Supplementary Materials for

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1 advances.sciencemag.org/cgi/content/full///e56/dc Supplementary Materials for Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on ph and hydrogen binding energy The PDF file includes: Jie Zheng, Wenchao Sheng, Zhongbin Zhuang, Bingjun Xu, Yushan Yan Published 8 March 6, Sci. Adv., e56 (6) DOI:.6/sciadv.56 Derivation of relationship between i and Epeak Cyclic voltammograms of,,, and at different ph values CVs and HOR/HER polarization curves at different ph values Cyclic voltammograms (CVs) Correlation of exchange current densities with Epeak Arrhenius plot CO stripping in electrolytes with different ph values Effect of anions on HOR/HER activities TEM images of supported metal electrocatalysts Procedure for calculating ph for buffer solutions and comparison between calculated and experimental values HOR/HER polarization curves on Vulcan XC-7 in electrolytes with different ph values Fig. S. CVs of,,, and at different ph values. Fig. S. CVs and HOR/HER polarization curves at different ph values on. Fig. S. CVs and HOR/HER polarization curves at different ph values on. Fig. S. CVs and HOR/HER polarization curves at different ph values on. Fig. S5. CVs and HOR/HER polarization curves at different ph values on. Fig. S6. CVs of,,, and in. M KOH. Fig. S7. Correlation of exchange current densities with Epeak. Fig. S8. Arrhenius plots of HOR/HER activities. Fig. S9. CO stripping profiles on at different ph values. Fig. S. CO stripping profiles on at different ph values. Fig. S. CO stripping profiles on at different ph values. Fig. S. CO stripping profiles on at different ph values. Fig. S. Effect of anions on HOR/HER activities.

2 Fig. S. TEM images and histograms of Pt, Ir, Pd, and. Fig. S5. Comparison of calculated and electrochemically measured ph of the electrolytes. Fig. S6. HOR/HER polarization curves on Vulcan XC-7. Table S. Particle size and surface area of,,, and. Table S. Effect of anions on Hupd peak potentials and HOR/HER exchange current densities.

3 Supplementary Materials. Derivation of relationship between i and Epeak Arrhenius equation i Ea K exp( ) (S.) RT where i is the exchange current density, K is the pre-exponential parameter, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin. Brønsted-Evans-Polanyi principle states that the activation energy is proportional to the reaction enthalpy for reactions in the same family, more specifically: E a E H (S.) where E is the activation energy of a reaction in the same family, is the enthalpy of reaction, and is a proportionality coefficient that characterizes the position of transition state along the reaction coordinate ( ). For HOR/HER ( H H e ), it consists of either a Tafel step ( H * Had ) or a Heyrovsky step ( H * Had H e ) followed by a Volmer step ( H ad H e *). If the Volmer step is the rate-determining step, the activation energy of HOR/HER is approximately the activation energy of the Volmer step E a Ea, Volmer E H H, desorption (S.) It has been derived earlier(5) that where H H, desorption H H, ads FE peak TSH (S.) H H, desorption is the enthalpy of H desorption, H H, ads Hupd desorption peak potential, S H is the standard entropy of H. is the enthalpy of H adsorption, E peak is the Hence, Ea E ( FE peak TSH ) (S.5) Substituting Eq. S.5 into Eq. S. yields i E K exp( ( FE peak RT E TSH Let A K exp( ) RT TS H ) E ) K exp( TS RT H FE ) exp( RT peak )

4 i FE peak Aexp( ) (S.6) RT If Ea, and Ea, are the activation energies of HOR/HER at ph and ph, respectively, while Epeak, and Epeak, are the Hupd desorption peak potentials at ph and ph, respectively, then according to Eq. S.5 E E F( E E ) (S.7) a, a, peak, peak.

5 . Cyclic voltammograms of,,, and at different ph values A. B. i (ma/cm Pt ) C i (ma/cm Pd ) M H PO (ph =.) phosphate buffer (ph =.9) phosphate buffer (ph = 7.) (bi)carbonate buffer (ph =.5). M KOH (ph =.78) -.5 HClO (ph =.9) phosphate buffer (ph =.66) phosphate buffer (ph = 6.9) phosphate buffer (ph = 9.9) phosphate buffer (ph =.8) -.5 i (ma/cm Ir ) D i (ma/cm Rh ) M H SO (ph =.) phosphate buffer (ph =.9) phosphate buffer (ph = 6.9) (bi)carbonate buffer (ph =.). M KOH (ph =.8) HClO (ph =.) phosphate buffer (ph =.5) phosphate buffer (ph = 6.7) (bi)carbonate buffer (ph =.) KOH (ph =.78) Fig. S. CVs of,,, and at different ph values. Cyclic voltammograms (CVs) of (A), (B), (C) and (D) at different phs (currents were normalized to ECSACV measured in. M KOH).

