Hydrogenases: Electrocatalysis and Implications for Future Energy Technology

1. Hydrogenases: Electrocatalysis and Implications for Future Energy Technology Fraser Armstrong Department of Chemistry Oxford

2. NASA uses hydrogen fuel to launch the space shuttles. H2(g) + O2(g)  H2O (liq) DH = -286 kJ/mol specific enthalpy -143 kJ/gram H2

3. Sunday Telegraph December 21 2003

4. The scientific challenges for a future H2-based energy economy synthesis from fossil fuels from solar energy conversion storage combustion fuel cell chemical, cryo, compression physical (electrolysis and photocells) Pt catalyst enzyme, organism or ‘smart’ catalyst (bioinspired) biological (fermentative and photosynthetic)

5. Chemical and Engineering News July 22,2002Volume 80, Number 29pp. 35-39

6. Juan Fontecilla-Camps Fe-only hydrogenase NiFe-hydrogenase

7. Active site of [NiFe]-hydrogenase An additional O-ligand is present in inactive states

9. Fuel cells can be as small as we like, even micro

10. H2 O2 CATHODE high E oxidases ? Ni-Fe oxidase electrons Power ? The future....fuel cells with cheap, inexpensive specific electrocatalysts, perhaps without a membrane ? Ideas from Nature ANODE Hydrogenase

11. H ........H Pt Pt On Pt, H2 is cleaved homolytically, Industry has to cope with CO H- H+ M base

12. At an enzyme, H2 is cleaved heterolytically. Spectroscopic studies reveal a Ni(III)-hydrido species as an intermediate (Ni is formally oxidised by H2) III

13. Structure of [NiFe]-hydrogenase from Desulfovibrio gigas H2 H+ Other [NiFe]-hydrogenases have similar sequences or spectroscopic properties

14. 1.23 V (0.82)pH7 O2 + 4H+ + 4e- 2H2O 0.00 V (-0.41)pH7 2H+ + 2e- H2 What we expect from chemistry alone Based on flat electrode, no hydrodynamic assistance maximum current for H2 oxidation is about 4 mA/cm2 at ambient temperature and 1 bar gas (from diffusion coefficient of H2). Similar figures for O2. Power expectation is then about 4 mW/cm2, whatever the catalyst is

15. Our mission.. How good are hydrogenases for Electrocatalysis ? How do they behave on an electrode ? What are the challenges for their widespread technological application?

16. Investigating hydrogenases by protein film voltammetry H+ H2 hydrogenase electrode surface Measure catalytic current = turnover rate Control chemistry by modulating electrode potential

17. Current = Turnover rate Protein Film Voltammetry: Catalytic action can produce a large current with characteristic dependence on potential Normalised current Potential/ Volts At steady-state, rate is function of potential, not time Guides to interpretation: Heering et al. J. Phys. Chem, B 102, 6889 (1998) Léger et al.Biochemistry40, 11234 (2001). Léger et al.J. Phys. Chem. B. 106, 13058 (2002). Léger et al.Biochemistry42, 8653 (2003).

18. Enzyme is adsorbed on rotating disc Pyrolytic Graphite ‘Edge’ electrode. Less than 10 femtomole of enzyme is addressed, and numerous consecutive experiments can be conducted on same sample. enzyme and co-adsorbate (polyamine)

19. potential controller / data collection electrode rotator – up to 3500 rpm o-ring seal onto electrode rotator saturated calomel reference electrode gas out septum for injection of liquids gas in water out Pt wire counter electrode water in water jacket for temperature control a few picomoles of protein adsorbed on spinning graphite electrode surface Special rotating-disc cell for studying gas enzymes designed by Kylie Vincent

21. Preparing the film: Stationary PGE electrode is potential-cycled in dilute H2ase solution ( < 1 mM) (in this case D.gigas NiFe enzyme)

22. Catalytic voltammograms for film of NiFe hydrogenase adsorbed at a rotating disc PGE electrode under 0.1 atm H2. (Pershad et alBiochemistry 38, 8992 (1999) Based on detectable coverage limit, kcat >> 1500 sec-1 at 30 oC [3Fe-4S]+/0 - 2 x [4Fe-4S]2+/+ 3800 rpm 15.0 3000 rpm H2 oxidation rate keeps increasing as rotation rate is increased. Scan rate 0.2 V/s 2000 rpm A m 1000 rpm Current / E (H+/H2) 5.0 i lim Buffer - 5.0 - 0.6 - 0.4 - 0.2 0 0.2 Potential vs SHE

23. Voltammograms for electrocatalytic Hydrogen oxidation Pt/PGE 100% H2 H2ase/PGE 100% H2 H2ase/PGE 10% H2 (Jones et al. Chem. Commun. 866 2002).

24. Levich plots for H2 oxidation by [NiFe]-H2ase and Pt catalyst bound at identical or same-sized electrodes (Jones et al. Chem. Commun. 866 2002). Turnover rate estimated in region of 10,000 sec-1 at 45 oC  NiFe hydrogenase on PGE Pt on PGE Pt on Au

25. solution adsorbed H2ase molecules graphite electrode

26. Inhibition by CO is easily reversed at [NiFe]-H2ase but not Pt

27. ‘100%- Bio’ hydrogen fuel cell : no chemical catalysts H2 O2 Nafion membrane

28. We have established.... • Active site of Allochromatium vinosum [NiFe]-hydrogenase can oxidise H2 at diffusion controlled rate, comparable to Platinum catalyst, over wide pH range. • CO inhibition is easily reversible • (and can be avoided completely, see later) • Challenges. • Need to control and stabilise enzyme adsorption. (Experiments with new porous carbon materials) • Hydrogenases inactivated by oxidants. • For all future applications of hydrogenases, we need to understand the mechanisms of inactivation and activation and, with collaboration from other experts, find solutions to these problems.

29. concentration max 0 surface O2 aerobic 5 m bacterio- chlorophyll depth 10 m anaerobic 15 m sulfide sediment 20 m Hydrogenases operate at low potentials Microorganisms have to cope with O2

30. Oxidation inactivates NiFe hydrogenases: a bridging ligand X blocks the site oxidants O2? reductants H2 ?