Optimization of bioelectrochemical systems for sustainable energy storage

Microbially catalyzed electrochemical systems for sustainable energy storage A smooth and complete transition towards renewable energy is challenged by the variability of generation of power from renewable resources, which cannot by themselves function as baseload generation. Under business-as-usual scenarios, the integration of renewables beyond 30% of total grid power generation is generally expected to require additional measures to guarantee baseload providence. Energy Storage Technologies (ESTs) which store variable generated renewable energy could effectively replace the current fossil fuel driven baseload generation, and are therefore widely seen as potential key players in the renewable energy transition. However, in anticipation of such an intensified future role for energy storage technologies, it becomes increasingly important for the production, operation and recommissioning of these storage facilities not to be in conflict with the sustainability endeavors which their intended use is part of. In this thesis, the use of Bioelectrochemical Systems (BESs) for energy storage is proposed as a possible technology to address this challenge. As is described in more detail in Chapter 1, microorganisms in BESs are able to sustain their growth and activity by using electrodes either as the electron acceptor or donor required for driving their cellular respiration. Microorganisms are able to do so by exploiting various mechanisms, transferring electrons between an electrode and the microbial cell. This way, a wide variety of complex electron transfer chains (ETCs) naturally present in microbial communities, carrying out the redox reactions occurring in microbial catabolism, from oxidation of a wide range of carbohydrates, ammonium and fatty acids to the reduction of nitrate, oxygen, carbon dioxide or sulphate, may be ported to the artificial environment of a man-made electrochemical cell. Bioelectrochemical systems (BES) hold potential both for conversion of electricity into chemicals through microbial electrosynthesis (MES), and the provision of electrical power by oxidation of organics using microbial fuel cells (MFC) and have been intensively studied with regard to these two separate applications. However, when the works covered in this thesis commenced, combining MES and MFC in a single system had not been done before, while obviously this would hold the premise of providing a power-to-power electrical storage technology. It was hypothesized that, by combining MES and MFC, microbially catalysed electrical energy storage may be possible, based on abundant, low-cost and environmentally-friendly chemicals. Chapter 2 of this thesis provides a proof-of-concept for a Microbial Rechargeable Battery (MRB) allowing storage of electricity by combining MES and a MFC in one system. For this first proof, hexacyanoferrate (II/III) was used as counter redox couple. Duplicate runs showed stable performance during 15 days, with acetate being the main energy carrier. An energy density of around 0.1 kWh/m3 (normalized to anode electrolyte volume) was achieved at full-cycle energy efficiency of 30 to 40 %, with a nominal power output during discharge of 190 W/m3 (normalized to anode volume). Prevention of methanogenesis in bioanodes A crucial aspect for a well-performing MRB, and for bioelectrochemical systems (BESs) in general, is the efficient oxidation of substrates by the bioanode, which is reflected in high Coulombic efficiency (CE). In bioanodes, the formation of methane from more reduced substrates impedes CE as these substrates are then no longer available for production of electrical current. As such, methanogenesis forms a major threat to CE in BESs, while core to the field of conventional anaerobic digestion (AD), and much wanted in methane producing biocathodes for power-to-gas technologies. To obtain high CEs and to prevent the formation of methane in the MRB, in the proof-of-concept the halogenated organic compound 2-bromoethanosulfonate (2-BES) was added as a known selective inhibitor for production of methane. However, the sustainability of this practice, both from environmental and technical perspective, may be disputed, as is discussed with more detail in Chapter 7. Instead of using (2-BES) to prevent methane formation, in Chapter 3 alternative approaches are explored to achieve high bioanode CE, by providing a competitive advantage to electrogens over methanogens. Factors identified that affect this competition in bioanodes are, amongst others, electrolyte composition (most notably substrate concentration, conductivity and pH) and anode potential. Focus is put on acetate as a substrate and the competition between methanogens and electrogens is analyzed from a thermodynamic and kinetic point of view. Based on a review of experimental data from earlier studies, it is proposed that low substrate loading in combination with a sufficiently high anode overpotential results in favorable growth kinetics of electrogens compared to methanogens. Thus, anode potential and substrate loading rate may be used as operational parameters for controlling CE at desirable levels, especially in open systems, or in semi-open systems where microbial composition cannot be adequately controlled for. To achieve high current density in combination with low substrate concentrations, it is essential to have a high specific anode surface area. New reactor designs with these features are essential for BESs to be successful in wastewater treatment in the future. As is discussed in more detail in chapter 7, the possible implications of the developed theory for the MRB would be that instead of using 2-BES for methane inhibition during discharging, a redox flow cell-like configuration could be adopted in which the bioanode gradually receives acetate at a loading rate ensuring proper CE development, while current density is modulated to optimize anode potential during discharge. Prevention of methanogenesis in biocathodes As a logical follow up to the work done on CE in bioanodes, Chapter 4 focuses on operational strategies for biocathodes