6 . CVs and HOR/HER polarization curves at different ph values A i (ma/cm Pt ) B i (ma/cm Pt ) i (ma/cm Pt ) i (ma/cm Pt ) i (ma/cm disk ) i (ma/cm disk ) Phosphate buffer (ph =.6) i (ma/cm Pt ) i (ma/cm Pt ). Phosphate buffer (ph =.6) i,in-situ =.89 ma/cm Pt i,ex-situ =.8 ma/cm Pt = C C C Phosphate buffer (ph = 6.8) E ir-free i,in-situ =.56 ma/cm Pt. - i -,ex-situ =. ma/cm Pt RF =.6 in-situ =.7 -. loading: 5 g/cm, RF disk ex-situ =.8 - Phosphate buffer (ph = 6.8) Phosphate buffer (ph = 6.8) i (ma/cm Pt ) M H PO (ph =.) loading: g/cm disk, RF in-situ =.9 RF ex-situ =. loading: 5 g/cm, disk -. RF ex-situ =.8 RF =. in-situ Phosphate buffer (ph =.6) -.5 A i (ma/cm disk ) B M H PO (ph =.) A i (ma/cm Pt ) i (ma/cm Pt ).. M H PO (ph =.) i,in-situ = 6. ma/cm Pt i,ex-situ =.6 ma/cm Pt = D D D (bi)carbonate buffer (ph =.) i,in-situ =.5 ma/cm Pt -. - i RF in-situ =.,ex-situ =.8 ma/cm Pt -. loading: 6 g/cm, =.6 RF disk ex-situ =.6 - (bi)carbonate buffer (ph =.) (bi)carbonate buffer (ph =.) i (ma/cm disk ). E E E. M KOH (ph =.8) E diffusion. i =.6 ma/cm - - Pt = loading: 7.5 g/cm, RF =.5 disk -.. M KOH (ph =.8). M KOH (ph =.8) i (ma/cm disk ) B i (ma/cm Pt ) Fig. S. CVs and HOR/HER polarization curves at different ph values on. CVs (A-E), HOR/HER polarization curves (A-E), and kinetic current densities with fittings based on the Butler- Volmer equation with αa + αc = (A-E) on in electrolytes at different ph values. CVs were measured in Ar at a scanning rate of 5 mv/s. HOR/HER polarization curves were collected in H- saturated electrolytes at a scanning rate of mv/s and a rotating speed of 6 rpm.

7 A i (ma/cm Ir ) i (ma/cm Ir ) i (ma/cm Ir ) (bi)carbonate buffer (ph =.) loading: g/cm, RF = 7.9 disk. M KOH (ph =.8) i (ma/cm disk ) M H SO (ph =.)..8. B B B i (ma/cm Ir ) D D D i (ma/cm Ir ) loading:.5 g/cm, RF ex-situ =.6 disk. M H SO (ph =.) loading: g/cm disk, Phosphate buffer (ph =.9) loading: g/cm disk, Phosphate buffer (ph = 6.9) loading: 8 g/cm disk, RF in-situ =.5 RF in-situ =. RF ex-situ =. RF in-situ =. RF ex-situ =.5 RF in-situ =. RF ex-situ = 5.9 A C. C C E i (ma/cm disk ) i (ma/cm disk ) i (ma/cm disk ) E i (ma/cm disk ) (bi)carbonate buffer (ph =.) Phosphate buffer (ph =.9)..... Phosphate buffer (ph = 6.9) M KOH (ph =.8) A i (ma/cm Ir ) i (ma/cm Ir ) i (ma/cm Ir ) i (ma/cm Ir ) E i (ma/cm Ir ) E M H SO (ph =.) i,in-situ =.76 ma/cm Pt i,ex-situ =.6 ma/cm Pt = Phosphate buffer (ph =.9) i,in-situ =.6 ma/cm Pt i,ex-situ =.96 ma/cm Pt =.8 Phosphate buffer (ph = 6.9) i,in-situ =.8 ma/cm Pt i,ex-situ =.75 ma/cm Pt =.58 (bi)carbonate buffer (ph =.) i,in-situ =. ma/cm Pt i,ex-situ =. ma/cm Pt = M KOH (ph =.8) i =. ma/cm Pt = Fig. S. CVs and HOR/HER polarization curves at different ph values on. CVs (A-E), HOR/HER polarization curves (A-E), kinetic current density with their fitting into the Butler-Volmer equation with αa + αc = (A-E) on in electrolytes with different ph. CVs were measured in Ar at a scanning rate of 5 mv/s, HOR/HER polarization curves were collected in H-saturated electrolytes at a scanning rate of mv/s and rotating speed of 6 rpm.

