Supporting Information. Structure Activity Correlations in a Nickel Borate Oxygen Evolution Catalyst

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1 Supporting Information Structure Activity Correlations in a Nickel Borate Oxygen Evolution Catalyst D. Kwabena Bediako, Benedikt Lassalle-Kaiser, Yogesh Surendranath, Junko Yano, Vittal K. Yachandra *, and Daniel G. Nocera, * Department of Chemistry, 6-335, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Physical Bioscience Division, Lawrence Berkeley National Laboratory, Berkeley, California, vkyachandra@lbl.gov (V.K.Y.); nocera@mit.edu (D.G.N.) Index EXAFS curve fitting Estimation of film thickness Figure S1. Galvanostatic anodization Figure S2. Anodization in 0.1 M KB i Figure S3. Coulometric analysis of film valency Figure S4. EXAFS spectra of an anodized Ni B i film held at 1.05 and 1.15V Figure S5. EXAFS spectra of Ni(OH) 2 and anodized Ni B i film held at 0.4 V Figure S6. EXAFS spectra of β-niooh, NaNiO 2, γ-niooh, and anodized Ni B i Figure S7. Models showing the effect of Jahn-Teller distortion on Ni O and Ni Ni scattering interactions. Table S1. EXAFS curve fitting parameters for NaNiO 2 Table S2. EXAFS curve fitting parameters for β-niooh Table S3. EXAFS curve fitting parameters for as-deposited Ni B i sample Page S2 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S1

2 EXAFS Curve Fitting The goodness of the fit was evaluated by the EXAFS R-factor (R f ) that represents the absolute difference (least-square fit) between theory and data. 1,2 For evaluation among different models, the reduced χ 2 (χ ν 2 ) was used: χ ν 2 = χ 2 /(N idp N var ) where N idp is the number of independent points in the measurement, and N var is the number of variable parameters in the fit. χ 2 is defined by the following equation: χ 2 = N idp N pts ε k 2 Npts i χ data calc [ i χ i ] 2 where ε k is the measurement uncertainty in k-space, and N pts is the number of points in the fitting range of the data. The measurement uncertainty was estimated by the root-mean-square (rms) average of χ(r) between 15 and 25 Å as described in the literature. 3 The uncertainty in the variables, scaled by the square root of χ ν 2, are estimated by fitting parameters using a Fourierfiltered spectrum of a relevant R range of absorber-backscatter distances. Estimation of film thickness We estimate the thickness of these films by noting that a Ni O distance of ~1.9 A has been identified in the EXAFS studies described here, and we therefore approximate that the primary coordination sphere of each Ni center occupies a cube of volume ~125 A 3. With this consideration, a monolayer of catalyst would constitute ~0.7 nmol of nickel centers per cm 2 of surface area, corresponding to a charge of ~65 µc/cm 2. The ITO-coated substrates are not atomically smooth, and these films are expected to grow by nucleation. Therefore, the term monolayer in this case more aptly approximates a monolayer s worth of material, and for such thin films, large regions of the ITO would be exposed. Bearing these qualifications in mind, we estimate a film thickness of 8 nm for a 1.0 mc/cm 2 film. A significant amount of O 2 evolution is coincident with film deposition and, as such, the amount of Ni centers should be less than that expected for the passage of 1.0 mc/cm 2 with 100% faradaic efficiency for deposition. Indeed, we find that the amount of Ni is ~63% of the charge passed in deposition (Table 1). This analysis only accounts for a Ni Ni interlayer distance of ~5 A, even though the interlayer distance in γ- NiOOH is ~7.2 Å. Thus, 8 nm is a reasonable upper limit for the thickness of these films. 1. Newville, M. J. Synchrotron Rad. 2001, 8, Ravel, B.; Newville, M. J. Synchrotron Rad. 2005, 12, Newville, M.; Boyanov, B. I.; Sayers, D. E. J. Synchrotron Rad. 1999, 6, S2

3 Figure S1. Galvanostatic anodization at 3.5 ma/cm 2 of a 1.0 mc/cm 2 Ni B i catalyst film in 1.0 M KB i ph 9.2 electrolyte. S3

