Mixed Culture Biocathodes for Production of Hydrogen, Methane, and Carboxylates.

Formation of hydrogen, methane, and organics at biocathodes is an attractive new application of bioelectrochemical systems (BESs). Using mixed cultures, these products can be formed at certain cathode potentials using specific operating conditions, of which pH is important. Thermodynamically, the reduction of CO2 to methane is the most favorable reaction, followed by reduction of CO2 to acetate and ethanol, and hydrogen. In practice, however, the cathode potential at which these reactions occur is more negative, meaning that more energy is required to drive the reaction. Therefore, hydrogen is often found as a second product or intermediate in the conversion of CO2 to both methane and carboxylates. In this chapter we summarize the inocula used for biocathode processes and discuss the achieved conversion rates and cathode potentials for formation of hydrogen, methane, and carboxylates. Although this overview reveals that BESs offer new opportunities for the bioproduction of different compounds, there are still challenges that need to be overcome before these systems can be applied on a larger scale. Graphical Abstract.

Hydrogen Gas Recycling for Energy Efficient Ammonia Recovery in Electrochemical Systems.

Recycling of hydrogen gas (H2) produced at the cathode to the anode in an electrochemical system allows for energy efficient TAN (Total Ammonia Nitrogen) recovery. Using a H2 recycling electrochemical system (HRES) we achieved high TAN transport rates at low energy input. At a current density of 20 A m-2, TAN removal rate from the influent was 151 gN m-2 d-1 at an energy demand of 26.1 kJ gN-1. The maximum TAN transport rate of 335 gN m-2 d-1 was achieved at a current density of 50 A m-2 and an energy demand of 56.3 kJ gN-1. High TAN removal efficiency (73-82%) and recovery (60-73%) were reached in all experiments. Therefore, our HRES is a promising alternative for electrochemical and bioelectrochemical TAN recovery. Advantages are the lower energy input and lower risk of chloride oxidation compared to electrochemical technologies and high rates and independence of organic matter compared to bioelectrochemical systems.

Competition between Methanogens and Acetogens in Biocathodes: A Comparison between Potentiostatic and Galvanostatic Control.

Microbial electrosynthesis is a useful form of 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 need strong improvement. Here we report on the effect 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 our results, it seems 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 may be exploited to steer product selectivity and reactor performance.

Low Substrate Loading Limits Methanogenesis and Leads to High Coulombic Efficiency in Bioelectrochemical Systems.

A crucial aspect for the application of bioelectrochemical systems (BESs) as a wastewater treatment technology is the efficient oxidation of complex substrates by the bioanode, which is reflected in high Coulombic efficiency (CE). To achieve high CE, it is essential to give a competitive advantage to electrogens over methanogens. Factors that affect CE in bioanodes are, amongst others, the type of wastewater, anode potential, substrate concentration and pH. In this paper, we focus on acetate as a substrate and analyze the competition between methanogens and electrogens from a thermodynamic and kinetic point of view. We reviewed experimental data from earlier studies and propose that low substrate loading in combination with a sufficiently high anode overpotential plays a key-role in achieving high CE. Low substrate loading is a proven strategy against methanogenic activity in large-scale reactors for sulfate reduction. The combination of low substrate loading with sufficiently high overpotential is essential because it results in favorable growth kinetics of electrogens compared to methanogens. 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.

Possibilities for extremophilic microorganisms in microbial electrochemical systems.

Microbial electrochemical systems exploit the metabolism of microorganisms to generate electrical energy or a useful product. In the past couple of decades, the application of microbial electrochemical systems has increased from the use of wastewaters to produce electricity to a versatile technology that can use numerous sources for the extraction of electrons on the one hand, while on the other hand these electrons can be used to serve an ever increasing number of functions. Extremophilic microorganisms grow in environments that are hostile to most forms of life and their utilization in microbial electrochemical systems has opened new possibilities to oxidize substrates in the anode and produce novel products in the cathode. For example, extremophiles can be used to oxidize sulfur compounds in acidic pH to remediate wastewaters, generate electrical energy from marine sediment microbial fuel cells at low temperatures, desalinate wastewaters and act as biosensors of low amounts of organic carbon. In this review, we will discuss the recent advances that have been made in using microbial catalysts under extreme conditions and show possible new routes that extremophilic microorganisms open for microbial electrochemical systems.

High rate copper and energy recovery in microbial fuel cells.

