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Tubular ceramic performance as separator for microbial fuel cell: A review
international journal of hydrogen energy xxx (xxxx) xxx
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Tubular ceramic performance as separator for microbial fuel cell: A review M.F. Hil Me, M.H. Abu Bakar* Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi, Selangor, 43600, Malaysia
highlights Performance of ceramic membrane is on par or better than conventional membrane. Ceramic membrane is ‘hardy’ against fouling and chemical scaling. Cost per energy produced using ceramic membrane is more profitable than conventional membrane. Tubular ceramic membrane is the most reported type of design. Previous studies use multiple tubular design instead of single big volume cell.
article info
abstract
Article history:
The depletion of unsustainable conventional energy sources and global warming issues
Received 15 May 2019
create world demand for green energy sources. The microbial fuel cell (MFC) technology
Received in revised form
with the capability to convert environmental waste to energy can be improved with cheap
25 July 2019
ceramic material. The ceramic is structurally porous, thus allow a direct exchange of
Accepted 15 August 2019
cation. The ceramic material also enhances stability thermally and chemically, non-ion
Available online xxx
selective characteristic, high mechanical strength, and easily washable. Commercially produced ceramic structures have been proven to reduce Chemical Oxygen Demand up to
Keywords:
92% and allow high power output. It is also comparatively durable in the long-term oper-
Microbial fuel cell
ation of MFC, compared to the commercially available membrane. The novelty of using
Ceramic
tubular design is the efficient use of space, which leads to the possibility of scaling up. As a
Separator
conclusion, a combination of both ceramic material and tubular design could be an
Design
excellent alternative separator for MFC.
Performance
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cost analysis
Introduction Traditional energy sources such as fossil fuels are used excessively, which in result led to its depletion. Fossil fuels must undergo combustion to produce energy. The burning, however, will produce carbon dioxide and other harmful pollutants as a side product, and therefore, this whole process
of creating energy is not clean and green. Because of the above issues, there is a need to search for a new alternative source of energy generation, which is both cheap and eco-friendly, such as fuel cell [1,2]. Microbial fuel cell (MFC) is a versatile renewable, eco-friendly green technology which can convert waste into energy through bio-electrochemical means and is practicable for wastewater treatment [3e5]. The technique combines biological catalytic activity with abiotic
* Corresponding author. E-mail address: [email protected] (M.H.A. Bakar). https://doi.org/10.1016/j.ijhydene.2019.08.115 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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electrochemical reaction and physics [6,7]. Dubbed renewable because this technology incorporates waste material, which is more favorable in terms of cost [6], as its fuel. The MFC is also eco-friendly because this technology can treat waste while producing enough energy to compensate its processes resulting in at least a zero net energy system. Non-utilization of expensive precious metals further reduced the cost [8]. Out of all technologies available, MFC can transform waste product as energy independent via microbes [9]. Therefore MFC can become biosensors to monitor certain aspects of the environment such as biochemical oxygen demand [10] and water quality monitoring [11e13]. The technology is flexible, can either stand on its own or merged with other technology [14] in the wastewater treatment and bio-energy production sectors [15]. MFC technology is also considered to be capable of processing municipal solid waste in the future, especially for fabrication of biohydrogen [16]. A report on the configuration done on MFC shows transformation to a microbial electrolysis cell (MEC) just by adding an external voltage source. The MEC is specially designed for biohydrogen production [17]. Advancement in MEC technology allows it to be merged with waste biorefinery to produce hydrogen gas as its additional product [18]. Chaturvedi & Verma described that an MFC is divided into two basic designs: single-chambered, both electrodes are in the same working sphere, and double-chambered, anode and cathode are in the separate working sphere, while further understanding and prediction are through modeling [19,20]. Microorganisms are inoculated and enriched in the anodic chamber MFC to yield electrochemically active bacteria (EAB) while organic waste or other carbon source used as fuel. EABs are the critical factor in this system as the process begins with the production of electrons by the EABs oxidizing substrates in the anodic chamber. The electrons and protons from biocatalytic fuel oxidation will traverse to cathode from anode via outer circuit and membrane respectively producing an electrical current. Therefore EAB functions also as a bridge for an electron to pass through to the current collector [21]. Studies concerning types of wastewater as fuel for the system has been reviewed [19]. Xafenias et al. reported that MFC could reduce heavy metal such as hexavalent chromium in wastewater from 10 mg L1 to 0.3 ± 0.3 mg L1 (97% reduction) in the first 45 h of operation at pH of 8 [22]. While in another report by Gangadaran & Nambi, 100 mg L1 hexavalent chromium was removed entirely (recovery up to 99.87% via precipitation on the cathode) in the first 48 h of operation while simultaneously producing a maximum power density of 767.01 mW m2 (2.08 mA m2) [23]. Both studies show that it is feasible to utilize MFC for water treatment beyond reducing only organic contaminants. As with many other advancing technologies, MFC is not without its own set of problem. Tee et al. reported a maximum power density of 55 mW m3 from a tubular up-flow MFC with air cathode [24]. As of now, the power problem remains one of the biggest hurdles to the progress of MFC technology apart from current instability, high internal resistance, high cost, fouling, and microbes limitation [25]. Finding a solution towards higher power output would be a big step in making use of this technology on a bigger scale and overall improve its performance [26]. One way of addressing this issue is to
