Supplementary information
Microwave plasma synthesized nitrogen-doped carbon nanotubes for oxygen reduction
ZurongDua, ShenggaoWanga, ChuixiongKonga, QuanrongDenga, GemingWanga, Chong Liangb, HaolinTangb,
aProvince Key Laboratory of Plasma Chemistry and Advanced Material, Wuhan Institute of Technology, Wuhan 430073, China
bState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430073, China
- Figtures and discussion
S1 RDEand corresponding K-L plots of CNTs (a), (b)and Pt-CNTs catalysts (c), (d).
Fig.S1 shows RDEand correspondingK-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 limitingcurrent 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 pure diffusion determined process when potential is lower than 0.55 V. The decreasedreductioncurrent for Pt-CNTs catalyst when potential is lower than 0.05 V is likelycaused 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 (jK) 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.
S2 RDE and K-L plots of NCNTs catalyst testedat thesecond time (a) (b), and the contrastivenvalue plotsondifferent 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 calculatedn matches well with thatin the formal paper (the dotted line in (c)). In addition, the n in several potentials on every sample modified electrode werealso presented in (c) to compare their O2conversion efficiency, concluding bared glass electrode (GC), CNTs/GC, NCNTs/GC and Pt-CNTs/GC.
S3 RDE and corresponding K-L plots of NCNTs-H (729 μg cm-2)catalyst (a), (b).The contrastivenvalue plots of nitrogen doped carbon nanotubescatalysts with different loading (c).
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 decreaseswith 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.
S4CV plots of NCNTs catalyst with the 2500th 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 cyclehave 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.
- Equations
S4 K-L equation
The Koutecky-Levich equation can be expressed as follows according to literature [3, 4]:
(1)
(2)
Where, j is the measured current, jK is the kinetic-limiting current,and jL is the diffusion-limiting current. ω is the angular velocity of the rotating disk electrode. n is the number of electron transferred per O2 molecule, F is the Faraday constant, C0 is the bulk concentration of O2, D0 is the coefficient of oxygen, ʋ is the kinematic viscosity of the electrolyte. Among them, ω= 2πN/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 jK: F = 96485 C mol-1, C0 = 1.2×10-6 mol cm-3, D0 = 1.9×10-5 cm2 s-1, ʋ = 0.01 cm2 s-1.
S5Tafel equation and jk correction
When reactant concentration doesn’t have apparent difference between electrode surface and bulk solution, the Butler-Volmer equation can be simplified as [5]:
(3)
Where, jK is the kinetic current, j0 is the exchange current, n0 is the number of electron transferred each O2 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), ifoverpotential is large for ORR, the oxidation current is low enoughand the later item in Butler-Volmer equation can be ignored. The Butler-Volmer equation can be simplified as:
(4)
When take the logarithm simultaneously toward the items in two sides of the equation, the equation can be transformed the following format:
(5)
The slope of E versuslgjKin equation (5) was called the Tafel slope. Since all other parameters in the Tafel slope are known, the parameters determining the Tafel slope are actuallyoverpotential and n0α. 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(jL) will reach. The mass transfer limiting currentcalibrations are applied to calculate jKas shownin equation (6). j in the equation (6) is the observed current andall currentare transformed to its absolute value for convenient calculation [5].
(6)
References
1.Markovi NM, Gasteiger HA, Ross (Jr.) (1996) Oxygen reduction on platinum low-index single-crystal surfaces in alkaline solution: Rotating ring diskPt(hkl) studies. J. Phys. Chem. 100:6715-6721.
2.Chen J, Li C, Shi GQ (2013) Graphene Materials for electrochemical capacitors. JPhys ChemLett. 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. ElectrochimicaActa. 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, Ni, and Sn) and PtMnMoX/C (X = Fe, Co, Ni, Cu and Sn) alloy catalysts. J Power Sources 236:311-320.
1