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Tolerance to oxygen of hydrogen enzyme electrodes
Electrochemistry Communications 8 (2006) 851–854 www.elsevier.com/locate/elecom
Tolerance to oxygen of hydrogen enzyme electrodes Sergey V. Morozov a, Oleg G. Voronin a, Elena E. Karyakina a, Nikolay A. Zorin b, Serge Cosnier c, Arkady A. Karyakin a,* a
Faculty of Chemistry, M.V. Lomonosov Moscow State University, Lenin Hills, GSP-3, 119992 Moscow, Russia b Institute of Basic biological problems RAS, Pushchino, Moscow Region, Russia c LEOPR, UMR CNRS 5630, Universite Joseph Fourier BP 53, 38041 Grenoble Cedex 9, France Received 9 February 2006; received in revised form 2 March 2006; accepted 6 March 2006 Available online 17 April 2006
Abstract Tolerance to oxygen of hydrogen enzyme electrodes was first shown. Despite hydrogenase activity is suppressed by O2, their active sites being wired to the electrode can be re-activated in the presence of sufficient amount of molecular hydrogen. As a result enzyme electrodes based on hydrogenase from Thiocapsa roseopersicina are active up to 20% of air content in hydrogen, which coincides with an explosion limit of H2–O2 mixture. Despite oxygen inhibition of hydrogen electrooxidation occurs, it is completely reversible, and enzyme electrodes restore 100% of their activity, when gas mixture is changed back to pure hydrogen. The observed tolerance to oxygen of hydrogen enzyme electrodes provides a possibility of their use in H2–O2 fuel cells improving efficiency of energy conversion compared with platinum based devices. 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen enzyme electrode; Hydrogenase; Oxygen; Tolerance; Stability
1. Introduction Modern human society is facing global energetic problem. The forecasts predict absolute maximum of oil and gas mining in 2010 [1], after that production of hydrocarbon fuels will be decreased, and their cost will rapidly grow up. Thus, a search for alternative energy source is among the most important tasks nowadays. Molecular hydrogen is presently considered as the most promising chemical fuel providing highest energy output per its molecular weight, and after combustion, a complete absence of environmental pollutants including carbon dioxide (CO2). To utilize energy of hydrogen combustion it is preferable to use electrochemical fuel cells. The latter provide the highest efficiency of energy conversion. *
Corresponding author. Tel.: +7 495 9394605; fax: +7 495 9394675. E-mail address: [email protected] (A.A. Karyakin).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.007
For transportation the only low-temperature hydrogen– oxygen fuel cells were found to be suitable. Fuel electrodes, in particular hydrogen ones, use platinum (or platinum group metals) as electrocatalysts. There are, however, certain important problems, which make impossible wide use of such fuel cells in future. The most crucial problem deals with platinum cost and availability. Every kilowatt producing platinum based fuel cell costs $2000–$3000 [2]. Hence, 50 kW engine costs from $100,000 to $150,000, which makes doubtful their commercialization. A possibility to decrease fuel cell costs by one order of magnitude are not true realistic, because a decrease of platinum loading dramatically affects operational characteristics of fuel electrodes [3]. Concerning platinum availability, its annual production is just 195 tons and world’s total resources are just 100,000 tons [4]. However, only to cover an annual car production (64 million [5]) equipping them with 50 kW engines, 6000
852
S.V. Morozov et al. / Electrochemistry Communications 8 (2006) 851–854
tons of platinum are required. Comparison of the above figures makes platinum fuel cell program unrealistic. We already addressed poisoning problems, which make hardly possible operation of platinum based fuel cells with cheap fuels [6,7]. A valuable alternative to catalysis by noble metals is biocatalysis. We already reported, that hydrogen fuel electrodes based on the enzymes do not face the above problems [8]. Indeed, the enzymes are products of biotechnology, which provide their completely renewable source, and low cost upon mass production. Concerning poisoning by fuel impurities, hydrogen enzyme electrodes are completely insensitive to the presence of sulfide anion [9]. Carbon monoxide decreases the current of hydrogen oxidation only starting from 1% of its content in fuel, and enzyme electrode completely restores its activity when the atmosphere is changed back to pure hydrogen [8]. There is another problem with platinum-based fuel cells to be considered. Separation of anode and cathode compartments in fuel cells is provided by a thin layer of polyelectrolyte to reduce internal battery resistance, which causes energy loss via heating. Such thin layers are unable to prevent penetration of hydrogen into oxygen electrode compartment and vise a versa. Since platinum metals catalyze both hydrogen oxidation and oxygen reduction, such penetration decreases energy output. Considering selectivity problem, enzymes are able to catalyze only their specific reaction: hydrogen oxidation for hydrogenases and oxygen reduction for oxidases (laccase, ascorbate oxidase etc.). Despite hydrogenases do not catalyze oxygen reduction, O2 is able to inactivate these enzymes oxidizing their active sites [10]. It is, thus, of great importance to investigate, whether hydrogen enzyme electrodes are able to operate under certain content of O2 in hydrogen. The finding of tolerance to oxygen of hydrogenase electrodes provides a real possibility of their use as fuel electrodes in H2–O2 fuel cells. Moreover, enzyme electrodes are advantageous over platinum based ones as their performance characteristics are less affected by the presence of air in hydrogen.
