Efficient Conversion of Lignin to Electricity Using a Novel Direct Biomass Fuel Cell Mediated by Polyoxometalates at Low Temperatures

New Application of Polyoxometalate Salts as Cathode Materials in Single Chamber MFC Using Wastewater for Bioenergy Production

Microbial fuel cells (MFCs) are a promising technology that can be applied in a bifunctional process in which wastewater treatment is used for renewable electric power generation. In this study, novel transition metal-modified Keggin-type lacunar polyoxometalate salts (L-POMs) Cs5PMo11M(H2O)O39 (M = Fe, Co), were synthesized and characterized by X-ray diffraction, SEM, EDX, IR, TGA/DSC, and UV-Vis/DSR spectroscopies to be tested, for the first time, as a cathode component in wastewater-fed air chamber MFCs. Both materials were tested in the presence and absence of light to evaluate their photocatalytic behavior. The best performance in terms of electricity production was obtained for the MFC containing the Co-modified POM-based cathode, which showed a maximum power of 418.15 mW/m2 equivalent to 331 mW per cubic meter of treated wastewater, and a maximum COD removal percentage of 97% after 96 h of MFC operation. Co- and Fe-modified POMs had outstanding optical behavior with lower energy gap values, 1.71 and 2.68 eV, respectively. The newly developed materials can be considered as promising alternative cathode catalysts in a new generation of MFC devices integrating full carbon removal from wastewater and a fast reduction of oxygen.

Redox Mediators in Homogeneous Co-electrocatalysis

Homogeneous electrocatalysis has been well studied over the past several decades for the conversion of small molecules to useful products for green energy applications or as chemical feedstocks. However, in order for these catalyst systems to be used in industrial applications, their activity and stability must be improved. In naturally occurring enzymes, redox equivalents (electrons, often in a concerted manner with protons) are delivered to enzyme active sites by small molecules known as redox mediators (RMs). Inspired by this, co-electrocatalytic systems with homogeneous catalysts and RMs have been developed for the conversion of alcohols, nitrogen, unsaturated organic substrates, oxygen, and carbon dioxide. In these systems, the RMs have been shown to both increase the activity of the catalyst and shift selectivity to more desired products by altering catalytic cycles and/or avoiding high-energy intermediates. However, the area is currently underdeveloped and requires additional fundamental advancements in order to become a more general strategy. Here, we summarize the recent examples of homogeneous co-electrocatalysis and discuss possible future directions for the field.

Oxidative pretreatment of lignocellulosic biomass for enzymatic hydrolysis: Progress and challenges

Deconstruction of cell wall structure is important for biorefining of lignocellulose to produce various biofuels and chemicals. Oxidative delignification is an effective way to increase the enzymatic digestibility of cellulose. In this work, the current research progress on conventional oxidative pretreatment including wet oxidation, alkaline hydrogen peroxide, organic peracids, Fenton oxidation, and ozone oxidation were reviewed. Some recently developed novel technologies for coupling pretreatment and direct biomass-to-electricity conversion with recyclable oxidants were also introduced. The primary mechanism of oxidative pretreatment to enhance cellulose digestibility is delignification, especially in alkaline medium, thus eliminating the physical blocking and non-productive adsorption of enzymes by lignin. However, the cost of oxidative delignification as a pretreatment is still too expensive to be applied at large scale at present. Efforts should be made particularly to reduce the cost of oxidants, or explore valuable products to obtain more revenue.

All-iron ions mediated electron transfer for biomass pretreatment coupling with direct generation of electricity from lignocellulose

A coupled process of biomass pretreatment for increasing cellulose digestibility and direct conversion of biomass to electricity has been developed with ferric or ferricyanide ions as the anode electron carriers, and Fe(NO₃)₃ activated by HNO₃ as the cathode electron carriers. Pretreated substrates are subjected to enzymatic hydrolysis for release of fermentable sugars, while the pretreatment liquor is employed as anolyte for electricity generation in a liquid flow fuel cell (LFFC). Pretreatment of sugarcane bagasse with 2 M FeCl₃ in anode reactor removes ∼ 100% hemicelluloses and obtains 76% enzymatic glucan conversion (EGC), while pretreatment with 0.1 M K₃[Fe(CN)₆] in 0.5 M KOH achieves 78% lignin removal, 95.8% EGC and 85.1% xylan conversion. From 1000 g bagasse, 171.3 g fermentable sugars is produced with co-generation of 101.4 W·h electricity based on FeCl₃ pretreatment, while 519 g fermentable sugars and 28.9 W·h electricity are obtained based on K₃[Fe(CN)₆] pretreatment.

Lignocellulose, Cellulose and Lignin as Renewable Alternative Fuels for Direct Biomass Fuel Cells

In recent years the use of renewable sources, such as lignocellulosic biomass (LCB), as the fuel for various types of fuel cells received growing interest. Different types of fuel cells, that is, operated at low temperatures (T<100 °C; microbial fuel cells (MFC), alkaline (AFCs) and flow fuel cells (FFCs)), intermediate temperatures (T in the range 150-300 °C, proton-conducting inorganic-organic composite membrane fuel cells), and high temperatures (T≥500 °C, direct carbon fuel cells (DCFCs)), have been used for the conversion of the chemical energy in LCB to electrical energy. The economic advantage of the direct use of LCB consists of avoiding the acid hydrolysis of cellulose to glucose for low-temperature fuel cells and the pretreatment at high temperatures necessary to convert biomass to biochar (pyrolysis) in the case of high-temperature fuel cells. In this Review, the characteristics of direct biomass fuel cells are presented and their performance is compared with that of indirect biomass fuel cells fed with glucose (low-temperature fuel cells) and biochar (high-temperature fuel cells).

