Superb water splitting activity of the electrocatalyst Fe3Co(PO4)4 designed with computation aid

For efficient water splitting, it is essential to develop inexpensive and super-efficient electrocatalysts for the oxygen evolution reaction (OER). Herein, we report a phosphate-based electrocatalyst [Fe3Co(PO4)4@reduced-graphene-oxide(rGO)] showing outstanding OER performance (much higher than state-of-the-art Ir/C catalysts), the design of which was aided by first-principles calculations. This electrocatalyst displays low overpotential (237 mV at high current density 100 mA cm−2 in 1 M KOH), high turnover frequency (TOF: 0.54 s−1), high Faradaic efficiency (98%), and long-term durability. Its remarkable performance is ascribed to the optimal free energy for OER at Fe sites and efficient mass/charge transfer. When a Fe3Co(PO4)4@rGO anodic electrode is integrated with a Pt/C cathodic electrode, the electrolyzer requires only 1.45 V to achieve 10 mA cm−2 for whole water splitting in 1 M KOH (1.39 V in 6 M KOH), which is much smaller than commercial Ir-C//Pt-C electrocatalysts. This cost-effective powerful oxygen production material with carbon-supporting substrates offers great promise for water splitting.

suggesting that not only Fe 2+ and Fe 3+ states but also Co 2+ and Co 3+ states kept almost same during the cycling. Nevertheless, though very small, L3,2-edge XAS spectra shifted very slightly to higher energy, indicating that Fe/Co is very slightly oxidized during the OER stability test. As a result, FeOx/CoOx or FeOOH/CoOOH could be slightly formed during the OER process as the reviewer pointed out. However, CoOOH is expected to be a less active site because of its higher overpotential than RuO2 (which has a much lower activity than ours) 8 and its less inductive effect of P towards Co sites in our catalyst. Phosphate sites are not active, as the reviewer addressed. However, metal sites are active. The DFT calculations demonstrate that the Fe-sites of Fe3Co(PO4)4 are active sites with overpotential 0.24 V in excellent agreement with the experiment.           Reference electrode calibration was carried in a three electrode system with Pt foil as working and counter electrode and Hg/HgO (1M NaOH) as reference electrode. The calibration was performed in high purity hydrogen saturated 1M KOH electrolyte. Steady-state linear-sweep voltammetry (LSV) was run at a scan rate of 0.5 mV s −1 and the potential at which current crosses zero was taken as thermodynamic potential (vs. Hg/HgO) for the hydrogen electrode (Supplementary Fig. 38).
The potential at which current crosses zero is -0.915 V vs Hg/HgO.  Fig. 29). The XRD peaks (except one peak at 32.8 0 ) of 4 match with the standard PDF card of Fe2P2O7@rGO (JCPDS 01-076-1762). The peak at 32.8 0 matches with the PDF card of Fe2PO5 (JCPDS 00-036-0084). The XRD pattern of 5 shows a mixture of CoFe2O4 and Fe2O3, in good agreement with the standard data (JCPDS 01-079-1744 for CoFe2O4 and 01-079-1744 for Fe2O3). Consequently, XRD patterns of 2-5 indicate the formation of desired products with the highest degree of crystallinity.
Core level XPS spectra of 1 (Fe3Co (PO4)4@rGO). The core level XPS spectrum of C 1s shows the main strong and sharp peak at 284.6 eV corresponds to graphitic carbon, while the peak located at 285.8 eV is assigned to C-O/C-P (Supplementary Fig. 20a) 35 . The spectrum of Co 2p displays the core-level XPS peaks at 782.3 eV (2p3/2) and 797.4 eV (2p1/2) with satellite peaks at 786.6, 790.7, and 803 eV corresponding to the cationic state of Co species (Supplementary Fig. 20b) 12 . The Fe 2p spectrum exhibits peaks of two different spin-orbits. The peaks located at binding energies of 713.2 and 725.3 eV with shakeup satellites (718.8 and 728 eV) are attributed to 2p3/2 and the 2p1/2 of Fe 3+ , while the peak located at binding energy of 711.6 eV with satellite peak of 715.3 eV correspond to Fe 2+ state (Supplementary Fig. 20c) 36,37,38 .
Core level XPS spectra of 2 (FeCo(PO4)2@rGO). In the core level XPS spectrum of C 1s, the peak centered at 284.5 eV corresponds to the graphitic carbon, while the peak located at 285.7 eV is assigned to C-O/C-P (Supplementary Fig. 30a) 35 . The core level XPS spectrum of P 2p shows typical peaks of phosphate species at binding energies of 133.7 and 134.6 eV (Supplementary Fig. 30b) 39 . The peaks of O 1s at binding energies of 531.3 and 532.3 eV correspond to the core level of O in phosphate group (Supplementary Fig. 30c) 39 . The XPS spectrum of Co 2p shows two core-level peaks at 782.2 eV (2p3/2) and 796.6 eV (2p1/2) with satellite peaks at 786.7, 790.9, and 802.2 eV corresponding to the cationic state of Co species (Supplementary Fig. 30d) 12 . The XPS spectrum of Fe 2p shows peaks of two different spinorbits. The peaks at binding energy of 713.9 and 725 eV with shakeup satellites (718.2 and 726.9 eV) are assigned to 2p3/2 and the 2p1/2 of Fe 3+ , while the peak located at binding energy of 712 eV with satellite peak of 715.2 eV correspond to Fe 2+ state (Supplementary Fig. 30e) 37,38 .