The pyrrole derivative, N-methyl-N 0 -(12-pyrrol-1-yldodecyl)-4,4 0 -bipyridinium ditetrafluoroborate (MPDBP) was synthesized as previously reported [12]. 2.2. Hydrogenase electrodes preparation Carbon filament material CFM (1 · 0.5 cm) (TVS300 M, ‘Alten’ Company, Moscow Region, Russia) was used as working electrode. CFM specific electrical resistance was 50–70 mX cm. Hydrogenase electrodes were prepared by enzyme adsorption from its aqueous solution (2–4 mg /ml) for 12 h at 4 C in 0.005 M K-phosphate buffer, pH = 7.0 onto the CFM electrodes modified with poly(MPDBP). The solution of (MPDBP) in CH3CN (2 mM, 100 ll) was deposited on CFM electrode and allowed to dry for 2 h in ambient conditions. Electrochemical polymerization was carried out by repeated potential cycling over the range 0.8 to +0.95 V vs. Ag/AgClj1 M KCl reference electrode with the sweep rate 50 mV/s for 40 min in 0.1 M LiClO4 solution pH = 6.0. After polymerization the electrodes were washed with phosphate buffer solution (0.05 M KH2PO4, 0.1 M KCl, pH = 7.0) for 10 min. The apparent surface coverage of the electropolymerized (MPDBP) (C 1.4 107 mol cm2) was determined from the charge recorded under the reversible peak system assigned to the one-electron reduction of the polymerized viologen group, leading to an electropolymerization yield of 39%. 2.3. Electrochemical characterization of hydrogenase electrodes A three-compartment electrochemical cell contained a platinum net auxiliary electrode and a platinum black hydrogen electrode in the same solution as a reference (reversible hydrogen electrode RHE). The cell construction allowed deaeration of the working electrode compartment. Electrochemical activity of electrodes was tested in galvanostatic mode using Solartron Electrochemical Interface 1286 (UK) with constant gas flow through solution and stirring. All experiments were made in phosphate buffer solution (0.05 M KH2PO4, 0.1 M KCl, pH = 7.0). The whole electrochemical cell was thermostated during experiments at 30 C.
2. Experimental 2.4. Tolerance to oxygen 2.1. Materials The [NiFe] hydrogenase from Thiocapsa roseopersicina strain BBS was purified according to procedure described in [11] to 90% of purity and characterized by activity of 600 lmol (H2) min1 mg of protein. The solutions throughout this work were prepared using distilled water from a Milli-Q system (Millipore, Bedford, MA). Methyl viologen chloride was 98% pure (Sigma). All other reagents used were of analytical reagent grade. The hydrogen was produced by water electrolysis (gas supplying system Eldis 15M, Russia).
The effect of O2 content on hydrogenase electrode activity was monitored at a fixed overvoltage of 200 mV. The current was recorded continuously until stationary with hydrogen or different air/H2 or argon/H2 mixtures flow through working electrode compartment. EG& G PAR 173 potentiostat/galvanostat with EG & G PAR 175 universal programmer and Hewlett Packard X-Y recorder HP 7046A were used in this study. The amount of oxygen in air–H2 mixtures was monitored with gaz analyzer PKG4-K (‘‘Praktik-NC’’ Company, Moscow Region, Russia) connected to gas outlet.
