Enzyme-assisted Catalysis on black Titania Substrate Electrodes

The widespread use of fuel cells, electrolyzers, and solar water splitting devices for energy generation and storage is limited by the dependence on noble metal catalysts. There is hence a need for the development of efficient electrocatalysts made of Earth-abundant elements. In nature, hydrogenases are metallo-enzymes that catalyze the reversible oxidation reaction of hydrogen to protons, with an activity comparable to that of Pt. The reaction sites of these hydrogenases usually contain Ni and Fe and for this reason they are known as [NiFe]-hydrogenases. However, there are two major challenges in the incorporation of hydrogenases into bioelectrical devices: 1) Appropriate attachment and electronic interaction with the conductive support and 2) oxygen sensitivity of the enzyme. EnCaSE addresses both issues by exploring the powers of nanotechnology and advanced characterization methods to immobilize, image and analyze the enzyme on the substrate. The protocol of production of active enzyme was optimized and implemented. Immunogold labeling combined with electron microscopy techniques were used to show that the enzyme molecules attach to the internal walls of the black titania nanotubes. Loading enzyme into the titania nanotubes revealed to be a challenging activity and the consortium developed a protocol to quantitatively evaluate enzyme infiltration. A proof-of-concept experiment was designed to test the full electrochemical cell able to produce hydrogen while mitigating oxygen inactivation of the enzyme catalyst. Finally, such a cell was combined with photosynthetic bacteria, which utilize hydrogen as feedstock, for CO2 conversion to methanol.

A facile, inexpensive and scalable method to synthesize inorganic platforms for hybrid electrodes was developed. The geometric, structural and electric characteristics of TiO2 nanotubes (TNTs) are tuned by the conditions of synthesis and subsequent thermochemical treatments. Neutral oxygen vacancies are the origin of the color centers and justify the metallic properties of black TNTs. The yield of recombinant enzyme was up to 6 mg per 1 L culture. The hydrogen evolution rate obtained was up to 2.8 µmol hydrogen min-1 mg-1. A critical aspect behind the development of the hybrid electrode is the loading of hydrogenase molecules into TNTs, which can be assisted by magnetic fields. The electrochemical behavior agrees with hydrogenase attachment to TNTs walls as attested by immuno-gold labelling. Direct electron transfer at the bio-inorganic interface demonstrated the EnCaSE proof-of-concept. This work plays a significant role in exploring new materials for bio-inorganic electrodes.

In this project a new class of electrodes for enzyme attachment and bio-assisted catalysis is developed based on the hydrogenated TiO2 or black titania. Black titania nanotubes ensure high electronic conductivity, hydrophilicity and high surface area. The project will assess whether the high aspect ratio morphology (tube length vs. pore diameter) can provide shielding of the hydrogenase (HydA), an O2-sensitive enzyme, as well as encasing for improved attachment and functionalisation. HydA is known for its selective catalytic activity for proton reduction and hydrogen evolution, which is comparable with that of Pt. EnCaSE replaces and alleviates the need for Pt-group metal electrocatalysts in water electrolysers. The project will test the novel cathode bioelectrode in a system of artificial photosynthesis. In this system, PV-assisted water electrolysis takes place in the same aqueous solution where bacteria are present. The bioreactor is fed with CO2, which bacteria assimilate together with the produced hydrogen, in order to complete the Calvin Cycle and produce biomass and alcohols. In EnCaSE inorganic-organic interfaces are engineered with the main aim to develop advanced and intrinsically safe biomaterials. The implementation of this interdisciplinary project is assured by a consortium where the partners have complementary expertise in state-of-the-art materials characterisation techniques, materials science and biotechnology. The project is a collaboration between SINTEF, University of Oslo and Oslo University Hospital. It runs for 3 years and recruits one researcher and one post-doctoral researcher. OUH monitors the environmental impact of the produced materials and ensures their safe disposal.