identify the best quality of wastewater used. Distillery wastewater has been reported to produce higher power output as the Chemical Oxygen Demand (COD) level increases [27]. Study on using digested fresh algal bloom as feed-in MFC has also been reported before with success [28]. Another critical aspect of MFC is the membrane or separator. MFC relies heavily on the ionic exchange between anode and cathode because it will be the primary driving force of electron through the outer circuit. A separator is also essential interphase for the flow of ions induced by electroosmotic effect from MFC electrical field [29]. The flow of ions allows for recovery and recycling of ion (for extraction and separation purposes), which is dependent on power performance [30]. The ability for ion recovery and recycle will increase in elemental extraction and the advantage of directly monitoring effluent quality done when the system efficiency increases [12]. At present, despite the various studies done on separators, the costly conventional membrane is still being used [31,32]. Hernandez-Flores et al. reported that power-to-cost ratio of Nafion™ 117 (0.23 mW US$1) is lower compared to agar-based alternative membranes (0.9e4.4 mW US$1), thereby highlighting the importance of research towards reducing the cost per unit of power produced [32]. For membranes used in MFC, one of the most crucial points that require consideration is that it is susceptible to fouling by the growth of biofilm on the membrane itself. According to Baranitharan et al., biofilm is an aggregation of microorganism that forms a layer of film on surfaces aided by other polymeric substances (EPS) [33]. The film formed when EPS binds the microorganism to surfaces, allowing the proliferation of the population. The formation process involves three stages, which are initial attachment, maturation, and detachment, sequentially one after another. It was reported that dispersal of detachment or slough-off of biofilm population, either as a single cell or as an aggregate, has the potential to become the foundation for new biofilm layer on other clean biofilm-free surfaces thus enlarging the biofilm layer as the process repeated [34]. In terms of MFC, the ever-growing biofilm layer poses a threat mainly to the membrane employed in an MFC system, which in result causes a reduction in performance of MFC. Recently, Daud et al. [35] reported that not only cation exchange membrane (CEM) but ceramic membrane are also affected by the buildup of biofilm on the membrane surface, also referred to as biofouling. The biofouling of the membranes causes a reduction of power production from a maximum of 1800 mW m2 to 1400 mW m2 for ceramic membrane and from 1200 mW m2 to 600 mW m2 for CEM after six months of operation. Reduction of power density is more severe when CEM is used as a membrane, proving that the ceramic membrane is more stable and viable for extended usage compared to CEM. Given the above issues, the ceramic or clay-based separator has been introduced as an alternative. Past study shows that ceramic as separator shows an almost equal performance, in terms of power density to conventional CEM [36]. Ceramic can be utilized because it is porous, thus allowing the ionic exchange to happen freely [37]. Furthermore, ceramic is cheap, durable, and high integrity material which could withstand fouling, therefore reducing the cost needed to service the system [38]. Many research on the ceramic membrane conducted using tubular design
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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membranes has been reported [39e46]. These reports relate the interest in tubular design due to it is promising for scaling up, high performance both electrochemically and for wastewater treatment [47,48]. Optimization of spacing between anode and membrane is possible due to the constant cross-section provided by the tubular design when scaling up [48,49]. Greenman & Ieropoulos described MFCs as a multicellular life form whereby Allometric law and Kleiber’s law were applied to understand the effect of scaling up on MFC from a biological organism viewpoint [50,51]. The implication of applying both laws in scaling-up MFC suggests that a better performing MFC would preferably be in a smaller operating volume, however, if the increment applied onto the stacking size, a modest increase in power output is possible. This idea was supported by Kim et al., who reported that total power density of individual tubular MFC is 0.3 W m3 higher compared to a tubular MFC stack consisting an equal number of individual units at low organic loading rate [49]. As interest in tubular design skyrocketed, new ideas in conjunction with a tubular design, MFC has popped out such as usage of tubular MFC inside the human body in the medical field [52] and rotating carbon brush anode in tubular MFC [53]. A new method of utilizing MFC to synthesize hydroxide coagulant in catholyte has been demonstrated by Gajda et al., which utilizes cave-shaped terracotta membrane as part of the experimental design [54]. Their method opens up another possibility for MFC as electro-synthesizer of coagulating factor. This review aims to analyze the usage of a tubular ceramic, specifically as a separator in MFC. Both ceramic and tubular design has their advantages, and combining both of them could answer the demand for better performing, longlasting, and cheap MFC. This review, comparatively against other reviews, is focused on tubular designed ceramic membrane rather than the usage of other design of ceramic membrane used in MFC.
Ceramic membrane for microbial fuel cell A bibliometric study done by Khudzuri et al. showed that the keyword ‘ceramic membrane’ has appeared up to five times when searched, with the average publication year of 2016, indicating the recent trend of using ceramic in MFC [55]. Ceramic has been reviewed [41,56,57] and reported [41] to be a promising membrane material for MFC owing to their proton conductivity. Pasternak et al. reported that the physical and chemical attributes of the ceramic membrane are directly related to MFC performance: the highest porosity of 1.8e2% and silica concentration of 64.54% pyrophyllite, generated highest power output of 6.16 mW m2 within one week of operation [41]. Daud et al. reported that MFC using zirconia ceramic filter as membrane maintained a high performance for 12 months, with a maximum power density of 1600 mW m2 while CEM MFC only lasts up to 8 months at a maximum power density of 600 mW m2 before declining [35]. In a field study by Ieropoulos et al., ceramic separator MFC was for the first time used in a public setting where the system powered by pee from urinals [39]. The first set of trial with 288 MFC units operated without supercapacitors, yielded a
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maximum power output of 160 mW and 400 mW while the second set with 432 MFC units, yielded a maximum power of 400 mW and 800 mW. The supercapacitor usually added to an MFC set up as a way to ensure a continuous supply of electric energy [58]. Recently, a study on power generation using ceramic MFC in supercapacitive mode recorded power production from a solution of 40.1 mS cm1 increased up to 170% [58]. The study showed harsh condition: high salinity and acidity, such as reported by Jannelli et al., had caused the Nafion membrane used to foul and deteriorated [59]. Therefore condition such as reported before by Ieropolus et al., whereby high salinity due to the usage of human urine, will not be profitable if Nafion became the membrane [39] whereas ceramic membrane can easily withstand the mentioned condition. Another study paring a ceramic MFC with alternative cathode binder of mixed polytetrafluoroethylene and chitosan to treat human urine resulted in the power production of 510 mW which is 60.3% of maximum power produced by when using polytetrafluoroethylene [60]. Application of clayware MFC was successful in reducing COD and nitrite up to 79% and 77% respectively in cow’s urine with maximum power production of 5.23 W m3 [61]. Combination of terracotta based MFC with algae as primary biomass fed to the anode [62] gives the possibility of controlling algal bloom through MFC technology.