Photo/Bio-Electrochemical Systems for Environmental Remediation and Energy Harvesting

Water and energy systems are interdependent: water is utilized in each stage of energy production, and energy is required to extract, treat, and deliver water for many uses. However, energy and water systems are usually developed and managed independently. In the quest to develop environmentally friendly and energy-efficient solutions for water and energy issues, photoelectrochemical (PEC) energy conversion and microbial electrochemical (MEC) systems show profound potential for addressing environmental remediation problems and harvesting energy simultaneously. Herein, PEC, MEC, and their variant hybrid systems toward energy conversion and environmental remediation are summarized and discussed.

Anthraquinone-Mediated Fuel Cell Anode with an Off-Electrode Heterogeneous Catalyst Accessing High Power Density when Paired with a Mediated Cathode

The development of processes for electrochemical energy conversion and chemical production could benefit from new strategies to interface chemical redox reactions with electrodes. Here, we employ a diffusible low-potential organic redox mediator, 9,10-anthraquinone-2,7-disulfonic acid (AQDS), to promote efficient electrochemical oxidation of H₂ at an off-electrode heterogeneous catalyst. This unique approach to integrate chemical and electrochemical redox processes accesses power densities up to 228 mW/cm² (528 mW/cm² with iR-correction). These values are significantly higher than those observed in previous mediated electrochemical H₂ oxidation methods, including those using enzymes or inorganic mediators. The approach described herein shows how traditional catalytic chemistry can be coupled to electrochemical devices.

Mediated Fuel Cells: Soluble Redox Mediators and Their Applications to Electrochemical Reduction of O2 and Oxidation of H2, Alcohols, Biomass, and Complex Fuels

Mediated fuel cells are electrochemical devices that produce power in a manner similar to that of conventional proton exchange membrane fuel cells (PEMFCs). They differ from PEMFCs in their use of redox mediators dissolved in liquid electrolyte to conduct oxidation of the fuel or reduction of the oxidant, typically O2, in bulk solution. The mediators transport electrons (and often protons) between the electrode and the catalysts or chemical reagents in solution. This strategy can help overcome many of the challenges associated with conventional fuel cells, including managing complex multiphase reactions (as in O2 reduction) or the use of challenging or heterogeneous fuels, such as hydrocarbons, polyols, and biomass. Mediators are also commonly used in enzymatic fuel cells, where direct electron transfer from the electrode to the enzymatic active site can be slow. This review provides a comprehensive survey of historical and recent mediated fuel cell efforts, including applications using chemical and enzymatic catalysts.

Low-Energy Catalytic Electrolysis for Simultaneous Hydrogen Evolution and Lignin Depolymerization

Here, a new proton-exchange-membrane electrolysis is presented, in which lignin was used as the hydrogen source at the anode for hydrogen production. Either polyoxometalate (POM) or FeCl₃ was used as the catalyst and charge-transfer agent at the anode. Over 90 % Faraday efficiency was achieved. In a thermal-insulation reactor, the heat energy could be maintained at a very low level for continuous operation. Compared to the best alkaline-water electrolysis reported in literature, the electrical-energy consumption could be 40 % lower with lignin electrolysis. At the anode, the Kraft lignin (KL) was oxidized to aromatic chemicals by POM or FeCl₃ , and reduced POM or Fe ions were regenerated during the electrolysis. Structure analysis of the residual KL indicated a reduction of the amount of hydroxyl groups and the cleavage of ether bonds. The results suggest that POM- or FeCl₃ -mediated electrolysis can significantly reduce the electrolysis energy consumption in hydrogen production and, simultaneously, depolymerize lignin to low-molecular-weight value-added aromatic chemicals.

Phosphomolybdic acid and ferric iron as efficient electron mediators for coupling biomass pretreatment to produce bioethanol and electricity generation from wheat straw

Phosphomolybdic acid (PMo₁₂) was used as an electron mediator and proton carrier to mediate biomass pretreatment for ethanol production and electricity generation from wheat straw. In the pretreatment, lignin was oxidized anaerobically by PMo₁₂ with solubilization of a fraction of hemicelluloses, and the PMo₁₂ was simultaneously reduced. In an external liquid flow cell, the reduced PMo₁₂ was re-oxidized with generation of electricity. The effects of several factors on pretreatment were investigated for optimizing the conditions. Enzymatic conversion of cellulose and xylan were about 80% and 45%, respectively, after pretreatment of wheat straw with 0.25M PMo₁₂, at 95°C for 45min. FeCl₃ was found to be an effective liquid mediator to transfer electrons to air, the terminal electron acceptor. By investigating the effects of various operation parameters and cell structural factors, the highest output power density of about 11mW/cm² was obtained for discharging of the reduced PMo₁₂.