The OER electrocatalytic properties are assessed firstly by linear sweep voltammetries (LSVs). Before further analyses, the mass ratio between Bulk-BP and Raw-Co
3O
4 (P/Co) is optimized (Fig. S10 in Supporting information). Noted that the superordinate onset potential (
E0) and prominent overpotential at the current density of 10 mA/cm
2 (
η10) reveal the best electrochemical performance of Co
3O
4/BP obtained at the P/Co of 2/5 (Experimental section in Supporting information).
Fig. 4a compares the LSVs of BP, Co
3O
4, Co
3O
4/BP in O
2-saturated 1.0 mol/L KOH, while employs RuO
2 as a benchmark. Remarkably, the Co
3O
4/BP heterojunction possesses the most positive
E0 (1.448 V) and lowest
η10 (253 mV) in comparison to those of BP and Co
3O
4 (Table S4 in Supporting information), and even exceeded than RuO
2, confirming the excellent OER catalytic activity of Co
3O
4/BP. Moreover, to the best of our knowledge, the OER catalytic activity of Co
3O
4/BP is one of the highest photocatalytic activities for other BP-based catalysts recently reported (Table S5 in Supporting information). Such an outstanding catalytic performance can be further verified by contrasting the corresponding Tafel slopes (
Fig. 4b). Co
3O
4/BP heterojunction exhibits a Tafel slope of 68 mV/dec (
Fig. 4c), which is slightly smaller than RuO
2, highlighting a more favorable OER kinetics of Co
3O
4/BP. The electrochemical impedance spectroscopy (EIS) and double-layer capacitance (
Cdl) tests are carried out to evaluate the electron transfer efficiency and electrochemical active surface area (ECSA).
Figs. 4d and
e and Fig. S11 (Supporting information) show the results of
Cdl and EIS measurements, in which the smallest nyquist semicircle and largest
Cdl imply the lowest electron transfer resistance and highest intrinsic oxygen electrocatalytic activity of the Co
3O
4/BP heterojunction compared with those of single-catalysts (BP and Co
3O
4). In addition to catalytic activity, the durability is also necessary for an excellent oxygen catalyst.
Fig. 4f exhibit the LSVs and chronoamperometric response of Co
3O
4/BP, in which the attenuations of
E0 and
η10 for the Co
3O
4/BP are almost negligible after 1000 continuous OER cycles. Moreover, the loss of current density for the Co
3O
4/BP is only around 24% after 10 h continuous OER operation (Fig. S12a in Supporting information), confirming the robust durability of developed Co
3O
4/BP heterojunction. The HRTEM image after the repetitive cycles reveals that Co
3O
4 nanoparticles still well dispersed on the lamellar BP substance, again demonstrating its good structure stability (Fig. S13 in Supporting information). The high-resolution XPS spectra of Co 2p and P 2p of Co
3O
4/BP before and after OER cycles so as to clarify the OER process (Fig. S14 and Table S6 in Supporting information). The shift towards lower binding energies of Co 2P
1/2 and Co 2P
3/2 indicates the oxidation process of Co
2+ to Co
3+, which also can be confirmed by the increased atomic ratio of Co
3+/Co
2+ after OER test (Tables S3 and S6 in Supporting information). Such results suggest that the low valent Co ions in Co
3O
4 provides the actual active sites for OER electrocatalysis. Additionally, the negligible increase in the P-O ratio from 11.8% to 19.4% results from the slight oxidation of BP during the OER process.