Molecular Level Picture of the Hydrogen Evolution Egill Skúlason, Gustav S. Karlberg, Jan Rossmeisl, Thomas Bligaard, Jeff Greeley, Hannes Jónsson, Jens K. Nørskov

1

Outline • Introduction – Electrolysis – Possible mechanisms of HER – HER and Had on Pt(111)

• DFT modeling & Results for HER on Pt(111) – Volmer, Tafel & Heyrovsky reactions – NEB & Ea vs U – The transfer coefficient

• The overall molecular-level picture of HER • Summary • Further studies

2

Electrolysis

Cathode: 2H++2e-  H2

Total:

Anode: H2O  1/2O2 +2H++2e-

H2O  1/2O2 +H2

ΔG0 =2.46 eV (1.23 eV/electron)

3

Elementary Reaction Steps in HER Overall reaction: 2(H+ + e-) -> H2 Volmer

Tafel

Heyrovsky

H+ + e- -> Had

2Had -> H2

Had + H+ + e- -> H2

or

H2

H+ Had

H+

H2 V

Had

e-

ePt 4

Developing a Molecular-Level Picture of HER • Experimentally difficult to probe the process – Most surface science methods cannot be used because of the electrolyte – Few in situ methods exist (IR, SFG, EXAFS)

• No consensus of the predominant reaction mechanism – Studies on Pt electrodes either suggest Tafel or Heyrovsky reaction – Markovic et al concluded [Ref: 1]: Tafel on Pt(110) but Heyrovsky on Pt(100) – They could not conclude the reaction mechanism on Pt(111)

• The Ea of HER on Pt(111) is ca. 0.2 eV at U = 0 V • Electronic structure calculations offer the possibility of providing a detailed molecular-level description of the process and perhaps conclude which mechanism has lower Ea: Tafel 2Had -> H2

or

Heyrovsky Had +

H+

+

e-

-> H2

??? 5

1 Markovic,

Grgur and Ross, J. Phys. Chem. B 101 (1997) 5405

Cyclic Voltammograms for Pt(111) in Different Electrolytes H2 oxidation

Had -> H+ + e-

[Ref: 2]

OH related

HClO4 related

H2SO4 related H2 evolution

H+ + e- -> Had 6

2 Markovic,

Gasteiger, and Ross, J. Electrochem. Soc., 144 (1997) 1591

Adsorption of H on Pt(111) • Two different adsorbed hydrogen species on Pt(111) have been invoked • Hupd: Initial H ads. in FCC 3-fold sites starting at ca. +0.35 V vs NHE at 300 K – In situ IRAS: 1000-1300 cm-1 [Refs: 3, 4, 5] – Our DFT: 1017 cm-1 [Ref: 6]

• Hopd: Adsorbed H at and below the U where HER becomes thermodynamically possible – Surface-Enhanced IR: 2100 cm-1 [Ref: 7] – Our DFT: 2200 cm-1 [Ref: 6]

• Hopd is suggested to be more reactive than Hupd and in on-top site 3

Baro, Ibach and Brushman, Surf. Sci. 88 (1979) 384 Richter and Ho, Phys. Rev. B 36 (1987) 9797 5 Reutt, Chabal and Christman, J. Electron. Spectrosc. Relat. Phenom. 44 (1987) 325 6 Skúlason et al, submitted (2007) 7 Kunimatsu, Senzaki, Tsushima and Osawa, Chemical Physics Letters 401 (2005) 451 4

7

Volmer Reaction H+ + e- -> Had

8

Low Additional Ea for Volmer Reaction ! Need detailed model

H+ + e- -> Had

Main contribution to the barrier: Initial proton transfer from H3O+ to H2O (IS -> TS) The proton transfer to the electrode is downhill in energy (TS -> FS) Low barrier -> Energetics given by the reaction energy -> This reaction is in equilibrium at 300K

9 Skúlason et al, submitted (2007)

Differential Ads. Energy as a Function of Hcov Volmer Reaction: H+ + e- -> Had

Ads. free energy of H:

ΔG = ΔEDFT + Δ(ZPE) - TΔS At standard conditions (298K, pH=0, 1bar H2): H+

+

e-

Sconf.= -kB*(θln(θ)+(1-θ)ln(1-θ)) dSconf/dθ = kB * ln ((1-θ)/ θ)

