A Porphyrinic Zirconium Metal-Organic Framework for Oxygen Reduction Reaction: Tailoring the Spacing between Active-Sites through Chain-Based Inorganic Building Units

Abstract

The oxygen reduction reaction (ORR) is central in carbon-neutral energy devices. While platinum group materials have shown high activities for ORR, their practical uses are hampered by concerns over deactivation, slow kinetics, exorbitant cost, and scarce nature reserve. The low cost yet high tunability of metal-organic frameworks (MOFs) provide a unique platform for tailoring their characteristic properties as new electrocatalysts. Herein, we report a new concept of design and present stable Zr-chain-based MOFs as efficient electrocatalysts for ORR. The strategy is based on using Zr-chains to promote high chemical and redox stability and, more importantly, tailor the immobilization and packing of redox active-sites at a density that is ideal to improve the reaction kinetics. The obtained new electrocatalyst, PCN-226, thereby shows high ORR activity. We further demonstrate PCN-226 as a promising electrode material for practical applications in rechargeable Zn-air batteries, with a high peak power density of 133 mW cm^-2. Being one of the very few electrocatalytic MOFs for ORR, this work provides a new concept by designing chain-based structures to enrich the diversity of efficient electrocatalysts and MOFs.

Figure 1

(a) Pawley fit of the experimental PXRD pattern (λ = 1.5406 Å) of PCN-226, which shows a good agreement. (b and d) Reconstructed 3D reciprocal lattice of PCN-226(Cu) and PCN-226(Co), respectively. (c and e) The particles from which the cRED data were collected. (f and g) The hk0 reciprocal planes of PCN-226(Cu) and PCN-226(Co), respectively, showing a similar intensity distribution. (h) HRTEM image of PCN-226(Cu) along the [010] direction that shows a rectangular pore packing with d₂₀₀ = 18.32 Å and d₀₀₂ = 16.18 Å (calculated d₂₀₀ = 18.53 Å and d₀₀₂ = 16.25 Å). Inset: Fourier transform of the image. (i) Symmetry imposed HRTEM image from the region highlighted by the red square in (h). The structural model of PCN-226(Cu) is superimposed in the image to show the good agreement.

Figure 2

(a) PCN-226 is formed by the linkage of infinite zigzag zirconium chains and TCPP. The infinite zigzag zirconium chains have the composition [ZrO(−COO)₂]_∞, with Zr atoms being hepta-coordinated. (b) The connection of PCN-226 showing a new topology, ztt, with Schlafli symbol {4·8²}{4²·6·8²·10} for the net. (c) The crystal structure of PCN-226 viewed along the b-axis showing the pore opening 7.2 Å × 4.8 Å and (d) the crystal structure viewed along the c-axis showing the 5.4 Å × 4.2 Å opening pores. Cyan capped octahedral, Zr; red spheres, O; black spheres, C, blue spheres, N; orange spheres, Cu.

Figure 3

(a) CV curves of PCN-226(Co) in 0.1 M KOH solution saturated by N₂ and O₂, respectively. (b) LSV curves, (c) electron transfer numbers (n), and (d) Tafel plots of PCN-226(Co), PCN-221(Co), PCN-222(Co), and Pt/C. (e) Durability test at 0.46 V (vs RHE) and 1600 rpm in O₂-saturated 0.1 M KOH solution, where j₀ is the initial current. It shows PCN-226(Co) has the lowest current loss (18%), followed by Pt/C (20%), PCN-221(Co) (48%), and PCN-222(Co) (50%). The MOFs were mixed with 50% carbon black.

Figure 4

(a) Schematic illustration of the Zn-air battery. (b) Discharge and charge cycling curves. (c) Discharging curves at j = 20 mA cm^–2. (d) Polarization curves and corresponding power density. (e) Long-term durability test of the Zn-air batteries at j = 2 mA cm^–2 assembled with PCN-226(Co) and commercial Pt/C+RuO₂. The loading of PCN-226(Co) is 6.25 times the amount that was used in the ORR test.

Figure 5

(a) A truncated molecular model of PCN-226. Color code: brown, Co; blue, N; gray, C; white, H. We varied the distance between the two porphyrin motifs to investigate the effect of the spacing on ORR. (b) Adsorption free energy of ORR intermediates as a function of the spacing. The two inherent spacings in PCN-226 are ca. 4 and 7 Å. The horizontal dashed lines indicate ΔG of an ideal catalyst. (c) Free energy diagram for the 7 Å spacing between porphyrin motifs (red) as compared to the ideal catalyst (black).