S.V. Morozov et al. / Electrochemistry Communications 8 (2006) 851–854
3. Results and discussion
853
3.1. Hydrogen enzyme electrode As was shown in our earlier study [8], hydrogen enzyme electrode in hydrogen saturating solution reaches hydrogen equilibrium potential. Current-potential curve of enzyme electrode is shown in Fig. 1. It is seen, that with the precision of millivolts the observed zero-current potential is indeed coincided with the potential of reversible hydrogen electrode (platinum black under H2 flow in the same solution). Anodic current at positive overvoltages can be attributed only to oxidation of molecular hydrogen [8], since in the absence of either hydrogenase or molecular hydrogen in this potential range just cathodic current can be registered. It was independently shown, that at negative overvoltages the observed cathodic current (Fig. 1) is due to hydrogen evolution [13].
% of initial activity
100
80
100
95
90
85
80
75
% of hydrogen Fig. 2. The remaining current of hydrogen oxidation at 200 mV overvoltage in the presence of impurities of (n) – oxygen and (n) – argon.
3.2. Tolerance to oxygen When hydrogen passing through the electrochemical cell was changed to hydrogen–air mixture the zero-current potential was shifted to positive region, and the anodic current was decreased. Current of hydrogen oxidation at hydrogenase electrode as a function of H2 content in hydrogen–air mixture is shown in Fig. 2. Experiment was carried out at anodic overvoltage of 200 mV. It can be surprised, that in Fig. 2 initial points do not correspond to 100% of hydrogen. A reason for this is a non-zero oxygen content even in ‘pure’ hydrogen noticed from O2-meter. As seen in Fig. 2, the anodic current is linearly decreased with the decrease of H2 content. However, when the flowing gas is changed back to hydrogen, 100% of the initial
1.6 1.4 1.2
electrode activity restores. Moreover, at every point in Fig. 1 the anodic current is completely stable even in the presence of 20% of air, which coincides with the explosion limit of H2–O2 mixture. Hence, there is no either long-term or short-term inactivation of hydrogenase electrode in the presence of air in hydrogen up to explosion limit. We note, that hydrogenases from different sources, and in particular, hydrogenase from Thiocapsa roseopersicina, loose 100% of their activity even in the presence of 1% of oxygen [14]. The mechanism of the short-term inactivation is an oxidation of hydrogenase active site, which switches off the enzyme. However, being wired to the electrode surface, hydrogenase can be activated electrochemically by reduction of its active site. Most probably, in the presence of some amount of oxygen the oxidized and thus inactivated enzyme molecules are re-activated by electron donation from the electrode. According to this, some amount of hydrogen reductive equivalents are used for re-activation of the oxidized enzyme. In this case oxygen inhibition should be stronger, than current decrease when hydrogen is diluted with inert gas. Indeed, as seen in Fig. 2, in H2– argon mixture current decrease is less.
I, mA/cm
2
1.0
4. Conclusion
0.8 0.6 0.4 0.2 0.0
-150
-100
-50
0 -0.2
50
100
150
200
250
Er, mV
Fig. 1. Steady-state current–voltage curve of hydrogen oxidation/evolution at hydrogen enzyme electrode based on hydrogenase from Thiocapsa roseopersicina immobilized on poly(MPDBP) (C = 1.4 ·107 mol cm2).
Despite hydrogenase from Thiocapsa roseopersicina is not an oxygen-independent one, and its activity is suppressed by the traces of O2, the corresponding hydrogen enzyme electrodes are not inactivated by molecular oxygen up to 20% of air content in hydrogen, which coincides with the explosion limit of H2–O2 mixture. Hence, such enzyme electrodes can be referred to as ‘oxygen tolerant’ ones. The finding of tolerance to oxygen of enzyme electrodes made on the basis of regular oxygen-sensitive hydrogenases opens a new area for their applications both in sensors and in H2–O2 fuel cells.