Bioelectrochemical systems (BESs) are a novel, promising technology for the recovery of metals. The prerequisite for upscaling from laboratory to industrial size is that high current and high power densities can be produced. In this study we report the recovery of copper from a copper sulfate stream (2 g L(-1) Cu(2+)) using a laboratory scale BES at high rate. To achieve this, we used a novel cell configuration to reduce the internal voltage losses of the system. At the anode, electroactive microorganisms produce electrons at the surface of an electrode, which generates a stable cell voltage of 485 mV when combined with a cathode where copper is reduced. In this system, a maximum current density of 23 A m(-2) in combination with a power density of 5.5 W m(-2) was produced. XRD analysis confirmed 99% purity in copper of copper deposited onto cathode surface. Analysis of voltage losses showed that at the highest current, most voltage losses occurred at the cathode, and membrane, while anode losses had the lowest contribution to the total voltage loss. These results encourage further development of BESs for bioelectrochemical metal recovery.

Fluidized capacitive bioanode as a novel reactor concept for the microbial fuel cell.

The use of granular electrodes in Microbial Fuel Cells (MFCs) is attractive because granules provide a cost-effective way to create a high electrode surface area, which is essential to achieve high current and power densities. Here, we show a novel reactor design based on capacitive granules: the fluidized capacitive bioanode. Activated carbon (AC) granules are colonized by electrochemically active microorganisms, which extract electrons from acetate and store the electrons in the granule. Electricity is harvested from the AC granules in an external discharge cell. We show a proof-of-principle of the fluidized capacitive system with a total anode volume of 2 L. After a start-up period of 100 days, the current increased from 0.56 A/m(2) with 100 g AC granules, to 0.99 A/m(2) with 150 g AC granules, to 1.3 A/m(2) with 200 g AC granules. Contact between moving AC granules and current collector was confirmed in a control experiment without biofilm. Contribution of an electro-active biofilm to the current density with recirculation of AC granules was limited. SEM images confirmed that a biofilm was present on the AC granules after operation in the fluidized capacitive system. Although current densities reported here need further improvement, the high surface area of the AC granules in combination with external discharge offers new and promising opportunities for scaling up MFCs.

Relating MEC population dynamics to anode performance from DGGE and electrical data.

The microbial electrolysis cell (MEC) is a promising system for H2 production, but little is known about the active microbial population in MEC systems. Therefore, the microbial community of five different MEC graphite felt anodes was analyzed using denaturing gradient gel electrophoresis (DGGE) profiling. The results showed that the bacterial population was very diverse and there were substantial differences between microorganisms in anolyte and anode samples. The archaeal population in the anolyte and at the anodes, and between the different MEC anodes, was very similar. SEM and FISH imaging showed that Archaea were mainly present in the spaces between the electrode fibers and Bacteria were present at the fiber surface, which suggested that Bacteria were the main microorganisms involved in MEC electrochemical activity. Redundancy analysis (RDA) and QR factorization-based estimation (QRE) were used to link the composition of the bacterial community to electrochemical performance of the MEC. The operational mode of the MECs and their consequent effects on current density and anode resistance on the populations were significant. The results showed that the community composition was most strongly correlated with current density. The DGGE band mostly correlated with current represented a Clostridium sticklandii strain, suggesting that this species had a major role in current from acetate generation at the MEC anodes. The combination of RDA and QRE seemed especially promising for obtaining an insight into the part of the microbial population actively involved in electrode interaction in the MEC.

Bioelectrochemical systems: an outlook for practical applications.

Bioelectrochemical systems (BESs) hold great promise for sustainable production of energy and chemicals. This review addresses the factors that are essential for practical application of BESs. First, we compare benefits (value of products and cleaning of wastewater) with costs (capital and operational costs). Based on this, we analyze the maximum internal resistance (in mΩ m(2) ) and current density that is required to make microbial fuel cells (MFCs) and hydrogen-producing microbial electrolysis cells (MECs) cost effective. We compare these maximum resistances to reported internal resistances and current densities with special focus on cathodic resistances. Whereas the current densities of MFCs still need to be increased considerably (i.e., internal resistance needs to be decreased), MECs are closer to application as their current densities can be increased by increasing the applied voltage. For MFCs, the production of high-value products in combination with electricity production and wastewater treatment is a promising route.

Capacitive bioanodes enable renewable energy storage in microbial fuel cells.

We developed an integrated system for storage of renewable electricity in a microbial fuel cell (MFC). The system contained a capacitive electrode that was inserted into the anodic compartment of an MFC to form a capacitive bioanode. This capacitive bioanode was compared with a noncapacitive bioanode on the basis of performance and storage capacity. The performance and storage capacity were investigated during polarization curves and charge-discharge experiments. During polarization curves the capacitive electrode reached a maximum current density of 1.02 ± 0.04 A/m(2), whereas the noncapacitive electrode reached a current density output of only 0.79 ± 0.03 A/m(2). During the charge-discharge experiment with 5 min of charging and 20 min of discharging, the capacitive electrode was able to store a total of 22,831 C/m(2), whereas the noncapacitive electrode was only able to store 12,195 C/m(2). Regarding the charge recovery of each electrode, the capacitive electrode was able to recover 52.9% more charge during each charge-discharge experiment compared with the noncapacitive electrode. The capacitive electrode outperformed the noncapacitive electrode throughout each charge-discharge experiment. With a capacitive electrode it is possible to use the MFC simultaneously for production and storage of renewable electricity.