Characteristic of a ceramic membrane According to Padaki et al., ceramic separator employs an absorption process [63]. Ceramic, as a separator, relies on its porosity to allow ionic exchange between anode and cathode compartments unlike other conventional membranes, which utilizes ion selection. Besides, the porous material shows better stability compared to CEM for short term operation and almost equal performance in terms of power density in longterm operation [36]. Porous terracotta has been shown to not restrict movement between the surface of the cathode and anode chamber [64] as a result of an electrical field generated by MFC [44]. Other than that, the porous character of the ceramic membrane may allow oxygen to crossover. Modification on ceramic membrane has been reported, such as layering the membrane with chitosan/montmorillonite nanocomposite to reduce oxygen crossover [65]. Their study shows that the modified ceramic membrane gave increased in maximum power and current density twice higher than a non-modified ceramic membrane, which suggests for more study in this area. Apart from porosity, Ortiz-martı´nez et al. briefly mentioned that the thickness of ceramic would also play a role in cell performance [66]. The advantages of the ceramic membrane are the high degree of protonic and ionic exchange formed by the narrow and well-defined pore size distribution, the high thermal stability in harsh environment and, cleaning with harsh material such as activated carbon [67] will not affect the membrane, unlike conventional membrane which is non-reusable when fouled. Advance growth of biofouling on the membrane can cause the cake layer to form, which may lower the performance of the membrane. The formed biofouling cake reduces the pore size and proton conductivity of the membrane over time [63,68]. Membrane fouling caused by deposition of foulant present in
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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wastewater on the membrane or porous matrix. Fouling itself is a significant concern for reducing membrane life-time and increasing capital as well as operational cost due to maintenance or in a worst-case scenario, replacement [57]. According to Winfield et al., ceramic membrane provides a bacteria friendly environment while improving the energy harvesting of MFC [38]. Depending on the design and positioning of the ceramic membrane, chemical scaling and biofouling may occur when the cathode is exposed to air [69], unlike when the cathode is concealed as exhibited by Ieropolous et al. [39]. Tremouli et al. reported the formation of struvite in air-cathode ceramic stack fuel cell fed with urine [70]. The struvite thought to block the ionic exchange, contributing to the low performance of their system. This occurrence proposes that dehydration due to access to open-air plays a role in chemical scaling. Pasternak et al. reported an increased in internal resistance after long term operation of ceramic membrane MFC with urine as feed [41]. They claimed the incident probably resulted from biofilm thickening over time, fouling of the pores and formation of uric salts, which may have caused the limited diffusion of nutrients. A study by Rago et al. found that terracotta membrane impedes the growth of biocathode in air-cathode MFC. Their bovine sewage cultured MFC systems recorded power up to 283 mW m2 when without terracotta membrane and 33 mW m2 when with the terracotta membrane. Similar inferiority in power observed when they used swine sewage cultured MFC systems: power obtained 333 mW m2 when without terracotta membrane and 67 mW m2 when with terracotta membrane [71]. In this study, the vast disparity of power produced is thought to be due to terracotta having microscopic pores of around 10e100 nm, causing the biocathodes not to form consistently and thus bringing down the overall performance of the system. This information suggested that not all ceramic-based membrane is suitable for MFC system, and thus depending on the aim, a different type of ceramic membrane needs to be used. Ceramic membrane MFC has also been reported to be able to produce alkaline catholyte, which increases in pH, the more power the system can produce [45]. It has been pointed out that to increase the power production of an MFC, lowering the inherent internal resistance presented by the membrane is a must [72]. One of the best ways to analyze the internal resistance is by using Electro Impedance Spectroscopy (EIS) which allows us to compute the loss of power caused by the internal interaction of MFC system [73]. Recently, several studies have started to include the impedance when comparing the performance between ceramic membranes with other types of membrane or separator [65,74e76]. According to Zhao et al., EIS allows to analyze the conductivity of the membrane, electrolyte, and electrode and also the various causes of impedance such as charge transfer, biofilm capacitance and crossover of organic substrate and oxygen [73]. It was reported that the biofilmic capacitance is due to the ability of cytochrome to store electron, which causes the withholding of electron catalyzed in anode from transferring to the anodic current collector and thus lessens the current and ultimately the performance of MFC itself [77]. Crossover and mixing of substrate and oxygen will cause the performance to dwindle, and thus a method to prevent crossover is necessary such as by adding a layer of chitosan/montmorillonite nanocomposite as a barrier to prevent crossover of oxygen [65]. Unlike Nafion, ceramic membrane relies on its pore for ions
to pass through and therefore, consideration towards water uptake by the membrane is critical. A report mentioned that ceramic with less silicone can absorb more water and has its power output increased by 64% compared to non-modified ceramic [74]. This high uptake of water leads to high ionic conductivity, eventually creating more uptake of ion through ceramic and less build-up in ohmic resistance. This condition gives a boost to MFC performance.
Tubular ceramic designed as separator In MFC, separators are usually designed either as a tubular structure or in the form of a planar surface. Tubular, or also known as caves or cylinder, is the more popular choice due to the structure itself is readily available in store, such as in the form of ceramic pot [78]. According to Janicek et al., tubular design limits dead liquid space compared to planar designs [48], therefore allowing an amply mixed and continuous flow of liquid in the system. The aspect of the tubular design is essential because the flow of mass at the electrode must not be limited. Previously, Jana et al. demonstrated that earthenware cylinder based up-flow MFC yielded a power density up to 0.24 W m2, an increment by 46% when compared to commercial Proton Exchange Membrane (PEM) [79]. Combination of power production and waste treatment using the tubular ceramic showed by Ghadge & Ghangrekar achieved up to 0.74 W m3 with 78% COD removal from a 26 L air-cathode MFC made of clayware cylinder [69]. As has been reported, tubular ceramic membrane demonstrated the capacity to produce high power production while allowing high COD removal (Table 1). For a real-world application, Ieropoulos et al. reported the first-ever attempt on charging a commercial mobile phone using a tubular ceramic-based membrane-less MFC [80]. They arranged 12 cylindrical MFCs in cascades of three units thus forming four stacks, with urine as feed. Their MFC arrangement produced a higher maximum voltage of around 3.7 V (2.2 mW), which is 10% above the cut off voltage for a mobile phone. Gajda et al. reported that their terracotta tubular MFC generated enough energy to power up an LED continuously over a week [29]. Gajda et al. also demonstrated that the terracotta cave MFC could charge mobile phone [44]. Their study shows the possibility of carbon capture and storage with the accumulation of alkaline catholyte. However, the effect of the electro-osmotic drag, which is responsible for the water extraction from the anolyte through the membrane requires further understanding. Another study on a urine-fed-closeended terracotta cave MFC by Ieropoulos et al. [39], reported maximum power density of 595.24 mW m3 (connected to capacitor bank then to LED light modules) and 992.06 mW m3 (connected directly to the LED light modules) while removing COD up to 95%. Their study showed the MFC performance dependency on the hydraulic retention time, which requires modification to cater for prolonged treatment time.
Tubular ceramic optimization as separator One of the main concerns of using ceramic membrane is the crossover of oxygen into the anode chamber, which may cause a detrimental effect to MFC. The tubular designed
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
Design
Inoculum
not mentioned
Na1
Na1
Activated sludge
Terracotta
880 mW
Na1
mixed anaerobic sludge collected from the septic tank bottom activated sewage sludge and urine
clayware
5.23 W m3
82 ± 4.1%
Jadhav et al. [61]
terracotta
1st trial: 160 mW, 2nd trial: 400 mW
1st trial: >90%, 2nd trial: 30%
Ieropoulos et al. [39]
50% activated sewage sludge þ50% fresh urine 50:50 activated sewage þ urine
fine fire clay c eramic cylinders
2.1 ± 0.19 mW
Na1
Jimenez et al. [81]
terracotta ceramic cylinders
1800 mW (urine þ sea mix)
50% of activated sewage s ludge þ 50% urine
mullite and terracotta
83e107 mW, 20.8e27 W m3 (mullite), 53e78 mW, 13e19.4 W m3 (terracotta)
18.1% (urine þ sea mix) Na1
Merinojimenez et al. [45] Tremouli et al. [70]
2015
Ceramic cylinder
2015
Terracotta caves with cathode surrounding the outer surface Clayware pot inside a plastic container
2017
2017
2018
Cylinder ceramic housed in cylinder acrylic chamber cascading air cathode MFC stack
Na1: Not Applicable.
Citation
89.6 ± 3.2%
Anaerobic mixed sludge
One ended terracotta caves wrapped with carbon veil cathode on the outside surface Cylinder ceramic housed in cylinder acrylic chamber.
