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).