↔ H* ↔ 1/2H2

The Hcov is given by µH determined by U vs NHE:

µH = -eU

[Ref: 8]

A direct link between U and Hcov since states with free energy:

ΔG < µH = -eU will tend to be filled Skúlason et al, submitted (2007)

8

Nørskov, Rossmeisl, Logadottir, Lindqvist, Kitchin, Bligaard and Jónsson, J. Phys. Chem. B 108 (2004) 17886

10

Coverage dependent U Scale Volmer Reaction: H+ + e- -> Had

ΔG = -eU 11 Skúlason et al, submitted (2007)

Tafel Reaction 2Had -> H2

12

Tafel Reaction Profile 2Had -> H2

13 Skúlason et al, submitted (2007)

Getting U dependent Ea for Tafel Reaction 2Had -> H2

14 Skúlason et al, submitted (2007)

Tafel Reaction 2Had -> H2

Transfer coefficient α = dEa/d(eU) = 0.64

15 Skúlason et al, submitted (2007)

Heyrovsky Reaction Had + H+ + e- -> H2

16

Heyrovsky Reaction Profile Had + H+ + e- -> H2

17 Skúlason et al, submitted (2007)

Creating Different Electrode Potential Need detailed model for: Had + H+ + e- -> H2

Surface charge ∝ Work Function -> U vs NHE

U = φ - φNHE

The model is realistic, and a similar system has been studied experimentally [Ref: 9] Skúlason et al, submitted (2007) 9Henderson, Surface Science Reports 46 (2002) 1

18

Heyrovsky Reaction:

Had + H+ + e- -> H2

Transfer coefficient α = dEa/d(eU) = 0.42

The α factor is in good agreement with measured values [Ref: 10] 10 Neyerlin,

Gu, Jorne, Gasteiger, submitted (2006)

19 Skúlason et al, submitted (2007)

Why do we calculate much higher Ea? Heyrovsky Reaction: Had + H+ + e- -> H2

Our methodology? Are there steps in the experiments? Experiments

20 Skúlason et al, submitted (2007)

Heyrovsky Reaction Had + H+ + e- -> H2 We confirm with explicit calculations: The electrochemical transfer coefficient ≅ Brønsted-Evans-Polanyi-type relation

21 Skúlason et al, submitted (2007)

Overall Molecular-Level Picture of HER on Pt(111) • Starting at positive values and lowering U -> H starts building up on the surface in FCC sites until Hcov = 1 ML around U = 0 V • Lowering U further does not increase Hcov until U < -0.5 V (H in on-top site) • Below U = 0 V, HER becomes thermodynamically possible • The Ea for Heyrovsky is found to be lower than Tafel, suggesting it to be the dominated one • The Heyrovsky reaction proceeds by a proton from solution attacking 22 a H atom in on-top position

Summary We have established quite detailed picture of the double layer during electrochemical reaction Our preliminary analysis for HER gives a molecular level picture of the process, in good agreement with several experimental observations

Strong U dependence of Ea for Tafel and Heyrovsky That slope gives access to a theoretical value for the transfer coefficient: ~0.65 for Tafel 23 ~0.4 for Heyrovsky

Summary Probably the Heyrovsky mechanism dominates in HER on Pt(111) but since Tafel mechanism is close in Ea we can not be sure, taken all errors into considerations Perhaps Tafel and Heyrovsky mechanism are working in parallel -> Is that the reason they cannot conclude anything experimentally about which mechanism it is on the Pt(111) elelctrode? We confirm with explicit calculations that the electrochemical transfer coefficient can be viewed as a manifestation of the BEP-type relationship

24

TS

Further Studies • HOR is the opposite reaction of HER -> We have all the data -> We need to analyze it

HER

HOR

H + + e-

H2

• Include more water in the model – Better solvation of the proton ? – Hydrogen bonding -> IS vs TS vs FS ?

• HER on other metals: Ru, Pd and Au – Trends, Volcano, Transfer coefficient

• HER on Pt(110) & Pt(100) • HER on Pt steps

H+ H

H2

e25