COD removal
51.65 mW
Baked clayware pot
2016
Highest Power
Red soil and Black soil Earthenware
2014
2016
Material
Ghadge et al. [42] Santoro et al. [43] Gajda et al. [44]
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Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
Table 1 e Some of the reported studies using tubular ceramic membrane from 2014 to 2018. Year
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The amount of porosity in the ceramic also contributes to the performance of ceramic as a separator in MFC. Fig. 1 on maximum power drop between forward sweep and reverse polarization sweep shows a decline of maximum power almost four times for CEM when compared to the other two types of porous membranes: ceramic and BioBag [36]. In another study, Daud et al. reported that the performance of ceramic membrane MFC operated in batch mode with porosity of 13.80% (2300 ± 45 mW m2) performed the highest compared to the 11.00% (1900 ± 20 mW m2) and 11.05% (2000 ± 50 mW m2) [40]. Their study shows that the increment in the porosity of ceramic membrane will affect the performance of MFC positively.
Fig. 1 e Shows the difference in terms of maximum power production of between CEM, Ceramic, and BioBag [36].
ceramic membrane will have the anode surrounded by the porous membrane; thus, the effect of oxygen crossover might be prominent. Bakar et al. reported that prolonged exposure of culture to 7.5 ppm dissolved oxygen will cause the performance of CEM MFC to deteriorate however upon introduction to anaerobic condition, the average performance of MFC is easily attainable [82]. Winfield et al. concurred by proving that the addition or introduction of normal atmospheric condition air into the system with the cylindrical ceramic membrane will cause no significant effect towards performance and health of biofilm as opposed to reported previously using CEM [83]. Introduction of atmospheric air that comes from the conventional water treatment process into the anodic chamber will cause the power production to drop by 1%, and upon stopping, the system will resume back to its normal capacity. The reported study shows the non-detrimental effect of air present in the closed-circuit system of a tubular design ceramic membrane in MFC. Apart from the air contact with the anode, the ceramic raw materials equally affecting the performance of MFC. For instance, the highest power density obtained is from pyrophyllite followed by earthenware, mullite, and alumina with a power value of 6.93 W m3, 6.85 W m3, 4.98 W m3 and 2.60 W m3 respectively [41]. The characteristic of ceramic can be improved as reported by Pasternak et al. They found that the highest power performance, which came from earthenware and pyrophyllite, showed a higher percentage of SiO2: 67.92% for earthenware, 64.54% for pyrophyllite, 36.65% for mullite and 2.02% for alumina [41]. Higher power performance observed with a higher percentage of SiO2 may suggest that a higher percentage of SiO2 in clay-based membrane could enhance MFC performance. The connection between the SiO2 and MFC performance, however, is still unknown. Table 2 e Cost of conventional PEM per m2. Types of conventional PEM Nafion™ Membrane N115 Nafion™ Membrane N117 Nafion™ Membrane N1110 CMI-7000S Membrane
Price £ m2
Source
1429.78 1733.11 2816.22 79.17
[84] [84] [84] [41]
Cost comparison between commercial membranes and ceramic as separator Although some membrane might boast superior characteristics above others, its high cost will induce impracticality for any foreseeable usage. Some research in scaling-up used expensive separator materials, even though other choices are cheaper [43]. One of the significant drawbacks of using PEM is high cost, as seen in Table 2, apart from fouling, high cation to proton transfer rate ratio and substrate and oxygen crossover [36,51,73,74]. The Nafion membrane then is seen as not suitable for scaling up purposes due to its expensive cost, and the membrane itself quickly deteriorates after a short period operation [55]. Because of the high material cost, effort to commercialize the application of MFCs has been continuously hindered [75]. According to Ieropoulos et al., one unit of MFC, inclusive of electrical component, made using available clay cylinder at maximum would cost around £ 1. This price is low, considering the clay cylinder could last a very long lifetime [80]. They summarised that MFC from ceramic material shows an advantage in the essential cost of running MFC. In a report by Pasternak et al., earthenware is a cheap (£ 4.14 m-2) available material for membrane (almost 19 times cheaper than CMI7000S membrane) and was able to generate the highest absolute power production of 78.1 mW compared to other ceramic tested (Mullite: 56.7 mW, Pyrophyllite: 44.4 mW, Alumina:29.7 mW) [41]. On the other hand, pyrophyllite as the most expensive material (£ 387.96 m-2) used in their study was more than three -times cheaper compared to the Nafion™ membrane, N115. From their cost and performance analysis, Pasternak et al. suggested the possibility for success when ceramic used in MFC in a large setup. However, further study into the durability of their performance and properties in long-term operation is much needed [41]. A previous report commented that a tubular up-flow fuel cell with CMI7000 membrane was not practical due to lower power production (maximum power density of 90 ± 11 W m3) and higher energy cost at more than 1000 times (capacity cost of £ 692.52 W-1) comparatively to other renewable energy sources [85].
Discussion and future prospects Over the years, interest in MFC propels the advance of MFC technology towards different applications, thus instigating
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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improvisation for improvement. Usage of the conventional membrane creates a high operational cost compared to ceramic, which is much cheaper due to the lower cost of ceramic preparation [41] and the inherent low cost of the material itself [80]. Ceramic is chosen as a suitable material for membrane because it is porous therefore to some extent, can reduce the restriction on the ionic exchange through a conventional membrane which has been discussed by Rahimnejad & Adhami [86]. On the other hand, ceramic is structurally durable and able to withstand changes induced by microbiota present in the system. This strength is invisible on conventional membranes and thus assist in solving issues of the conventional membrane such as being resistant to fouling. The interrelation between ceramic-based materials with the performance of MFC as of now has not been verified. The non-verification is due to not all ceramic able to produce high performance in terms of power output and COD removal when becoming separator in MFC [41]. Tubular ceramic has been reported much more frequently compared to planar ceramic. Understandably, the tubular design allows for a more robust flow of biomass [48] as well as optimal membrane electrode assembly, thus causes a surge of increase in tubular-designed MFC study. The successful attempt of field study [39] and practical demonstration [29] using a tubular ceramic membrane, has further strengthened the trend of using tubular ceramic. There is a need for a more aggressive approach towards assessing tubular ceramic MFC in a domestic setting. This view is based on MFC capability for water treatment on its own without the need for other complicated systems. Furthermore, the MFC technology has been evaluated as an economical alternative for activated sludge system [87] and therefore presented the possibility of success when implemented in a more basic domestic setting.
Conclusion Ceramic is not only cheap but also shows comparable performance with conventional membranes. It is structurally ionic and protonic conductive and therefore employed as a suitable separator in MFC. High power output and COD removal have been associated with tubular ceramic separator. Tubular ceramic has been proven to be useable for practical usage through field testing [39] and powering up the small electrical appliance, apart from being easily accessible compared to conventional membranes [80]. The tubular design is favorable due to having unobstructed plug-flow, which enables for better mass transport to the electrodes, enhancing the performance of MFC. However, the lack of research in planar design ceramic MFC causes difficulty for design comparison.
Acknowledgments The authors gratefully acknowledge the support given for this work by GGPM-2017-012 from Universiti Kebangsaan Malaysia (UKM).
