Air-Cathodes

C H A P T E R

1.5 Air-Cathodes Xin Wang1, Nan Li2 1

MOE Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin, China; 2School of Environmental Science and Engineering, Tianjin University, Tianjin, China

1.5.1 INTRODUCTION Fuel cell technologies have been successfully introduced into an emerging area called “bioelectrochemical systems” [1], “microbial electrochemical technologies” [2], or “microbial fuel cells (MFCs)” [3] in the past decade. Using this bacterial catalyzed system, it is demonstrated possible to recover energy (electricity [4], hydrogen, and methane [2]) from organic wastes (such as wastewater [5] and solid biomass [6]) or salinity gradient to desalinate water [7], detect contamination [3], produce chemicals [2], and remedy contaminations [8]. With the exception of external energy required systems, all of these spontaneous electron transfer processes are constructed on the basis of potential difference between the anode and the cathode. In order to harvest more energy from wastes, a more positive cathode potential and a more negative anode potential are required when external loading is applied. The anode potential is determined by the redox potential and the ratio of NADþ/NADH produced by exoelectrogenic bacteria [9]. As anode surface areas and exoelectrogens are usually abundant in welldesigned MFCs, the anode potential can be stabilized at about 0.2 V versus a standard hydrogen electrode mainly affected by the temperature, the pH, and bacterial kinetics [2,10]. According to the recent results, the power density of MFCs increased approximately in proportion with the cathode specific area (m2/m3) over a few orders of magnitude [11], indicating that the cathode mainly determines the performance of an MFC. Oxygen in air had been widely used in MFCs as an ideal electron acceptor because it is free, green, and sustainable. However, the high overpotential of oxygen reduction reaction (ORR) and the continuous supplement of oxygen are two principle problems to solve. ORR in electrolyte occurs following two possible pathways, including a complete four electrons reduction to H2O and a two electrons reduction to H2O2. It was found that the neutral pH, low buffer capacities, and low ionic concentrations strongly restrict the performance of

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TABLE 1.5.1

Possible Oxygen Reduction Reactions (ORRs) at the Cathode of Microbial Fuel Cells (MFCs)

ORRs O2 þ 4Hþ þ 4e / 2H2 O þ



O2 þ 2H þ 2e / H2 O2 þ



H2 O2 þ 2H þ 2e / 2H2 O 



O2 þ 2H2 O þ 4e / 4OH

Thermodynamic Electrode Potential at Standard Conditions (V)

Thermodynamic Electrode Potential at MFC Conditions (V)

1.229

0.805

0.70

0.328

1.76

1.375

0.401

0.805

ORR cathodes [12]. As neutral pH and room temperature are required for the growth of anodic bacteria, thermodynamic redox potentials of ORRs need to be normalized to 25 C and pH of 7 (Table 1.5.1). Comparing these potentials to those at standard condition, they 4F 0 are obviously affected by the pH according to E0 ¼ E0 þ RT lnðPO2 Þ 2:3RT F pH, where E is 0 the electrode potential at the practical condition, E is the cathode potential at the standard condition, F is the Faraday constant, R is the gas constant, T is the temperature in Kelvin, PO2 is the partial pressure of oxygen in air. However, visible potential drops are usually observed on real cathode potentials of MFCs compared with those thermodynamic values listed in Table 1.5.1, which is mainly due to the high activation energy barrier of ORR (also described as activation overpotential). Therefore, the catalyst is needed to overcome this activation barrier. In some early studies, oxygen was supplied by air pumps in the cathode chamber [13,14]. Although oxygen can be easily delivered to the cathode in this system, the energy consumption during aeration (w3 W) is well over the energy recovered from wastes (w1 mW), making it unsustainable. Air-cathode is an improved design by compressing the cathode chamber into a hydrophilic thin layer on the surface of the cathode, so that the oxygen in air can passively diffuse through the cathodic matrix to the ORR site without aeration. The aircathode was initially introduced into MFCs by Park and Zeikus in 2003 to decrease the cost and simplify the configuration by using a window-mounted cathode instead of cathode chamber [15]. This archetype of air-cathode is made by applying a 2-mm thick porcelain septum (100% kaolin) on graphite electrode and baking. Despite the utilization of Mn4þ as the anodic mediator and Fe3þ as the cathodic mediator, the simple design of this novel single-chambered system exhibited short-term excellent power density of 788 mW/m2, demonstrating that the air-cathode is a promising oxygen supplement design for the cathode.

