Supporting Information

Supporting Information Bio-Inspired Engineering of Cobalt-Phosphonate Nanosheets for Robust Hydrogen Evolution Reaction Zhong-Sheng Cai, 1, Yi Shi, 2, Song-Song Bao, 1 Yang Shen, 1 Xing-Hua Xia,*,2 and Li-Min Zheng*,1 1 State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, P. R. China 2 State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China Corresponding authors: lmzheng@nju.edu.cn; xhxia@nju.edu.cn These authors contributed equally. S1

Table S1. Crystallographic data for 1Co. Temperature 298 K V (Å 3 ) 889.39(11) Empirical formula C 7 H 9 CoO 5 P Z 4 Fw 263.04 ρ calcd (g cm -3 ) 1.964 Crystal system monoclinic F(000) 532 Space group P2 1 /n GOF 1.007 a (Å) 4.8553(4) R1, wr2 [I >2σ(I)] a 0.0339, 0.0658 b (Å) 32.281(2) R1, wr2 (all data) a 0.0559, 0.0721 c (Å) 5.6773(4) ( ρ) max, ( ρ) min / e Å -3 0.446, -0.405 β (deg) 91.788(1) CCDC number 1529373 a R 1 = Σ F o - F c /Σ F o. wr 2 = [Σw(F 2 o -F 2 c ) 2 /Σw(F 2 o ) 2 ] 1/2 Table S2. Selected bond lengths [Å] and angles [deg] for 1Co. Co1-O1 2.2078(18) O1-Co1-O2B 97.91(7) Co1-O1W 2.113(2) O1-Co1-O1C 160.26(7) Co1-O2 2.2778(18) O1W-Co1-O2 87.90(7) Co1-O3A 2.0550(18) O1W-Co1-O3A 174.13(8) Co1-O2B 2.0831(18) O1W-Co1-O2B 93.99(7) Co1-O1C 2.0740(18) O1C-Co1-O1W 91.37(7) P1-O1 1.5479(19) O2-Co1-O3A 87.18(7) P1-O2 1.5453(19) O2-Co1-O2B 164.07(7) P1-O3 1.507(2) O1C-Co1-O2 94.02(7) O1-Co1-O1W 89.03(7) O2B-Co1-O3A 89.87(7) O1-Co1-O2 66.27(6) O1C-Co1-O3A 92.17(7) O1-Co1-O3A 86.05(7) O1C-Co1-O2B 101.75(7) Symmetry codes: A: 1+x, y, z; B: 1/2+x, 1/2-y, -1/2+z; C: 1/2+x, 1/2-y, 1/2+z Table S3. Fitting parameters of the PXRD patterns of compounds 1Mn, 1Fe, 1Ni and 1Cu. Compound 1Mn 1Fe 1Ni 1Cu Space group P2 1 /n P2 1 /n P2 1 /n P2 1 /n Scale 0.44286 0.90408 0.69302 0.33654 a (Å) 4.86786 4.97707 4.79185 4.84012 b (Å) 30.96581 32.08457 32.52412 30.25552 c (Å) 5.72908 5.49016 5.54410 5.84598 β (deg) 90.70572 92.81849 91.4872 91.54907 V (Å 3 ) 863.52167 875.65047 863.76335 855.77485 GOF 1.10 3.54 1.17 1.05 R wp 3.86 19.49 6.91 11.58 S2

Table S4. HER activities of various catalysts in neutral solution. Catalysts Overpot ential/ mv Tafel slope/ mv dec 1 η/ mv TOF/ s -1 Reference 1Co-ns 84 48 250 0.023 In this work Ni-S film 1 550 77 N/A N/A J. Mater. Chem. A, 2014, 2, 19407 Co-P-B/rGO 2 168 82 N/A N/A J. Mater. Chem. A, 2014, 2, 18420 Co-S film 3 43 93 187 0.017 J. Am. Chem. Soc., 2013, 135, Co 2 L 2 (SO 4 ) 2 ( H 2 O) 6 4 17699 340 N/A N/A N/A CrystEngComm., 2014, 16, 8492 FePS 3 rgo 5 55 N/A N/A N/A ACS Energy Lett., 2016, 1, WP NAs/CC 6 100 125 N/A N/A ACS Appl. Mater. Interfaces, 367 2014, 6, 21874 FeP/CC 7 46 70 N/A N/A ACS Appl. Mater. Interfaces, 2014, 6, 20579 H 2 -CoCat 8 50 140 N/A N/A Nature Mater., 2012, 11, 802 Cu 2 MoS 4 /FT O 9 8912 160 95 N/A N/A Energy Environ. Sci., 2012, 5, Co-MoS 3 film 10 84 87 N/A N/A Chem. Sci., 2012, 3, 2515 MoS 3 film 10 176 86 N/A N/A Chem. Sci., 2012, 3, 2515 Co 2 P/NPG 11 40 79 N/A N/A Nano lett., 2016, 16, 4691 S3

