Fig. 3. High-resolution XPS spectrum of CoFe-CoxN@NC:
(a) C 1s, (b) N 1s, (c) Fe 2p, (d) Co 2p
3.2 Electrocatalytic performance of
CoFe-CoxN@NC
The OER electrocatalytic activity
of CoFe-CoxN@NC catalysts as investigated in a standard
three-electrode system in 1.0 M KOH solution. The effects of the
pyrolysis temperature, urea dosage, Fe/Co metal ratio and metal/lignin
ratio during synthesis process on the OER electrocatalytic activity of
CoFe-CoxN@NC were shown in Fig. 4 and Fig. S9-10. As the
pyrolysis temperature increased, the overpotential required to achieve a
current density of 10 mA·cm-2 (η10) using
CoFe-CoxN@NC catalyst decreased at first and then
increased at the temperature of 700℃, which was related to the degree of
defects and graphitization of the carbon material. The low pyrolysis
temperature leaded to incomplete carbonization of the sample, thus
showed poor electrical conductivity that seriously affected the OER
electrocatalytic activity. Excessive pyrolysis temperature leaded to the
metal agglomeration and pore blockage, thus reduced the OER
electrocatalytic activity. When the optimal ratio of urea and
EHL-COOH-CoFe precursor was 1:1, CoFe-CoxN@NC exhibited
the lowest over-potential η10 of 270 mV, that was much lower than that
of CoFe@C catalyst without the doping of urea (340 mV). The active sites
of pyridine N and graphite N could promote the synergistic interaction
between nitrogen and CoFe alloy, the CoxN composition in
CoFe-CoxN@NC would increase from the XRD patterns (Fig.
1d) with the increasing dosage of urea during the in situ pyrolysis
process, therefore, the composition ratio of CoFe alloy and
CoxN heterojunction decreased that caused the inferior
OER electrocatalytic activity of CoFe-CoxN@NC.
Monolithic Co-based catalyst was insufficient to deliver the ideal OER
electrocatalytic activity and stability. Rational synthesis of
bimetallic CoFe alloy was investigated through the partial substitution
with Fe, and the metal composition content in
CoFe-CoxN@NC determined by ICP-OES was shown in Table
S3. With the increase of substitution with Fe based on the atomic ratio
of Fe and Co to 21%, the overpotential η10 was decreased to 270 mV,
while that of Co@NC was 340 mV; further increased the Fe composition
content, Fe became the major active composition, CoxN
content in CoFe-CoxN@NC decreased and ever disappeared
verified by the XRD patterns in Fig. 1b, thus decreased the
overpotential η10 to 416 mv with the Fe substitution of 96%, while that
of Fe@NC was 448 mV. With the increase of the ratio of metal moles and
lignin to 8 mM/g in the self-assembly coordination with
Fe3+ and Co2+ process,
CoFe-CoxN@NC showed the optimal OER performance. Based
the results above, the optimal synthesis parameters were the pyrolysis
temperature of 700℃, the ratio of metal moles and lignin of 8 mM/g, the
atomic ratio of Fe:Co = 2:6, the ratio of urea and EHL-COOH-CoFe
precursor of 1:1, and the metal content of the corresponding
CoFe-CoxN@NC was 1.45 wt% of Fe and 5.60 wt% of Co.
The overpotential η10 of CoFe-CoxN@NC material was 270
mV with a Tafel slope of 85 mV·dec−1, which could be
comparable to the performance of commercial Ir/C catalyst (252 mV@10
mA·cm−2, 82 mV·dec−1).