SUPPLEMENTARY INFORMATION

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1 In the format provided by the authors and unedited. SUPPLEMENTARY INFORMATION ARTICLE NUMBER: DOI: /NENERGY Photocatalytic hydrogen generation from hydriodic acid using methylammonium lead iodide in dynamic equilibrium with aqueous solution Sunghak Park 1, Woo Je Chang 2, Chan Woo Lee 1, Sangbaek Park 1, Hyo-Yong Ahn 1, and Ki Tae Nam 1,2* 1 Department of Materials Science and Engineering, Seoul National University, Seoul , Korea 2 Interdisciplinary Program for Bioengineering, Seoul National University, Seoul , Korea These authors contributed equally to this work * To whom correspondence should be addressed: Ki Tae Nam, Ph.D. Department of Materials Science and Engineering, Seoul National University, Seoul , Korea (Republic of) Tel: , Fax: nkitae@snu.ac.kr NATURE ENERGY 1

2 Supplementary Figure 1 Scanning Electron Microscope (SEM) images of MAPbI3 prepared from organic solvent before (a) and after (b-d) dipping in a saturated solution. The dipping times were 1 min (b), 10 min (c), and 30 min (d). NATURE ENERGY 2

3 Supplementary Figure 2 SEM images of the as-prepared MAPbI3 powder formed in saturated HI solution. a, Image of MAPbI3 powders with wide size distributions. b, Magnified image of one MAPbI3 powder. NATURE ENERGY 3

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5 Supplementary Figure 3 X-ray diffraction (XRD) patterns of the precipitates with various [I - ] and [H + ] concentrations. The black, red, blue, magenta, green, royal blue, purple, and violet lines indicate log[i - ]=1, 0.5, 0, -0.2, -0.4, -0.5, -0.6 and The black dashed line indicates the peak position of the tetragonal MAPbI3 phase, the sky blue line indicates the peak position of the monohydrate phase, the blue line indicates the peak position of the dihydrate phase and the dark yellow line indicates the peak position of the PbI2 phase. The star indicates the pattern of KI. a, The ph is b, The ph is c, The ph is d, The ph is e, The ph is f, The ph is 0. g, The ph is 0.5. h, The ph is 1. NATURE ENERGY 5

6 Supplementary Figure 4 Kubelka-Munk equation applied to the absorbance spectrum of the MAPbI3 Powder. The energy is determined from the wavelength of the absorbance spectrum. The optical absorption coefficient, F(α) = A 2 / 2(1-A), is calculated where A is absorbance, h is the planck constant ( m 2 kg s -1 ), and ν is the frequency of light at a specific wavelength. The multiplying factor of P at (F(α)hν) P is confirmed as 2 due to its linear drop line, which indicates the direct band gap character of the MAPbI3 powder. By extrapolating the drop line to zero, the band gap can be determined to be 1.53 ev. NATURE ENERGY 6

7 Supplementary Figure 5 Photocatalytic HI splitting by the MAPbI3 powder in saturated solution at various conditions. a, Photocatalytic HI splitting reaction of MAPbI3 powder in a saturated solution under both light and dark conditions. b, Photocatalytic HI splitting reaction of the saturated solution in the presence and absence of MAPbI3 powder. NATURE ENERGY 7

8 Supplementary Figure 6 Electrochemical H2 evolution from an MAPbI3 electrode in a saturated solution system. a, Linear sweep voltammetry curve of an MAPbI3-loaded carbon electrode in a saturated solution system. It shows significant electrocatalytic H2 evolution activity. In contrast, an unmodified carbon electrode shows minimal H2 evolution activity in aqueous HI. The carbon electrode also shows little H2 evolution activity in a saturated solution. b, Theoretical amount of evolved H2 and measured amount of H2 after bulk electrolysis using the MAPbI3 electrode. The bulk electrolysis was conducted at -0.9 V versus saturated calomel electrode (vs. SCE), and the evolved H2 was analysed by GC. NATURE ENERGY 8

9 Supplementary Figure 7 Absorbance of the standard I3 - solution for I3 - quantification. The standard solutions containing specific amounts of I3 - were characterized by UV-Vis absorption spectroscopy. The inset displays the absorbance curve of the standard at 353 nm. The concentration of I3 - in an unknown solution could be determined by substituting the value of the absorbance at 353 nm into the I3 - standard curve at 353 nm. The slope of the standard curve was , and intercept was The R 2 value was NATURE ENERGY 9