used in Microbial electrosynthesis (MES). Apart from being indispensable to the concept of the MRB, MES is a useful technology for the renewable production of organic commodities from biologically catalyzed reduction of CO2. However, for the technology to become applicable, process selectivity, stability and efficiency needs further improvements. The effects were analysed of different electrochemical control modes (potentiostatic/galvanostatic) on both the start-up characteristics and steady-state performance of biocathodes using a non-enriched mixed culture inoculum. Based on results obtained, it is concluded that kinetic differences exist between the two dominant functional microbial groups (i.e. homoacetogens and methanogens) and that by applying different current densities, these differences can be exploited to steer product selectivity and reactor performance. This is due mainly to the observation that at high hydrogen partial pressures, acetogens may outgrow methanogens. We pose that, through future optimizations by these means, acetate producing MES can be sustained at high production rates and product selectivity without the need of methanogen-inhibiting chemical additives or pre-enrichment of inoculum, albeit at the (slight) cost of voltage efficiency. As is discussed more extensively in chapter 7, this theory could be implemented to the concept of the MRB – in case required - by increased current densities upon charging, while keeping solid retention time low. Studying microbial growth dynamics in BES: looking at the biofilm Where chapters 3 and 4 established qualitative relationships between operational parameters and system performance indicators, a broadly shared demand for quantitative data exists within BES research. Chapter 5 stresses the need of studying in more detail the microbial growth dynamics in bioelectrochemical systems, in order to allow for their proper design and operation. To address this need, Optical Coherence Tomography (OCT) was applied as a tool for in situ and non-invasive quantification of biofilm growth on electrodes (bioanodes). An experimental platform is designed and described in which transparent electrodes are used to allow for real-time, three-dimensional biofilm imaging. The accuracy and precision of the developed method is assessed by relating OCT results to well-established standards for biofilm quantification (COD and Total N) and show high correspondence to these standards. Biofilm thickness as observed by OCT ranged between 3 and 90 μm for experimental durations ranging from 1 to 24 days. This translated to growth yields between 38 and 42 mg CODbiomass/g CODacetate at an anode potential of -0.35 V vs. Ag/AgCl. Time-lapse observations of an experimental run performed in duplicate show high reproducibility in obtained microbial growth yield using the developed method. We identify OCT as a powerful tool for conducting in-depth characterizations of microbial growth dynamics in BESs. Additionally, the presented platform allows concomitant application of this method with various optical and electrochemical techniques. Making the MRB more sustainable: two cathodes put to the test The proof-of-concept described in Chapter 1 shows that bioelectrochemical CO2 reduction and subsequent product oxidation may successfully be combined in one integrated system. However, the ferricyanide/ferrocyanide counter electrode used in this first experiments proofed unstable under the conditions tested after prolonged testing periods. For further development of the MRB, a suitable alternative counter electrode needs to be found. In Chapter 6 , two alternative counter electrodes types are put to the test – namely (i) oxygen/water and (ii) a capacitive electrode - for use in the MRB platform. During daily charge/discharge cycling over periods of 11 to 15 days, experimentally obtained energy efficiencies of 25 and 3.7 % were reported when using the capacitive and the oxygen/water electrodes, respectively. Large overpotentials, resulting in a voltage efficiency of 15 % and oxygen crossover leading to Coulombic efficiencies of 25 % caused the considerably lower efficiency for the oxygen/water systems, despite the theoretical higher voltage efficiency. Although the capacitive electrode equipped systems performed better, energy density is limited by the operational potential window within which capacitive systems can operate reliably. Go/No Go? A comparative analysis of the MRB and guide to future designs Despite the disappointing results regarding so-far tested counter electrodes, in Chapter 7, the microbially catalyzed carbon dioxide/acetate redox couple is identified as a promising anode for future implementation in Aqueous Organic Redox Flow Batteries (AORFB). It is concluded that the bidirectional carbon dioxide/acetate bioanode may be competitive to, or even outperforms most currently investigated redox chemistries for AORFBs. The bioelectrode described in this thesis may prove to be a welcome addition to the existing world of aqueous organic redox flow batteries. That said, the impracticalities encountered so far with tested counter electrodes underline the importance of accounting for charge transport and separation of compounds between anode and cathode in system design, and as most important design considerations are identified (a) membrane type, (b) buffer selection and (c) proton coupled electron transfer at the counter electrode reaction. It is emphasized that, based partially on standing practice in hydrogenotrophic fermentation, charge density for the bioelectrode as high as 185 Ah·L-1 may be reached. This would translate to total system energy densities of over 50Wh/L to be feasible, once a compatible counter electrode is identified and a fully integrated system is developed. Considering energy capacities required for domestic/small community use (10-1000 kWh) the storage volume required would then be within realistic proportions (0.2-200 m3). Provided the many reservations made throughout this thesis, a carbon dioxide/acetate based AORFB may show to be a sustainable, safe and appropriate energy storage technology for decentralized use in the future.