Professor Elisabeth Lojou, National Center for Scientific Research (CNRS), Marseilles, France.
Enzymatic Catalysis for H2/O2 Fuel Cells: From Self-assembled Monolayers to (nano)carbon Materials.
One of the challenges in the large scale development of H2/O2 fuel cells is the replacement of platinum catalysts required to accelerate both hydrogen oxidation and oxygen reduction. One alternative is the use of enzymes as biocatalysts. We previously explored the biodiversity and ability of some organisms to survive in extreme environmental conditions, and demonstrated the feasibility of the use of O2- and CO-tolerant hydrogenase for H2 oxidation, and multicopper bilirubin oxidases for O2 reduction into water in H2/O2 enzymatic biofuel cells . Although very efficient compared to platinum, the nanometric size of the enzyme with an active site isolated in the protein moiety imposes to develop interfaces with: i) suitable chemical functionalization for proper orientation of the enzyme required for fast interfacial electron transfer rate, ii) large surface/volume ratio to enhance the amount of enzymes participating to the catalysis.
Distribution of orientation of enzymes can be determined by studying enzyme immobilization on Self-Assembled-Monolayers on gold electrodes, while carbon nanotube (CNT) networks may serve as platforms to enhance the catalytic currents. In this work we discuss how the molecular basis for proper orientation obtained on SAMs can be extended to mesoporous conductive networks. We illustrate this issue by examining the intriguing behavior of two different hydrogenases for H2 oxidation, and two different bilirubin oxidases (BODs) for O2 reduction. We focus on their functional immobilization on CNTs presenting surface chemistry with different functionalities and charges.
Aquifex aeolicus is a hyperthermophile ancestral bacterium that harbors an O2- and CO-tolerant membrane-bound hydrogenase. Molecular dynamic and electrochemistry coupled to PMIRRAS and Surface Plasmon Resonance provided the tools to determine the required functionalities for efficient direct electron transfer, and to propose a model for the orientation of the enzyme as a function of hydrophobicity and charges . Accordingly, the enzyme immobilized on CNTs exhibited the expected catalytic efficiency according to CNT functionality. Ralstonia eutropha is another bacterium which also harbors an O2-and CO-tolerant hydrogenase which shares more than 60% homology with A. aeolicus hydrogenase. However, a very different behavior was observed on charged CNTs, that we will discuss on the basis of detergent content in the membrane-bound enzyme sample . We also determined the molecular determinant for functional orientation of BOD from the fungus Myrothecium verrucaria. As for A. aeolicus hydrogenase, the key parameters obtained on SAMs were suitable to get a proper orientation on functionalized CNTs . However, BOD from the bacterium Bacillus pumilus, that is preferable than M. verrucaria BOD because it is thermostable, failed in direct electrical connection to the CNT-based electrode in the same conditions as the M. verrucaria BOD. To establish the molecular basis for such different behaviors, we undertook an in-depth multidisciplinary study of the interactions between the two BODs and the various CNTs. Comparative modeling of the enzymes allowed to point out the differences in surface charges near T1 copper center, the entry point of electrons, as well as in dipole moment of the two BODs as a function of pH. Catalytic current for O2 reduction by BODs immobilized on the surface of different CNTs was then followed by electrochemistry. A discussion on the proportion of well oriented BOD for direct electron transfer and forces governing this orientation can be drawn from these studies . Finally, the induced performances of H2/O2 biofuel cells will be reported .
 de Poulpiquet et al., Electrochem. Commun. (2014); Mazurenko et al., Sust. Energ Fuels (2017).
 Ciaccafava et al., Angew. Chem. (2012); Oteri et al., Phys. Chem. Chem. Phys. (2014).
 Monsalve et al., ChemElectroChem. (2016).
 Gutierrez-Sanchez et al., ACS Catalysis (2016).
 Mazurenko et al., ACS Appl. Mater. Interfaces (2016).  Mazurenko et al., Energ. Environm. Sci. (2017)
ABOUT THE PRESENTER
Professor Lojou is currently Research Director at the National Center for Scientific Research (CNRS), Marseilles, France. Her main research topics include: electrochemical interface functionalization for direct and mediated electron transfer on redox enzymes; hydrogen biofuel cell development. As an electrochemist, her topics have evolved from industrial research devoted to the development of new liquid cathodes for high power lithium cells to more fundamental queries concerning long range electron transfer within biological macromolecules. Her current interest focuses on the functional immobilization of enzymes from extremophilic bacteria onto electrodes. She has over 90 publications. Distinctions awarded include: 2015 Award of Public Research for Environment and Energy, 2012 ADEME Award for Innovation and Environment, 1989 Innovation Award of SAFT-Leclanché Company. Administrative functions include: 2014-18 Vice-chair of the International Society of Electrochemistry, division Bioelectrochemistry; 2012-14 French National Chemistry Institute Committee; 2008-16 CNRS Evaluation Committee, Electrochemistry division; 2015 Vice-President of French Chemical Society, SCF, Electrochemistry Division; 2010 Secretary of the French Group of Bioelectrochemistry (GFB).
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