8 A i (ma/cm Pd ) i (ma/cm Pd ) ~.6 V ~.5 V Phosphate buffer (ph = 6.9) i (ma/cm disk ) M HClO (ph =.) V B B B. V E ir-free E diffusion i (ma/cm Pd ) C i (ma/cm Pd ) V.5 V. M HClO (ph =.) -. loading: g/cm disk Phosphate buffer (ph =.7).6 D D. D. i (ma/cm Pd ) E RF in-situ =. -.6 loading: g/cm RF disk ex-situ =. -.8 Phosphate buffer (ph =.). M KOH (ph =.8) RF = 5.9 A i (ma/cm disk ) C i (ma/cm disk ) i (ma/cm disk ) E i (ma/cm disk ) Phosphate buffer (ph =.7) Phosphate buffer (ph = 6.9) Phosphate buffer (ph =.). M KOH (ph =.8)... A i (ma/cm Pd ) i (ma/cm Pd ) C i (ma/cm Pd ) i (ma/cm Pd ) E i (ma/cm Pd ) E-.. E-.. E-. M HClO (ph =.) i,in-situ =.8 ma/cm Pd i,ex-situ =.77 ma/cm Pd = Phosphate buffer (ph =.7) i,in-situ =.5 ma/cm Pd i,ex-situ =.5 ma/cm Pd =.78 E Phosphate buffer (ph = 6.9) i,in-situ =. ma/cm Pd i,ex-situ =.8 ma/cm Pd = Phosphate buffer (ph =.) i,in-situ =.9 ma/cm Pd i,ex-situ = ma/cm Pd = M KOH (ph =.8) i =. ma/cm Pd = Fig. S. CVs and HOR/HER polarization curves at different ph values on.cvs (A-E), HOR/HER polarization curves (A-E), kinetic current density with their fitting into the Butler-Volmer equation with αa + αc = (A-E) on in electrolytes with different ph. CVs were measured in Ar at a scanning rate of 5 mv/s, HOR/HER polarization curves were collected in H-saturated electrolytes at a scanning rate of mv/s and rotating speed of 6 rpm.

9 A i (ma/cm Rh ) i (ma/cm Rh ) i (ma/cm Rh ) - -. (bi)carbonate buffer (ph =.) loading: g/cm, RF = 6. disk -.5. M KOH (ph =.8) i (ma/cm disk ) M HClO (ph =.) D D D E ir-free i (ma/cm Rh ) loading: 9 g/cm disk, RF in-situ =.7 RF ex-situ = 6. Phoshpate buffer (ph = 6.7) loading: g/cm disk, RF in-situ =.8 RF ex-situ = 6.5 i (ma/cm Rh )... M HClO (ph =.) i,in-situ = 6.7 ma/cm Rh i,ex-situ =.77 ma/cm Rh = B. B B Phoshpate buffer (ph =.5) E ir-free E diffusion -. - i -. RF = 7.5,in-situ =.8 ma/cm Rh in-situ -.5 loading: 9 g/cm, - i,ex-situ =.8 ma/cm Rh RF disk ex-situ = 6. = Phoshpate buffer (ph =.5) Phoshpate buffer (ph =.5) i (ma/cm Rh ) C E loading: g/cm disk, RF in-situ =. RF ex-situ =.9. M HClO (ph =.) A i (ma/cm disk ) C i (ma/cm disk ) i (ma/cm disk ) E i (ma/cm disk ) Phoshpate buffer (ph = 6.7) (bi)carbonate buffer (ph =.) M KOH (ph =.8) A i (ma/cm Rh ) C i (ma/cm Rh ) i (ma/cm Rh ) E i (ma/cm Rh ).... E Phoshpate buffer (ph = 6.7) i,in-situ =.7 ma/cm Rh i,ex-situ =.8 ma/cm Rh =.59 (bi)carbonate buffer (ph =.) i,in-situ =.7 ma/cm Rh i,ex-situ =. ma/cm Rh = M KOH (ph =.8) i =. ma/cm Rh = Fig. S5. CVs and HOR/HER polarization curves at different ph values on. CVs (A-E), HOR/HER polarization curves (A-E), kinetic current density with their fitting into the Butler-Volmer equation with αa + αc = (A-E) on in electrolytes with different ph. CVs were measured in Ar at a scanning rate of 5 mv/s, HOR/HER polarization curves were collected in H-saturated electrolytes at a scanning rate of mv/s and rotating speed of 6 rpm.

