Zurong Du a
, Shenggao Wang a
, Chuixiong Kong a
, Quanrong Deng a
, Geming Wang a
,
Chong Liang b
, Haolin Tang b
, a Province Key Laboratory of Plasma Chemistry and Advanced Material, Wuhan
Institute of Technology, Wuhan 430073, China b
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430073, China
S1 RDE and corresponding K-L plots of CNTs (a), (b) and Pt-CNTs catalysts (c),
(d).
Fig. S1 shows RDE and corresponding K-L plots of CNTs catalyst and Pt-CNTs catalyst. The RDE plot of CNTs catalyst shows that the reduction current increases with the increase of overpotential and the increase of electrode rotating rates when potential is lower than 0.8 V, indicating the mixed kinetics-diffusion control process of oxygen reduction, and the pure diffusion limiting current section can’t reach in the employed potential window. However, the Pt-CNTs catalyst presents kinetics-diffusion determined process in the potential range of 0.85 V – 0.55 V and
1

pure diffusion determined process when potential is lower than 0.55 V. The decreased reduction current for Pt-CNTs catalyst when potential is lower than 0.05 V is likely caused by the hydrogen absorption [1]. The linear K-L plots of these two catalysts indicate first order reaction kinetics of oxygen reduction on them, which is same as
NCNTs catalyst. But the difference in the number of electron transferred ( n ) and kinetic current ( j
K
) in each potential can be calculated from the slopes and the vertical axis intercepts of K-L curves according to K-L equation as shown below.
2

S2 RDE and K-L plots of NCNTs catalyst tested at the second time (a) (b), and the contrastive n value plots on different samples (c).
Fig. S2 shows the RDE and K-L plots of NCNTs catalyst which has been tested at the second time to verify the reproducibility of the n presented in the formal paper. It can be found that the two plots here are almost identical with those in the formal paper. As a result, the calculated n matches well with that in the formal paper (the dotted line in
(c)). In addition, the n in several potentials on every sample modified electrode were also presented in (c) to compare their O
2
conversion efficiency, concluding bared glass electrode (GC), CNTs/GC, NCNTs/GC and Pt-CNTs/GC.
3

S3 RDE and corresponding K-L plots of NCNTs-H (729 μg cm -2 ) catalyst (a), (b).
The contrastive n value plots of nitrogen doped carbon nanotubes catalysts with different loading (c).
4

Fig. S3(c) indicates the general tendency that the n increases with increased catalyst loading until the loading of 729 μg cm
-2
, thereafter the n decreases with further increase of loading. Therefore, the optimal loading of NCNTs in terms of n is 729 μg cm
-2
, and this conclusion is consistence with the LSVs analysis in the formal paper.
5

S4 CV plots of NCNTs catalyst with the 2500 th cycles.
The stability of NCNTs catalyst has also been verified using consecutive CV testing of 2500th repetitions. The representative CV plots in the 1st, 500th and 2500th cycle have been shown. The slight decay in capacitance can be observed from the 1st cycle to 500th cycle [2]. Even so, the oxygen reduction peak potential and current almost keep constant from the 1st cycle to 2500th cycle. Likewise, this result indicates the excellent stability of NCNTs catalyst.
6

S4 K-L equation
The Koutecky-Levich equation can be expressed as follows according to literature [3,
4]:

1 j


1 j
K


1 j
L


1 j
K


1
B

1 / 2
(1)
B

0 .
62 nFC
0
( D
0
) 2 / 3
 
1 / 6 (2)
Where, j is the measured current, j
K
is the kinetic-limiting current, and j
L
is the diffusion-limiting current.
ω
is the angular velocity of the rotating disk electrode. n is the number of electron transferred per O
2
molecule, F is the Faraday constant, C
0
is the bulk concentration of O
2
, D
0
is the coefficient of oxygen, ʋ
is the kinematic viscosity of the electrolyte. Among them,
ω
= 2π N /60 has been adopted and N represents the electrode rotating numbers per minute.
Basing on the experimental temperature of 24 °C, the following constant are employed to calculate the n and j
K
: F = 96485 C mol
-1
, C
0
= 1.2
× 10
-6
mol cm
-3
, D
0
=
1.9
× 10
-5
cm
2
s
-1
, ʋ
= 0.01 cm
2
s
-1
.
S5 Tafel equation and j k
correction
When reactant concentration doesn’t have apparent difference between electrode surface and bulk solution, the Butler-Volmer equation can be simplified as [5]:
7

j
K
 j
0

 e n
0

FE
RT
 e n
0

1
  
FE
RT

 (3)
Where, j
K
is the kinetic current, j
0
is the exchange current, n
0
is the number of electron transferred each O
2
molecules reaction, α is the transfer coefficient, F is the Faraday constant, E is the overpotential, R is the gas constant and T is the reaction temperature in Kelvin.
Basing equation (3), if overpotential is large for ORR, the oxidation current is low enough and the later item in Butler-Volmer equation can be ignored. The
Butler-Volmer equation can be simplified as: j
K
 j
0 e n
0

FE
RT (4)
When take the logarithm simultaneously toward the items in two sides of the equation, the equation can be transformed the following format:
E

2 .
31 RT n
0

F log j
K

2 .
31 RT n
0

F log j
0
(5)
The slope of E versus lg j
K in equation (5) was called the Tafel slope. Since all other parameters in the Tafel slope are known, the parameters determining the Tafel slope are actually overpotential and n
0
α
. The higher the Tafel slope is, the faster the overpotential increase with the current. Therefore, in order to obtain a high current at low overpotential section, an electrochemical reaction should exhibit relatively low
Tafel slopes [6].
When RDE testing is conducted on enough high overpotential, the mass transfer limiting current ( j
L
) will reach. The mass transfer limiting current calibrations are applied to calculate j
K as shown in equation (6). j in the equation (6) is the observed
8

current and all current are transformed to its absolute value for convenient calculation
[5]. j
K
 j
L jj
L
 j
(6)
References
1. Markovi NM, Gasteiger HA, Ross (Jr.) (1996) Oxygen reduction on platinum low-index single-crystal surfaces in alkaline solution: Rotating ring disk
Pt(hkl)
studies.
J. Phys. Chem. 100:6715-6721.
2. Chen J, Li C, Shi GQ (2013) Graphene Materials for electrochemical capacitors. J
Phys Chem Lett. 4:1244-1253.
3. Ratso S, Kruusenberg I, Vikkisk M (2014) Highly active nitrogen-doped few-layer graphene/carbon nanotube composite electrocatalyst for oxygen reduction reaction in alkaline media. Carbon 73:361-370.
4. Wong WY, Daud WRW, Mohamad AB (2014) The impact of loading and temperature on the oxygen reduction reaction at nitrogen-doped carbon nanotubes in alkaline medium. Electrochimica Acta. 129:47-54.
5. Song C, Zhang J, (2008) PEM fuel cell electrocatalysts and catalyst layers: fundamental and application. In: Zhang J (eds). Springer, Berlin.
6. Ammam M, Easton EB (2013) Oxygen reduction activity of binary PtMn/C, ternary
PtMnX/C (X = Fe, Co, Ni, Cu, Mo and, Sn) and quaternary PtMnCuX/C (X = Fe, Co,
9

Ni, and Sn) and PtMnMoX/C (X = Fe, Co, Ni, Cu and Sn) alloy catalysts. J Power
Sources 236:311-320.
10