Fig.3 Device and performance test for generating
H2O2.(a) Schematic diagram of
h-type device generating H2O2,
specification: length × width × height =7cm×5cm×5cm, electrolytic liquid
volume 20ml; (b) At the electrode Gr0.5CB0.5/PTFE0.5, the net
generation rate of H2O2 changes with the
throughput of O2 under the conditions of current
150mA•cm-2, 0.5 mol•L–1Na2SO4 solution and pH=1.0. (c)The net generation rate and Faraday current efficiency of
H2O2 changed with the current density of
electrodes with different compositions under the condition of
O2 flow rate of 30ml•min-1, 0.5
mol•L–1 Na2SO4solution, pH=1.0; (d) At the electrode Gr0.5CB0.5/PTFE0.5, the
net generation rate of H2O2 varies with
the pH of the electrolyte under the conditions of O2flow rate of 30ml•min-1, current
150mA•cm-2 and 0.5 mol•L–1Na2SO4 solution; (e) Electrode
Gr0.5CB0.5/PTFE0.5, under the condition of O2 flow rate
of 30ml•min-1, 0.5 mol•L–1Na2SO4 solution, pH=1.0, the cumulative
concentration of H2O2 changes with
reaction time at different current density; (f) Electrode
Gr0.5CB0.5/PTFE0.5 the Faraday current efficiency of generating
H2O2 with different current density
changes with reaction time under the condition of O2flow rate of 30ml•min-1, 0.5
mol•L–1 Na2SO4solution, pH=1.0.
Fig.3a shows the schematic diagram of the hydrogen peroxide generating
unit. Oxygen passes through the gas diffusion electrode integrating the
diffusion layer and the reaction layer, and the excess oxygen is
discharged from the hydrogen peroxide solution in the cathode chamber.
Fig.3b shows the relationship between the hydrogen peroxide generation
rate and the oxygen flow rate at the current density of
150mA•cm-2. It can be seen that when the
O2 flow rate is no less than
30ml•min-1, the influence of O2throughput on the generation of H2O2 can
be excluded. Unless otherwise stated, the oxygen flow rate in the paper
is 30ml•min-1. The investigation results of some
influencing factors such as the type of pore-making agent, partial
pressure of oxygen, and addition ratio of carbon materials are shown in
Fig.S3.
It can be seen from Fig.3c and Fig.S4c that no matter what kind of
electrode, the hydrogen peroxide generation rate increases as the
current density increases, but the increasing slope becomes smaller and
smaller. After the current density exceeds 200mA•cm-2,
it gradually becomes stable. The maximum hydrogen peroxide generation
rate is about 2.5mmol•cm-2•h-1, and
the corresponding Faraday efficiency also decreases gradually. Comparing
different electrodes, it can be found that the hydrogen peroxide
generation rate increases with the increase of CB content. When the mass
ratio of CB/Gr≥1, the electrode performance does not increase
significantly. Compared with Gr0.5CB0.5/PTFE0.5 and CB/PTFE0.5, the pore
volumes are significantly different, but the electrochemical active
surface area is similar. We believe that the production rate of hydrogen
peroxide depends on the electrochemical active surface area. Using
carbon materials such as carbon black, which are much cheaper than
graphene and carbon nanotubes, and simply adding PTEF, gas diffusion
electrodes with high hydrogen peroxide rate can be obtained compared
with other carbon materials.
As pH goes from 0 to 2, the formation rate of
H2O2 increases linearly, but when pH=2
to 4, the reaction rate is basically independent of pH. With different
pH range, the corresponding speed control steps of hydrogen peroxide
generation may be different.
Fig.3e shows that with the increase of the current density, the
generation rate of H2O2 increases, and
the maximum cumulative concentration can be reached in a shorter time.
At the current density of 50mA•cm-2 and
O2 flow rate of 30ml•min-1, after
780min, the maximum cumulative concentration of
H2O2 reaches 117g•L-1,
i.e.
3.44mol•L-1, or
about 11.8% of the mass fraction. The maximum cumulative concentration
is related to the decomposition of hydrogen peroxide. When the
decomposition rate is increased to equal to the rate of hydrogen
peroxide formation, the hydrogen peroxide concentration will remain
constant for a long time if there is no mechanical damage to the
electrode.
Fig.3f is the average Faraday efficiency of hydrogen peroxide
concentration accumulated to a certain point. It can be seen that the
electrodes are activated at the beginning. The Faraday current
efficiency decreases rapidly with the increase of hydrogen peroxide
concentration, reflecting that the decomposition of hydrogen peroxide
into water will affect the overall electrode efficiency. When the
hydrogen peroxide concentration is no longer rising, the instantaneous
Faraday efficiency drops to zero. In addition, too high current density
will reduce the service life of the electrode, and the surface of the
electrode where the micro-pore structure is damaged has hydrogen
generation, which will also reduce the Faraday efficiency of hydrogen
peroxide generation.
3.4 Simulating the polarization curve under real reaction
conditions based on Bulter-Volmer
equation