Fig.2 Characterization results of chemical property parameters of
different electrode materials. (a) Raman diagram of different
electrode materials at excitation source wavelength of
532nm;(b) Raman diagram of electrode Gr/PTFE0.5 under the
condition of O2 flow rate of
30ml•min-1, current of 150 mA•cm-2,
0.5mol•L-1 Na2SO4solution, pH=1.0, before and after reaction for 5h; (c) Fourier
infrared spectra of different electrode materials. The electrochemical
test is carried out with Ag/AgCl as the reference electrode, Pt
electrode as the counter electrode, and rotating disk electrode (RDE) as
the working electrode in O2 saturated 0.5
mol•L-1 Na2SO4solution with pH=1.0; (d) Double-layer capacitance test of
different electrode materials at RDE speed of 1600rpm at different
scanning rates.
Fig.2a shows the Raman spectrum of different electrode materials. Peak D
(1350cm-1) in the figure reflects the disordered
structure of the material. Peak G (1580cm-1 ) reflects
the ordered structure of carbon materials. The larger the ratio of peak
D to peak G strength, the more defect sites there are. From the Raman
test results, it can be seen that the
ID/IG (0.07) of Gr is far less than that
of CB (1.01), indicating that CB structure has a large degree of
disorder and a large number of surface defect sites. Before and after
the addition of PTFE, the ID/IG of the
material itself was almost unchanged, which indicated that no new
defects are generated after the addition of PTFE, and the original
crystal structure of Gr and CB materials is not changed. It can be seen
from the Fig.2b graphite gas diffusion electrode after using 5h,
ID/IG value is similar to carbon
materials, carbon black and disorder have no difference, showing that
graphite crystal structure in the electrochemical oxygen enriched
environment is destroyed by corrosion, and surface defects increase
dramatically. In Fig. S2, a new absorption peak appeares in the IR
spectrum of Gr/PTFE0.5 after reaction 5h at 1451cm-1,
which also indicates that the surface of graphite material is oxidized
to form carboxylic acid groups.
In order to investigate the changes in chemical composition of different
electrode materials, the functional groups of the as-prepared samples
are characterized by FT-IR(Fig2c). Fig.2c shows that the peak of
~ 3435cm-1 is the stretching vibration
peak of -OH [67], and the absorption peak of
~ 2925, 1630 and 1380cm-1 respectively
belongs to the stretching vibration of C-H of CB[70], C=C[71] and C-OH
group. In addition, C-O-C absorption (~
1100cm-1) and -COO absorption (~
1460cm-1) signal peaks appear in FT-IR spectrum[28]. Compared to CB and Gr in Fig. 2c, it can be
seen that under the condition of the same determination of CB is better
than the absorption of Gr signals, showing that CB surface more oxygen
containing functional groups, and oxygen in the CB mainly -OH, -COC- and
-O-C=O. According to report [43],O2 adsorption on oxygen-containing functional groups
(such as -COO-, -COC-) on the surface of C for the Pauling model. This
Pauling model is conducive to the generation of *OOH intermediate during
the 2e- oxygen reduction process, which is the active site for the
generation of hydrogen peroxide. After PTFE is added, C-F vibration
absorption peaks appear in the FT-IR spectrum in Fig.2c at
1220cm-1 and 1153cm-1, and the
positions and intensity of the infrared absorption peaks of
oxygen-containing functional groups don’t change. It should be pointed
out that these results are only static characterization. For reference,
the study of Chuan Xia [72] shows that surface
modification of PTFE can improve oxygen adhesion, and the oxygen
adsorbed near the active site during the reaction process will affect
the adsorption and desorption intensity of the intermediate species in
the electrochemical reaction, thus affecting the generation rate and
selectivity of products.
In order to further explore the 2e-ORR performance of electrode
materials, CHI760E electrochemical workstation is used to characterize
the electrode materials through linear scanning voltammetry curve (LSV)
and cyclic voltammetry curve (CV). The test results are listed in
Table.1.
Table.1 Characterization results
of different electrode materials