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Correlating the hydrogen evolution reaction activity in alkaline electrolytes

with the hydrogen binding energy on monometallic surfaces

Article  in  Energy & Environmental Science · April 2013

DOI: 10.1039/C3EE00045A

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aDepartment of Chemical and Biomolecular

Academy Street, Newark, DE 19716, USA. EbDepartment of Chemical Engineering, Colu

New York, NY 10027, USA. E-mail: jc3972@

† Electronic supplementary information (procedures. See DOI: 10.1039/c3ee00045a

Cite this: Energy Environ. Sci., 2013, 6,1509

Received 6th January 2013Accepted 28th March 2013

DOI: 10.1039/c3ee00045a

www.rsc.org/ees

This journal is ª The Royal Society of

Correlating the hydrogen evolution reaction activity inalkaline electrolytes with the hydrogen binding energyon monometallic surfaces†

Wenchao Sheng,a MyatNoeZin Myint,a Jingguang G. Chen*b and Yushan Yan*a

Broader context

Electrochemical energy conversion devices such as electrolysers, fuel cells,and solar hydrogen devices can provide a safe, clean and sustainablehydrogen based energy system, and have been in the spotlight of researchfor decades. The alkaline nature of hydroxide exchange membranesenables the utilization of earth-abundant non-precious metal catalysts,and thus makes these devices more commercially viable. However, theslower kinetics of the hydrogen evolution/oxidation reaction (HER/HOR)even on the most active Pt catalyst in alkaline compared to that in acidicmedia hinders the development of these low temperature alkaline elec-trochemical devices. Enhancement of the cell efficiency therefore requiresdesign and development of highly active electrocatalysts for the HER/HORin alkaline electrolytes. Nevertheless, the principle that governs the HER/HOR in alkaline media remains unclear and has not gained enoughattention. In this paper, the HER in alkaline solutions is used as the probereaction to examine the relationship between the exchange currentdensity and the calculated hydrogen binding energy (HBE). The results

The slow reaction kinetics of the hydrogen evolution and oxidation

reactions (HER/HOR) on platinum in alkaline electrolytes hinders the

development of alkaline electrolysers, solar hydrogen cells and

alkaline fuel cells. A fundamental understanding of the exchange

current density of the HER/HOR in alkaline media is critical for the

search and design of highly active electrocatalysts. By studying

the HER on a series of monometallic surfaces, we demonstrate that

the HER exchange current density in alkaline solutions can be

correlatedwith the calculated hydrogen binding energy (HBE) on the

metal surfaces via a volcano type of relationship. The HER activity

varies by several orders of magnitude from Pt at the peak of the plot

toWandAu located on the bottomof each side of the plot, similar to

the observation in acids. Such a correlation suggests that the HBE can

be used as a descriptor for identifying electrocatalysts for HER/HOR

in alkaline media, and that the HER exchange current density can be

tuned by modifying the surface chemical properties.

show that the exchange current density on monometallic surfaces can becorrelated with their HBE values via a volcano plot. This new nding isexpected to help provide guidance to the future search for more efficientelectrocatalysts for the HER/HOR in alkaline media.

1 Introduction

The alkaline nature of hydroxide exchange membranes enablesthe utilization of earth-abundant non-precious metal catalystsfor electrolysers,1 solar hydrogen generators,2 and fuel cells,3,4

and this new generation of electrochemical devices can providea safe, clean and sustainable hydrogen based energy system. Incontrast to the hydrogen evolution/oxidation reaction (HER/HOR) in proton exchange membrane based electrolysers andfuel cell systems, where the HER/HOR on Pt are fast with anegligible overpotential,5,6 the HER/HOR are much slower on Ptin alkaline electrolytes.7 Such slow kinetics would signicantlyreduce the cell efficiency, and consequently lead to a higherloading of the Pt catalyst. It is therefore critical to investigate the

