For ORR reaction, CB has a higher initial potential than Gr, indicating
that CB has better catalytic activity than Gr. After the addition of
PTFE, the initial potential of oxygen reduction increases and moves
towards the direction of thermodynamic equilibrium potential, indicating
that the catalytic activity of PTFE is improved. In Fig.S2e, the open
circuit potential of Gr under the condition of saturated dissolved
oxygen is 0.147v, which is far less than that of CB (0.825v), indicating
that the surface of Gr has less oxygen adsorption and basically no
oxygen reduction catalytic activity, which is consistent with the
previously characterized small specific surface area of Gr and fewer
defects. For CB, due to the large specific surface area and more
defects, the oxygen adsorbed on the surface of CB has a greater tendency
to undergo reduction reaction, and 0.825v is higher than the standard
potential of 0.69v of 2e-reduction mechanism of oxygen, indicating the
existence of 4e-reduction mechanism for CB.
After adding PTFE, the open circuit potential of Gr becomes 0.328v, but
it is still far away from the standard potential of 0.7v, with poor
activity. CB is compounded with Gr and added with PTFE
(Gr0.5CB0.5/PTFE0.5), with an open-circuit potential of 0.786V, which is
close to the oxygen 2e-reduction potential. Peer’s study[73] has also shown that the addition of PTFE can
change the surface polarity of the material and increase the affinity
adsorption capacity of oxygen. Whether the open circuit potential is
closer to the standard potential and the selectivity of hydrogen
peroxide generation is better still needs further experimental study.
In addition to increasing the activity of active sites, there are
strategies to increase the number of active sites that affect the rate
of hydrogen peroxide formation. By observing the variation trend of the
capacitance reflecting the surface roughness of the electrode in Fig.S2d
and the electrochemical active surface area in Table.1, we are surprised
to find that the electrochemical active surface area with PTFE
significantly increases, even though the electrochemical active surface
area of PTFE with Gr is higher than that of the original CB material,
indicating that the addition of PTFE significantly increases the number
of active sites. This shows the important role of adding PTFE in
promoting the formation of hydrogen peroxide. It can also be considered
that the addition of PTFE resulted in the formation of hierarchical pore
structure, and the presence of polymer phase also improved the affinity
of catalyst surface to gas. More three-phase interfaces are constructed
by the two kinds of interaction. This structure is conducive to oxygen
transfer, oxygen adsorption and 2e-activation, thus improving the
hydrogen peroxide generation rate and Faraday efficiency.
3.3 The performance of synthesis
H2O2 in a H-type
cell