1.5.2 THE STRUCTURE OF AIR-CATHODES As illustrated in context, the archetype of air-cathodes is made by the combination of a conductive matrix (graphite) and a porcelain septum layer (Fig. 1.5.1A). The porcelain septum layer here is marked as the catalyst layer (CL) because this hydrophilic kaolin film

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1.5.2 THE STRUCTURE OF AIR-CATHODES

(A) Catalyst Layer Water Side

Matrix Air Side

(B)

(C)

Catalyst Layer Proton Exchange Membrane

Catalyst Layer

Water Side

Water Side

Air Side

Matrix

Matrix

(E)

(D)

Gas Diffusion Layer

Catalyst Layer

(F)

Catalyst Layer

Carbon Base Layer Separator

Water Side

Matrix

Air Side

Air Side

Water Side

Matrix

Gas Diffusion Catalyst Layer Layer Carbon Base Layer Air Side

Gas Diffusion Layer

Water Side

Air Side

Matrix

FIGURE 1.5.1 The development of air-cathode structures, with a sketch where the matrix, the catalyst layer, the gas diffusion layers, the carbon base layer, and the separator are. Air cathodes include the original design using porcelain septum (A), the membrane electrode assembly (B), the air-cathode without (C) and with (D) the diffusion layer, the air-cathode with separators (E) and the air-cathode integrating catalyst layer and diffusion layer into the matrix (F).

allows water diffusion toward the graphite matrix to facilitate proton hydrate supplement. In order to avoid the water leakage and increase ORR performance without mediator addition, this structure is substituted by a more reasonable design of a porous conductive matrix and a CL, with a proton exchange membrane (PEM) hot pressed (140  C at 1780 kPa for 3 min) on the surface of CL (Fig. 1.5.1B) [4]. Protons can easily transfer across the hydrophilic canals formed by sulfonated isopoly in PEM to meet electrons at the catalyst such as Pt to complete ORR. Although large pores in matrix facilitate oxygen transport, the presence of PEM limits the rate of proton transfer, which has been described as the internal resistance electrochemically [16]. Liu and Logan initially reported that the removal of PEM from CL lead to an 89% increase of the maximum power density from 262  10 mW/m2 (6.6  0.3 W/m3) to 494  21 mW/m2 (12.5  0.5 W/m3) (Fig. 1.5.1C). However, the exposure of the porous matrix directly to the air results in excess oxygen diffusion to this system and even a potential risk of water leakage, which had been demonstrated by the substantial decrease of coulombic efficiency (CE) from 40%e55% to 9%e12% [16]. Different from chemical fuel cells, excess oxygen through the porous matrix and CL in MFCs incurs not only a decrease in CE but also an undesirable aerobic growth of bacteria (most of them are believed less related with electricity generation), and therefore the proton diffusion is limited by the coverage of thick biofilm on the cathode [17]. Reversely, if the oxygen is blocked by additional layers, covering either the CL or the matrix, the internal resistance is simultaneously increased due to the diffusion resistance. Thus, the balance of oxygen diffusion by an optimized structure is one of the most critical problems to the performance of an air-cathode. Brushing additional layers to the air-facing side followed by heating had been demonstrated as an effective method to moderate the oxygen overload (Fig. 1.5.1D). Cheng et al. found that the potential of air-cathode reached 117 mV compared with <10 mV associated I. MICROBIAL ELECTROCHEMICAL TECHNOLOGY (MET) - BASICS