Engineering, University of Delaware, 150

-mail: yanys@udel.edu

mbia University, 500 West 120th Street,

columbia.edu

ESI) available: Detailed electrochemical

Chemistry 2013

HER/HOR in an alkaline environment in order to obtain afundamental understanding and to develop more efficientelectrocatalysts. It has been generally accepted that the reactionactivity of the HER in terms of the exchange current density inan acid correlates with the chemisorption energy of hydrogenon the metallic surfaces by a volcano plot, revealing that anoptimal hydrogen binding energy (not too strong and not tooweak) would lead to the highest activity.8–10 Greeley et al.11,12

have demonstrated that the exchange current densities on a Pdmonolayer deposited on a series of substrates in an acid followthe volcano relationship with HBE. The bi- or tri-metallicsurface conguration modies the HBE such that Pd/Pt and Pd/PtRu surfaces show one order of magnitude enhancement ofthe exchange current density over Pd in an acid electrolyte.12

Furthermore, recent ndings13,14 show that MoS2 has a higherHER exchange current density than Mo, ascribed to its weakerHBE.14,15 Esposito et al.16,17 have found that Pt monolayers on

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WC and Pt exhibit almost identical HER activity, attributed totheir similar HBE. These observations suggest that the HERactivity can be tuned by modifying the hydrogen bindingstrength of the electrocatalysts, and therefore HBE can be usedas a screening tool for catalyst design. However, it is unclearwhether HBE can be used as a descriptor for the HER/HOR inalkaline solution, which should be determined rst in order todevelop more efficient electrocatalysts in alkaline media.

Due to the fact that not many metals except Pt-group metalsshow appreciable HOR activity in alkaline electrolytes, we usedthe HER as a probe reaction to examine the catalytic activity of aseries of monometallic surfaces, and investigated the relation-ship between the HER exchange current density and the HBE ofthe corresponding surfaces. A volcano type of correlationapplies in alkaline electrolytes. This indicates that the HBE canbe used as a useful descriptor for the HER in bases, similar tothe case in acids, which suggests the possibility of tuning theelectrochemical activity by modifying the surface chemicalproperties.

2 Experimental and theoretical methods2.1 Electrochemical measurements

All of the metal samples are polycrystalline disks with a diam-eter of 5 mm. Prior to the electrochemical measurements, thedisks were polished using 0.05 mm alumina powder and cleanedwith de-ionized water. The HER measurements on non-Ptsurfaces were performed in a H2-saturated 0.1 M KOH solutionusing linear sweep voltammetry (LSV). The HER/HOR on Pt weremeasured using cyclic voltammetry (CV). The polarizationcurves were corrected for solution resistance, which wasmeasured by AC impedance (ESI†). Exchange currents wereacquired by extrapolation of the current density between�1 mAcmdisk

�2 and �5 mA cmdisk�2 to the reversible hydrogen

potential (non-Pt metal) or tting the kinetic currents of theHER/HOR to the Butler–Volmer equation (Pt).

The electrochemical surface areas (ESAs) of Pt, Ni, Ag, Cu, Auand Pd were determined using CV by the H adsorption–desorp-tion (Pt),18,19 OH desorption (Ni),20,21 oxide formation (Ag22 andCu23), oxide reduction (Au24 and Pd25) and capacitance methods(Co).26 The measurements were made in an Ar-saturated 0.1 MKOH solution. Detailed experimental procedures and ESAcalculations for all the metal surfaces are provided in the ESI.†

Table 1 Calculated hydrogen binding energy on monometallic surfaces and meas

DEH (eV) H adsorption site log(i0 (A

Ag 0.25 FCC �7.3 � 0Au 0.14 FCC �6.2 � 0Cu �0.20 FCC �5.8 � 0Pt �0.46 FCC �3.20 �Pd �0.62 FCC �3.9Co �0.51 FCC �5.5 � 0Ni �0.51 FCC �5.1 � 0Fe* �0.59 Four-fold hollow �4.9 � 0W* �0.80 Two-fold bridge �7.2 � 0

1510 | Energy Environ. Sci., 2013, 6, 1509–1512

2.2 DFT calculations

The HBE values were calculated using the Vienna ab initioSimulation Package (VASP).27–29 The exchange–correlationenergy was approximated using the PW91 functional within thegeneralized gradient approximation (GGA) with a basis set ofplane waves up to an energy cutoff of 396 eV. In all cases, themodel surface consisted of a periodic 3 � 3 unit cell with fourlayers of metal atoms corresponding to the most close-packedcongurations separated by six equivalent layers of vacuum.The two bottom layers of the slab were xed while the top twolayers were allowed to relax to reach the lowest energy cong-uration. Spin-polarization was included for all surfaces. Thebinding energy was calculated using the equation:9