Table S5. Comparative overpotentials of HER activity on different electrocatalysts in the neutral solution and the seawater. Catalyst Neutral solution Seawater Reference 1Co-ns 84 mv 205 mv In this work cobalt-sulfide film 43 mv ~500 mv J. Am. Chem. Soc., 2013, 135, 17699 Co P B/rGO 168 mv ~200 mv J. Mater. Chem. A, FePS 3 395 mv @ j =-10 ma cm -2 rgo-feps 3 220 mv @ j = -10 Co/N-codoped nanocarbon ma cm -2 240 mv @ j = -10 ma cm -2 2014, 2, 18420 673 mv @ j=-10 ACS Energy Lett., 2016, ma cm -2 1, 367 467 mv @ j=-10 ACS Energy Lett., 2016, ma cm -2 1, 367 340 mv @ j=-10 Nanoscale, 2015, 7, ma cm -2 2306 S4

Calculation of TOF The turn over number (TOF) was calculated from the HER polarization curve in Figure 3a. The total number of active sites was determined from the ICP-OES result. Number of active sites (in mol) for 1Co-ns, m = 3.26 x 10-8 mol From the number of active sites, the per-site turnover frequency (in s -1 ) was calculated using the following equation: TOF=J A/2 F m (1) where J stands for the anodic current density, A stands for the electrode surface area, F is the Faraday constant (96485 C mol -1 ) and m is the mole amount of Co 2+ ions. The factor 1/2 in the equation represents that two electrons are required to form one hydrogen molecule from two protons (2H + +2e - = H 2 ). TOF for 1Co-ns = 0.023 s -1 @ -0.24 V vs. RHE (2) S5

Figure S1. Calibration of the Ag/AgCl electrode. Variation in the potential difference between the Ag/AgCl and RHE reference electrodes with time in 0.1 M N 2 -saturated tris- HNO 3 (ph = 7.4) neutral aqueous solution. Figure S2. SEM image of 1Co bulk. S6

Figure S3. Left: Light floccules obtained after the reaction of Co(CH 3 COO) 2 and 3-moppH 2 in water at room temperature under sonication. Right: Gel-like state of the floccules after the addition of a small amount of water. Figure S4. SEM image of the floccules. Up panel shows the element mapping images of Co, P, C, and O in the floccules. S7

Figure S5. The energy dispersive X-ray spectroscopy (EDS) pattern (left) of the floccules and the corresponding element amount (right). Figure S6. XPS spectra of 1Co. S8

Figure S7. XPS spectra of the floccules of 1Co (1Co-ns). The chemical nature of the sub-monolayer sample was investigated by X-ray photoelectron spectroscopy (XPS). Each element has characteristic binding energies associated with electronic transitions from each of its core atomic orbitals, which together with small shifts from the chemical environment, give rise to a characteristic set of peaks in the XPS spectrum. As observed, the XPS spectra of 1Co and 1Co-ns show the presence of cobalt, phosphorus and oxygen (Figures S6 and S7). The P2p region of both samples exhibits two sharp peaks with 133.0 ev and 134.0 ev binding energies (area ratio of 2:1) corresponding to the 2p3/2 and 2p1/2 core levels of the central phosphorus atoms in the phosphate species. In the Co region, two broad sets of signals corresponding to the 2p3/2 (781.9 ev) and 2p1/2 (797.8 ev) core levels are observed. The O1s signals are centered for both materials at 531.6 ev. The P/Co/O atomic ratios for Co-ns (7.09:6.09:29.58) are similar to those of Co-bulk (7.75:6.52:32.23), indicating the same atomic composition. As the Co2p, P 2p and O1s signals of the 1Co and 1Co-ns are nearly in the same range as those of the cobalt phosphate inorganic compound, we tentatively conclude that the epitaxial organic skeleton helps optimize the electronic states of this coordination, further influencing the chemical nature and potential applications. S9