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Tubular ceramic performance as separator for microbial fuel cell: A review M.F. Hil Me, M.H. Abu Bakar* Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi, Selangor, 43600, Malaysia
highlights Performance of ceramic membrane is on par or better than conventional membrane. Ceramic membrane is ‘hardy’ against fouling and chemical scaling. Cost per energy produced using ceramic membrane is more profitable than conventional membrane. Tubular ceramic membrane is the most reported type of design. Previous studies use multiple tubular design instead of single big volume cell.
article info
abstract
Article history:
The depletion of unsustainable conventional energy sources and global warming issues
Received 15 May 2019
create world demand for green energy sources. The microbial fuel cell (MFC) technology
Received in revised form
with the capability to convert environmental waste to energy can be improved with cheap
25 July 2019
ceramic material. The ceramic is structurally porous, thus allow a direct exchange of
Accepted 15 August 2019
cation. The ceramic material also enhances stability thermally and chemically, non-ion
Available online xxx
selective characteristic, high mechanical strength, and easily washable. Commercially produced ceramic structures have been proven to reduce Chemical Oxygen Demand up to
Keywords:
92% and allow high power output. It is also comparatively durable in the long-term oper-
Microbial fuel cell
ation of MFC, compared to the commercially available membrane. The novelty of using
Ceramic
tubular design is the efficient use of space, which leads to the possibility of scaling up. As a
Separator
conclusion, a combination of both ceramic material and tubular design could be an
Design
excellent alternative separator for MFC.
Performance
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cost analysis
Introduction Traditional energy sources such as fossil fuels are used excessively, which in result led to its depletion. Fossil fuels must undergo combustion to produce energy. The burning, however, will produce carbon dioxide and other harmful pollutants as a side product, and therefore, this whole process
of creating energy is not clean and green. Because of the above issues, there is a need to search for a new alternative source of energy generation, which is both cheap and eco-friendly, such as fuel cell [1,2]. Microbial fuel cell (MFC) is a versatile renewable, eco-friendly green technology which can convert waste into energy through bio-electrochemical means and is practicable for wastewater treatment [3e5]. The technique combines biological catalytic activity with abiotic
* Corresponding author. E-mail address: [email protected] (M.H.A. Bakar). https://doi.org/10.1016/j.ijhydene.2019.08.115 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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electrochemical reaction and physics [6,7]. Dubbed renewable because this technology incorporates waste material, which is more favorable in terms of cost [6], as its fuel. The MFC is also eco-friendly because this technology can treat waste while producing enough energy to compensate its processes resulting in at least a zero net energy system. Non-utilization of expensive precious metals further reduced the cost [8]. Out of all technologies available, MFC can transform waste product as energy independent via microbes [9]. Therefore MFC can become biosensors to monitor certain aspects of the environment such as biochemical oxygen demand [10] and water quality monitoring [11e13]. The technology is flexible, can either stand on its own or merged with other technology [14] in the wastewater treatment and bio-energy production sectors [15]. MFC technology is also considered to be capable of processing municipal solid waste in the future, especially for fabrication of biohydrogen [16]. A report on the configuration done on MFC shows transformation to a microbial electrolysis cell (MEC) just by adding an external voltage source. The MEC is specially designed for biohydrogen production [17]. Advancement in MEC technology allows it to be merged with waste biorefinery to produce hydrogen gas as its additional product [18]. Chaturvedi & Verma described that an MFC is divided into two basic designs: single-chambered, both electrodes are in the same working sphere, and double-chambered, anode and cathode are in the separate working sphere, while further understanding and prediction are through modeling [19,20]. Microorganisms are inoculated and enriched in the anodic chamber MFC to yield electrochemically active bacteria (EAB) while organic waste or other carbon source used as fuel. EABs are the critical factor in this system as the process begins with the production of electrons by the EABs oxidizing substrates in the anodic chamber. The electrons and protons from biocatalytic fuel oxidation will traverse to cathode from anode via outer circuit and membrane respectively producing an electrical current. Therefore EAB functions also as a bridge for an electron to pass through to the current collector [21]. Studies concerning types of wastewater as fuel for the system has been reviewed [19]. Xafenias et al. reported that MFC could reduce heavy metal such as hexavalent chromium in wastewater from 10 mg L1 to 0.3 ± 0.3 mg L1 (97% reduction) in the first 45 h of operation at pH of 8 [22]. While in another report by Gangadaran & Nambi, 100 mg L1 hexavalent chromium was removed entirely (recovery up to 99.87% via precipitation on the cathode) in the first 48 h of operation while simultaneously producing a maximum power density of 767.01 mW m2 (2.08 mA m2) [23]. Both studies show that it is feasible to utilize MFC for water treatment beyond reducing only organic contaminants. As with many other advancing technologies, MFC is not without its own set of problem. Tee et al. reported a maximum power density of 55 mW m3 from a tubular up-flow MFC with air cathode [24]. As of now, the power problem remains one of the biggest hurdles to the progress of MFC technology apart from current instability, high internal resistance, high cost, fouling, and microbes limitation [25]. Finding a solution towards higher power output would be a big step in making use of this technology on a bigger scale and overall improve its performance [26]. One way of addressing this issue is to