EHatomic ¼ EH–slab � Eslab � 0.5 � EH2(g)

where EHatomic is the binding energy of atomic hydrogen on thegiven slab, EH–slab is the energy of the slab with 1/9 ML hydrogenadsorbed, Eslab is the energy of the slab in a vacuum, and EH2(g)

is the energy of hydrogen in the gas phase. Different adsorptionsites, such as atop, bridge, fcc, and hcp, were calculated, butonly the values corresponding to the most stable binding siteare included in Table 1.

3 Results and discussion

Fig. 1a shows the LSVs of the HER on a series of monometallicsurfaces aer iR-correction in 0.1 M KOH. These monometallicsurfaces show different overpotentials for the HER reaching acurrent density of �1 mA cmdisk

�2, varying from �150 mV forPd to �500 mV for Ag. The curves between �1 mA cmdisk

�2 and�5 mA cmdisk

�2 were used to obtain the exchange current at thereversible potential by extrapolation of the Tafel plots (Fig. 1b)to 0 V vs. RHE. However, Pt exhibits a very small overpotential of��30mV at�1 mA cmdisk

�2 due to the relatively fast kinetics ofthe HER on Pt compared to other metals. It is therefore notappropriate to acquire the exchange current by extrapolation ofthe Tafel plot as the Tafel equation is indeed a simplied formof the Butler–Volmer equation with a signicantly large over-potential. The exchange current of the HER on Pt was thereforeobtained by tting the kinetic currents of the HER and HOR tothe Butler–Volmer equation as suggested in a previous study.7

Fig. 1c demonstrates the polarization curves of the HER/HOR

ured exchange current densities, log(i0), roughness factors and Tafel slopes

cmmetal�2)) Roughness factor

Tafel slopes ofthe HER (mV dec�1)

.3 1.7 �134 � 9

.6 2.2 �168 � 9

.2 6.1 �226 � 250.01 1.6 �113 � 1

1.7 �210.4 2.5 �126 � 6.5 1.9 �135 � 32.4 — �131 � 12.4 — �90 � 7

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on Pt obtained at 10 mV s�1 and a rotation speed of 2500 rpmbefore and aer iR-correction. Fig. 1d illustrates the HER/HORkinetic currents and their ts to the Butler–Volmer equation.The total HER exchange currents on the monometallic surfaceswere subsequently normalized to the ESAs to obtain theexchange current densities. Due to the difficulty in measuringthe real surface areas of Fe and W electrochemically, the datareported for these two surfaces were based on the geometricdisk area (5 mm in diameter). The exchange current densities(i0), the Tafel slopes and the roughness factors, dened as theratio between the ESA and the geometric disk area, are listed inTable 1. The i0 for Pt was determined to be 0.62 � 0.01 mAcmPt

�2, in good agreement with a previous study.7 The mono-metallic surfaces demonstrate the exchange current densityvarying within four orders of magnitude and different Tafelslopes ranging from�90mV dec�1 to�226mV dec�1, reectingdifferent current responses to the applied overpotential.

In order to explore the relationship between the HER activityand HBE, we plot the HER exchange current densities as afunction of calculated HBE values of these metallic surfaces(Fig. 2).

It is clear that Pt is still the best catalyst for the HER inalkaline media showing the highest exchange current density,similar to the case in acidic media.9,10 W, Fe, Ni, Co and Pd arelocated on the le-branch of the volcano curve because theybind to hydrogen too strongly while the opposite is true for Cu,

Fig. 1 (a) Linear sweep voltammograms (LSVs) of monometallic surfaces collected incurves were recorded at a sweep rate of 10 mV s�1 and a rotation rate of 2500 rpm; (bHER/HOR on Pt before (dotted red line) and after (solid black line) iR-correction in H2

and a rotation rate of 2500 rpm (positive-going sweep); (d) HER/HOR kinetic currentsblue line, aa ¼ 0.48). The HER/HOR kinetic currents were acquired from the iR-correc

This journal is ª The Royal Society of Chemistry 2013

Ag and Au. As the HBE of a metal increases or decreases fromthat of Pt, its HER activity decreases by orders of magnitude. Aspointed out by the Sabatier principle and many previousstudies,8–10 neither too strong nor too weak binding wouldfavour the overall reaction because strong or weak binding leadsto either difficulty in removing the nal product or pooradsorption of the reactant. This principle appears to apply inboth basic and acidic electrolytes where Pt has the optimal HBE.This volcano curve strongly suggests that the HBE can be auseful descriptor for identifying HER electrocatalysts.