10 . Cyclic voltammograms (CVs) A. B. i (ma/cm Pt ) - -. i (ma/cm Ir ) M KOH, Ar, 5 mv/s -. E vs. RHE (V) -.5. M KOH, Ar, 5 mv/s -. E vs. RHE (V) C. D. i (ma/cm Pd ) M KOH, Ar, 5 mv/s -. E vs. RHE (V) i (ma/cm Rh ) M KOH, Ar, 5 mv/s -. E vs. RHE (V) Fig. S6. CVs of,,, and in. M KOH. Cyclic voltammograms of (A), (B), (C) and (D) measured in Ar-saturated. M KOH with a scanning rate of 5 mv/s. The dash line represents CV scans in a small potential range to avoid the contribution of oxide reduction current to the Hupd adsorption current. The electrochemical surface areas of, and in this work were determined from the Hupd adsorption and desorption peaks while that of was determined from PdO reduction peak, as described in the experimental section in manuscript and listed in Table S as ECSACV. The surface areas can also be determined from CO-stripping peak corresponding to a charge density of μc/cm,(6) which were listed in Table S as ECSACO-stripping. ECSACV and ECSACO-stripping of,, and (except ECSACO-stripping of ) are smaller than their corresponding volume/surface averaged surface area (Sv/a TEM ), probably due to the partial covering of the particles by the carbon support. The ECSACO-stripping is larger than ECSACV for all four supported metals (Table S). Similar results were obtained by Durst et al., and they suggested to normalize the HOR/HER kinetic currents by surface area determined from CO-stripping.() However, since we are studying the trends of activities vs ph or HBE, normalizing the activities by ECSACV will not alter our conclusion as long as we follow the same protocol for all experiments. In addition, the difference between ECSACO-stripping and ECSACV is less than a factor of, which is not significant compared with the two-orders of magnitude difference of HOR/HER activity in acid and base.

11 5. Correlation of exchange current densities with Epeak A B i (ma/cm Pt ).. Phosphate buffer Citrate buffer Acetate buffer (bi)carbonate buffer Borate buffer KOH E peak, ir-free i (ma/cm Ir ).. HClO H SO Phosphate buffer Citrate buffer Acetate buffer (bi)carbonate buffer Borate buffer KOH E peak, ir-free C i (ma/cm Pd ).. HClO H SO Phosphate buffer Citate buffer Acetate buffer (bi)carbonate buffer Borate buffer KOH E peak, ir-free D i (ma/cm Rh ).. HClO H SO Phosphate buffer Citrate buffer Acetate buffer (bi)carbonate buffer Borate buffer KOH E peak, ir-free Fig. S7. Correlation of exchange current densities with Epeak. Exchange current densities of HOR/HER on (A), (B), (C) and (D) as a function of underpotential deposited hydrogen (Hupd) desorption peak potential (Epeak) from CVs. Gray dash lines in the four figures are linear fittings of the data.

12 6. Arrhenius Plot ln(i ) (ln(ma/cm Metal )) /T (/K) Fig. S8. Arrhenius plots of HOR/HER activities. Arrhenius plot of HOR/HER activities on (square), (diamond), (up-triangle) and (down-triangle) in. M KOH.

13 7. CO stripping in electrolytes with different ph values. M HClO(pH=.8) -. M HPO(pH =.9) -.. M HPO(ml)+ml M KOH (ph =.) M HPO(ml)+ml 6M KOH (ph = 5.85). M HPO(ml)+ml 6M KOH (ph = 7.69). M HPO(ml)+ml 6M KOH (ph =.5) M HPO(ml)+ml M KOH (ph =.69 ). M KOH (ph =.78) -. vs. RHE (V) Fig. S9. CO stripping profiles on at different ph values.

