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The fluidized capacitive bio-anode as a novel reactor concept for the microbial fuel cell Alexandra Deeke, Tom H.J.A. Sleutels, Tim F.W. Donkers, Hubertus V.M. Hamelers, Cees J.N. Buisman, and Annemiek Ter Heijne Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es503063n • Publication Date (Web): 16 Dec 2014 Downloaded from http://pubs.acs.org on December 23, 2014

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Environmental Science & Technology

120x146mm (150 x 150 DPI)

ACS Paragon Plus Environment


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The fluidized capacitive bio-anode as a novel reactor concept for

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the microbial fuel cell

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Alexandra Deeke a, b, Tom H. J. A. Sleutels b, Tim F.W. Donkersa,b, Hubertus V. M. Hamelers

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b

, Cees J. N. Buisman a, b, Annemiek Ter Heijne a,*

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a

Sub-Department of Environmental Technology, Wageningen University, Bornse Weilanden

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9, P.O. Box 8129, 6708 WG Wageningen, The Netherlands

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b

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8900 CC Leeuwarden, The Netherlands

Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, P.O. Box 1113,

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* Corresponding author. Phone: +31 317 483447: [email protected]

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Abstract

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The use of granular electrodes in Microbial Fuel Cells (MFCs) is attractive because granules

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provide a cost-effective way to create a high electrode surface area, which is essential to

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achieve high current and power densities. Here, we show a novel reactor design based on

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capacitive granules: the fluidized capacitive bio-anode. Activated carbon (AC) granules are

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colonized by electrochemically-active microorganisms, which extract electrons from acetate

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and store the electrons in the granule. Electricity is harvested from the AC granules in an

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external discharge cell. We show a proof-of-principle of the fluidized capacitive system with a

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total anode volume of 2 L. After a start-up period of 100 days, the current increased from

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0.56 A/m2 with 100 g AC granules, to 0.99 A/m2 with 150 g AC granules, to 1.3 A/m2 with 200

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g AC granules. Contact between moving AC granules and current collector was confirmed in

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a control experiment without biofilm. Contribution of an electro-active biofilm to the current

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density with recirculation of AC granules was limited. SEM images confirmed that a biofilm

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was present on the AC granules after operation in the fluidized capacitive system. Although

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current densities reported here need further improvement, the high surface area of the AC

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granules in combination with external discharge offers new and promising opportunities for

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scaling up MFCs.

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1. Introduction

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With the increase in world population and the resulting increase in energy consumption,

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together with the need to keep our planet healthy, the search is on for renewable energy

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production technologies. One of these technologies is the Microbial Fuel Cell (MFC), in which

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electrochemically-active microorganisms convert the chemical energy in biodegradable

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organics into electrical energy, so that electricity is directly recovered from the decomposition

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of organics in wastewater. In the past decade, many crucial elements in the MFC have been

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studied, from electrode materials

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communities 8, electron transfer mechanisms

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types of wastewater 14–16, alternative cathode reactions 17–19, and scaling up 20,21.

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Two important parameters for operation of Microbial Fuel Cells (MFCs) are the current

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density and the power output. The current density is directly related to the conversion rate of

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organic matter, and thus determines the rate at which wastewater is cleaned. The power

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density expresses the rate at which energy is recovered from wastewater, and determines

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the efficiency of the conversion from organics to electricity. During the past decade, the

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research on MFCs has intensified considerably, and the highest reported power densities

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increased from 0.01 W/m² in 2002

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time, we see in the past few years, reported power densities seem to have stagnated around

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2-3 W/m2. Typical power densities, when normalized to volume, range between 1 and 200

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W/m3

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technologies: anaerobic digestion, in which an energy recovery of 1,000 W/m3 can be

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achieved 25,28.

25–27

1

, to membranes

22,23

2–4

, reactor design

9,10

, mass transport

up to 3.1 W/m2 in 2009

5–7

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, microbial

, start-up strategies

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12,13

,

(Figure 1). At the same

. This power density is still too low to be competitive with one of the competing


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Figure 1 Development of reported power densities in literature over the past decade.

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Although these power densities have been demonstrated at lab-scale, the question is how

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and if these systems will show the same performance when operated at a larger scale. In a

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recent review

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(reactors larger than 1L) are discussed. The main limitations in these scaled-up systems are

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related to a high internal resistance, caused by electrode spacing or limited conductivity of

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the electrodes, especially when granular materials are used in a tubular design.

