2 Experimental
2.1 Preparation of gas diffusion
electrode
(1) Pretreatment of stainless steel mesh:
Firstly, the stainless steel mesh (40 mesh, Runshi metal products
factory) is cut into length × width =5cm×5cm specification, and soaked
in 5% of the mass of PTFE emulsion (PTFE) for 5-10s. Then we take it
out and put it in a oven at 105℃ to dry. Repeating the above operation
several times, until the weight of stainless steel mesh is doubled, and
dry for 1h.
(2) Preparation of carbon black, pore-forming agent and binder paste:
According to the dry weight ratio, we weigh the carbon materials(Vulcan
XC-72R, AR, Cabot, USA; Graphite powder, AR, Tianjin chemical reagent
factory), binder (60wt % of PTFE, Japan Daikin company) and
pore-forming agent (NH4HCO3, AR, Tianjin
wind ship chemical technology co., LTD.). Then, we mix them evenly, and
add an appropriate amount of ethanol (AR, characters). Ultrasonic shocks
for 20min and disperses them evenly, and volatilizes the ethanol at 40℃
to a thick paste, which is carbon black, pore-forming agent and binder
paste.
The typical formula is carbon material and PTFE mass ratio of 2:1, at
this case, electrode abbreviation is Gr/PTFE0.5 or CB/PTFE0.5, Gr: CB:
PTFE= 1:1:1, electrode abbreviation is Gr0.5CB0.5 /PTFE0.5. Where: Gr
and CB are graphite powder and carbon black (Vulcan XC-72R) powder.
(3) Preparation and molding of gas diffusion electrode
The paste is evenly coated on the hydrophobic stainless steel mesh and
pressed at 6MPa pressure for 20min to obtain the initial gas diffusion
electrode. The initial gas diffusion electrode is placed in a muffle
furnace and calcined at 330℃ for 1h. After cooling, the electrode is
taken out and soaked in deionized water for 2h.
2.2 Characterization and analytical
methods
(1) Nitrogen adsorption and desorption test (BET)
The pore size distribution and specific surface area of electrode
materials are measured by ASAP2460 gas adsorption analyzer.
(2) Raman spectral representation
Conducted in the SENTERRA Raman Scope (BRUKER) tester, the excitation
source wavelength is 532nm and the power is about 2mW.
(3) Fourier-transform infrared spectroscopy (FTIR)
The vibration absorption characteristics of chemical bonds or functional
groups in molecules are used to test the molecular structure and
functional groups of electrode materials by FTIR.
(4) Scanning electron microscopy (SEM)
SEM is used to observe the structure, material distribution and
microscopic pore distribution of the electrode surface.
(5) Electrochemical Tests
The performance of selective oxygen reduction is assessed in a standard
three-electrode system controlled by a potentiostat (CHI 760E) with a
RDE setup. A glassy carbon (GC) disc (RRDE-3A; Pine Instrument Company;
0.0707cm2 of disc area) is employed as the working
electrode. The Pt wire and saturated Ag/AgCl electrode are used as the
counter and reference electrode, respectively. All the reported
potentials are referred to as the reversible hydrogen electrode (RHE)
potential.
The catalyst ink is prepared by dispersing 15mg of carbon materials
catalyst in 800μL of isopropyl alcohol, 800μL of deionized water and
14μL of 5wt% Nafion solution, followed by a sonication of 30min. After
that, 9.6μL of the catalyst ink is deposited on the GC part of the RDE
electrode and dried to form a uniform thin film at 95℃ keeping 12h. An
O2-saturated 0.5 M
Na2SO4 (pH=1.0) aqueous solution is used
as an electrolyte for measurement. The working electrode is rotating at
1600rpm.
Linear sweep voltammetry (LSV) is carried out to compare the
electrochemical behavior during H2O2production, and are performed by the electrochemical workstation at a
scan rate of 5mV•s-1 in the three-electrode cell
system. The prepared cathode is used as working electrode. The Pt wire
and saturated Ag/AgCl electrode are used as the counter and reference
electrode, respectively. All the reported potentials are referred to as
the reversible hydrogen electrode (RHE) potential. The LSV curves of the
working electrodes are obtained in the potential range of 0.0–0.9V in
O2-saturated Na2SO4solution
(pH=1.0),
ambient temperature.
2.3 Experimental methods and
devices
Gas diffusion electrode is assembled in the removable H-type experiment
device, the typical conditions of the preparation of hydrogen peroxide
are the cathode chamber volume 7 cm3, the thickness of
1cm, anode platinum electrode, Proton Exchange Membrane (PEM),
electrolyte, 0.5 mol•L–1Na2SO4 solution ( pH=1.0), current 1.05A
, O2 flow rate 30ml•min– 1,
circulating pump flow 20ml•min- 1, reaction, every
10min sampling, the determination of
H2O2 concentration.
2.4 Analysis and calculation
methods
(1) Determination of H2O2 concentration
The concentration of H2O2 is determined
by colorimetry, using 0.1 mol•L–1Ti(SO4)2 as chromogenic agent. The
ultraviolet spectrophotometer detects that its maximum absorption
wavelength is at 407nm. Diluted with 30wt%
H2O2 into a series of concentration
gradients of 1-60mg•L-1, under 407nm absorption
wavelength, the standard curve of absorbance (A) and
H2O2 concentration (C) is made, which
meets the following requirements: A = 0.023C. concentration of
experimental sample can be calculated according to the standard curve of
absorbance-H2O2 concentration.
(2)The current efficiency of the H2O2electrosynthesis was calculated using Eq. (1)
\(FE=\frac{\text{nFCV}}{\text{It}}\times 100\%\) (1)
Where n is the number of transferred electrons in the reduction
of O2 to H2O2 (n=2),F is the Faraday constant
(96485C•mol−1),C is the concentration of
H2O2 (mol•L–1),V is the volume of electrolyte (L), I is the current
intensity (A), and t is the electrocatalysis time (s).
(3) Calculation of H2O2 generation rate
The apparent generation rate of H2O2 is
the difference between the net generation rate of
H2O2 and the decomposition rate. We
usually use the net generation rate of
H2O2 to describe the electrochemical
activity of the electrode. According to the relationship between the
concentration of H2O2 (C ) and the
reaction time (t ), the apparent formation rate of
H2O2 is r = dC / dt . After
reasonable derivation and calculation, the influence of
H2O2 decomposition is eliminated, and
when r is extrapolated to t=0 min, the net generation rate of
H2O2 (rt=0 ) is
obtained to reflect the electrochemical activity of the electrode.