Fig. 4: First-principles calculations of CPFB bulk and surface structures and the resulting band alignments.
a Atomic bulk structures of stoichiometric (Co4Fe4K4(CN)24), non-stoichiometric Co rich (Co6Fe2K4(CN)24), and defective-Co vacancy (Co3Fe4(H2O)6(CN)18) CPFB, and the corresponding calculated PDOS for each element. The vacuum energy level (Evacuum) is shown in dotted lines in PDOS. Individual atomic surface structures are shown in Supplementary Fig. 25. The Fermi level (EFermi), which is calculated from DFT calculations and represents the highest energy level that is occupied by electrons at zero K temperature, is shown in dashed lines in PDOS. Enlarged plots of the PDOS around the VBM of defective CFPB are shown to present the unoccupied states (mostly from Fe3+) due to the Co vacancy. b Band alignments between different CFPB based on the vacuum level and the corresponding Standard Hydrogen Electrode (SHE) potential, where Eabs = ESHE + 4.44 V. The shaded regions represent the bands that are occupied with electrons, green for Fe and blue for Co contributions (obtained from PDOS). The empty regions around the VBM of non-stoichiometric Co deficient and defective CFPB mean the unoccupied bands that are capable of accepting electrons. The reduction potential of water (1.23 V vs. SHE), oxygen (0.7 V vs. SHE), and H2O2 to form hydroxyl radical (0.3 V vs. SHE), are shown in the dash-dotted line, dotted line, and dashed line, respectively. c Illustration shows the mechanism using the active sites-isolated CFPB catalyst for the tandem reaction (water oxidation reaction, ORR, and Fenton-like reaction). The innate feature of inter-charge transfer in CFPB can provide aid in renewing the electron configuration of different active sites after the catalysis reactions.