The HER in a base proceeds in the following sequence ofVolmer–Heyrovsky or Volmer–Tafel mechanisms:30

Volmer: H2O + e� + * 4 Had + OH�

Heyrovsky: Had + H2O + e� 4 H2 + OH� + *

Tafel: 2Had 4 H2 + 2*

where * represents the hydrogen adsorption sites. Compared tothe HER in acids, the reactant in bases is water instead ofhydronium (H3O

+). In the above elementary reaction steps,adsorbed hydrogen (Had) appears to be the only reaction inter-mediate on the electrode surface, similar to the case in theacidic electrolyte. Consequently adsorption and removal ofadsorbed H atoms become competitive processes. Thiscompetition occurs in both acids and bases in the presence of

H2-saturated 0.1 M KOH after iR-correction at room temperature. The polarization) Tafel plots of the HER on the monometallic surfaces; (c) polarization curves of the

-saturated 0.1 M KOH at room temperature collected at a sweep rate of 10 mV s�1

on a Pt surface (solid black line) and their fits to the Butler–Volmer equation (dashedted polarization curve, and the HOR branch was corrected for H2 mass transport.

Energy Environ. Sci., 2013, 6, 1509–1512 | 1511

Fig. 2 Exchange current densities, log(i0), on monometallic surfaces plotted as afunction of the calculated HBE. The i0s for non-Pt metals were obtained byextrapolation of the Tafel plots between �1 and�5 mA cmdisk

�2 to the reversiblepotential of the HER and then normalization by the ESAs of these metal surfaces.All data are listed in Table 1. The dashed lines are guides for the eye.

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anions such as ClO4�, SO4

2� and Cl� in acids or OH� anions inbases, although the absolute HBE values are most likely modi-ed by the surrounding anion species.31 Therefore, an optimalHBE is still desired to maintain the balance between theadsorption and removal of the H intermediate in bases.

4 Conclusions

In summary, the correlation between the HER exchange currentdensities and the HBE values has been established in an alka-line medium via a volcano plot, similar to the case in acids,which suggests that the HER activity could be tailored by tuningthe surface chemical properties such that an optimal HBE valuecan be obtained. Further studies are in progress to apply thisknowledge to design and develop monometallic and bimetallicmaterials as more efficient electrocatalysts for alkaline-basedelectrolysers, solar hydrogen and fuel cell devices.

Acknowledgements

This work was supported by the ARPA-E program of the USDepartment of Energy (DOE DE-AR0000009). JGC and MMacknowledge support from the US Department of Energy (DOEDE-FG02-13ER16381).

Notes and references

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D. M. Kolb, ChemPhysChem, 2006, 7, 1032–1035.13 J. D. Benck, Z. B. Chen, L. Y. Kuritzky, A. J. Forman and

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15 B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen,J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Norskov,J. Am. Chem. Soc., 2005, 127, 5308–5309.

16 D. V. Esposito and J. G. Chen, Energy Environ. Sci., 2011, 4,3900–3912.

17 D. V. Esposito, S. T. Hunt, Y. C. Kimmel and J. G. Chen, J. Am.Chem. Soc., 2012, 134, 3025–3033.

18 F. C. Nart and W. Vielstich, Handbook of Fuel Cells:Fundamentals, Technology and Applications, John Wiley &Sons, New York, 2003.

19 T. J. Schmidt, H. A. Gasteiger, G. D. Stab, P. M. Urban,D. M. Kolb and R. J. Behm, J. Electrochem. Soc., 1998, 145,2354–2358.

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24 S. Trasatti and O. A. Petrii, Pure Appl. Chem., 1991, 63, 711–734.