854
S.V. Morozov et al. / Electrochemistry Communications 8 (2006) 851–854
Acknowledgements The financial supports through INTAS grant (03-56102), Russian Federal Science and Innovation agency contract 02.434.11.5003, and Russian Science Support foundation are greatly acknowledged. References [1] www.hubbertpeak.com/summary.htm. [2] FY 2003 Progress Report for Hydrogen, Fuel Cells, and Infrastructure Technologies Program, U.S. Department of Energy, Washington, 2003. [3] P.J. Ferreira, G.J. la O’, Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H.A. Gasteigerc, J. Electrochem. Soc. 152 (2005) 2256. [4] www.platinum.matthey.com/analyst_index.html. [5] www.oica.net/htdocs/Main.htm.
[6] M. Murthy, M. Esayian, A. Hobson, S. MacKenzie, W.-K. Lee, J.W. Van Zee, J. Electrochem. Soc. 148 (2001) 1141. [7] T.J. Schmidt, H.A. Gasteiger, R.J. Behm, J. Electrochem. Soc. 146 (1999) 1296. [8] A.A. Karyakin, S.V. Morozov, E.E. Karyakina, S.D. Varfolomeyev, N.A. Zorin, S. Cosnier, Electrochem. Comm. 4 (2002) 417. [9] A.A. Karyakin, S.V. Morozov, E.E. Karyakina, N.A. Zorin, V.V. Perelygin, S. Cosnier, Biochem. Soc. Trans. 33 (2005) 73. [10] F.A Armstrong, Curr. Op. Chem. Biol. 8 (2004) 133. [11] N.A. Zorin, O.N. Pashkova, I.N. Gogotov, Biochem. (Moscow) 60 (1995) 515. [12] S. Cosnier, B. Galland, C. Innocent, J. Electroanal. Chem. 433 (1997) 113. [13] S.V. Morozov, P.M. Vignais, L. Cournac, N.A. Zorin, E.E. Karyakina, A.A. Karyakin, S. Cosnier, Int. J. Hydrogen En. 27 (2002) 1501. [14] I.N. Gogotov, N.A. Zorin, L.T. Serebriakova, E.N. Kondratieva, BBA 523 (1978) 335.
Tolerance to oxygen of hydrogen enzyme electrodes Sergey V. Morozov a, Oleg G. Voronin a, Elena E. Karyakina a, Nikolay A. Zorin b, Serge Cosnier c, Arkady A. Karyakin a,* a
Faculty of Chemistry, M.V. Lomonosov Moscow State University, Lenin Hills, GSP-3, 119992 Moscow, Russia b Institute of Basic biological problems RAS, Pushchino, Moscow Region, Russia c LEOPR, UMR CNRS 5630, Universite Joseph Fourier BP 53, 38041 Grenoble Cedex 9, France Received 9 February 2006; received in revised form 2 March 2006; accepted 6 March 2006 Available online 17 April 2006
Abstract Tolerance to oxygen of hydrogen enzyme electrodes was first shown. Despite hydrogenase activity is suppressed by O2, their active sites being wired to the electrode can be re-activated in the presence of sufficient amount of molecular hydrogen. As a result enzyme electrodes based on hydrogenase from Thiocapsa roseopersicina are active up to 20% of air content in hydrogen, which coincides with an explosion limit of H2–O2 mixture. Despite oxygen inhibition of hydrogen electrooxidation occurs, it is completely reversible, and enzyme electrodes restore 100% of their activity, when gas mixture is changed back to pure hydrogen. The observed tolerance to oxygen of hydrogen enzyme electrodes provides a possibility of their use in H2–O2 fuel cells improving efficiency of energy conversion compared with platinum based devices. 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen enzyme electrode; Hydrogenase; Oxygen; Tolerance; Stability
1. Introduction Modern human society is facing global energetic problem. The forecasts predict absolute maximum of oil and gas mining in 2010 [1], after that production of hydrocarbon fuels will be decreased, and their cost will rapidly grow up. Thus, a search for alternative energy source is among the most important tasks nowadays. Molecular hydrogen is presently considered as the most promising chemical fuel providing highest energy output per its molecular weight, and after combustion, a complete absence of environmental pollutants including carbon dioxide (CO2). To utilize energy of hydrogen combustion it is preferable to use electrochemical fuel cells. The latter provide the highest efficiency of energy conversion. *