identify the best quality of wastewater used. Distillery wastewater has been reported to produce higher power output as the Chemical Oxygen Demand (COD) level increases [27]. Study on using digested fresh algal bloom as feed-in MFC has also been reported before with success [28]. Another critical aspect of MFC is the membrane or separator. MFC relies heavily on the ionic exchange between anode and cathode because it will be the primary driving force of electron through the outer circuit. A separator is also essential interphase for the flow of ions induced by electroosmotic effect from MFC electrical field [29]. The flow of ions allows for recovery and recycling of ion (for extraction and separation purposes), which is dependent on power performance [30]. The ability for ion recovery and recycle will increase in elemental extraction and the advantage of directly monitoring effluent quality done when the system efficiency increases [12]. At present, despite the various studies done on separators, the costly conventional membrane is still being used [31,32]. Hernandez-Flores et al. reported that power-to-cost ratio of Nafion™ 117 (0.23 mW US$1) is lower compared to agar-based alternative membranes (0.9e4.4 mW US$1), thereby highlighting the importance of research towards reducing the cost per unit of power produced [32]. For membranes used in MFC, one of the most crucial points that require consideration is that it is susceptible to fouling by the growth of biofilm on the membrane itself. According to Baranitharan et al., biofilm is an aggregation of microorganism that forms a layer of film on surfaces aided by other polymeric substances (EPS) [33]. The film formed when EPS binds the microorganism to surfaces, allowing the proliferation of the population. The formation process involves three stages, which are initial attachment, maturation, and detachment, sequentially one after another. It was reported that dispersal of detachment or slough-off of biofilm population, either as a single cell or as an aggregate, has the potential to become the foundation for new biofilm layer on other clean biofilm-free surfaces thus enlarging the biofilm layer as the process repeated [34]. In terms of MFC, the ever-growing biofilm layer poses a threat mainly to the membrane employed in an MFC system, which in result causes a reduction in performance of MFC. Recently, Daud et al. [35] reported that not only cation exchange membrane (CEM) but ceramic membrane are also affected by the buildup of biofilm on the membrane surface, also referred to as biofouling. The biofouling of the membranes causes a reduction of power production from a maximum of 1800 mW m2 to 1400 mW m2 for ceramic membrane and from 1200 mW m2 to 600 mW m2 for CEM after six months of operation. Reduction of power density is more severe when CEM is used as a membrane, proving that the ceramic membrane is more stable and viable for extended usage compared to CEM. Given the above issues, the ceramic or clay-based separator has been introduced as an alternative. Past study shows that ceramic as separator shows an almost equal performance, in terms of power density to conventional CEM [36]. Ceramic can be utilized because it is porous, thus allowing the ionic exchange to happen freely [37]. Furthermore, ceramic is cheap, durable, and high integrity material which could withstand fouling, therefore reducing the cost needed to service the system [38]. Many research on the ceramic membrane conducted using tubular design
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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membranes has been reported [39e46]. These reports relate the interest in tubular design due to it is promising for scaling up, high performance both electrochemically and for wastewater treatment [47,48]. Optimization of spacing between anode and membrane is possible due to the constant cross-section provided by the tubular design when scaling up [48,49]. Greenman & Ieropoulos described MFCs as a multicellular life form whereby Allometric law and Kleiber’s law were applied to understand the effect of scaling up on MFC from a biological organism viewpoint [50,51]. The implication of applying both laws in scaling-up MFC suggests that a better performing MFC would preferably be in a smaller operating volume, however, if the increment applied onto the stacking size, a modest increase in power output is possible. This idea was supported by Kim et al., who reported that total power density of individual tubular MFC is 0.3 W m3 higher compared to a tubular MFC stack consisting an equal number of individual units at low organic loading rate [49]. As interest in tubular design skyrocketed, new ideas in conjunction with a tubular design, MFC has popped out such as usage of tubular MFC inside the human body in the medical field [52] and rotating carbon brush anode in tubular MFC [53]. A new method of utilizing MFC to synthesize hydroxide coagulant in catholyte has been demonstrated by Gajda et al., which utilizes cave-shaped terracotta membrane as part of the experimental design [54]. Their method opens up another possibility for MFC as electro-synthesizer of coagulating factor. This review aims to analyze the usage of a tubular ceramic, specifically as a separator in MFC. Both ceramic and tubular design has their advantages, and combining both of them could answer the demand for better performing, longlasting, and cheap MFC. This review, comparatively against other reviews, is focused on tubular designed ceramic membrane rather than the usage of other design of ceramic membrane used in MFC.
Ceramic membrane for microbial fuel cell A bibliometric study done by Khudzuri et al. showed that the keyword ‘ceramic membrane’ has appeared up to five times when searched, with the average publication year of 2016, indicating the recent trend of using ceramic in MFC [55]. Ceramic has been reviewed [41,56,57] and reported [41] to be a promising membrane material for MFC owing to their proton conductivity. Pasternak et al. reported that the physical and chemical attributes of the ceramic membrane are directly related to MFC performance: the highest porosity of 1.8e2% and silica concentration of 64.54% pyrophyllite, generated highest power output of 6.16 mW m2 within one week of operation [41]. Daud et al. reported that MFC using zirconia ceramic filter as membrane maintained a high performance for 12 months, with a maximum power density of 1600 mW m2 while CEM MFC only lasts up to 8 months at a maximum power density of 600 mW m2 before declining [35]. In a field study by Ieropoulos et al., ceramic separator MFC was for the first time used in a public setting where the system powered by pee from urinals [39]. The first set of trial with 288 MFC units operated without supercapacitors, yielded a
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maximum power output of 160 mW and 400 mW while the second set with 432 MFC units, yielded a maximum power of 400 mW and 800 mW. The supercapacitor usually added to an MFC set up as a way to ensure a continuous supply of electric energy [58]. Recently, a study on power generation using ceramic MFC in supercapacitive mode recorded power production from a solution of 40.1 mS cm1 increased up to 170% [58]. The study showed harsh condition: high salinity and acidity, such as reported by Jannelli et al., had caused the Nafion membrane used to foul and deteriorated [59]. Therefore condition such as reported before by Ieropolus et al., whereby high salinity due to the usage of human urine, will not be profitable if Nafion became the membrane [39] whereas ceramic membrane can easily withstand the mentioned condition. Another study paring a ceramic MFC with alternative cathode binder of mixed polytetrafluoroethylene and chitosan to treat human urine resulted in the power production of 510 mW which is 60.3% of maximum power produced by when using polytetrafluoroethylene [60]. Application of clayware MFC was successful in reducing COD and nitrite up to 79% and 77% respectively in cow’s urine with maximum power production of 5.23 W m3 [61]. Combination of terracotta based MFC with algae as primary biomass fed to the anode [62] gives the possibility of controlling algal bloom through MFC technology.