14 M HClO (ph=.8). M HPO (ph =.5). M HPO(ml)+ml M KOH(pH=.9).. M HPO(ml)+6ml M KOH (ph = 5.9). M HPO(ml)+ml M KOH(pH = 7.5). M HPO (ml) +.5ml M KOH (ph = 9.86). M HPO (ml) + ml M KOH (ph =.). M KOH (ph =.78) vs. RHE (V) Fig. S. CO stripping profiles on at different ph values.

15 ... M HClO (ph =.7) M HPO (ph =.9). M HPO(ml)+ ml M KOH (ph =.66). M HPO(ml)+6 ml M KOH (5.97) M HPO(ml)+ ml M KOH (ph =.5). M HPO(ml)+ ml M KOH(.)... M KOH (.78). -. vs. RHE (V) Fig. S. CO stripping profiles on at different ph values.

16 M HClO(pH =.7). M HPO(pH =.6)... M HPO(ml)+ ml M KOH (ph =.7).. M HPO(ml)+6 ml M KOH (ph = 6.) -.. M HPO(ml)+ ml M KOH (ph = 7.8) M HPO(ml)+ ml M KOH (ph =.) M HPO(ml)+ ml M KOH (ph =.66) M KOH(pH =.78) vs. RHE (V) Fig. S. CO stripping profiles on at different ph values.

17 8. Effect of anions on HOR/HER activities A C E.. -. Ar, 5 mv/s Pt disk Pt disk Pt disk Ar, 5 mv/s E vs RHE (V). M KOH. M KOH +. M KCl. M KOH + M KCl. M KOH + M KCl. M KOH. M KOH +. M K SO. M KOH + saturated K SO (~.6 M) E vs RHE (V). M KOH. M KOH +. M NaClO. M KOH + (<) M NaClO E vs RHE (V) Ar, 5 mv/s B i (ma/cm disk ) D i (ma/cm disk ) F i (ma/cm disk ) - - Pt disk. M KOH, mv/s. M KOH +. M KCl, mv/s. M KOH + M KCl, 5 mv/s. M KOH + M KCl, 5 mv/s H, 6 rpm Pt disk Pt disk vs. RHE (V). M KOH. M KOH +. M K SO. M KOH + saturated K SO (~.6 M) H, mv/s, 6 rpm vs. RHE (V). M KOH. M KOH +. M NaClO. M KOH + (<) M NaClO H, 6 rpm, mv/s vs RHE (V) Fig. S. Effect of anions on HOR/HER activities. CVs and the HOR/HER polarization curves on a Pt disk in. M KOH with additional salts of (A, B) KCl, (C, D) KSO, and (E, F) NaClO.

18 9. TEM images of supported metal electrocatalysts Fig. S. TEM images and histograms of Pt, Ir, Pd, and. TEM images of (A), (B), (C) and (D) and their corresponding histograms of particle size (E-H), the particle size is.9 ±. nm for,.9 ±.8 nm for,. ±.6 nm for and. ±.7 nm for. The number averaged particle size (dn) was calculated as follows

19 n d i i d n (S9.) n where di is the individual particle diameter, and n is the number of particles measured. The volume/area averaged particle size (dv/a) is more accurate to represent the specific surface area than the number averaged particle size, which can be calculated as follows n i d i d v / a (S9.) n d i i The surface area determined from d / ( TEM v a TEM S v / a ) was calculated by S TEM v a 6 / d (S9.) where is the density of the metal (.5 g/cm for Pt,.56 g/cm for Ir,. g/cm for Pd, and. g/cm TEM for Rh), d is the particle size in nm, S / is in unit of m /g. v a

20 . Procedure for calculating ph for buffer solutions and comparison between calculated and experimental values The buffer solutions were prepared by adding x ml ( < x < ) M KOH into ml. M HPO, ml. M KHCO, ml. M citric acid,. M acetic acid and. M boric acid to obtain phosphate buffer solutions, carbonate/bicarbonate buffer solutions, citrate buffer solutions, acetate buffer solutions and borate buffer solutions, respectively. Their final ph (or [H + ]) can be calculated as follows using phosphoric acid as an example: Phosphoric acid (HPO) H H H PO PO pka =.5 PO H HPO H pka = 7. H PO HPO pka =.5 Mass balance: [ H PO ] [ H PO ] [ H PO ] [ HPO ] [ PO ] (S.) Charge balance: [ K ] [ H ] [ OH ] [ H PO ] [ HPO ] [ PO ] (S.) K a [ H ][ H PO ] (S.) [ H PO ] [ H ][ HPO ] K a (S.) [ H PO ] [ H ][ PO ] K a (S.5) [ HPO ] K w [ H ][ OH ] (S.6) We know [ H PO ] and [ K ], the six unknowns [ H PO ], [ H PO ], [ HPO ], [ PO ], [ H ] and [ OH ] can be obtained by solving Eqs S.-S.6. The pka values for KHCO, citric acid, acetic acid and boric acid are as follows: Potassium bicarbonate (KHCO) H H CO HCO pka = 6. H CO HCO pka =. Citric acid pka =., pka =.76, pka = 6. Acetic acid (CHCOOH) pka =.8