Core level XPS spectra of 3 (Fe2Co(PO4)3@rGO). The core level XPS spectrum of C 1s shows the peak of graphitic carbon at binding energy of 284.4 eV and C-O/C-P at binding energy of 285.3 eV (Supplementary Fig. 31a) 35 . The high resolution XPS spectrum of P 2p shows the phosphate peaks at binding energies of 133.2 and 134 eV (Supplementary Fig.  31b) 39 . Similarly, the XPS spectrum of O 1s in phosphate group shows the peaks at binding energies of 531.3 and 532.3 eV (Supplementary Fig. 31c) 39 . The spectrum of Co 2p shows the peaks of 2p3/2 and 2p1/2 at binding energies of 781.8 eV and 796.8 eV with shakeup satellite peaks at 785.7, 789.8, and 803.6 eV (Supplementary Fig. 31d) 12 . The Fe 2p XPS spectrum exhibits the peaks of Fe +3 at binding energies of 713.6 and 724.2 eV with satellites peaks of (719.9 and 726 eV) and peak of Fe 2+ at binding energy of 711.3 eV with satellites peak of 715.6 and 717.4 eV (Supplementary Fig. 31e) 37,38 .
Core level XPS spectra of 4 (Fe2P2O7@rGO). The XPS spectrum of C 1s in FeP2O7 shows the peak of graphitic carbon at binding energy of 284.4 eV and C-O/C-P at binding energy of 285.8 eV (Supplementary Fig. 32a) 35 . The peaks of phosphate in FeP2O7 is located at binding energies of 133.2 and 134.2 eV (Supplementary Fig. 32b) 39 . The P-O in phosphate group is located at binding energies of 531.4 and 532.5 eV (Supplementary Fig. 32c) 39 . The Fe 2p in FeP2O7 exhibits 2p3/2 and the 2p1/2 of Fe 3+ at binding energies of 714.7 and 725.3 eV with shakeup satellites (717.7 and 727.8 eV) and Fe 2+ at binding energies of 712.1 and 720.8 eV (Supplementary Fig. 32d) 37,38,40 .
Core level XPS spectra of 5 ((CoFe2O4)(Fe2O3)@rGO). The XPS spectrum of C 1s in (CoFe2)O4-GO shows the peak of graphitic carbon and C-O at binding energies of 284.4 and 285.8, respectively (Supplementary Fig. 33a) 35 . The high resolution XPS spectrum of oxygen shows three peaks at binding energies of 529.6, 531.6, and 532.9 eV, which can be assigned to metal-oxygen bond, metal-hydroxides, and adsorbed oxygen species (Supplementary Fig.  33b) 41,42 . The XPS spectrum of Co 2p displays core-level peaks at binding energies of 780.9 eV (2p3/2) and 795.6 eV (2p1/2) with satellite peaks at 784.1, 787.5, and 790.8, eV corresponding to the Co 2+ species in (CoFe2)O4 (Supplementary Fig. 33c) 43,44 . The high resolution XPS spectrum of Fe 2p in (CoFe2)O4 exhibits peaks of two different spin-orbits. The peaks at binding energies of 710.1 and 723.9 eV corresponding to Fe3O4, while the peak located at binding energy of 712 eV with satellite peaks of 718.7 and 726.2 eV corresponds to Fe 3+ state (Supplementary Fig. 33d) 37,38,40 .
Core level XPS spectra of Catalysts 6-9. The Supplementary Figs. 34-37 show the core level XPS spectra of C 1s, P 2p, O 1s, Co 2p and Fe 2p of catalysts 6-9. In all these catalysts 6-9, the C Is have the peaks of graphitic carbon (284.3-284.5 eV) and C-O/C-P (285.3 or 285.6 eV) (Supplementary Figs. 34a-37a) 35 . Similarly, the XPS spectra of O 1s in catalysts 6-9 show the peaks at binding energies which can be assigned to phosphate group (Supplementary Figs.  34c-37c) 39 . The core-level XPS spectra of P 2p, Co 2p and Fe 2p of catalysts 7-9 have almost similar binding energy to that of catalyst 1 in which the XPS spectra of P 2p show the typical peaks of phosphate species at binding energies of 133.1-133.3 eV and 134-34.2 eV (Supplementary Fig. 35b-37b) 39 . The binding energies of Co and Fe spectra ( Supplementary  Figs. 35d, e -37d, e) show that Co and Fe in catalysts 7-9 have similar cationic states to that of catalyst 1. However, in catalyst 6 we note that the core level XPS spectra of P 2p, Co 2p and Fe 2p have some different peaks compared to catalyst 1. For example, the core level XPS spectrum of P 2p in catalyst 6 shows two different states of peaks (Supplementary Fig. 34b), one can be assigned to phosphide (unresolved doublet centered at 129.6 eV) 45 and the other to phosphate (resolved doublet centered at 133.1 eV and 134 eV) 39 . The high-resolution XPS spectrum of Co 2p in catalyst 6 shows two pairs of peaks (Supplementary Fig. 34d). The peaks located at binding energies of 778.9 and 794.3 eV are assigned to metallic Co in CoP 46 , while the peaks at binding energies of 782.4 and 799.2 eV with shakeup satellites (787.6 and 804.4 eV) correspond to cationic cobalt in metal phosphate 12 . Similarly, the core level XPS of Fe 2p in catalyst 6 shows zero valence state peaks at binding energies of 707.2 and 720 eV 46 and cationic state peaks at binding energies of 711.8, 714.3 and 725.1 eV with shakeup satellite peaks at 716.6 and 728.7 eV, which are attributed to metal phosphide and metal phosphate, respectively (Supplementary Fig. 34e).