Characteristic of a ceramic membrane According to Padaki et al., ceramic separator employs an absorption process [63]. Ceramic, as a separator, relies on its porosity to allow ionic exchange between anode and cathode compartments unlike other conventional membranes, which utilizes ion selection. Besides, the porous material shows better stability compared to CEM for short term operation and almost equal performance in terms of power density in longterm operation [36]. Porous terracotta has been shown to not restrict movement between the surface of the cathode and anode chamber [64] as a result of an electrical field generated by MFC [44]. Other than that, the porous character of the ceramic membrane may allow oxygen to crossover. Modification on ceramic membrane has been reported, such as layering the membrane with chitosan/montmorillonite nanocomposite to reduce oxygen crossover [65]. Their study shows that the modified ceramic membrane gave increased in maximum power and current density twice higher than a non-modified ceramic membrane, which suggests for more study in this area. Apart from porosity, Ortiz-martı´nez et al. briefly mentioned that the thickness of ceramic would also play a role in cell performance [66]. The advantages of the ceramic membrane are the high degree of protonic and ionic exchange formed by the narrow and well-defined pore size distribution, the high thermal stability in harsh environment and, cleaning with harsh material such as activated carbon [67] will not affect the membrane, unlike conventional membrane which is non-reusable when fouled. Advance growth of biofouling on the membrane can cause the cake layer to form, which may lower the performance of the membrane. The formed biofouling cake reduces the pore size and proton conductivity of the membrane over time [63,68]. Membrane fouling caused by deposition of foulant present in
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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wastewater on the membrane or porous matrix. Fouling itself is a significant concern for reducing membrane life-time and increasing capital as well as operational cost due to maintenance or in a worst-case scenario, replacement [57]. According to Winfield et al., ceramic membrane provides a bacteria friendly environment while improving the energy harvesting of MFC [38]. Depending on the design and positioning of the ceramic membrane, chemical scaling and biofouling may occur when the cathode is exposed to air [69], unlike when the cathode is concealed as exhibited by Ieropolous et al. [39]. Tremouli et al. reported the formation of struvite in air-cathode ceramic stack fuel cell fed with urine [70]. The struvite thought to block the ionic exchange, contributing to the low performance of their system. This occurrence proposes that dehydration due to access to open-air plays a role in chemical scaling. Pasternak et al. reported an increased in internal resistance after long term operation of ceramic membrane MFC with urine as feed [41]. They claimed the incident probably resulted from biofilm thickening over time, fouling of the pores and formation of uric salts, which may have caused the limited diffusion of nutrients. A study by Rago et al. found that terracotta membrane impedes the growth of biocathode in air-cathode MFC. Their bovine sewage cultured MFC systems recorded power up to 283 mW m2 when without terracotta membrane and 33 mW m2 when with the terracotta membrane. Similar inferiority in power observed when they used swine sewage cultured MFC systems: power obtained 333 mW m2 when without terracotta membrane and 67 mW m2 when with terracotta membrane [71]. In this study, the vast disparity of power produced is thought to be due to terracotta having microscopic pores of around 10e100 nm, causing the biocathodes not to form consistently and thus bringing down the overall performance of the system. This information suggested that not all ceramic-based membrane is suitable for MFC system, and thus depending on the aim, a different type of ceramic membrane needs to be used. Ceramic membrane MFC has also been reported to be able to produce alkaline catholyte, which increases in pH, the more power the system can produce [45]. It has been pointed out that to increase the power production of an MFC, lowering the inherent internal resistance presented by the membrane is a must [72]. One of the best ways to analyze the internal resistance is by using Electro Impedance Spectroscopy (EIS) which allows us to compute the loss of power caused by the internal interaction of MFC system [73]. Recently, several studies have started to include the impedance when comparing the performance between ceramic membranes with other types of membrane or separator [65,74e76]. According to Zhao et al., EIS allows to analyze the conductivity of the membrane, electrolyte, and electrode and also the various causes of impedance such as charge transfer, biofilm capacitance and crossover of organic substrate and oxygen [73]. It was reported that the biofilmic capacitance is due to the ability of cytochrome to store electron, which causes the withholding of electron catalyzed in anode from transferring to the anodic current collector and thus lessens the current and ultimately the performance of MFC itself [77]. Crossover and mixing of substrate and oxygen will cause the performance to dwindle, and thus a method to prevent crossover is necessary such as by adding a layer of chitosan/montmorillonite nanocomposite as a barrier to prevent crossover of oxygen [65]. Unlike Nafion, ceramic membrane relies on its pore for ions
to pass through and therefore, consideration towards water uptake by the membrane is critical. A report mentioned that ceramic with less silicone can absorb more water and has its power output increased by 64% compared to non-modified ceramic [74]. This high uptake of water leads to high ionic conductivity, eventually creating more uptake of ion through ceramic and less build-up in ohmic resistance. This condition gives a boost to MFC performance.
Tubular ceramic designed as separator In MFC, separators are usually designed either as a tubular structure or in the form of a planar surface. Tubular, or also known as caves or cylinder, is the more popular choice due to the structure itself is readily available in store, such as in the form of ceramic pot [78]. According to Janicek et al., tubular design limits dead liquid space compared to planar designs [48], therefore allowing an amply mixed and continuous flow of liquid in the system. The aspect of the tubular design is essential because the flow of mass at the electrode must not be limited. Previously, Jana et al. demonstrated that earthenware cylinder based up-flow MFC yielded a power density up to 0.24 W m2, an increment by 46% when compared to commercial Proton Exchange Membrane (PEM) [79]. Combination of power production and waste treatment using the tubular ceramic showed by Ghadge & Ghangrekar achieved up to 0.74 W m3 with 78% COD removal from a 26 L air-cathode MFC made of clayware cylinder [69]. As has been reported, tubular ceramic membrane demonstrated the capacity to produce high power production while allowing high COD removal (Table 1). For a real-world application, Ieropoulos et al. reported the first-ever attempt on charging a commercial mobile phone using a tubular ceramic-based membrane-less MFC [80]. They arranged 12 cylindrical MFCs in cascades of three units thus forming four stacks, with urine as feed. Their MFC arrangement produced a higher maximum voltage of around 3.7 V (2.2 mW), which is 10% above the cut off voltage for a mobile phone. Gajda et al. reported that their terracotta tubular MFC generated enough energy to power up an LED continuously over a week [29]. Gajda et al. also demonstrated that the terracotta cave MFC could charge mobile phone [44]. Their study shows the possibility of carbon capture and storage with the accumulation of alkaline catholyte. However, the effect of the electro-osmotic drag, which is responsible for the water extraction from the anolyte through the membrane requires further understanding. Another study on a urine-fed-closeended terracotta cave MFC by Ieropoulos et al. [39], reported maximum power density of 595.24 mW m3 (connected to capacitor bank then to LED light modules) and 992.06 mW m3 (connected directly to the LED light modules) while removing COD up to 95%. Their study showed the MFC performance dependency on the hydraulic retention time, which requires modification to cater for prolonged treatment time.
Tubular ceramic optimization as separator One of the main concerns of using ceramic membrane is the crossover of oxygen into the anode chamber, which may cause a detrimental effect to MFC. The tubular designed
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
Design
Inoculum
not mentioned
Na1
Na1
Activated sludge
Terracotta
880 mW
Na1
mixed anaerobic sludge collected from the septic tank bottom activated sewage sludge and urine
clayware
5.23 W m3
82 ± 4.1%
Jadhav et al. [61]
terracotta
1st trial: 160 mW, 2nd trial: 400 mW
1st trial: >90%, 2nd trial: 30%
Ieropoulos et al. [39]
50% activated sewage sludge þ50% fresh urine 50:50 activated sewage þ urine
fine fire clay c eramic cylinders
2.1 ± 0.19 mW
Na1
Jimenez et al. [81]
terracotta ceramic cylinders
1800 mW (urine þ sea mix)
50% of activated sewage s ludge þ 50% urine
mullite and terracotta
83e107 mW, 20.8e27 W m3 (mullite), 53e78 mW, 13e19.4 W m3 (terracotta)
18.1% (urine þ sea mix) Na1
Merinojimenez et al. [45] Tremouli et al. [70]
2015
Ceramic cylinder
2015
Terracotta caves with cathode surrounding the outer surface Clayware pot inside a plastic container
2017
2017
2018
Cylinder ceramic housed in cylinder acrylic chamber cascading air cathode MFC stack
Na1: Not Applicable.
Citation
89.6 ± 3.2%
Anaerobic mixed sludge
One ended terracotta caves wrapped with carbon veil cathode on the outside surface Cylinder ceramic housed in cylinder acrylic chamber.