Corresponding author. Tel.: +7 495 9394605; fax: +7 495 9394675. E-mail address: [email protected] (A.A. Karyakin).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.03.007
For transportation the only low-temperature hydrogen– oxygen fuel cells were found to be suitable. Fuel electrodes, in particular hydrogen ones, use platinum (or platinum group metals) as electrocatalysts. There are, however, certain important problems, which make impossible wide use of such fuel cells in future. The most crucial problem deals with platinum cost and availability. Every kilowatt producing platinum based fuel cell costs $2000–$3000 [2]. Hence, 50 kW engine costs from $100,000 to $150,000, which makes doubtful their commercialization. A possibility to decrease fuel cell costs by one order of magnitude are not true realistic, because a decrease of platinum loading dramatically affects operational characteristics of fuel electrodes [3]. Concerning platinum availability, its annual production is just 195 tons and world’s total resources are just 100,000 tons [4]. However, only to cover an annual car production (64 million [5]) equipping them with 50 kW engines, 6000
852
S.V. Morozov et al. / Electrochemistry Communications 8 (2006) 851–854
tons of platinum are required. Comparison of the above figures makes platinum fuel cell program unrealistic. We already addressed poisoning problems, which make hardly possible operation of platinum based fuel cells with cheap fuels [6,7]. A valuable alternative to catalysis by noble metals is biocatalysis. We already reported, that hydrogen fuel electrodes based on the enzymes do not face the above problems [8]. Indeed, the enzymes are products of biotechnology, which provide their completely renewable source, and low cost upon mass production. Concerning poisoning by fuel impurities, hydrogen enzyme electrodes are completely insensitive to the presence of sulfide anion [9]. Carbon monoxide decreases the current of hydrogen oxidation only starting from 1% of its content in fuel, and enzyme electrode completely restores its activity when the atmosphere is changed back to pure hydrogen [8]. There is another problem with platinum-based fuel cells to be considered. Separation of anode and cathode compartments in fuel cells is provided by a thin layer of polyelectrolyte to reduce internal battery resistance, which causes energy loss via heating. Such thin layers are unable to prevent penetration of hydrogen into oxygen electrode compartment and vise a versa. Since platinum metals catalyze both hydrogen oxidation and oxygen reduction, such penetration decreases energy output. Considering selectivity problem, enzymes are able to catalyze only their specific reaction: hydrogen oxidation for hydrogenases and oxygen reduction for oxidases (laccase, ascorbate oxidase etc.). Despite hydrogenases do not catalyze oxygen reduction, O2 is able to inactivate these enzymes oxidizing their active sites [10]. It is, thus, of great importance to investigate, whether hydrogen enzyme electrodes are able to operate under certain content of O2 in hydrogen. The finding of tolerance to oxygen of hydrogenase electrodes provides a real possibility of their use as fuel electrodes in H2–O2 fuel cells. Moreover, enzyme electrodes are advantageous over platinum based ones as their performance characteristics are less affected by the presence of air in hydrogen.
