Combination of bioelectrochemical systems and electrochemical capacitors: Principles, analysis and opportunities

© 2019 The Authors Bioelectrochemical systems combine electrodes and reactions driven by microorganisms for many different applications. The conversion of organic material in wastewater into electricity occurs in microbial fuel cells (MFCs). The power densities produced by MFCs are still too low for application. One way of increasing their performance is to combine them with electrochemical capacitors, widely used for charge storage purposes. Capacitive MFCs, i.e. the combination of capacitors and MFCs, allow for energy harvesting and storage and have shown to result in improved power densities, which facilitates the up scaling and application of the technology. This manuscript summarizes the state-of-the-art of combining capacitors with MFCs, starting with the theory and working principle of electrochemical capacitors. We address how different electrochemical measurements can be used to determine (bio)electrochemical capacitance and show how the measurement data can be interpreted. In addition, we present examples of the combination of electrochemical capacitors, both internal and external, that have been used to enhance MFC performance. Finally, we discuss the most promising applications and the main existing challenges for capacitive MFCs

Optimisation of Microbial Fuel Cells (MFCs) through bacterial-robot interaction

For over 100 years, Microbial Fuel Cells (MFCs) have been developed as eco-friendly alternatives for generating electricity via the oxidation of organic matter by bacteria. In the early 2000s, collectives of MFCs were proven fea-sible energy providers for low-power robots such as Gastrobot and EcoBots. Even though individual MFC units are low in power, significant progress has been achieved in terms of MFC materials and configurations, enabling them to generate higher output levels. However, up to this date, MFCs are produced and matured using conventional laboratory methods that can take up to three months to bring the MFCs to their maximum power aptitudes. In this work, an approach to use a low-cost (£1.5k) RepRap liquid handling robot called EvoBot was employed with the aim to automate the maturing process of MFCs and allow them to reach their maximum power ability in a shorter time span. Initially, the work focused on establishing an interface and interconnection between the living cells (in the MFC) and the robotic platform, and investigating whether the MFC voltage can trigger a feedback loop feeding mechanism. It was shown that the robot successfully matured the MFCs in just 6 days and, they were also 1.4 times more powerful than conventionally matured MFCs (from 19.1 mW/m2 to 26.5 mW/m2). This work took a rounded approach in improving the overall MFC perfor-mance. 3D-printable materials that can emerge from EvoBot were investi-gated for fabricating MFCs. MFCs employing these materials improved their power output by almost 50% (from 66μW to 130 μW) compared to the ones based on conventional, fluorinated materials. Furthermore, EvoBot was able to improve the fuel supply frequency and composition using evolutionally algorithms. For the first time, this project has demonstrated that the fabrica-tion, maintenance and power generation of MFCs can be optimised via the interaction and support of a dedicated robotic system

Towards an Autonomous Algal Turf Scrubber: Development of an Ecologically-Engineered Technoecosystem

The development of an autonomous and internally-controlled technoecological hybrid is explored. The technoecosystem is based on an algal turf scrubber (ATS) system that combines engineered feedback control programming with internal feedback patterns within the ecosystem. An ATS is an engineered, high-turbulent aquatic system to cultivate benthic filamentous algae for the removal of pollutants from an overlying water stream. This research focuses on designing a feedback control system to control the primary production of algae in an ATS through monitoring of the algal turf metabolism and manipulation of the turbulence regime experienced by the algae. The primary production of algae in an ATS, and thus the potential of the waste treatment process, is known to be directly related to the level of turbulence in the flowing water stream resulting from the amplitude and frequency of the wave surge. Experiments are performed to understand the influence of turbulence on the biomass production rate of algae in an ATS. These results show that biomass production is correlated with wave surge amplitude at a constant frequency. Further, the influence of turbulence on the net ecosystem metabolism of an algal turf in an ATS was investigated. Results showed that both net primary production and respiration, measured through the diurnal change of inorganic carbon, follow a subsidy-stress relationship with increasing wave surge frequency, although some of this trend may be explained by the transfer of metabolic gases across the air-water interface. A feedback control algorithm, developed to monitor the net primary production and manipulate a controlling parameter, was found to converge quickly on the state of maximum primary production when the variance of the input data was low, but the convergence rate was slow at only moderate levels of input variance. The elements were assembled into a physical system in which the feedback control algorithm manipulated the turbulence of the flow in an ATS system in response to measured shifts in ecosystem metabolism. Results from this testing show that the system can converge on the maximum algal productivity at the lowest level of turbulence--the most efficient state from an engineering perspective--but in practice the system was often confounded by measurement noise. Investigation into the species composition of the dominant algae showed shifting relative abundance for those units under automated control, suggesting that certain species are more suited for utilizing the technological feedback pathways for manipulating the energy signature of their environment

Artificial symbiosis in EcoBots

Truly autonomous robotic systems will be required to abstract energy from the environment in order to function. Energetic autonomy refers to the ability of an agent, to maintain itself in a viable state for long periods of time. Its behaviour must be stable in order not to yield to an irrecoverable debt in any vital resource, i.e. it must not cross any of its lethal limits [1, 2]. With this in mind, our long-term goal is the creation of a robot, which can collect energy for itself. This energy must come from the robot's environment and must be sufficient to carry out tasks, which require more energy than that available at the start of the mission. In this respect our definition of an autonomous robot is more akin to Stuart Kauffman's definition of an autonomous agent, a self-reproducing system able to perform at least one thermodynamic work cycle [3] - but without the burden of self-reproduction! Building automata is certainly not something new. The first recorded example of an automaton dates back to the first century A.D. when Heron of Alexandria constructed a self-moving cart driven by a counter weight attached to the wheel base [4]. In more recent times, there are of course, some real robots, which already comply with this definition. For example, robots such as NASA's 'Spirit' [5] employ solar panels to power their explorations of Mars and have demonstrated their impressive ability to be self-sustaining. However, there will be numerous domains in which solar energy will not be available such as in underwater environments, sewers or when constrained to operate only in the dark. We are, therefore, interested in a class of robot system, which demonstrates energetic autonomy by converting natural raw electron-rich organic substrate (such as plant or insect material) into power for essential elements of robotic behaviour including motion, sensing and computation. This requires an artificial digestion system and concomitant artificial metabolism or, as in the case of EcoBots-I and -II, a rapprochement between an engineered artefact and a biological system - the robot symbiot.</p

Energy and metabolism

Energy resulting from metabolism is essential for any living system-from single-cell to multicellular organisms. This also applies to symbiotic robots (SymBots), which function utilizing the energy (electricity) generated by living microorganisms. In the context of living technologies, artificial symbiosis between the living and the artificial entities of the machine becomes vital for the whole system. If the living entity stops generating energy, the mechatronic system ceases to work yet it is the mechatronic system that provides the microbes with food, and gets rid of their waste. This chapter presents and discusses SymBots, based on EcoBots that operate using Microbial Fuel Cells as onboard living energy devices. The interface between science and engineering is exemplified through the study and optimization of MFCs, producing the necessary data for technological implementation. Biological inspiration stems from living organisms metabolizing and adapting to the environment (homeostasis), which is the main process transferred to engineering