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.