfer at the N-rGO/Cu3N modified electrode-solution interface. Such enhanced charge transfer can be ascribed to the synergistic effect of Cu3N and NrGO. The synergistic effect facilitates the electron transfer kinetics for ORR. Lack of durability and anode fuel intolerance are two major problems with traditional catalysts. The durability of the N-rGO/Cu3N was examined with amperometry by holding the potential at -0.15 V and monitoring the time-depended current response. For comparison the 20% Pt/C modified electrode was also subjected to such durability test at an identical condition. Interestingly, N-rGO/Cu3N could retain 83% of its initial current even after 6 h. The traditional 17 ACS Paragon Plus Environment


20% Pt/C could retain only 65% (Figure S11A). To further evaluate the durability, the polarization curves were recorded at 1600 rpm in O2 saturated electrolyte before and after amperometry durability test for 6 h. Significant negative shift in the onset potential (>20 mV) and E1/2 (>30 mV) was observed with the traditional Pt/C (Figure S11 B, C). However, only small negative shift in onset potential and E1/2 (5-10 mV) was observed with N-rGO/Cu3N, further demonstrating its high durability. Methanol tolerance was tested in presence of 1 M methanol in O2 saturated 0.1 M KOH. No change in the voltammetric signature and onset potential was observed after addition of methanol (Figure S11D), suggesting that the NrGO/Cu3N is tolerant to methanol. The electrocatalytic activity of N-rGO/Cu3N can be explained by considering the synergistic effect of N-rGO and Cu3N. Moreover, in metal nitrides, the nitrogen atom increase the energy density on the metal nitrides.54 The d10 configuration of Cu(I) weakens the strong adsorption of intermediate and such electronic effect enhances the overall performance of the composite catalyst.

Summary In summary, we have developed a new solvothermal approach for the synthesis of electrocatalytically active Cu3N for reduction of oxygen. Unlike the traditional methods, our approach does not require NH3 atmosphere. Our synthetic approach involves the solvothermal reduction of Cu(II) and the subsequent nitridation of Cu(I) using HMT as both reducing and nitridating agent in argon atmosphere. The in situ generated formaldehyde and ammonia function as reducing and nitridating agents, respectively. The composite material derived from assynthesized cubic Cu3N nanoparticles and N-rGO efficiently catalyzes the four-electron reduction of oxygen to water in alkaline medium. The performance of the N-rGO/Cu3N is superior to that of the as-synthesized Cu3N and undoped rGO/Cu3N based materials. The 18 ACS Paragon Plus Environment

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possible synergistic effect of N-rGO and Cu3N enhances the ORR activity of N-rGO/Cu3N. The kinetic analysis shows that the % of HO2‾ is very small and the catalytic disproportionation of in situ generated HO2‾ is negligible. The N-rGO/Cu3N is durable and ~10 mV negative shift in the half-wave potential and 6% decrease in the limiting current density were obtained after holding the potential for 6 h at -0.15 V, highlighting the durability. It is demonstrated that the catalyst support has strong control over the overall electrocatalytic performance of Cu3N.

Associated Content Supporting Information. FTIR spectral profile of GO, rGO, N-rGO (Figure S1), XPS of N-rGO (Figure S2). TEM image of rGO/Cu3N (Figure S3). Cyclic voltammogram for ECSA measurement (Figure S4). RDE and RRDE polarisation curves and their consequences (Figure S5-S9). Impedance spectra and durability test (Figure S10-S11). Summarised electrochemical parameters (Table S1).

Author Information Corresponding Author *E-mail: [email protected].

ORCID C. Retna Raj: 0000-0002-7956-0507

Notes The authors declare no competing financial interest.



Acknowledgement This work was financially supported by the Science and Engineering Research Board (SERB) (Grant No. EMR/2016/002271), New Delhi and IIT Kharagpur.


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Table of Content Graphics


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Copper Nitride Nanostructure for the Electrocatalytic Reduction of

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