The pyrrole derivative, N-methyl-N 0 -(12-pyrrol-1-yldodecyl)-4,4 0 -bipyridinium ditetrafluoroborate (MPDBP) was synthesized as previously reported [12]. 2.2. Hydrogenase electrodes preparation Carbon filament material CFM (1 · 0.5 cm) (TVS300 M, ‘Alten’ Company, Moscow Region, Russia) was used as working electrode. CFM specific electrical resistance was 50–70 mX cm. Hydrogenase electrodes were prepared by enzyme adsorption from its aqueous solution (2–4 mg /ml) for 12 h at 4 C in 0.005 M K-phosphate buffer, pH = 7.0 onto the CFM electrodes modified with poly(MPDBP). The solution of (MPDBP) in CH3CN (2 mM, 100 ll) was deposited on CFM electrode and allowed to dry for 2 h in ambient conditions. Electrochemical polymerization was carried out by repeated potential cycling over the range 0.8 to +0.95 V vs. Ag/AgClj1 M KCl reference electrode with the sweep rate 50 mV/s for 40 min in 0.1 M LiClO4 solution pH = 6.0. After polymerization the electrodes were washed with phosphate buffer solution (0.05 M KH2PO4, 0.1 M KCl, pH = 7.0) for 10 min. The apparent surface coverage of the electropolymerized (MPDBP) (C 1.4 107 mol cm2) was determined from the charge recorded under the reversible peak system assigned to the one-electron reduction of the polymerized viologen group, leading to an electropolymerization yield of 39%. 2.3. Electrochemical characterization of hydrogenase electrodes A three-compartment electrochemical cell contained a platinum net auxiliary electrode and a platinum black hydrogen electrode in the same solution as a reference (reversible hydrogen electrode RHE). The cell construction allowed deaeration of the working electrode compartment. Electrochemical activity of electrodes was tested in galvanostatic mode using Solartron Electrochemical Interface 1286 (UK) with constant gas flow through solution and stirring. All experiments were made in phosphate buffer solution (0.05 M KH2PO4, 0.1 M KCl, pH = 7.0). The whole electrochemical cell was thermostated during experiments at 30 C.
2. Experimental 2.4. Tolerance to oxygen 2.1. Materials The [NiFe] hydrogenase from Thiocapsa roseopersicina strain BBS was purified according to procedure described in [11] to 90% of purity and characterized by activity of 600 lmol (H2) min1 mg of protein. The solutions throughout this work were prepared using distilled water from a Milli-Q system (Millipore, Bedford, MA). Methyl viologen chloride was 98% pure (Sigma). All other reagents used were of analytical reagent grade. The hydrogen was produced by water electrolysis (gas supplying system Eldis 15M, Russia).
The effect of O2 content on hydrogenase electrode activity was monitored at a fixed overvoltage of 200 mV. The current was recorded continuously until stationary with hydrogen or different air/H2 or argon/H2 mixtures flow through working electrode compartment. EG& G PAR 173 potentiostat/galvanostat with EG & G PAR 175 universal programmer and Hewlett Packard X-Y recorder HP 7046A were used in this study. The amount of oxygen in air–H2 mixtures was monitored with gaz analyzer PKG4-K (‘‘Praktik-NC’’ Company, Moscow Region, Russia) connected to gas outlet.
S.V. Morozov et al. / Electrochemistry Communications 8 (2006) 851–854
3. Results and discussion
853
3.1. Hydrogen enzyme electrode As was shown in our earlier study [8], hydrogen enzyme electrode in hydrogen saturating solution reaches hydrogen equilibrium potential. Current-potential curve of enzyme electrode is shown in Fig. 1. It is seen, that with the precision of millivolts the observed zero-current potential is indeed coincided with the potential of reversible hydrogen electrode (platinum black under H2 flow in the same solution). Anodic current at positive overvoltages can be attributed only to oxidation of molecular hydrogen [8], since in the absence of either hydrogenase or molecular hydrogen in this potential range just cathodic current can be registered. It was independently shown, that at negative overvoltages the observed cathodic current (Fig. 1) is due to hydrogen evolution [13].
% of initial activity
100
80
100
95
90
85
80
75
% of hydrogen Fig. 2. The remaining current of hydrogen oxidation at 200 mV overvoltage in the presence of impurities of (n) – oxygen and (n) – argon.