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with a 171% increase in CE (from 19% to 32%) when one carbon base layer and optimized four layers of polytetrafluoroethylene (PTFE) were added as gas diffusion layer (GDL) [18]. The measurable water loss can also be prevented. To further reduce the excess oxygen to increase CE, separators, especially those inexpensive noncation selective membranes, were applied on the surface of CL [19]. It can also be a nonconducting spacer to sandwich between the anode and the cathode when the electrode spacing is extremely reduced (called membrane/cloth electrode assembly; MEA/CEA, Fig. 1.5.1E). Polycarbonate, cellulose nitrate, and nylon membranes were initially utilized as separators to minimize the electrode spacing in mini-MFCs [20]. A simpler J-cloth covered on the surface of the air-cathode was reported as an effective barrier for oxygen diffusion to the electrolyte with an obvious high power density of 1010 W/m3 and high CE of 71% based on a tiny reactor volume (2.5 mL) [21]. It had also been shown that power output decreased with the increase of layer number, indicating that the use of separators sacrificed the proton transfer ability to weaken the oxygen invasion. The aerobic biofouling of the air-cathode can only be removed by refreshing the less expensive separator [17]. Recently, a larger MFC (30 mL) with the same CEA structure was constructed using nonwoven fabric as separator and obtained a maximum power density of 4.30 W/m2 at a current density up to 16.4 A/m2 (2.87 kW/m3 at 10.9 kA/m3) [22], showing that the aircathode can be used for large-scale application in the future. However, the complexity of the structure and the use of the Pt catalyst limited the scale-up of this cathode. A novel structure was introduced by direct pressing a mixture of activated carbon (AC) and PTFE to a conductive metal matrix (nickel mesh) at 150 bar [23]. This air-cathode is free of metal catalyst and carbon cloth, with a higher power density of 1220 mW/m2 than 1060 mW/m2 of Pt/C catalyzed brushing air-cathode. However, similar as in Fig. 1.5.1C, this simple structure has a risk of water leakage. It was further improved by individually rolling conductive GDL and CL films to a metal matrix [stainless steel mesh (SSM)], as indicated in Fig. 1.5.1F [24]. Both GDL and CL contain carbon powder with different types and functions. The conductive but low specific surface area of carbon black (not AC) was added into GDL to enhance the conductivity, whereas a porous AC was utilized as the catalyst in CL with a much larger carbon/PTFE ratio than the GDL. The maximum power density of this air-cathode increased from 802 to 1355  26 mW/m2 when the plain AC was adopted and the heating of CL was avoided [25]. As the fabrication of both GDL and CL needs a roll squeezer, the size of this air-cathode can be easily enlarged using a roll squeezer of industrial grade. Besides, the price is approximately 30e60 $/m2, which is much lower than 1400 $/m2 of Nafion/Pt/carbon cloth (Fig. 1.5.1D) [25], indicating that it is very promising for the largescale application in wastewater treatment plants or water desalination plants. Overall, although the configuration of air-cathodes has always varied and improved, the CL is always functioned as the key component because it is the site where ORRs take place. The CL consists of the catalyst and the binder.

1.5.3 OXYGEN REDUCTION REACTION CATALYSTS The catalyst in CL determines the ORR pathway and the cathode performance. In the viewpoint of energy conversion, an excellent catalyst desires a four-electron reduction of oxygen to water as incomplete reduction to hydrogen peroxide decreases the energy efficiency. However, because the alkaline solution of hydrogen peroxide is a valuable industrial product I. MICROBIAL ELECTROCHEMICAL TECHNOLOGY (MET) - BASICS

1.5.3 OXYGEN REDUCTION REACTION CATALYSTS

O2

O2 ads

H2O2 ads

103

H2O

H2O2

FIGURE 1.5.2

Scheme of oxygen reduction reactions taken place with parallel two- and four-electron reaction

pathways.

which is possibly used for bleaching [2], two-electron reduction is also interesting with high additional value despite energy recovery. Very different from catalysts in most chemical fuel cells, the “ideal” ORR catalysts in MFCs should be highly active at neutral pH, biostable (or antibiocontamination), as well as inexpensive.

