Modelling the HCOOH/CO 2 Electrochemical Couple: When Details Are Key

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1 C1 Chemistry Modelling the HCOOH/CO 2 Electrochemical Couple: When Details Are Key Stephan N. Steinmann Laboratoire de Chimie ENS Lyon France

2 Storing Energy in Molecules ElectroCat. HCOOH + ½ O 2 CO 2 + H 2 O OCV=1.48 V + Liquid, safe + Good voltage - Volumetric energy density 2100 Wh L -1 (40 % that of methanol) Lack of mechanistic understanding Intermediates, mechanism, barriers, overpotential? 2

3 The Challenge Electrochemical potential Complex environment Detailed in situ analytics are challenging Small interface buried in complex solution What do we learn from DFT studies? How realistic does a computational model need to be? 3 Image:

4 Electrocatalysis Thermodynamics Electrons are exchanged with electrode at potential U Grand canonical ensemble with e - at potential U Charge conservation and neutrality requires a counterion Derive E(U) of electron + cation from thermodynamics: ½ H 2 H + + e - ΔG = 0 at ph=0, U=0 V vs. SHE 4 Energy of H 2 and H + are constants Energy of e - is fully determined by U

5 Overall thermodynamics neglects: Modified reactivity due to: Solvent Electrolyte (and ph) Polarization of the electrode Electric double layer Heterogeneous Electrocatalysis Reactivity Full system: Heart of heterogeneous electrocatalysis Solvent, electrolyte and electrochemical potential Too complex for realistic DFT computations 5

6 Computational Heterogeneous Catalysis Computational Hydrogen Electrode (CHE) [1] Surface Charging (SC) [2] A A A q=0 A A A q=f(u,a) U ΔG(U)=ΔG DFT χ + qu No polarization of electrode Coupled proton-electron transfer Variable potential (workfunction) No solvent ΔG(U)=ΔG DFT χ (U) + qu Polarization of electrode Uncoupled electron transfer Reaction at constant potentials Implicit solvent [3] 6 [1] Norskov et al., JPCB 2004, 108, [2] Neurock et al., PRB 2006, 73, ; Filhol, PCCP 2011, 13, [3] Hennig et al., JCP 2014, 140,

7 Computational Hydrogen Electrode Surface Charging Model Comparison: CO 2 adsorption on Ni(111) CO 2 Adsorption Exothermic! Differential charge injection 7 p(3x3) unit cell PBE, cut-off: 400 ev, VASP+VASPsol accepted in ChemPhysChem

8 Computational Hydrogen Electrode Surface Charging Model Comparison: CO 2 adsorption on Ni(111) CO 2 adsorption Exothermic! O C O Differential charge injection 8 p(3x3) unit cell PBE, cut-off: 400 ev, VASP+VASPsol accepted in ChemPhysChem

9 Formic Acid Electro-Oxidation CHE model, U = 0 V, formate path, Ni(111) HCOOH HCOO+H + +e - CO 2 +2H + +2e - 9 accepted in ChemPhysChem

10 Formic Acid Electro-Oxidation SC model, U = 0 V, formate path, Ni(111) HCOOH HCOO+H + +e - CO 2 +2H + +2e - > 0.3 ev 10 accepted in ChemPhysChem

11 HCOOH HCOO+H + +e - CO 2 +2H + +2e - > 0.3 ev SC model, U = 0 V, formate path, Ni(111) 11 accepted in ChemPhysChem

12 Formic Acid Electro-Oxidation CO 2 (g) Significant potential dependence of chemical steps Reorientation of formate Desorption of CO 2 12 accepted in ChemPhysChem

13 Formic Acid Electro-Oxidation Change of rate limiting transition state HCOOH HCOO+H + +e - CO 2 +2H + +2e ev RLTS re-orientation desorption 13 accepted in ChemPhysChem

14 HCOOH HCOO+H + +e - CO 2 +2H + +2e - Charge injection (0.3 e - ) 14 accepted in ChemPhysChem

15 Workfunction and Potential Dependence Changes in workfunction induce deviations from CHE Differences up to +/- 0.3 ev/v 15 accepted in ChemPhysChem

16 Electrocatalysis complex for first principles simulations Insight from simple methods Keeping potential constant influences reaction profiles Conclusions Barriers on Ni(111): desorption/adsorption of CO 2, formate re-orientation For CO 2 reduction, need to lower chemisorption barrier! 16

17 Acknowledgements Philippe Sautet Carine Michel Jean-Sebastien Filhol Renate Schwiedernoch Computational Resources: 17

18 Formic Acid Electro-Oxidation Equilibrium potential : 0.18 V vs SHE (exp 0.2 V) Thermodynamic overpotential: 0.17 V (CHE 0.42 V) Kinetic overpotential : 0.32 V (CHE 0.52 V) HCOOH HCOO+H + +e - CO 2 +2H + +2e ev RLTS re-orientation desorption 18 accepted in ChemPhysChem

19 Surface charging I Uncorrected energies Workfunction elec abs z N 0 e abs G ( U ) EDFT ( N0) EDFT ( Ne) EDFT ( N0) q Va ( Ne)d Ne U ( Ne N0) Z N0 Implicit DMF Factor due to metallic screening Mamatkulov, Filhol, PCCP 2011, 13, Potential of vacuum GC ensemble Free Energy 19

20 Surface charging II Free (electronic) energy Number of electrons Implicit DMF Continuous functions by interpolation Sometimes extrapolation is needed 20