Effect of operational parameters on Coulombic efficiency in bioelectrochemical systems.

To create an efficient bioelectrochemical system, a high Coulombic efficiency is required. This efficiency is a direct measure for the competition between electrogens and methanogens when acetate is used as substrate. In this study the Coulombic efficiency in a microbial electrolysis cell was investigated. As a result of an increase in substrate concentration from 1 to 35 mM, the current density increased to 21.1A/m(2), while the Coulombic efficiency decreased to 52%. As a result of an increase in anode potential from -450 to -0.250 mV, the current density increased to 20.9A/m(2), while the Coulombic efficiency increased 21%. Knowledge about the substrate affinity and growth kinetics is crucial to control the Coulombic efficiency. Further research is required to optimize strategies to find a balance between the Coulombic efficiency, current density and removal rate of organic material.

Effect of mass and charge transport speed and direction in porous anodes on microbial electrolysis cell performance.

The use of porous electrodes like graphite felt as anode material has the potential of achieving high volumetric current densities. High volumetric current densities, however, may also lead to mass transport limitations within these porous materials. Therefore, in this study we investigated the mass and charge transport limitations by increasing the speed of the forced flow and changing the flow direction through the porous anode. Increase of the flow speed led to a decrease in current density when the flow was directed towards the membrane caused by an increase in anode resistance. Current density increased at higher flow speed when the flow was directed away from the membrane. This was caused by a decrease in transport resistance of ions through the membrane which increased the buffering effect of the system. Furthermore, the increase in flow speed led to an increase of the coulombic efficiency by 306%.

Reduction of pH buffer requirement in bioelectrochemical systems.

Low pH buffer capacity of waste streams limits further development of bioelectrochemical systems (BES) because accumulation of protons potentially leads to acidification of the anodic biofilm. Here we introduce a system that makes it possible to recover alkalinity in an extra recovery compartment. The system consisted of this extra compartment which was located between anode and cathode compartment. The compartment was separated from the anode by a cation exchange membrane and from the cathode by an anion exchange membrane, which made clean hydrogen production possible. To compensate for the charge movement as a result of the flow of electrons, both cations and hydroxyl ions moved into the new recovery compartment. When a synthetic waste stream was fed through this recovery compartment, both pH and conductivity increased. When this stream is then fed to the anode of the BES, no additional buffer was required to produce the same current (3.5 A/m(2)) at an applied voltage of 1 V.

New applications and performance of bioelectrochemical systems.

Bioelectrochemical systems (BESs) are emerging technologies which use microorganisms to catalyze the reactions at the anode and/or cathode. BES research is advancing rapidly, and a whole range of applications using different electron donors and acceptors has already been developed. In this mini review, we focus on technological aspects of the expanding application of BESs. We will analyze the anode and cathode half-reactions in terms of their standard and actual potential and report the overpotentials of these half-reactions by comparing the reported potentials with their theoretical potentials. When combining anodes with cathodes in a BES, new bottlenecks and opportunities arise. For application of BESs, it is crucial to lower the internal energy losses and increase productivity at the same time. Membranes are a crucial element to obtain high efficiencies and pure products but increase the internal resistance of BESs. The comparison between production of fuels and chemicals in BESs and in present production processes should gain more attention in future BES research. By making this comparison, it will become clear if the scope of BESs can and should be further developed into the field of biorefineries.

Microbial electrolysis cells for high yield hydrogen gas production from organic matter.

The use of electrochemically active bacteria to break down organic matter, combined with the addition of a small voltage (> 0.2 V in practice) in specially designed microbial electrolysis cells (MECs), can result in a high yield of hydrogen gas. While microbial electrolysis was invented only a few years ago, rapid developments have led to hydrogen yields approaching 100%, energy yields based on electrical energy input many times greater than that possible by water electrolysis, and increased gas production rates. MECs used to make hydrogen gas are similar in design to microbial fuel cells (MFCs) that produce electricity, but there are important differences in architecture and analytical methods used to evaluate performance. We review here the materials, architectures, performance, and energy efficiencies of these MEC systems that show promise as a method for renewable and sustainable energy production, and wastewater treatment.