Figure S8. AFM image of 1Co-ns dispersed in water with the corresponding height values. Figure S9. The 3D visualization of the AFM image in Figure S8. Height / nm 0.8 0.0 0.8 0.0 0.8 0.0 0.8 0.0 0.8 0.0 1.54 nm 1.48 nm 1.49 nm 1.49 nm 1.48 nm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Distance / µm 1 2 3 4 5 Height / nm 0.8 0.0 0.8 0.0 0.8 0.0 0.8 0.0 0.8 0.0 1.48 nm 1.48 nm 1.55 nm 1.52 nm 1.51 nm 0.0 0.5 1.0 1.5 2.0 2.5 Distance / µm Figure S10. The corresponding height profile with the same number labeling as shown in Figure S8. 6 7 8 9 10 S10

Figure S11. PXRD patterns for the 1Co samples obtained by routes a and route b. Transmittance / a.u. 1Co-route Co-bulk-route a a 1Co-route Co-bulk-route b b 4000 3000 2000 1000 Wavenumber / cm -1 Figure S12. IR spectra for the 1Co samples obtained by routes a and route b. S11

Figure S13. (A) HER polarization curves in tris-hno 3 solution (ph=7.4) obtained on 1Co-ns with different loading amount as indicated. (B) Tafel plots of 1Co-ns with the corresponding loading amounts derived from the early stages of the HER polarization curves. Figure S14. HER polarization curves in tris-hno 3 solution (ph=7.4) obtained on several catalysts as indicated in the figure. S12

Figure S15. (A) HER polarization curves obtained on 1Co-ns in D 2 O and H 2 O solution of tris-hno 3 (ph=7.4) as indicated in the figure. (B) Tafel plots of 1Co-ns in the corresponding solution derived from the early stages of the HER polarization curves. Figure S16. HER polarization curves of the 1Co-ns catalyst in 0.5 M H 2 SO 4 solution and tris- HNO 3 solution (ph=7.4). S13

Figure S17. HER polarization curves of the 1Co-ns catalyst in tris-hno 3 solution at different ph values (as indicated in the figure). Figure S18. HER polarization curves in artificial seawater obtained on 1Co-ns with different loading amounts as indicated. S14

Figure S19. (A) Electrochemical device used in the gas chromatography (GC) measurements. (B) GC measurements of products catalyzed by 1Co-ns in the artificial seawater. H 2 -standard: pure H 2 collected from the gas cylinder; H 2 -product: gas collected from the working electrode. Transmittance / a.u. Mn-bulk Fe-bulk Co-bulk Ni-bulk Cu-bulk 4000 3000 2000 1000 Wavenumber / cm -1 Figure S20. IR spectra of 1Mn, 1Fe, 1Co, 1Ni, and 1Cu. S15

100 90 Weight / % 80 70 Mn-bulk Co-bulk Ni-bulk Fe-bulk Co-ns Cu-bulk 100 200 300 400 500 T / o C Figure S21. TG curves of 1Mn, 1Fe, 1Co, 1Ni and 1Cu. S16

S17 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 2,200 2,100 2,000 1,900 1,800 1,700 1,600 1,500 1,400 1,300 1,200 1,100 1,000 900 800 700 600 500 400 300 200 100 0-100 -200 hkl_phase 0.00 % 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 17,000 16,000 15,000 14,000 13,000 12,000 11,000 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0-1,000-2,000-3,000-4,000-5,000-6,000-7,000-8,000-9,000 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 4,000 3,800 3,600 3,400 3,200 3,000 2,800 2,600 2,400 2,200 2,000 1,800 1,600 1,400 1,200 1,000 800 600 400 200 0-200 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 1,500 1,400 1,300 1,200 1,100 1,000 900 800 700 600 500 400 300 200 100 0-100 -200 hkl_phase 0.00 % Figure S22. The PXRD patterns of 1Mn, 1Fe, 1Co, 1Ni and 1Cu. 1Mn 1Fe 1Ni 1Cu

Figure S23. XPS spectra of 1Mn. Figure S24. XPS spectra of 1Fe. Figure S25. XPS spectra of 1Ni. S18

Figure S26. XPS spectra of 1Cu. Figure S27. (A) HER polarization curves obtained on several catalysts with different metal centers in 0.5 M H 2 SO 4 solution at 20 mvs -1. (B) Relationship between the HER electrocatalytic activity and the number of d electrons of the center transition metal (Mn II, Fe II, Co II, Ni II, and Cu II ). The current densities were obtained at -0.45 V (vs. RHE). The dashed volcano line is shown for guidance only. S19

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