1. Introduction

Electrochemical water splitting powered by solar, wind and tidal renewable energies is an ideal green and cost-effective hydrogen production path, and there are no carbon-containing compounds discharged1,2. Water splitting consists of two half reactions: hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode. The OER process suffers from multiple steps of proton-coupled electron transfer, thereby causing very sluggish reaction kinetics and high overpotential 3,4. Up to now, noble metals and metal oxides, such as Ir and Ru, are the state-of-the-art catalysts for the OER process, however, their scarcity, high costs and poor stability has severely hindered their widespread commercialization 5-7. Therefore, it is highly desirable to search and develop highly efficient, stable, and cost-effective OER electrocatalysts 3,8.
Owing to the intrinsic electron structure, special orbitals and high carrier mobility, transition metals Co-based and Ni-based catalysts have been widely investigated toward OER and perform great promise for potential applications 9-11. A wide variety of Co-based catalysts have been well investigated for OER, including metal oxides 12,13, layered double hydroxides14,15, bimetallic alloys 16-20, metallic nitrides 21,22 and phosphides23. Co-based oxides and layered double hydroxides are limited toward electrocatalytic performance for their poor electrical conductivity, which hinder the electron transfer11,24. Doping Fe element in Co species adjusts the local electronic structure and enhances the active sites, which can boost the OER electrocatalytic activity 18,25-29. In addition, bimetallic alloys and CoxN nitrides exhibit high OER catalytic activities, attributed to their superior electrical conductivity 22,30,31. However, Co-based monometallic or bimetallic catalysts are usually unstable and oxidized during the OER process in alkaline solution 32. What’s more, bimetallic alloys show the strong binding with oxygen intermediates toward OER, that require the high overpotential to overcome the reaction barriers 25.
Carbon-based nanomaterials have shown great potential in the design of metal based electrocatalysts for their cost-efficient, high specific surface area, excellent conductivity, and favorable durability in a harsh environment 27,28,33-37. However, the inert surface of carbon materials usually leads to the weak interaction between the metal nanoparticles and supports, which make it difficult to effectively regulate the electronic structure of metal nanoparticles and improve their catalytic activity 27,28. Biorefinery lignin is the most promising carbon precursor instead of petroleum and coal resources, owing to its second largest natural organic polymer after cellulose 38,39. In addition, with a three-dimensional network structure formed by phenylpropane units, lignin is rich in phenolic hydroxyl groups, alcoholic hydroxyl groups and part of carboxyl groups, and its carbon content is as high as 60%40,41. Moreover, lignin-derived carbon have been reported as the catalyst support in the applications of electrocatalysis42,43, photocatalysis degradation44,45, Fischer-Tropsch synthesis46,47 and other fields 48,49 in the last few years, but most of the lignin-derived carbon catalyst are prepared using direct carbonization of lignin followed by impregnation of metal active ingredients, of which was easily dissoluble and difficult to effectively inhibit their agglomeration, causing the inferior activity and durability 50.
In this work, making full use of the inherent properties and abundant carboxyl/hydroxyl functional groups of lignin biomacromolecule to precisely coordinate with transition metal ions with Co2+ and Fe3+ by aqueous self-assembly process, and form the lignin-based CoFe bimetallic supramolecules, then co-doped with nitrogen precursor urea in situ pyrolyzing at high temperature to obtain the desired CoFe biometallic active nano particles. The effects of the pyrolysis temperature, urea dosage, Fe/Co metal ratio and metal/lignin ratio during synthesis process on the structure and OER electrocatalytic activity were investigated. To our delight, CoxN component was in situ generated other than CoFe alloy during the pyrolysis and doping of urea revealed by the systematic characterization, and coupled with CoFe alloy to form the CoFe-CoxN heterojunction encapsulated by the graphic carbon layers and located on the nitrogen-doped lignin-derived shell-core carbon support (CoFe-CoxN@NC), that optimize the adsorption/desorption strength between the OER intermediates and metallic active site, disperse and decrease the size of CoFe-CoxN heterojunction, and prevent their aggregation, endowing the as-synthesized catalyst CoFe-CoxN@NC exhibited excellent OER electrocatalytic activity and stability in an alkaline medium.