Hydrogenases: Electrocatalysis and Implications for Future Energy Technology Fraser Armstrong Department of Chemistry Oxford H2(g) + O2(g) H2O (liq) DH = -286 kJ/mol specific enthalpy -143 kJ/gram H2 NASA uses hydrogen fuel to launch the space shuttles. Sunday Telegraph December 21 2003 The scientific challenges for a future H2-based energy economy synthesis from fossil fuels from solar physical (electrolysis and photocells) storage chemical, cryo, compression biological (fermentative and photosynthetic) energy conversion combustion fuel cell Pt catalyst enzyme, organism or ‘smart’ catalyst (bioinspired) Chemical and Engineering News July 22, 2002 Volume 80, Number 29 pp. 35-39 RENEWABLE Hydrogen bubbles form inside flask of sulfur-deprived green algae. PHOTO BY MICHAEL BARNES, UNIVERSITY OF CALIFORNIA Juan Fontecilla-Camps Fe-only hydrogenase NiFe-hydrogenase Active site of [NiFe]-hydrogenase The active site of hydrogenases con center coordinate invariant Cys thi which are termin An additional O-ligand is present in inactive states O The other two Cy bridge the Ni and centers. The latte coordinated by o two CNs. In the oxidized fo a putative oxo lig coordination of F octahedral and t square pyramida Identified by crystallography and FTIR Chemical and Engineering News July 22, 2002 Volume 80, Number 29 CENEAR 80 29 pp. 35-39 ISSN 0009-2347 CAN WE EXPLOIT HYDROGENASES? Insights into nature's substitute for platinum may lead to the design of inexpensive catalysts MICHAEL FREEMANTLE, C&EN LONDON ACTIVE SITE Oxford University Ph.D. student Sophie E. Lamle investigates activity of hydrogenase docked on an electrode. PHOTO BY MICHAEL FREEMANTLE Fuel cells can be as small as we like, even micro The future....fuel cells with cheap, inexpensive specific electrocatalysts, perhaps without a membrane ? Ideas from Nature ANODE Hydrogenase H2 Ni-Fe O2 CATHODE high E oxidases ? oxidase electrons Power ? On Pt, H2 is cleaved homolytically, Industry has to cope with CO H ........H H- H+ Pt M base Pt At an enzyme, H2 is cleaved heterolytically. Spectroscopic studies reveal a Ni(III)-hydrido species as an intermediate (Ni is formally oxidised by H2) + H Cys65 H S Ni III Cys530 S CN - CN Fe S S CO Cys68 Cys 533 Structure of [NiFe]-hydrogenase from Desulfovibrio gigas H2 H+ Fe Ni -Subunit (contains the active site) -Subunit (contains the electron relay) [4Fe-4S]dist [3Fe-4S] [4Fe-4S]prox Other [NiFe]-hydrogenases have similar sequences or spectroscopic properties What we expect from chemistry alone O2 + 4H+ + 4e- 2H+ + 2e- 2H2O 1.23 V (0.82)pH7 H2 0.00 V (-0.41)pH7 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 whatever the catalyst is 4 mW/cm2, Our mission.. How good are hydrogenases for Electrocatalysis ? How do they behave on an electrode ? What are the challenges for their widespread technological application? Investigating hydrogenases by protein film voltammetry H2 H+ hydrogenase electrode surface Measure catalytic current = turnover rate Control chemistry by modulating electrode potential Protein Film Voltammetry: Catalytic action can produce a large current with characteristic dependence on potential Normalised current e- 1 O R Potential/ Volts 0 -0.3 0 0.3 -1 -2 -3 -4 product substrate Current = Turnover rate At steady-state, rate is function of potential, not time -5 -6 Guides to interpretation: Heering et al. J. Phys. Chem, B 102, 6889 (1998) Léger et al. Biochemistry 40, 11234 (2001). Léger et al. J. Phys. Chem. B. 106, 13058 (2002). Léger et al. Biochemistry 42, 8653 (2003). 