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Granular materials are attractive because they provide a cost-effective way to create a high

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electrode surface area, which is essential to achieve high current and power densities.

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Granular materials, usually in the form of graphite or activated carbon, have mostly been

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used in a fixed bed reactor, in which the electric current was harvested via a current

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collector, which is a graphite rod

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was demonstrated that electricity could be generated in an anaerobic fluidized bed MFC,

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where the active carbon particles could flow freely in the anaerobic column so that better

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mixing could be achieved

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, the tubular and flat plate designs that have been tested on larger scale

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or a Ti-wire or mesh

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. To enhance mass transport, it

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. In this setup, the electricity was also harvested in-situ via a



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graphite rod inserted in the column. Both granular activated carbon, and graphite granules

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were tested, and it was shown that with the use of a mediator (neutral red), a power density

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up to 1.1 W/m2 could be produced 32.

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Another approach is to integrate capacitive materials in the bio-anode of an MFC33. When a

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capacitive activated carbon layer was cast onto an electrode, it was shown that 53% more

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charge could be recovered compared to a non-capacitive flat plate electrode. The thickness

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of the capacitive layer was shown to affect the current and power density34. Recently, Liu et

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al

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that were charged by exoelectrogenic biofilms inside a 7 mL anode chamber and discharged

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at a current collector integrated in the anode chamber. Mixing of the particles resulted in

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improved performance compared to a fixed bed setup.

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The objective of this study was to study a new reactor concept where electrons are stored

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inside AC granules in a charging column, and the electricity is harvested in an external

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electrochemical cell instead of in-situ in the anaerobic reactor. We tested this fluidized

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capacitive system with a total anolyte volume of 2.1 L and we analyzed the performance of

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this system in terms of current density, COD removal, and coulombic efficiency.

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have shown that electricity could be harvested intermittently from capacitive particles

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2. Materials and methods

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2.1 System setup.

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The principle of the fluidized capacitive system is shown in Figure 2.


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Figure 2: The setup of the fluidized capacitive system. In the charging reactor, electrochemically-active microorganisms attach to the activated carbon (AC) granules. While converting acetate to CO2 and protons, electrons are stored in the AC granules. Electricity is harvested in the discharge cell where the AC granules release their stored electrons to the current collector (anode). AC granules are recirculated through the system driven by a nitrogen gas flow.

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The setup consisted of a charging column (2.08 L), made of glass, and a discharge cell

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(0.022 L), made of Perspex, with a total anode volume of 2.1 L. Details of the reactor can be

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found in Figure S1A in the Supporting Information. In the charging column, the AC granules

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are transported upward by a flow of nitrogen gas. After the upward transport, the granules



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are transported through a glass tube and settle through the discharge cell. From the

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discharge cell, the granules are transported through a glass tube to the charging reactor. The

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discharge cell consisted of a current collector (anode) and a cathode, both flat graphite

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plates (Müller & Rössner GmbH, Troisdorf, Germany) of 11 cm2 surface area (Figure S1B

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Supporting Information). Both electrodes are separated by two plates with a flow channel,

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similar to the ones used in Ter Heijne et al. (2008)

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contact area of the electrodes of 11 cm2 and a thickness of 2 cm, resulting in 22 mL anode

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and cathode compartment volume in the discharge cell. A cation exchange membrane

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(Ralex, Mega, Straz pod Ralskem, Czech Republic) was used to separate anode and

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cathode compartment. Total circulation volume of the cathode was 1.02 L, consisting of the

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discharge cathode compartment of 22mL plus a storage vessel for the catholyte of 1L that

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was located next to the discharge cell.

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2.2 Electrode preparation and analysis. The anode granules consisted of activated carbon

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(AC) granules (bulk density: 510 kg/m³, fraction 1620 of GAC 1240, Norit, Amersfoort, The

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Netherlands). According to the specifications, the size of the majority of the granules was

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between 0.84 and 1.18 mm, with less than 4% of the particles 1.70 mm. 100 g (dry weight) of the AC granules were soaked in tap-water prior

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to inoculation. The granules were first placed in a flat plate cell and inoculated with effluent of

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another MFC operated on acetate

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used for the flat-plate cell was 500Ω. After 2 weeks, the flat-plate cell was opened and the

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granules were transferred into the fluidized bed reactor. On day 130 and 180, an additional

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50g of pre-inoculated granules were added from the flat plate cell to the fluidized bed reactor,

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resulting in a final amount of 200g AC granules.

