2. Experimental section
2.1 Materials
Enzymatic hydrolysis lignin (EHL) isolated from the corncob bio-refinery
residue through alkali dissolving, acid precipitation, water washing,
drying, ball milling and sieving, was provided by Shandong Longli
Biotechnology Co., Ltd. Iron chloride hexahydrate
(FeCl3·6H2O, AR 99%), cobalt nitrate
hexahydrate
(Co(NO3)2·6H2O, AR
99%), urea
(CO(NH2)2,
AR 99%), ammonia (NH3·H2O, AR
25-28%), ethanol
(C2H5OH, AR 99.7%), commercial iridium
carbon (20 wt%) were purchased from Shanghai Aladdin Chemical Co.,
Ltd., monochloro acetic acid
(C2H3ClO2, AR 98%) was
purchased from Shanghai Macklin Biochemical Technology Co., Ltd.,
and acetone
(C3H6O, AR 98%), hydrochloric acid
(HCl, 36.0-37.0%), nitric acid (HNO3, 65.0-68.0%),
sodium hydroxide (NaOH, AR 96%), potassium hydroxide (KOH, AR 85%)
were produced by Guangzhou Chemical Reagent
Factory. Nafion solution (D520, 5
wt%) was supplied from DuPont. Carbon papers (TGP-H-060, 20 cm×20 cm,
0.19 mm) were produced by Toray Corporation (Japan).
2.2 Preparation of lignin-based CoFe
supramolecules
Carboxylated enzymatic hydrolyzed lignin (EHL-COOH) was prepared by the
aqueous method according to our previous report 51.
Then 1.0 g EHL-COOH was dissolved in 100 mL pure water, while
FeCl3·6H2O and
Co(NO3)2·6H2O
(nFe/nCo = 8:0, 7:1, 6:2, 4:4, 2:6, 1:7,
0:8) were added into 50 mL pure water under vigorous stirring. The
mixture was added drop by drop into the aqueous solution of EHL-COOH and
stirred for 10 min. The pH of above solution was adjusted to 8 by
NH3·H2O and HCl. After aging for 12 h,
the precipitate was centrifuged, washed with ultra-pure water, dried at
80℃ for 24 h, and ground to obtain the lignin-based CoFe supramolecules
(EHL-COOH-CoFe).
2.3 Preparation of lignin-derived carbon
CoFe-CoxN@NC
Firstly, 1.0 g of EHL-COOH-CoFe was ground and mixed with a certain
amount of urea, then was heated at a rate of 5 ℃·min-1and pyrolyzed at high temperature for 2 h under the argon atmosphere.
The products were washed with 1 mol·L−1 HCl and
ultra-pure water, respectively, and dried at 80℃ for 12 h to obtain the
bimetal-based functionalized nitrogen-doped lignin-derived carbon
materials, denoted as
CoFe-CoxN@NC. The
effects of metal ratio, pyrolysis temperature, the ratio of metal to
lignin, and nitrogen content on the structure and electrocatalytic
performance of CoFe-CoxN@NC were systematically
investigated. 1.0 g of EHL-COOH was mixed and ground with 1.0 g of urea,
then pyrolyzed at 700 ℃ to obtain the metal free nitrogen-doped
lignin-derived carbon materials (NC) as comparative sample.
2.4 Structural characterization of
CoFe-CoxN@NC
The
crystal structure of the electrocatalyst was characterized by Rigaku
Miniflex-600 XRD instrument, which used a Cu-Kα radiated X-ray source
with a scanning range of\(\mathrm{2}\mathrm{\theta}\mathrm{\ }\mathrm{=}\mathrm{\ }\mathrm{5}\mathrm{-80}\)and a scanning rate of 10 °·min−1. The morphology and
elemental composition were characterized by American FEI Tecnai
G2 F20 TEM, and its operating voltage was 200 kV. The
degree of order and graphitization were characterized by LabRAM HR
Evolution Raman spectrometer, with an excitation wavelength of 532 nm.
Nitrogen adsorption/desorption isotherms were measured at 77 K with ASAP
2460 surface area and porosity analyzer. The elemental composition and
valence state were characterized by Thermo Scientific K-Alpha + X-ray
photoelectron spectrometer (XPS). All the peaks were calibrated
according to the standard position of C 1s peak (284.8 eV). The
percentage of doped metal in the sample was calculated by Varian 720
inductively coupled plasma OES spectrometer (ICP-OES). The band gap
width of carbon materials was determined by PE Lambda 950 UV-Vis Diffuse
Reflectance Spectrometer. The valence band and conduction band of carbon
materials were tested by UV photoelectron spectrometer (Thermo Fischer
Escalab 250 Xi).
2.5 Electrochemical
measurements
All
electrochemical measurements were performed in a three-electrode system
using the Gamry Interface 1010 electrochemical workstation. Graphite rod
and Hg/ HgO were used as the counter electrode and reference electrode,
respectively. The working
electrode was prepared as follows, 4 mg of carbon powder was added into
200 μL of 0.25% nafion-ethanol solution. After ultrasonic dispersion of
the powder, 50 μL of the slurry was dropped onto the treated carbon
paper, and then the carbon paper was held by electrode clamp as the
working electrode. The loading capacity of the catalyst was 4
mg·cm−2.
The electrochemical performances were investigated by using linear sweep
voltammetry (LSV) and cyclic voltammetry (CV) techniques at room
temperature. 1 mol·L−1 KOH solution was used as
electrolyte, and the scanning rate recorded by all the polarization
curves was 1 mV·s−1. Before LSV testing, the working
electrode with loading catalyst was firstly activated by several CV
cycles. All the measured data of the polarization curve were corrected
by IR, and all the potentials were calculated according to the
reversible hydrogen electrode (RHE). The expression is as follows:
\(\mathrm{E}\mathrm{\ }\left(\mathrm{\text{vs.}}\mathrm{\ }\mathrm{\text{RHE}}\right)\mathrm{\ =\ E}\mathrm{\ }\left(\mathrm{\text{vs.}}\mathrm{\ }\mathrm{Hg/HgO}\right)\mathrm{\ +\ 0.098\ +\ 0.0591\ \times\ }\mathrm{\text{pH}}\).
2.6 Density functional theory (DFT)
calculations
DFT calculations were carried out using the Vienna ab Initio Simulation
Package (VASP) 52.
The Perdew-Burke-Ernzerhof (PBE)
model in the form of the generalized gradient approximation (GGA) was
used to describe the exchange-correlation potential. For all
optimization calculations, the energy and force convergence criteria
were set to 10-5 eV and 0.05 ev·Å-1,
respectively. Total energy was calculated using the Projector
Augmented-Wave (PAW) method with a plane-wave cutoff energy of 400 eV. A
gamma-centered K-point of 3×3×1 was chosen to describe the Brillouin
zone.