## 3 Results and Discussion  
### Cyclic Voltammetry Test  
To evaluate the electrochemical activity and selectivity of ht-pDA/ACF, CV was firstly tested in an undivided cell from 0 V vs. Ag/AgCl to 1.0 V vs. Ag/AgCl. The CV curves of CF, acid-treated CF and ht-pDA/ACF at a scan rate of 10 mV/s are shown in Figure 2a.  
Figure 1: The preparation scheme of ao-pDA/ACF and ht-pDA/ACF.  
As we can see, the responding current is obviously enhanced after dopamine modifying, from 0.13 uA to 4.2 uA, suggesting an improvement of electrocatalytic activity for htpDA/ACF. Moreover, a pair of redox peaks appear on the curve of acid-treated CF at the 0.52 V vs. Ag/AgCl and 0.37 V vs. Ag/AgCl (Figure 2a, the red line 2), while the line of raw CF shows no obvious peak (Figure 2a, the black line 1). Similarly, the CV curve of ht-PDA/ACF shows a couple of well-defined peaks at 0.35 V vs. Ag/AgCl and 0.62 V vs. Ag/AgCl (Figure 2a, the blue line 3), which could be attributed to the 2e\({}^{-}\) ORR and its reverse reaction occurred on the surface of the ht-pDA/ACF cathode [16]. Additionally, the covered areas and the 2e\({}^{-}\) ORR peak current of CV curves of CF, ACF and ht-pDA/ACF cathode gradually increase, indicating the increasing electrons and mass transferability in the reaction [17]. The results prove that the activity of the ht-pDA/ACF cathode for the 2e\({}^{-}\) ORR significant improved compared with raw CF and acid-treated CF, implying that ht-pDA/ACF could be a promising alternative for the 2e\({}^{-}\) ORR and that pDA plays an important role during the reaction.  
We also investigated the change of CV curves of the ht-pDA/ACF cathode at different scan rates, as shown in Figure 2b. The CV curve of the ht-pDA/ACF cathode at a scan  
Figure 2: (**a**) The CV curve of carbon felt (curve 1), ACF (curve 2) and ht-pDA/ACF (curve 3) cathode at a scan rate of 10 mV/s from 0 V vs. Ag/AgCl to 1.0 V vs. Ag/AgCl in the electrolyte of 0.05 M H\({}_{2}\)SO\({}_{4}\) (pH = 1) with saturated O\({}_{2}\) (**b**) The CV curve of ht-pDA/ACF cathode at different scan rates from 1 mV/s to 100 mV/s (curve 4-curve 8).  
rate of 1 mV/s shows two indistinctive reductive peaks with similar intensity at 0.38V vs. Ag/AgCl and 0.44V vs. Ag/AgCl (Figure 2b, the black line 4), which represent the 4e\({}^{-}\) ORR and 2e\({}^{-}\) ORR, respectively [16; 18]. As the scan rate rises to 5 mV/s (Figure 2b, the red line 5), both peaks become more negative, which is due to the reaction polarization effect at a higher scan rate, and the intensity of the peak at 0.41 V vs. Ag/AgCl for H\({}_{2}\)O\({}_{2}\) production is obviously stronger than that at 0.26 V vs. Ag/AgCl for the 4e\({}^{-}\) ORR. Moreover, the peak that originated from the 4e\({}^{-}\) ORR disappears when the scan rate reaches 10 mV/s (Figure 2b, the blue line 6), whereas the peak attributed to the 2e\({}^{-}\) ORR appears clearly at 0.35 V vs. Ag/AgCl, suggesting that the 2e\({}^{-}\) ORR is more favorable during the reaction when ht-pDA/ACF was used as the cathode [18]. Such results imply that the ht-pDA/ACF cathode could have good selectivity and activity for H\({}_{2}\)O\({}_{2}\) production. When the scan rate continues rising to 50 mV/s and 100 mV/s (Figure 2b, the green line 7 and the pink line 8), the peak of the 2e\({}^{-}\) ORR also blurred and disappeared gradually as the peak of the 4e\({}^{-}\) ORR does.