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) H O H3N+ HO H NH3+ H H H NH3+ H O OH O + H3 N H H H3N+ H H HO Special rotating-disc cell for studying gas enzymes designed by Kylie Vincent potential controller / data collection electrode rotator – up to 3500 rpm saturated calomel reference electrode o-ring seal onto electrode rotator gas out septum for injection of liquids gas in water out Pt wire counter electrode water in a few picomoles of protein adsorbed on spinning graphite electrode surface water jacket for temperature control Preparing the film: Stationary PGE electrode is potential-cycled in dilute H2ase solution ( < 1 mM) (in this case D.gigas NiFe enzyme) 1.5 Re-oxidation of H2 produced by H+ reduction 1.0 i / mA 0.5 0.0 -0.5 H+ reduction -1.0 -1.5 -0.6 -0.4 -0.2 E / V vs SHE 0 0.2 Catalytic voltammograms for film of NiFe hydrogenase adsorbed at a rotating disc PGE electrode under 0.1 atm H2. (Pershad et al Biochemistry 38, 8992 (1999) Based on detectable coverage limit, kcat >> 1500 sec-1 at 30 oC 2x [4Fe-4S]2+/+ +/0 [3Fe-4S] 3800 rpm 15.0 3000 rpm Current / mA 2000 rpm 1000 rpm 5.0 E (H+/H2) i -0.4 -0.2 keeps increasing as rotation rate is increased. lim Buffer -5.0 -0.6 H2 oxidation rate 0 Potential vs SHE 0.2 Scan rate 0.2 V/s Voltammograms for electrocatalytic Hydrogen oxidation Pt/PGE 100% H2 H2ase/PGE 100% H2 4 3 2 1 4 3 2 1 0 -0.4 -0.05 c) Current Density /(mA cm-2) b) Current Density /(mA cm-2) Current Density /(mA cm-2) a) H2ase/PGE 10% H2 0.5 0.4 0.3 0.2 0.1 0 0.3 Potential /V vs. SHE -0.4 -0.05 0 0.3 Potential /V vs. SHE -0.4 -0.05 0.3 Potential /V vs. SHE (Jones et al. Chem. Commun. 866 2002). 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 D Pt on Au 4 3 2 1 0 0 30 60 rotation rate1/2 /(rpm 1/2) 1 % Hydrogen Current Density /(mA cm-2) 10 % Hydrogen Current Density /(mA cm-2) Current Density /(mA cm-2) 100 % Hydrogen 0.6 0.4 0.2 0 0 30 60 rotation rate1/2 /(rpm 1/2) 0.06 0.04 0.02 0 0 30 60 rotation rate1/2 /(rpm 1/2) solution adsorbed H2ase molecules graphite electrode 4 3 Platinum on Gold 2 1 0 0 200 400 600 Tim e /s Current Density /(mA cm-2) Inhibition by CO is easily reversed at [NiFe]-H2ase but not Pt Current Density /(mA cm-2) CO CO 3 2 AvH2ase 1 0 0 200 Time /s 400 ‘100%- Bio’ hydrogen fuel cell : no chemical catalysts H2ase (NiFe enzyme) on PGE electrode laccase (Cu enzyme) on PGE electrode O 2 Nafion membrane Power (micro Watts) H2 Max power output 80 60 40 20 0 -1 -20 1 3 5 log[R] (R in k ohm) 7 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. Hydrogenases operate at low potentials Microorganisms have to cope with O2 concentration surface 0 5m 10 m 15 m 20 m max O2 aerobic bacteriochlorophyll anaerobic sulfide sediment Oxidation inactivates NiFe hydrogenases: a bridging ligand X blocks the site NiIII INACTIVE (OXIDISED) CN Cys65 X S Ni III Cys530 S S CO S oxidants O2 ? CN Fe Cys68 Cys ACTIVE (REDUCED) 533 H2 ? NiII CN Cys65 S CN Ni Cys530 S Fe S S Cys CO Cys68 533 reductants