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2.3 Operation and Measurements. The fluidized capacitive anode was continuously fed

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with 5.2 mL/min of fresh medium and the anolyte and catholyte were recirculated at a rate of

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500mL/min. The HRT of the medium in the anode compartment was 6.7 h. The influent to the

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fluidized capacitive anode was a mixture of 10 mM sodium acetate with a 10 mM phosphate

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, but smaller. The flow channel had a

to achieve biofilm growth on the granules. The load


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buffer solution at pH 7. Additionally, 10 mL/L of a macronutrient, 1 mL/L of a micronutrient

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and 1 mL/L of a vitamin solution was supplied to the anolyte4. The catholyte (discharging cell)

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consisted of a 10 mM ferricyanide solution. The charging column was constantly flushed with

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nitrogen gas at a rate of 9 L/h from the bottom to the top to drive the upward transport of the

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AC granules. The average granule recirculation rate was 45 s, meaning that it took each

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granule in average 45s to pass one time through the reactor. The current collector was

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constantly controlled at a potential of -0.300V vs. Ag/AgCl (+0.2V vs. NHE, Pro Sense,

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Oosterhout, The Netherlands). This anode potential was chosen based on performance of

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capacitive bio-anodes during charge-discharge experiments in an earlier study33. The anode

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potential was controlled with a potentiostat (n-stat, Ivium Technologies, Eindhoven, The

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Netherlands). The pH of the anolyte was constantly measured using a pH-electrode

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(Liquisys, Endress + Hauser, Naarden, The Netherlands). Voltage and current were

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constantly recorded.

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Two types of control experiments were performed. The objective of the first experiment was

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to study if electrons could be transferred between non-inoculated AC granules and the

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current collector (anode). The objective of the second experiment was to study if a biofilm

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could grow on the current collector (anode) when granules were recirculated.

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To study electron transfer between non-inoculated AC granules and the current collector, we

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studied the charge/discharge behavior of the fluidized bed system both without granules and

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with 50g of non-inoculated granules in recirculation. The anolyte and catholyte were the

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same as used during the rest of the experiment. The potential of anode current collector was

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controlled in three steps: first two charging steps at an anode potential of -0.3 V vs Ag/AgCl

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during 30 s and -0.5 V vs Ag/AgCl during 30 s, then one discharging step at an anode

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potential of -0.1 V vs Ag/AgCl during 30 s. The anode potential of -0.3 V vs Ag/AgCl was

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chosen in the first charging step to charge the particles at the same potential used

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throughout the experiments; the second charging step was performed at more negative

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anode potential (-0.5 V vs Ag/AgCl) to increase the driving force for charge transfer. The



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same procedure of two charging steps and one discharging step was repeated after addition

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of 50 g AC granules to the anode compartment, to study if additional charge would be

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transferred when the AC granules were recirculated.

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To study the contribution of an electro-active biofilm on the current collector to the total

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current produced by the discharge cell, we operated the fluidized bed system without

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granules. The system was inoculated with effluent of another MFC operated on acetate and

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the current collector (anode) of the discharge cell was controlled at -0.3 V vs Ag/AgCl. After a

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stable current was reached, 50 g of AC granules were added to the system and recirculation

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started. Current was continuously monitored. Finally, the current was monitored after

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recirculation of AC granules was stopped.

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2.4 Analyses and calculations.

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During the first 112 days of operation the COD of the anode compartment was monitored in

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the influent and in the effluent of the cell. From these measurements the COD-removal rate

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and the coulombic efficiency were determined. The COD was measured using a standard

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test kit (Nr 414, Hach Lange, Tiel, The Netherlands).

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The specific surface area of the AC granules without biofilm was determined using BET-

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Analysis as described in Deeke et al. (2012)

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1989.4 ± 236.2 m²/g, which is a typical value for AC granules 38.

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The current density was calculated from the produced current through; i = I/A in A/m², where

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A is the projected surface area of the anode, cathode or membrane (0.0011 m2).