21 Boric acid (HBO) B( OH ) BO( OH ) H pka = 9. ( ) BO ( OH) H pka =. BO OH BO ( OH ) BO H pka =. The ph values of the other four buffer solutions were calculated in the same matter as in the case of phosphate buffer solutions. erimentally measured ph values of the electrolytes from HOR/HER equilibrium potentials for HOR/HER polarization curves according to Eq. S in the manuscript match well with those theoretically calculated by method mentioned above (Fig. S5), indicating the reliability of ph determination using electrochemical method.

22 ph ph ph ph ph phosphate buffer 6 8 (bi)carbonate buffer calculated 6 8 volume of M KOH into ml. M KHCO (ml) citric acid/citrate buffer 6 8 volume of M KOH into ml. M citric acid (ml) acetic acid/acetate buffer 6 8 calculated calculated volume of M KOH into ml. M acetic acid (ml) boric acid/borate buffer calculated volume of M KOH into ml. M H PO (ml) calculated 6 8 volume of M KOH into ml. M boric acid (ml) Fig. S5. Comparison of calculated and electrochemically measured ph of the electrolytes. Calculated (black line) and experimental measured phs (hollow square) as a function of the volume of M KOH (ml) added into ml. M HPO, KHCO, citric acid, acetic acid and boric acid.

23 . HOR/HER polarization curves on Vulcan XC-7 in electrolytes with different ph values A.6.,. M KOH B M KOH (ph =.7) Phosphate buffer (ph = 7.). M HClO (ph =.).5 M H SO (ph =.5) E vs RHE (V) Vulcan XC M KOH (ph =.7) Phosphate buffer (ph = 7.). M HClO (ph =.).5 M H SO (ph =.5) E vs RHE (V) Fig. S6. HOR/HER polarization curves on Vulcan XC-7. (A) Polarization curves on Vulcan XC-7 in. M KOH (ph =.7) (solid black line), phosphoric acid/phosphate buffer (ph = 7.) (solid green line),. M HClO (ph =.) (solid red line) and.5 M HSO (ph =.5) (solid blue line) measured in H at a scanning rate of mv/s and a rotating speed of 6 rpm. The loading of carbon on the glassy carbon electrode is. mg/cmdisk. The dash black line represents HOR/HER polarization curve on in H-saturated. M KOH at a scanning rate of mv/s and a rotating speed of 6 rpm for comparison. The loading of Pt is 8 µg/cm disk. (B) Zoom in polarization curves on Vulcan XC-7 in (A). The HOR/HER on carbon supported Pt, Ir, Pd and Rh are contributed by the precious metals since carbon alone show negligible HOR/HER activity in electrolytes with ph range from about to (Fig. S6).

24 Table S. Particle size and surface area of,,, and. Number averaged (dn TEM ) and volume/area averaged (dv/a TEM ) particle size from TEM, surface area calculated from dv/a TEM (Sv/a TEM ) and electrochemical surface area determined from CV in. M KOH (ECSACV) and CO stripping in. M KOH (ECSACO-stripping) of,, and ECSACV ECSACO-stripping (nm) (nm) (m /g) (m /g) (m /g).9 ±.. 6 ± 9 ± 6.9 ± ± 9 7. ± ± ± ± ± 8 dn TEM dv/a TEM Sv/a TEM Table S. Effect of anions on Hupd peak potentials and HOR/HER exchange current densities. Surface area measured from Hupd adsorption and desorption peaks, potentials for Pt() and Pt() and the corresponding exchange current densities (i) in. M KOH with addition of various salts Electrolyte Surface area (cm ) Peak()(V) Peak()(V). M KOH M KOH +. M KCl M KOH + M KCl M KOH +. M KSO M KOH + saturated (~.6 i (ma/cm Pt) M )KSO M KOH +. M NaClO M KOH + (<) M NaClO