Two-Dimensional Metal-Organic Framework Nanosheets with Cobalt-Porphyrins for High-Performance CO2 Electroreduction

The electrochemical reduction of CO2 presents a promising strategy to mitigate the greenhouse effect and reduce excess carbon dioxide emission to realize a carbon-neutral energy cycle, but it suffers from the lack of high-performance electrocatalysts. In this work, catalytic active cobalt porphyrin [TCPP(Co)=(5,10,15,20)-tetrakis(4-carboxyphenyl)porphyrin-Co-II] was precisely anchored onto water-stable 2D metal-organic framework (MOF) nanosheets (Zr-BTB) to obtain ultrathin 2D MOF nanosheets [TCPP(Co)/Zr-BTB] with accessible catalytic sites for the CO2 reduction reaction. Compared with molecular cobalt porphyrin, the TCPP(Co)/Zr-BTB exhibits an ultrahigh turnover frequency (TOF=4768 h(-1) at -0.919 V vs. reversible hydrogen electrode, RHE) owing to high active-site utilization. In addition, three post-modified 2D MOF nanosheets [TCPP(Co)/Zr-BTB-PABA, TCPP(Co)/Zr-BTB-PSBA, TCPP(Co)/Zr-BTB-PSABA] were obtained, with the modifiers of p-(aminomethyl)benzoic acid (PABA), p-sulfobenzoic acid potassium (PSBA), and p-sulfamidobenzoic acid (PSABA), to change the micro-environments around TCPP(Co) through the tuning of steric effects. Among them, the TCPP(Co)/Zr-BTB-PSABA exhibited the best performance with a faradaic efficiency (FECO) of 85.1 %, TOF of 5315 h(-1), and j(total) of 6 mA cm(-2) at -0.769 V (vs. RHE). In addition, the long-term durability of the electrocatalysts is evaluated and the role of pH buffer is revealed.