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The COD removal of the fluidized bed anode was determined between day 55 and 112. The

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COD removal efficiency (rCOD in %) was calculated according to:

࢘࡯ࡻࡰ =

33

. The specific surface area was found to be

ሺ࡯ࡻࡰ࢏࢔ − ࡯ࡻࡰ࢕࢛࢚ ሻ ࡯ࡻࡰ࢏࢔


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where CODin is the concentration of COD (mol/L) measured in the inflow of the fluidized bed

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anode and CODout is the concentration of COD (mol/L) measured in the outflow of the

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fluidized bed anode.

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With the COD removal and the average current density (A/m²), the coulombic efficiency (CE)

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was calculated according to:

‫= ܧܥ‬

‫ܯ‬ைଶ × ‫ܫ‬ ݊ × ‫ܦܱܥ∆ × ܳ × ܨ‬

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where MO2 is the molecular weight of oxygen (32 g/mol), I is the average current, n = 4 is the

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number of electrons exchanged per mole of oxygen, F is the Faraday constant (96485

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C/mol), Q is the volumetric influent rate (L/s) and ∆COD is the difference in COD between

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the influent and the effluent (g/L).

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3. Results and discussion

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In this work, we studied a novel reactor system in which AC granules are used in a fluidized

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reactor to store electrons derived from wastewater by electrochemically-active

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microorganisms, and in which the electricity is harvested in an external discharge cell.

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3.1 Increase in current production with increasing loading of AC particles

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During the first 100 days after addition of 100g pre-inoculated AC granules in the system,

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current density in the discharge cell (at -0.3 V vs Ag/AgCl) was around 0.17 A/m2. After 100

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days, an increase in current density was observed to 0.56 A/m2. After addition of another 50

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g AC granules, the current density first decreased but then gradually increased to 0.99 A/m2.

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After addition of another 50 g AC granules, a further increase to 1.3 A/m2 was observed on

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day 218. This increase in current density with increasing loading of pre-inoculated AC

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granules indicates that the electrochemically-active microorganisms on the AC granules are

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able to store electrons, and that these electrons can be harvested in the discharge cell.



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Figure 3 The development of the current density of the fluidized bed anode. The current density increased from 0.18 A/m² at day 96 to 1.3 A/m² at day 218. During the experimental period the AC granule loading was increased from 100g at the start, to 150 g on day 153, to 200 g on day 185.

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To monitor the microbial activity during start-up of the fluidized capacitive anode, the COD

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removal was measured and coulombic efficiency was determined between day 51 and 112.

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From day 51 to day 105, the current density was stable around 0.17 A/m2, and thereafter an

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exponential increase in current was observed to 0.56 A/m2. The COD removal efficiency

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increased in time from 20-40% in the beginning, to 65-72% in the days that a sharp increase

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in current density was found. The coulombic efficiency increased as well, from 7% on day 51

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to 27 % on day 112. This reflects that the electrochemically-active microorganisms indeed

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became more active during the last weeks before the increase in current was observed. Still,

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large part of the substrate was not converted into electrons, indicating loss of substrate to

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either biomass growth or methanogens.

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3.2 Role of AC granules in current production (abiotic test)


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To study in more detail if the AC granules indeed contributed to transfer of electrons, we

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tested, in an abiotic control if the granules made electrical contact with the current collector.

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An abiotic charge-discharge experiment was performed in the system, on the current

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collector alone and with 50 g AC granules using the standard anolyte medium, but without

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inoculation. We tested two charging steps at -0.3 and -0.5 V vs Ag/AgCl, where electrons

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where transferred from current collector to AC granules, followed by one discharging step at -

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0.1 V vs Ag/AgCl, where electrons were transferred from AC granules to current collector.