10 Supplementary Figure 8 Absorbance of 6.06 mol L -1 HI and the HI solution with added H3PO2. Visible light could be absorbed by the 6.06 mol L -1 HI solution. In contrast, almost no visible light could be absorbed by the H3PO2 added HI solution. NATURE ENERGY 10

11 Supplementary Figure 9 Linear sweep voltammetry measurement of H3PO2 aqueous solution with different ions conditions. The applied anodic potential only oxidize I - ions into I3 - ions in the solution. Concomitant I3 - generation was visually observed by the formation of dark brown colour at Pt working electrode during the sweep. H3PO2 remains stably in the measured potential range. NATURE ENERGY 11

12 Supplementary Figure 10 XRD patterns of MAPbI3 powder before and after photocatalytic HI splitting reaction. Tetragonal MAPbI3 phase was maintained after photocatalytic HI splitting reaction NATURE ENERGY 12

13 Supplementary Figure 11 MAPbI3 powder cycling test. Each evacuation process was performed with Ar after following 5 h of photocatalytic reaction. This cycling was repeated three times. NATURE ENERGY 13

14 Supplementary Figure 12 Photocatalytic H2 evolution using various amounts of MAPbI3 powder. The rate of H2 evolution becomes sluggish at 1,000 mg of MAPbI3 due to the irregular dispersion of the powder. NATURE ENERGY 14

15 Supplementary Figure 13 Photocatalytic H2 evolution of MAPbI3 powder with different light irradiation areas at a light power of 100 mw cm -2. NATURE ENERGY 15

16 Supplementary Figure 14 XRD patterns of the MAPbI3 powder and thermally annealed MAPbI3 powder in a polar solvent atmosphere. The intensities of the annealed MAPbI3 powder show higher values than the untreated MAPbI3 powder. NATURE ENERGY 16

17 Material (Cocatalyst) Solar HI splitting efficiency (%) Experimental Condition MAPbI3 (Pt) [This work] 0.81 % Photocatalyst 100 mw cm -2 (λ > 475nm) MAPbI3 [This work] 0.44 % Photocatalyst 100 mw cm -2 (λ > 475nm) [This work] WSe % Photocatalyst 100 mw cm -2 (λ > 475nm) Silicon (Pt) 1 0.6% PEC cell 100 mw cm -2 (Full solar light) WSe 2 (Pt) 2 4.2% PEC cell 100 mw cm -2 (Full solar light) Si (PEDOT:PSS) 3 3.7% PEC cell 100 mw cm -2 (Full solar light) GaAs (Pt) 4 2.6% PV-EC system 100 mw cm -2 (Full solar light) Supplementary Table 1 Solar HI splitting efficiency table. The MAPbI3 based photocatalysis system shows an efficiency that is comparable to PEC and PV-EC type HI splitting systems. NATURE ENERGY 17

18 Material (Cocatalyst) Amount of hydrogen evolved for 1h from 1 mol of MAPbI3 (mmolh2 molmapbi3-1 h -1 ) MAPbI3 (X) 6.94 Thermally annealed MAPbI3 in DMF atmosphere (X) 13.3 Thermally annealed MAPbI3 in DMSO atmosphere (X) 22.7 Thermally annealed MAPbI3 in DMSO atmosphere (Pt) 33.4 Supplementary Table 2 Hydrogen evolution activity of the MAPbI3 photocatalyst with various treatments under a light irradiation of 100 mw cm -2. The mol to mol comparison was conducted based on the hydrogen evolved after 5 h as shown in Fig. 5a. NATURE ENERGY 18

19 Supplementary References 1 Ardo, S., Park, S.H., Warren, E.L. & Lewis, N.S. Unassisted solar-driven photoelectrosynthetic HI splitting using membrane-embedded Si microwire arrays. Energy Environ. Sci. 8, (2015). 2 McKone, J.R., Potash, R.A., DiSalvo, F.J. & Abruña, H.D. Unassisted HI photoelectrolysis using n-wse2 solar absorbers. Phys. Chem. Chem. Phys. 17, (2015). 3 Mubeen, S., Lee, J., Singh, N., Moskovits, M. & McFarland, E.W. Stabilizing inorganic photoelectrodes for efficient solar-to-chemical energy conversion. Energy Environ. Sci. 6, (2013). 4 Khaselev, O. & Turner, J.A. Photoelectrolysis of HBr and HI using a monolithic combined photoelectrochemical/photovoltaic device. Electrochem. Solid-State Lett. 2, (1999). NATURE ENERGY 19