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27 G. Kresse and J. Furthmuller, Phys. Rev. B: Condens. MatterMater. Phys., 1996, 54, 11169–11186.

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29 G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater.Phys., 1993, 47, 558–561.

30 K. J. Vetter, Electrochemical Kinetics: Theoretical andExperimental Aspects, Academic Press, New York, 1967.

31 A. N. Frumkin, in Advances in Electrochemistry andElectrochemical Engineering, ed. P. Delahay, John Wiley &Sons Inc., New York, 1963, p. 319.

This journal is ª The Royal Society of Chemistry 2013

Supplemental Materials for

Correlating Hydrogen Evolution Reaction Activity in Alkaline

Electrolyte to Hydrogen Binding Energy on Monometallic Surfaces

Wenchao Sheng, a MyatNoeZin Myint, a Jingguang G. Chen, b* and Yushan Yan a*

†Department of Chemical and Biomolecular Engineering

University of Delaware, 150 Academy Street, Newark, DE 19716

$Department of Chemical Engineering

Columbia University, 500 West 120th Street, New York, NY 10027

Corresponding Authors:

Jingguang G. Chen: jc3972@columbia.edu

Yushan Yan: yanys@udel.edu

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

S1. Hydrogen evolution reaction (HER) on monometallic surfaces using the linear

sweep voltammetry (LSV)

The HER measurements were performed in 0.1 M KOH prepared from KOH

pellets (99.99% trace metals basis, Sigma Aldrich) using de-ionized water (17.5 MΩ•cm).

Prior to the HER measurements, the metal disk electrodes (5 mm in diameter, Pine

Instruments) were polished with 0.05 µm alumina powder, cleaned with de-ionized water,

and then mounted onto a rotator (Pine Instruments) as the working electrode. Two spiral

Pt wires (Pine Instruments) served as the reference and counter electrodes respectively.

With H2 bubbling throughout the HER measurements, the potential was indeed on the

reversible hydrogen electrode (RHE) scale. The polarization curves were taken using

LSV, scanning from 0 vs. RHE (−0.2 V for W and −0.1 V for Fe) to a negative potential

at a sweep rate of 10 mV/s and a rotation speed of 2500 rpm, which was applied to

remove the H2 bubbles generated during the HER. The LSVs were repeated for a few

times until the steady state was reached. The steady state LSVs after solution resistance

(iR) correction (see below for the measurement of the cell resistance) were used for data

analysis of the HER. However, due to the strong hydrogen absorption property of Ni, Co

and Pd, the LSVs kept degrading, and therefore the first LSVs without obvious oxide

reduction peaks were used because they closely resemble the metal surface response to

the HER. For the measurements of the HER on different metal electrodes, a fresh

electrolyte was used and the measurement for each metal disk was within 20 minutes to

minimize the contamination from glass corrosion in the alkaline solution. Representative

HER LSVs before and after iR-correction of Au and extrapolation of the Tafel plot to the

reversible hydrogen potential are shown in Fig. S1.

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

Figure S1. (a) Polarization curve of the HER before (dotted red line) and after (solid black

line) solution resistance (iR) correction on Au in H2-saturated 0.1 M KOH at room

temperature. The data were collected at a sweep rate of 10 mV/s and a rotation rate of 2500

rpm. (b) Tafel plot of the HER on Au (solid black line) and its linear fit and extrapolation to

the hydrogen reversible potential (dashed blue line).

S2. HER/HOR on Pt using the rotating disk electrode (RDE) method

The HER/HOR measurement of Pt was taken in the same setup except that a

double-junction Ag/AgCl (3.5 M KNO3) electrode (Analytical Sensor Inc.) was used as

the reference electrode, but the potential was referenced to RHE in this paper. The

HER/HOR polarization curves were recorded from ~−0.05 V to ~1.0 V vs. RHE at a

sweep rate of 10 mV/s and a rotation speed of 2500 rpm. After iR-correction, the kinetic

current of the HER/HOR, ik, was calculated based on the Koutecky-Levich equation

(1) Dk iii

111

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

where i is the measured current and iD is the limiting current.1 Subsequently, the kinetic

currents of the HER/HOR were fitted to the Butler-Volmer equation to obtain the

exchange current at 0 vs. RHE.