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The charging current is represented by a negative current; the discharging current is

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represented by a positive current. The results are shown in Figure 4 and summarized in

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Table 1. At -0.1 V vs Ag/AgCl, the current collector alone showed a small peak, after which

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the current quickly dropped to almost zero. At -0.3 V vs Ag/AgCl, a similar trend was

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observed but current stayed slightly higher. At -0.5 V vs Ag/AgCl, a discharge peak can be

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seen where the electrons were transported from the charged AC granules to the current

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collector. When repeating the test with 50 g AC granules, we see a similar pattern, but with a

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considerable higher current density at each potential, indicating that the AC granules do

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exchange electrons with the current collector. This effect was more pronounced for the

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charging steps than for the discharging steps, which is also reflected in the total transferred

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charge (Table 1). For the current collector alone, in the three cycles, 61-100 % of the

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electrons that were transferred during charging, were recovered in the discharge step. For

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the current collector with 50 g AC granules, the percentage of charge recovered in the

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discharge step was lower, between 12 and 16%. These results shows that it is possible to

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charge and discharge the AC granules on the current collector in this fluidized capacitive

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system, although part of the electrons charged on the particles were not recovered in the

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discharge step. It is at this point not clear why only part of the electrons were recovered

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when using 50 g AC granules. Explanations could be that the discharge time is not

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sufficiently long, that too few particles have been charged during the charging step, or that

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the driving force for discharge (at -0.1 V vs Ag/AgCl) was too low. While the results show that

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electron transfer between AC granules and current collector does occur, the results do not



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show what the mechanism of charge storage in the actual MFC system is. Are the electrons

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stored inside the AC granules, or can they also be stored in the biofilm

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is stored, and how this influences the performance of a fluidized capacitive bioanode, is an

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aspect that needs further study.

41

? Where the charge

2000 50 g AC granules

Current collector

Current (µΑ)

1000

0 -0.1

-1000

-0.1 -0.3

-0.3

-0.1

-0.3 -0.5

-0.5

-0.5

-2000 0

100

150

200

250

300

Time (s)

252 253 254 255 256

50

Figure 4 Charge-discharge behavior of the current collector and the current collector with 50 g AC granules. The current and stored charge with 50 g AC granules is higher than for the current collector alone, indicating that charge is transferred between current collector and AC granules.

257 258 259 260

Table 1 Overview of the transferred charge for the current collector alone and for the current collector with 50 g AC granules that are recirculated via the charging column, at different potentials

Transferred charge (mC) Potential (mV vs Ag/AgCl) -300 (charge step 1) -500 (charge step 2) Total charging -100 (discharge) % of charge recovered

Current collector Cycle 1 -0.43 -1.75 -2.18 2.29 105

50 g AC particles


-4.54 -7.94 -12.48 1.52 12

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-300 (charge step 1) -500 (charge step 2) Total charging -100 (discharge) % of charge recovered -300 (charge step 1) -500 (charge step 2) Total charging -100 (discharge) % of charge recovered

Cycle 2 -1.05 -1.82 -2.87 1.76 61 Cycle 3 -1.01 -1.89 -2.90 2.17 75

-3.68 -6.99 -10.67 1.50 14 -3.73 -6.61 -10.34 1.72 17

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3.3 Contribution of an electro-active biofilm on the current collector when non-inoculated AC

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granules are recirculated

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Figure 3 shows that an increase in particle loading resulted in an increase in current density.

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It is possible, though, that part of the current was produced by electrochemically-active

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microorganisms attached to the current collector (anode) of the discharge cell. To study the

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possible contribution of a biofilm on the current collector to current density, we performed a

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second (biotic) control experiment, in which we operated the discharge cell as a Microbial

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Fuel Cell (MFC), first without recirculation of AC granules. After inoculating the anolyte with

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electrochemically-active microorganisms and controlling the anode potential of the discharge

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cell at -0.3 V vs Ag/AgCl, the current collector (bio-anode) produced a maximum current of

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2.7 A/m2 after two weeks. After addition of 50 g non-inoculated granules to the anolyte, that

273

were continuously recirculated via the discharge cell, the current gradually decreased more

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than 5-fold to 0.51 A/m2 after 21 hours. Thus, during continuous flow of AC granules along

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the current collector, the activity of the electro-active biofilm on the current collector

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diminishes. After recirculation of the AC granules was ended, the current produced by the

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electro-active biofilm on the current collector gradually restored. These results show that

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biofilm formation on the current collector is possible and can contribute to the total current,

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however, it seems that the AC granules remove the biofilm, or result in inactivity of the

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biofilm, when they come in contact with the current collector. In our experiments, therefore,



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part of the current may originate from the electro-active biofilm on the current collector itself,

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but it is likely that most of the current originates from the electrochemically-active

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microorganisms on the AC granules: current density increased with increasing loading of AC

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granules, and the recirculation of non-inoculated AC granules in combination with an electro-

285

active biofilm on the current collector resulted in a 5-fold reduced current density of the

286

discharge cell.