COD removal
51.65 mW
Baked clayware pot
2016
Highest Power
Red soil and Black soil Earthenware
2014
2016
Material
Ghadge et al. [42] Santoro et al. [43] Gajda et al. [44]
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Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
Table 1 e Some of the reported studies using tubular ceramic membrane from 2014 to 2018. Year
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The amount of porosity in the ceramic also contributes to the performance of ceramic as a separator in MFC. Fig. 1 on maximum power drop between forward sweep and reverse polarization sweep shows a decline of maximum power almost four times for CEM when compared to the other two types of porous membranes: ceramic and BioBag [36]. In another study, Daud et al. reported that the performance of ceramic membrane MFC operated in batch mode with porosity of 13.80% (2300 ± 45 mW m2) performed the highest compared to the 11.00% (1900 ± 20 mW m2) and 11.05% (2000 ± 50 mW m2) [40]. Their study shows that the increment in the porosity of ceramic membrane will affect the performance of MFC positively.
Fig. 1 e Shows the difference in terms of maximum power production of between CEM, Ceramic, and BioBag [36].
ceramic membrane will have the anode surrounded by the porous membrane; thus, the effect of oxygen crossover might be prominent. Bakar et al. reported that prolonged exposure of culture to 7.5 ppm dissolved oxygen will cause the performance of CEM MFC to deteriorate however upon introduction to anaerobic condition, the average performance of MFC is easily attainable [82]. Winfield et al. concurred by proving that the addition or introduction of normal atmospheric condition air into the system with the cylindrical ceramic membrane will cause no significant effect towards performance and health of biofilm as opposed to reported previously using CEM [83]. Introduction of atmospheric air that comes from the conventional water treatment process into the anodic chamber will cause the power production to drop by 1%, and upon stopping, the system will resume back to its normal capacity. The reported study shows the non-detrimental effect of air present in the closed-circuit system of a tubular design ceramic membrane in MFC. Apart from the air contact with the anode, the ceramic raw materials equally affecting the performance of MFC. For instance, the highest power density obtained is from pyrophyllite followed by earthenware, mullite, and alumina with a power value of 6.93 W m3, 6.85 W m3, 4.98 W m3 and 2.60 W m3 respectively [41]. The characteristic of ceramic can be improved as reported by Pasternak et al. They found that the highest power performance, which came from earthenware and pyrophyllite, showed a higher percentage of SiO2: 67.92% for earthenware, 64.54% for pyrophyllite, 36.65% for mullite and 2.02% for alumina [41]. Higher power performance observed with a higher percentage of SiO2 may suggest that a higher percentage of SiO2 in clay-based membrane could enhance MFC performance. The connection between the SiO2 and MFC performance, however, is still unknown. Table 2 e Cost of conventional PEM per m2. Types of conventional PEM Nafion™ Membrane N115 Nafion™ Membrane N117 Nafion™ Membrane N1110 CMI-7000S Membrane
Price £ m2
Source
1429.78 1733.11 2816.22 79.17
[84] [84] [84] [41]
Cost comparison between commercial membranes and ceramic as separator Although some membrane might boast superior characteristics above others, its high cost will induce impracticality for any foreseeable usage. Some research in scaling-up used expensive separator materials, even though other choices are cheaper [43]. One of the significant drawbacks of using PEM is high cost, as seen in Table 2, apart from fouling, high cation to proton transfer rate ratio and substrate and oxygen crossover [36,51,73,74]. The Nafion membrane then is seen as not suitable for scaling up purposes due to its expensive cost, and the membrane itself quickly deteriorates after a short period operation [55]. Because of the high material cost, effort to commercialize the application of MFCs has been continuously hindered [75]. According to Ieropoulos et al., one unit of MFC, inclusive of electrical component, made using available clay cylinder at maximum would cost around £ 1. This price is low, considering the clay cylinder could last a very long lifetime [80]. They summarised that MFC from ceramic material shows an advantage in the essential cost of running MFC. In a report by Pasternak et al., earthenware is a cheap (£ 4.14 m-2) available material for membrane (almost 19 times cheaper than CMI7000S membrane) and was able to generate the highest absolute power production of 78.1 mW compared to other ceramic tested (Mullite: 56.7 mW, Pyrophyllite: 44.4 mW, Alumina:29.7 mW) [41]. On the other hand, pyrophyllite as the most expensive material (£ 387.96 m-2) used in their study was more than three -times cheaper compared to the Nafion™ membrane, N115. From their cost and performance analysis, Pasternak et al. suggested the possibility for success when ceramic used in MFC in a large setup. However, further study into the durability of their performance and properties in long-term operation is much needed [41]. A previous report commented that a tubular up-flow fuel cell with CMI7000 membrane was not practical due to lower power production (maximum power density of 90 ± 11 W m3) and higher energy cost at more than 1000 times (capacity cost of £ 692.52 W-1) comparatively to other renewable energy sources [85].
Discussion and future prospects Over the years, interest in MFC propels the advance of MFC technology towards different applications, thus instigating
Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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improvisation for improvement. Usage of the conventional membrane creates a high operational cost compared to ceramic, which is much cheaper due to the lower cost of ceramic preparation [41] and the inherent low cost of the material itself [80]. Ceramic is chosen as a suitable material for membrane because it is porous therefore to some extent, can reduce the restriction on the ionic exchange through a conventional membrane which has been discussed by Rahimnejad & Adhami [86]. On the other hand, ceramic is structurally durable and able to withstand changes induced by microbiota present in the system. This strength is invisible on conventional membranes and thus assist in solving issues of the conventional membrane such as being resistant to fouling. The interrelation between ceramic-based materials with the performance of MFC as of now has not been verified. The non-verification is due to not all ceramic able to produce high performance in terms of power output and COD removal when becoming separator in MFC [41]. Tubular ceramic has been reported much more frequently compared to planar ceramic. Understandably, the tubular design allows for a more robust flow of biomass [48] as well as optimal membrane electrode assembly, thus causes a surge of increase in tubular-designed MFC study. The successful attempt of field study [39] and practical demonstration [29] using a tubular ceramic membrane, has further strengthened the trend of using tubular ceramic. There is a need for a more aggressive approach towards assessing tubular ceramic MFC in a domestic setting. This view is based on MFC capability for water treatment on its own without the need for other complicated systems. Furthermore, the MFC technology has been evaluated as an economical alternative for activated sludge system [87] and therefore presented the possibility of success when implemented in a more basic domestic setting.
Conclusion Ceramic is not only cheap but also shows comparable performance with conventional membranes. It is structurally ionic and protonic conductive and therefore employed as a suitable separator in MFC. High power output and COD removal have been associated with tubular ceramic separator. Tubular ceramic has been proven to be useable for practical usage through field testing [39] and powering up the small electrical appliance, apart from being easily accessible compared to conventional membranes [80]. The tubular design is favorable due to having unobstructed plug-flow, which enables for better mass transport to the electrodes, enhancing the performance of MFC. However, the lack of research in planar design ceramic MFC causes difficulty for design comparison.
Acknowledgments The authors gratefully acknowledge the support given for this work by GGPM-2017-012 from Universiti Kebangsaan Malaysia (UKM).
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Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115
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Please cite this article as: Me MFH, Bakar MHA, Tubular ceramic performance as separator for microbial fuel cell: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.115