RT

F

RT

F

k

aa

eeii

)1(

0 (2)

In Eq (2), i0 is the exchange current, αa is the transfer coefficient, F is the Faraday’s

constant (96485 C/mol), R is the universal gas constant (8.314 J/mol/K), T is temperature

and η is the overpotential.2 i0 values with the best fitting were used in the data analysis.

S3. Electrochemical surface area (ESA) measurements using cyclic voltammetry and

capacitance methods

The ESA measurements were made in Ar-saturated 0.1 M KOH solution (prepared

from KOH pellets, Sigma Aldrich, with de-ionized water). A double junction Ag/AgCl

(3.5 M KNO3) electrode and a spiral Pt wire served as the reference and counter

electrodes respectively. All the potentials were referenced to the RHE scale.

Pt: The CV was scanned from ~0.03V to 1.0 V vs. RHE at a sweep rate of 50 mV/s.

The hydrogen adsorption/desorption region between 0.05 V and ~0.4 V vs. RHE (the

onset of the double layer region) was used to calculate the Pt surface area, assuming a

charge density of 210 µC/cm2

Pt for one monolayer of hydrogen coverage.3, 4

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

Ni: The CV was scanned from ~−0.15 to ~0.56 V vs. RHE at a sweep rate of 50

mV/s. The reduction peak at ~0.05 V vs. RHE corresponds to the reduction of Ni(OH)2.

The charge density is 514 µC/cm2

Ni.5, 6

Ag: The CV was scanned from ~0 to ~1.6 V vs. RHE at a sweep rate of 100 mV/s.

The peak at ~1.25 V vs. RHE can be attributed to the formation of one monolayer of

AgOH or Ag2O, which corresponds to a charge density of ~400 µC/cm2

Ag.7

Cu: The CV was scanned from ~−0.5 V to ~1.65 V vs. RHE at a sweep rate of 20

mV/s. The broad anodic peak between ~0.5 V and ~0.71 V corresponds to the formation

of one monolayer of Cu2O with a charge density of 360 µC/cm2Cu.

8

Au: The CV was scanned from ~0 to ~1.6 V vs. RHE at a sweep rate of 100 mV/s.

The reduction peak centered at ~1.1 V is due to the reduction of AuO, which corresponds

to a charge density of 390 µC/cm2

Au.9

Pd: The CV was scanned from ~0.16 V to ~1.25 V vs. RHE at a sweep rate of 50

mV/s. The oxide reduction peak located at ~0.75 V corresponds to a charge density of

424 µC/cm2

Pd.10

All the CVs are shown in Fig. S2.

Co: The ESA of Co was determined using specific capacitance method. The CVs

were recorded in Ar-saturated 0.1 M KOH at sweep rates of 10, 20, 30, 40, and 50 mV/s

between 0.87 V and 0.97 V vs. RHE. The current densities (per geometric surface area) at

0.92 V were plotted as a function of the scanning rate. The slope of the straight line,

divided by the specific capacitance (60 µF/cm2),

11 gives the roughness factor of the

electrode (Fig. S3). The real surface area was then calculated by geometric area (5 mm in

diameter) times the roughness factor.

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

Figure S2. (a) Cyclic voltammograms (CVs) of (a) Pt with two different upper potential

limits, (b) Ni, (c) Ag, (d) Cu, (e) Au and (f) Pd in Ar-saturated 0.1 M KOH.

Figure S3. CVs of Co in Ar-saturated 0.1 M KOH at different sweep rates. The inset shows

the linear relationship between the current density and the sweep rate at 0.92 V. The

roughness factor of the Co surface can be calculated by dividing the slope by the specific

capacitance of Co.

Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

S4. Impedance measurements

The solution resistances were measured right after the LSV measurements of the

HER at a constant potential with a 10 mV voltage perturbation applied. The ac spectra

were taken from 200 kHz to 1 Hz, where the real part of the resistance at 1 kHz was used

as the solution resistance.12

The HER/HOR polarization curves were then corrected to

obtain iR-free potential of the working electrode.

References

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Electronic Supplementary Material (ESI) for Energy & Environmental ScienceThis journal is © The Royal Society of Chemistry 2013

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