(2ff) Laccase-Mediated Oxygen Reduction in Liquid Flow Fuel Cells for Efficient Oxidation of Biomass Derived Aldehydes with Co-Generation of Electricity

I have obtained a master's degree in biochemistry and molecular biology. I am currently a doctoral student majoring in chemical engineering and technology. I expect to graduate in July 2024. My research interests mainly focus on the modification of bacterial strains through genetic engineering to improve the robustness of the strains while producing high value-added products.

Abstract

As the depletion of traditional fossil fuels and increasing global warming and environmental pollution, development of renewable energy and chemicals has attracted more and more attention. Lignocellulose, the most abundant renewable biomass in nature, is a form of energy conversion of solar energy and carbon fixation by plants and algae through photosynthesis, and it is also the most important link of the carbon cycle in nature. Therefore, the development of new industrial biocatalytic technologies for processing and utilization of renewable biomass to produce various chemicals can not only effectively promote the recycling of carbon, but also reduce carbon dioxide emissions, which is also a great boost to the global goal of carbon neutrality.

At present, the three major structural components of lignocellulose have been used to produce many chemicals and small molecule platform compounds through different catalytic technologies. For instance, hemicellulose-derived pentose degradation products, e.g., furfural, is an important platform compound, which can be used to synthesize furfural resin, to produce furoic acid, and furfuryl alcohol by hydrogenation, and the latter can account for more than 60% of the furfural market. The oxidation product of furfural, e.g., furoic acid, is a versatile chemical for the production of a variety of pharmaceutical drugs, fragrances, biofuels etc. Some metal oxides(e.g., Pt, Pd, Au ) have been developed to catalyze the oxidation of furfural by oxygen with furoic acid yield of 80%–99% depending on the special catalysts and severe reaction condition. However, the high price of noble metals and the scarcity of reserves limit their industrial application prospects. It is still necessary to develop efficient non-noble metals catalyst to achieve quick transfer of electrons to oxygen in order to improve the kinetic rate of furfural oxidation under mild conditions.

Laccase (EC 1.10.3.2) is a structurally stable multi-copper oxidase, widely derived from fungi, plants and bacteria, which have a broad spectrum of substrates with O₂ as an electron acceptor and generate water. And the fungal laccase is a highly oxidizing biological enzyme with a high redox potential (E^0’) between 500-800 mV vs NHE (standard hydrogen electrode). In addition, usually it is necessary to use low-molecular-weight mediators to achieve and accelerate electron transfer between substrate and enzyme due to the limitation of the active pocket of laccase.

A fuel cell is a device that converts chemical energy to electricity through a redox reaction. Biomass fuel cell is a device that uses biomass as fuel to generate electricity by catalyzing redox reactions. During the discharging process, the biomass molecular substances lose electrons in the oxidation reaction at the anode, while oxidants, usually oxygen receive electrons on the cathode and get reduced. However, with the development of fuel cells, it seems that these biomass fueled fuel cells are still of low efficiency, and thus promoting the cathodic efficiency is important to improve the fuel cell performance.

Hence, based on the advantages of biomass fuel cells and efficiency of laccase to mediate oxygen reduction, in this work we aimed to develop a fuel cell system to oxidize biomass derived aldehydes, for example furfural, to produce corresponding acids with co-generation of electricity. Firstly, Laccase was produced by heterologous expression of gene lacc6 from Pleurotus otreatus HAUCC 162. Under the optimal culture conditions, a supernatant laccase activity of 211.8 U/L was obtained. And then laccase was employed as an efficient catalyst to increase the kinetics of oxygen reduction reaction in a liquid flow fuel cell (LFFC). An electron transport chain (ETC) was constructed in the LFFC with Ag₂O as an anode electrocatalyst, and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) and laccase as the cathode electron carriers. The LFFC could well convert furfural to furoic acid and co-generation of electricity. Under the optimized cathodic conditions of 40 ℃, pH 4.0, 50 mM ABTS and laccase loading of 300 U/mmol ABTS, P_max of 70.8 mW/cm² could be obtained. For furoic acid production, 91% yield could be obtained under the anodic condition of 100 mM furfural and 1.5 M KOH. The electrons released from oxidation of furfural on the Ag₂O catalyst could be well transferred to that external circuit with a Faraday efficiency of 96.2%.

This novel system thus may provide a new route for clean production of biomass-based platform chemicals with co-generation of electricity by conversion of chemical energy to electric energy via combination of biological, chemical and electrochemical approaches.