Katharina Herkendell

Wastewater treatment plants (WWTPs) are among the most energy‐intensive public infrastructure services. The largest energy sink in WWTPs is the conventional activated sludge (CAS) process, which relies on aerobic degradation of organic matter. At the same time, anaerobic digestion (AD) of sludge produced during wastewater treatment is used to recover energy in the form of biogas composed mainly of methane and carbon dioxide. Nevertheless, the overall chemical energy in wastewater remains significantly underutilized. Bioelectrochemical systems (BES), that use specialized microorganisms to recover energy from wastewater, could offer a promising solution. Among them, microbial fuel cells (MFCs) can generate electricity by aeration‐free degradation of organic matter, while electromethanogenesis (EMG) can enhance methane production from sludge using external energy. This study quantitatively compares the energy performance of conventional and BES‐integrated WWTPs. Placing MFC upstream of the CAS process significantly reduces external energy demand from 0.043 to 0.014 kWh pe ⁻¹ d ⁻¹ by lowering aeration requirements. While MFC‐only configuration achieves the lowest energy external energy demand, the configuration combining upstream MFC with in situ EMG (to enhance AD) downstream leads to stable performance with fluctuating influent COD conditions.

Enzymatic fuel cells (EFCs) offer renewable energy conversion via highly selective electrode reactions using enzymes as natural catalysts even under mild conditions. Electrochemical impedance spectroscopy (EIS) is a valuable tool for evaluating EFC performance, providing insights into substrate mass transport, enzyme kinetics, and electrode stability. Despite its acknowledged importance, the use of EIS coupled with distribution of relaxation times (DRT) analysis in EFCs research is limited. Our study addresses this gap by employing EIS and DRT analysis to investigate enzyme‐based anodic processes, focusing on the bioelectrocatalytic oxidation of glucose catalyzed by glucose oxidase (GOx). Through careful variation of multiple parameters, it was possible to identify three distinct regions in the DRT plot. Each region has been subsequently associated with a key anodic process. The first region (R₁) is associated with high‐frequency phenomena occurring at the electrodes, primarily due to ionic conduction in the electrolyte. Intermediate‐frequency processes are associated to charge transfer kinetics in region 2 (R₂). Region 3 (R₃) is linked to diffusion processes occurring at low frequencies. This thorough examination offers an insight into the functioning of enzymatic bioelectrodes, which in turn drives improvements in the design and components of biofuel cells to increase their power output.

The adoption of microbial fuel cell (MFC) technology hinges on the development of efficient proton‐exchange membranes (PEMs), which significantly influences fuel cell performance and cost. PEMs have a critical role in preventing oxygen crossover, maintaining electrochemical neutrality, and supporting microorganisms within MFCs. Nafion, the current industry‐standard PEM, grapples with environmental, cost, and performance issues. Although improvements to Nafion have been reported using additives, immersion in heteropolyacids, different pretreatment methods, and UV irradiation, many of the challenges still remain. Herein, the recent developments in the area of alternative PEMs are reviewed and analyzed. Among them, sulfonated aromatic hydrocarbons, particularly sulfonated polyether ether ketone, have emerged as top contenders in terms of scale up and commercial viability. At the same time, membranes based on polyvinyl alcohol, ionic liquids, and natural materials are also being actively researched for various MFC applications. Since most studies are short term and lab scale, there is a need evaluate long‐term stability and economic cost of PEMs in terms of standardized parameters such as power‐to‐cost and normalized energy recovery. Additionally, for emerging low‐energy‐density MFC applications like biosensors and in vivo power sources, PEM properties and design need to be tailored carefully.

Ordered mesoporous carbon CMK-3 sieves with a hexagonal structure and uniform pore size have recently emerged as promising materials for applications as adsorbents and electrodes. In this study, using sucrose as the sustainable carbon source and SBA-15 as a template, CMK-3 sieves are synthesized to form bioelectrocatalytic immobilization matrices for enzymatic biofuel cell (EFC) electrodes. Their electrochemical performance, capacitive features, and the stability of enzyme immobilization are analyzed and compared to commercially available multi-walled carbon nanotubes (MWCNT) using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The anodic reaction in the presence of glucose oxidase (GOx) and ferrocene methanol (FcMeOH) on the sustainably sourced CMK-3-based electrodes produces bioelectrocatalytic current responses at 0.5 V vs. saturated calomel electrode (SCE) that are twice as high as on the MWCNT-based electrodes under saturated glucose conditions. For the cathodic reaction, the MWCNT-based cathode performs marginally better than the CMK-3-based electrodes in the presence of bilirubin oxidase (BOD) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS²⁻). The CMK-3-based EFCs assembled from the GOx anode and BOD cathode results in a power output of 93 μW cm⁻². In contrast, the output power of MWCNT-based EFCs is approximately 53 μW cm⁻². The efficiency of CMK-3 as a support material for biofuel cell applications is effectively demonstrated.

Bioelectrochemical systems (BES) employ enzymes, subcellular structures or whole electroactive microorganisms as biocatalysts for energy conversion purposes, such as the electrosynthesis of value-added chemicals and power generation in biofuel cells. From a bioelectrode engineering viewpoint, customizable nanostructured carbonaceous matrices have recently received considerable scientific attention as promising electrode supports due to their unique properties attractive to bioelectronics devices. This review demonstrates the latest advances in the application of nano- and micro-structured carbon electrode assemblies in BES. Specifically, in view of the gradual increase in the commercial applicability of these systems, we aim to address the stability and scalability of different BES designs and to highlight their potential roles in a circular bioeconomy.

Enzyme-functionalized magnetic nanoparticles are channeled by magnetic field gradients onto modified electrodes to activate enzymatic cascades for multi-substrate power generation.