287

To study if, at the end of the experiment, the AC granules were colonized by

288

microorganisms, the granules were analyzed by Scanning Electron Microscopy (SEM) before

289

and on day 112. Indeed, a biofilm was observed on the AC granules on day 112 (Figure S2,

290

Supporting Information).

291

3.4 Discussion and perspectives

292

We show a proof-of-principle of the fluidized capacitive anode, where AC granules with

293

electrochemically-active microorganisms are charged in a charging column, and electricity is

294

harvested in an external discharge cell. Advantages of the fluidized capacitive anode

295

compared to standardized MFC set-ups are that a high electrode surface can be achieved in

296

a cost-effective way, that ohmic losses during discharge are minimized, and that the system

297

is well-mixed so that mass transport limitations at the bio-anode are reduced.

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A high electrode surface area can be achieved with the AC granules, while the discharge cell

299

can be of relatively small size. When high currents can be achieved in the discharge cell, this

300

may offer an economically attractive alternative for scaling up microbial electrochemical

301

systems as the need for the expensive constituents of such a cell (membranes, cathodes

302

with catalyst) are reduced.

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The second advantage is that, when charged, it is likely that cations surround the AC

304

granules in the electrical double layer. In this way, the generally low conductivity of the

305

wastewater does not pose a major limitation in the charging step. Our hypothesis is that

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during discharge, these cations are also released, leading to locally high conductivity and

307

therefore minimal ohmic losses in the discharge cell. This can offer advantages compared to


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the limited conductivity in MFC designs where wastewater is directly converted into electricity

309

and not stored in AC granules.

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Because the granules are fluidized, the diffusion layers of acetate towards the biofilm, and

311

protons away from the biofilm, are minimized, and mass transfer is increased. An important

312

issue to address in later study is that, for practical application, mixing should be achieved

313

with minimum energy (gas sparging).

314

The maximum current density achieved in this fluidized capacitive system was 1.3 A/m2. This

315

is lower than the current density that has been achieved with a bio-anode on a flat electrode,

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up to 4.5 A/m2 at -0.3 V vs Ag/AgCl

317

possible to increase the granule loading because this resulted in clogging. It is likely that a

318

further increase in loading rate will result in a further increase in current density, and further

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study with a new reactor design will reveal to what extent the performance of the fluidized

320

bed reactor can be improved. Another study on fluidized anode granules that were

321

discharged in-situ, reported current densities up to 2.4 A/m2 at -0.4 V vs Ag/AgCl35. In fact,

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electro-active microorganisms on the current collector may even have contributed to the

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current density, though AC granules touching the current collector seem to prevent biofilm

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formation on the current collector. Visual inspection of the current collector at the end of the

325

experiment showed that the surface was slightly eroded by the AC granules, an erosion not

326

previously observed for the same material operated as a bio-anode without recirculation of

327

granules. It is likely, though, that there were small zones where a biofilm could be formed.

328

Similarly, the biofilm may also detach from the AC granules when touching the current

329

collector, an aspect that we have not studied in more detail here. An interesting question in

330

this respect is how the AC granules can discharge when covered with an electro-active

331

biofilm. Possibly, the electro-active biofilm is conductive, and electrons may be transported

332

via the biofilm to the current collector

333

clear and needs to be addressed in further study.

36

. In the configuration used in this study, it was not

42

. The exact mechanism, however, is at this point not



334

In addition to enhancing biofilm attachment to the AC granules, several other strategies, like

335

increased particle loading rate, and optimizing retention and cycling times, need to be

336

explored to show the feasibility and applicability of this novel reactor concept for MFCs.

337 338

5. Acknowledgements

339

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of

340

Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the

341

Dutch Ministry of Economic Affairs, the European Union European Regional Development

342

Fund, the Province of Fryslân, the city of Leeuwarden and by the EZ-KOMPAS Program of

343

the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of

344

the research theme “Resource Recovery” for the fruitful discussions and their financial

345

support. Also the authors would like to thank Jan Tuinstra and Razan Rosli for the support in

346

the lab.

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