In situ construction of Co/N/C-based heterojunction on biomass-derived hierarchical porous carbon with stable active sites using a Co-N protective strategy for high-efficiency ORR, OER and HER trifunctional electrocatalysts

Abstract The facile designs and fabrication of noble metal-free electrocatalysts are highly required to achieve multifunctional catalytic activity with excellent stability in Zn-air batteries, fuel cells and water splitting systems. Herein, a heterostructure engineering is applied to construct the high performance Co, N-containing carbon-based multifunctional electrocatalysts with the feature of isotype (i.e. n-n type Co2N0.67-BHPC) and anisotype (i.e. p-n type Co2O3-BHPC) heterojunctions for ORR, OER and HER. The n-n type Co2N0.67-BHPC, in which biomass (e.g. mushroom)-derived hierarchical porous carbon (BHPC) incorporated with nonstoichiometric active species Co2N0.67, is fabricated by using an in situ protective strategy of macrocyclic central Co-N4 from CoTPP (5,10,15,20-tetrakis(phenyl) porphyrinato cobalt) precursor through the intermolecular π-π interactions between CoTPP and its metal-free analogue H2TPP. Meanwhile, an unprotected strategy of macrocyclic central Co-N4 from CoTPP can afford the anisotype Co2O3-BHPC p-n heterojunction. The as-prepared n-n type Co2N0.67-BHPC heterojunction exhibited a higher density of Co-based active sites with outstanding stability and more efficient charge transfer at the isotype heterojunction interface in comparison with p-n type Co2O3-BHPC heterojunction. Consequently, for ORR, Co2N0.67-BHPC exhibits the more positive onset and half-wave potentials of 0.93 and 0.86 V vs. RHE, respectively, superior to those of the commercial 20 wt% Pt/C and most of Co-based catalysts reported so far. To drive a current density of 10 mA cm-2, Co2N0.67-BHPC also shows the lower overpotentials of 0.34 and 0.21 V vs. RHE for OER and HER, respectively. Furthermore, the Zn-air battery equipped with Co2N0.67-BHPC displays higher maximum power density (109 mW cm-2) and charge-discharge cycle stability. Interestingly, the anisotype heterojunction Co2O3-BHPC as trifunctional electrocatalyst reveals evidently photoelectrochemical enhancement compared with the photostable Co2N0.67-BHPC. That is to say, isotype heterojunction material (n-n type Co2N0.67-BHPC) is equipped with better electrocatalytic performance than anisotype one (p-n type Co2O3-BHPC), but the opposite is true in photoelectrochemical catalysis. Meanwhile, the possible mechanism is proposed based on the energy band structures of the Co2N0.67-BHPC and Co2O3-BHPC and the cocatalyst effects. The present work provides much more possibilities to tune the electrocatalytic and photoelectrochemical properties of catalysts through a facile combination of heterostructure engineering protocol and macrocyclic central metal protective strategy.