3.2. Tolerance to oxygen When hydrogen passing through the electrochemical cell was changed to hydrogen–air mixture the zero-current potential was shifted to positive region, and the anodic current was decreased. Current of hydrogen oxidation at hydrogenase electrode as a function of H2 content in hydrogen–air mixture is shown in Fig. 2. Experiment was carried out at anodic overvoltage of 200 mV. It can be surprised, that in Fig. 2 initial points do not correspond to 100% of hydrogen. A reason for this is a non-zero oxygen content even in ‘pure’ hydrogen noticed from O2-meter. As seen in Fig. 2, the anodic current is linearly decreased with the decrease of H2 content. However, when the flowing gas is changed back to hydrogen, 100% of the initial
1.6 1.4 1.2
electrode activity restores. Moreover, at every point in Fig. 1 the anodic current is completely stable even in the presence of 20% of air, which coincides with the explosion limit of H2–O2 mixture. Hence, there is no either long-term or short-term inactivation of hydrogenase electrode in the presence of air in hydrogen up to explosion limit. We note, that hydrogenases from different sources, and in particular, hydrogenase from Thiocapsa roseopersicina, loose 100% of their activity even in the presence of 1% of oxygen [14]. The mechanism of the short-term inactivation is an oxidation of hydrogenase active site, which switches off the enzyme. However, being wired to the electrode surface, hydrogenase can be activated electrochemically by reduction of its active site. Most probably, in the presence of some amount of oxygen the oxidized and thus inactivated enzyme molecules are re-activated by electron donation from the electrode. According to this, some amount of hydrogen reductive equivalents are used for re-activation of the oxidized enzyme. In this case oxygen inhibition should be stronger, than current decrease when hydrogen is diluted with inert gas. Indeed, as seen in Fig. 2, in H2– argon mixture current decrease is less.
I, mA/cm
2
1.0
4. Conclusion
0.8 0.6 0.4 0.2 0.0
-150
-100
-50
0 -0.2
50
100
150
200
250
Er, mV
Fig. 1. Steady-state current–voltage curve of hydrogen oxidation/evolution at hydrogen enzyme electrode based on hydrogenase from Thiocapsa roseopersicina immobilized on poly(MPDBP) (C = 1.4 ·107 mol cm2).
Despite hydrogenase from Thiocapsa roseopersicina is not an oxygen-independent one, and its activity is suppressed by the traces of O2, the corresponding hydrogen enzyme electrodes are not inactivated by molecular oxygen up to 20% of air content in hydrogen, which coincides with the explosion limit of H2–O2 mixture. Hence, such enzyme electrodes can be referred to as ‘oxygen tolerant’ ones. The finding of tolerance to oxygen of enzyme electrodes made on the basis of regular oxygen-sensitive hydrogenases opens a new area for their applications both in sensors and in H2–O2 fuel cells.
854
S.V. Morozov et al. / Electrochemistry Communications 8 (2006) 851–854
Acknowledgements The financial supports through INTAS grant (03-56102), Russian Federal Science and Innovation agency contract 02.434.11.5003, and Russian Science Support foundation are greatly acknowledged. References [1] www.hubbertpeak.com/summary.htm. [2] FY 2003 Progress Report for Hydrogen, Fuel Cells, and Infrastructure Technologies Program, U.S. Department of Energy, Washington, 2003. [3] P.J. Ferreira, G.J. la O’, Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H.A. Gasteigerc, J. Electrochem. Soc. 152 (2005) 2256. [4] www.platinum.matthey.com/analyst_index.html. [5] www.oica.net/htdocs/Main.htm.
[6] M. Murthy, M. Esayian, A. Hobson, S. MacKenzie, W.-K. Lee, J.W. Van Zee, J. Electrochem. Soc. 148 (2001) 1141. [7] T.J. Schmidt, H.A. Gasteiger, R.J. Behm, J. Electrochem. Soc. 146 (1999) 1296. [8] A.A. Karyakin, S.V. Morozov, E.E. Karyakina, S.D. Varfolomeyev, N.A. Zorin, S. Cosnier, Electrochem. Comm. 4 (2002) 417. [9] A.A. Karyakin, S.V. Morozov, E.E. Karyakina, N.A. Zorin, V.V. Perelygin, S. Cosnier, Biochem. Soc. Trans. 33 (2005) 73. [10] F.A Armstrong, Curr. Op. Chem. Biol. 8 (2004) 133. [11] N.A. Zorin, O.N. Pashkova, I.N. Gogotov, Biochem. (Moscow) 60 (1995) 515. [12] S. Cosnier, B. Galland, C. Innocent, J. Electroanal. Chem. 433 (1997) 113. [13] S.V. Morozov, P.M. Vignais, L. Cournac, N.A. Zorin, E.E. Karyakina, A.A. Karyakin, S. Cosnier, Int. J. Hydrogen En. 27 (2002) 1501. [14] I.N. Gogotov, N.A. Zorin, L.T. Serebriakova, E.N. Kondratieva, BBA 523 (1978) 335.