3. Results and discussion

3.1 Synthesis and characterization of CoFe-CoxN@NC

The CoFe-CoxN@NC catalyst was synthesized by self-assembly and pyrolysis process, as shown in Fig. 1a. In brief, biorefinery lignin was carboxylated to improve its solubility and coordinated with Co2+ and Fe3+ to form the lignin-based CoFe supramolecules, followed by co-doping with urea and in situ pyrolyzing at high temperature to obtain the CoFe-CoxN heterojunction on the nitrogen-doped lignin-derived carbon.
The crystal phase structure and graphitization degree of CoFe-CoxN@NC samples were shown in Fig. 1 and Fig. S1. From the XRD patterns of the metal free nitrogen-doped lignin-derived carbon materials (NC) and CoFe-Co5.47N@NC, it could be observed that the characteristic peaks of NC at the position of 24° and 43°, respectively, which belonged to the (002) and (100) crystal planes of graphitic carbon53. When the Fe/Co ratio gradually decreased from 7:1 to 1:7, the XRD peaks of (110) and (200) planes of Fe (PDF#06-0696) moved to a smaller angle and disappeared until the (111) and (200) planes of Co (PDF#15-0806) appeared, and all the samples had a broad peak around 26° attributed to the (002) plane of graphitic carbon, as shown in Fig. 1b. The dominant phase is bimetallic CoFe alloy with diffraction peaks centered at 44.9° and 65.3°, which can be ascribed to the (110) and (200) crystal planes of CoFe alloy (PDF#49-1568). The diffraction peaks of 43.7°, 50.8° and 74.9° corresponding to the (111), (200) and (220) crystal planes of Co5.47N (PDF#41-0943) were also observed 54. The XRD peak of CoFe-Co5.47N@NC moved towards a higher angle (~ 0.4°) relative to Co5.47N, which meant that the lattice expansion was due to the partial substitution of Co by Fe 55. With the increase of pyrolysis temperature, the diffraction peak intensity belonging to the (002) crystal plane of graphitic carbon increased, the graphitization degree gradually increased as well as the electronic conductivity. With the increase of the urea dosages during the in situ pyrolysis process, the diffraction peak intensity of 43.7° belong to the (111) plane of Co5.47N increased continuously, indicating that the coordination between metal and nitrogen gradually increased, and nitrogen was successfully introduced into the lignin-derived carbon support. The optimal Fe/Co ratio and urea dosage was beneficial to the formation of CoFe alloy and CoxN heterojunction.