4 Figure S2. Anodization at 1.1 V (vs. NHE) of a 1.0 mc/cm 2 Ni B i catalyst film in 100 mm KB i, ph 9.2 electrolyte. S4

5 Figure S3. Coulometric analysis of an anodized Ni B i catalyst film prepared by passage of 1.0 mc/cm 2 during deposition. S5

6 Figure S4. FT EXAFS spectra and k 3 -weighted EXAFS spectra (inset) of an anodized Ni B i catalyst film maintained at 1.05 ( ) and 1.15 V ( ). S6

7 Figure S5. FT EXAFS spectra and k 3 -weighted EXAFS spectra (inset) of Ni(OH) 2 ( ) and an anodized Ni B i catalyst film held at 0.4 V( ). S7

8 Figure S6. FT EXAFS spectra and k 3 -weighted EXAFS spectra (inset) of an anodized Ni B i catalyst film held at 1.05 ( ) V, γ-niooh ( ), NaNiO 2 ( ), and β-niooh ( ). S8

9 Figure S7. Models of the first and second shell scattering paths in (a) a structure where all Ni O (lime green) and Ni Ni (black) paths are equivalent, such as that found in γ-niooh and (b) a Jahn-Teller distorted structure where there exists two sets of non-equivalent Ni O (lime green) and Ni Ni (black) distances, such as that found in β-niooh or NaNiO 2. S9

10 Table S1. NaNiO 2 EXAFS Curve Fitting Parameters a Fit Path R / Å N σ 2 /Å 2 ΔE 0 / ev R f / % 1 Ni O 1.90 (0.02) 6.0 b (0.002) Ni Ni 2.87 (0.02) 6.0 b (0.001) 2 Ni O 1.92 (0.01) 4.0 (0.4) c (0.002) d Ni O 2.19 (0.04) 2.0 (0.4) c (0.002) d Ni Ni 2.90 (0.01) 4.4 (0.4) c (0.001) d Ni Ni 3.15 (0.03) 1.6 (0.4) c (0.001) d a Fitting region 1.2 R / Å 3.2, 2.69 k / Å Values in parentheses indicate uncertainties (see page S2). b fixed values. c Sum of the N values for the short and long interactions were fixed to 6. d σ 2 values were set to be equal for the short and long interactions. S10

11 Table S2. β-niooh EXAFS Curve Fitting Parameters a Fit Path R / Å N σ 2 /Å 2 ΔE 0 / ev R f / % 1 Ni O 1.92 (0.03) 6.0 b (0.002) Ni Ni 2.82 (0.04) 6.0 b (0.003) 2 Ni O 1.92 (0.05) 3.9 (0.5) c (0.002) d Ni O 2.12 (0.05) 2.1 (0.5) c (0.002) d Ni Ni 2.86 (0.02) 3.3 (0.2) c (0.001) d Ni Ni 3.09 (0.02) 2.7 (0.2) c (0.001) d a Fitting region 1.0 R / Å 3.6, 2.55 k / Å Values in parentheses indicate uncertainties (see page S2). b fixed values. c Sum of the N values for the short and long interactions were fixed to 6. d σ 2 values were set to be equal for the short and long interactions. S11

12 Table S3. As-deposited Ni B i Curve Fitting Parameters a Fit Path R / Å N σ 2 /Å 2 E 0 / ev R f / % 1 Ni O 1.92 (0.01) 6.0 b (0.001) Ni Ni 2.81 (0.01) 6.0 b (0.001) 2 (in text) Ni O 1.90 (0.01) 4.0 (0.4) c (0.001) d Ni O 2.10 (0.03) 2.0 (0.4) c (0.001) d Ni Ni 2.83 (0.01) 4.6 (0.35) c (0.001) d Ni Ni d 3.05 (0.03) 1.4 (0.35) c (0.001) d a Fitting region 1.0 R / Å 3.6, 2.55 k / Å Values in parentheses indicate uncertainties (see page S2). b fixed values. c Sum of the N values for the short and long interactions were fixed to 6. d σ 2 values were set to be equal for the short and long interactions. S12