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