Performance of air-cathode stacked microbial fuel cells systems for wastewater treatment and electricity production

Synthesis and performance of a cathode catalyst derived from Bauhinia accuminata seed pods in single and stacked microbial fuel cell

The cathode catalyst in microbial fuel cell (MFC) plays a crucial role in scaling up. Activity of biomass-derived activated carbon catalysts with appropriate precursor selection in a natural clay membrane-based MFC of 250 mL was studied. The performance of scaled up MFC of 1.5 L capacity with two different configurations was monitored. Rod-shaped particles with slit-type pores and amorphous graphitic nature with a surface area of 800.37 m²/g was synthesized. The intrinsic doping of heteroatoms N and P in the catalyst was with atomic weight percentages of 4.5 and 3.5, respectively and the deconvolution of N1 spectra confirmed pyridinic N and graphitic N content of 17.3% and 34.1% validating its suitability as a cathode catalyst. Electrochemical characterization of the catalyst coated SS mesh electrode confirmed that a loading of 5 mg/cm² rendered higher catalytic activity compared to bare SS mesh. The maximum power density in catalyst modified cell was 0.91 W/m³ compared to 0.02 W/m³ as obtained in a plain stainless steel electrode cell at a COD removal efficiency of 93.3%. Series, parallel, and parallel-series combinations of 6 cells showed a maximum voltage of 4.15 V when connected in series and a maximum power density of 1.54 W/m³ when connected in parallel. System with multielectrode assembly achieved better power and current density (0.84 W/m³ and 1.97 A/m³) than the mixed parallel series circuitry (0.7 W/m³ and 0.57 A/m³). These performance results confirm that the catalyst is effective in both stacked and hydraulically connected system.

Organic Waste Substrates for Bioenergy Production via Microbial Fuel Cells: A Key Point Review

High-energy consumption globally has raised questions about the low environmentally friendly and high-cost processes used until now for energy production. Microbial fuel cells (MFCs) may support alternative more economically and environmentally favorable ways of bioenergy production based on their advantage of using waste. MFCs work as bio-electrochemical devices that consume organic substrates in order for the electrogenic bacteria and/or enzyme cultures to produce electricity and simultaneously lower the environmental hazardous value of waste such as COD. The utilization of organic waste as fuels in MFCs has opened a new research path for testing a variety of by-products from several industry sectors. This review presents several organic waste substrates that can be employed as fuels in MFCs for bioenergy generation and the effect of their usage on power density, COD (chemical oxygen demand) removal, and Coulombic efficiency enhancement. Moreover, a demonstration and comparison of the different types of mixed waste regarding their efficiency for energy generation via MFCs are presented. Future perspectives for manufacturing and cost analysis plans can support scale-up processes fulfilling waste-treatment efficiency and energy-output densities.

Comparative evaluation of simultaneous nitritation/denitritation and energy recovery in air-cathode microbial fuel cells (ACMFCs) treating low C/N ratio wastewater

Air-cathode microbial fuel cells (ACMFCs) can extract available electrons from the low C/N ratio wastewater (LCNW) for pollutant degradation and power generation. However, the multiple effects of operating parameters and their relationship between the performances and parameters are still lacking. In this study, several ACMFCs for simultaneous nitritation/denitritation (SND) and energy recovery were constructed and evaluated in terms of chemical oxygen demand (COD), NH₄⁺-N, C/N ratio, phosphate buffer solution (PBS), and external resistance (R_ext), and several derived parameters (e.g., organic loading rate (OLR), nitrogen loading rate (NLR)). Results indicated that ACMFCs could be used to treat LCNW successfully with high pollutant removal rates and sustainable current generation. Maximum removal efficiencies of 94% COD, 92% NH₄⁺-N, and 92% total nitrogen (TN) were achieved. A maximum power density of 1400 mW m^-2 and columbic efficiency of 69.2% were also obtained at a low C/N ratio of 1.7-2.6. Low C/N ratios promoted SND by balancing nitritation and denitritation. The microbial community and their predicated function results showed considerable nitrifiers and denitrificans were enriched in the ACMFCs, contributing to SND and power recovery. Further analyses showed that the NH₄⁺-N could inhibit SND, but PBS and R_ext had no obvious effects on this outcome. Co-occurrence network analysis demonstrated that power is positively correlated with COD and R_ext; strong correlations between organic removal and COD, and between nitrogen removal and ammonia, conductivity, and C/N ratio were also noted. Overall, the appropriate control of such parameters is necessary to achieve efficient SND in ACMFCs for LCNW treatment.

Effect of cell structure and heat pretreating of the microorganisms on performance of a microbial fuel cell

While microbial fuel cells are being considered as a tool for energy saving in wastewater treatment facilities, such applications in oil refineries pose a challenge due to harder acclimation of microorganisms. In this research, the effect of heat pretreating mixed culture microorganisms (MCM), and cell cross section, on the performance of a novel cell design with two cross sections (single chamber microbial fuel cells, with circular: SCMFC_CC and rectangular: SCMFC_RC cross section) fed batched with refinery wastewater were investigated. First, using original and heat pretreated MCM, the performance of SCMFC_CC in terms of chemical oxygen demand (COD) removal and electricity production was investigated. Then, using only the heat pretreated MCM, the electricity production of SCMFC_RC was measured and compared with that of SCMFC_CC. Heat pretreatment of MCM improved maximum open circuit voltage (OCV) and maximum power density generated by 14% and 16%, respectively. However, heat pretreatment reduced COD removal by about 4%. The performance of SCMFC_CC in terms of maximum OCV and power density compared to SCMFC_RC was improved by 41% and 279%, respectively. Heat treatment of MCM increases the electricity generation of the cell, while reducing the performance of COD reduction due to decreasing the microorganism varieties in the MCM.

Platinum-free, graphene based anodes and air cathodes for single chamber microbial fuel cells

Microbial fuel cells (MFCs) exploit the ability of microorganisms to generate electrical power during metabolism of substrates. However, the low efficiency of extracellular electron transfer from cells to the anode and the use of expensive rare metals as catalysts, such as platinum, limit their application and scalability. In this study we investigate the use of pristine graphene based electrodes at both the anode and the cathode of a MFC for efficient electrical energy production from the metabolically versatile bacterium Rhodopseudomonas palustris CGA009. We achieve a volumetric peak power output (PV) of up to 3.51 ± 0.50 W m-3 using graphene based aerogel anodes with a surface area of 8.2 m2 g-1. We demonstrate that enhanced MFC output arises from the interplay of the improved surface area, enhanced conductivity, and catalytic surface groups of the graphene based electrode. In addition, we show a 500-fold increase in PV to 1.3 ± 0.23 W m-3 when using a graphene coated stainless steel (SS) air cathode, compared to an uncoated SS cathode, demonstrating the feasibility of a platinum-free, graphene catalysed MFCs. Finally, we show a direct application for microwatt-consuming electronics by connecting several of these coin sized devices in series to power a digital clock.