1.5.3.1 Pt as the Oxygen Reduction Reaction Catalyst So far, Pt is the most common ORR catalyst in MFCs because it is widely used in chemical fuel cells to effectively reduce the overpotential. The ORR mechanism of Pt had been most widely investigated but not well known until now. According to the activated complex theory, the catalysis starts from the adsorption of oxygen to the active center on the surface of Pt (as indicated by the subscript of “ads” in Fig. 1.5.2). The adsorbed oxygen (O2 ads) is directly reduced to water or to the adsorbed hydrogen peroxide (H2O2 ads) as the intermediate. The H2O2 ads can be further reduced to water, desorbed into electrolyte, and/or decomposed to O2 ads. As indicated by rotating ring-disk electrode measurements in previous study, ORR on Pt is a major four-electron reaction in both acid and alkaline conditions [26]. Although the Pt seems like an ideal catalyst for ORR in MFCs, it is not economically viable for a large-scale application for wastewater treatment. The substitution of Pt in MFCs is much easier than in some chemical fuel cells. In the cathode of polymer electrolyte membrane fuel cells (PEMFCs), for example, the acidic and oxidizing environment constrains upon the selection of catalyst material. Thus, noble metal, such as Pt, is hard to be substituted due to their thermodynamical stability at a potential higher than 0.9 V. Different from PEMFCs, the pH of the electrolyte in MFCs is approximate 7, whereas the cathode potential is lower than 0.5 V, therefore the Pt loading can be substantially reduced and more nonnoble metal catalysts can be applied. For example, only a slight decline of cathode potential (20e40 mV) was observed when the Pt loading was reduced from 2 to 0.1 mg/cm2 [27]. Besides, it is reported that the sulfide produced during the metabolism of sulfate by anaerobic bacteria poisons Pt [28], showing that new catalyst resistance to biological pollution are needed for MFCs.

1.5.3.2 Nonnoble Metal Catalysts Transition metals chelating with macrocyclic ligands containing 4 N atoms (N4) are normally used as ORR catalyst of air-cathodes as N4 ligands normally have higher activities than N2O2, N2S2, O4, N2S1, N2S2, and S4 according to previous studies (Fig. 1.5.3) [29]. The

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1.5. AIR-CATHODES

(A)

(B)

N

N

N

N

N

Metal N N

Metal

N N

N N

N

(C)

O

(D)

N N O

N Metal N N

O

N

N N N N Metal O O Metal N N N N N N

O

FIGURE 1.5.3 The schematic diagram for molecular structures of porphyrin (A), phthalocyanine (B), tetramethoxyphenylporphyrin (C) and TAPH (D).

ORR activity depends on the species of metals, macrocyclic ligands, and the size of p electron system. Transition metals including Co, Fe, and Mn and macrocyclic ligands including porphyrin (Fig. 1.5.3A), phthalocyanine (Pc; Fig. 1.5.3B), and their derivatives (such as tetramethoxyphenylporphyrin, TMPP, Fig. 1.5.3C) had been applied on air-cathodes by painting a mixture of the catalyst and Nafion to carbon paper as indicated in Fig. 1.5.1D [30,31]. Febased N4 catalysts always exhibit a desirable four-electron ORR pathway as Pt, whereas Co-based N4 complexes promote ORR through two-electron pathway to H2O2 [32]. Taking metal porphyrin complexes as an example, the ORR electron transfer numbers in aqueous trifluoromethane sulfonic acid (0.1 M) are as follows: 4.0 of Fe complex > 2.49 of Mn complex > 2.20 of Co complex [33]. However, the power outputs using these compounds as catalysts decrease slightly different following the order of FePc (634 mW/m2) > CoTMPP (483 mW/m2) > MnPc (353 mW/m2) when biological metabolic substances exist in

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1.5.3 OXYGEN REDUCTION REACTION CATALYSTS

105

electrolyte at neutral pH [29,31]. The maximum power density obtained using FePc was even higher than that of Pt (593 mW/m2) in MFCs [31,32]. It is further shown that the crossover of substrate and metabolites from the anode compartment to the cathode has a less negative effect on FePc than Pt, indicating that metal macrocyclic ligands have the advantage of antipollution over Pt in MFCs [28]. The utilization of CoTMPP in MFCs only slightly enhances the cathode performance over a current density of 0.6 mA/cm2 but decreases the electrode potential at a lower current density compared with the Pt, probably because the intermediate of H2O2 needs higher current density to be further reduced to water [27]. Although singlenuclear Coebased N4 takes a two-electron pathway of ORR, binuclear Coebased N4 complexes perform a different behavior of four-electron pathway (such as Co2TAPH, TAPH is 6,7,8,9,12,19,20,21,22,25-decahydro-8,8,10,21,21,23,-hexamethyl-5,26:13,18-bis(azo)-dibez(1,2, 6,7,12,13,17,18)oxa-azacyclodocosine, Fig. 1.5.3D), which depends on the distance between both Co atoms [34]. The performance of this catalyst needs to be tested in real MFCs. As these metal complexes are reversibly adsorbed on glassy carbon, it is also demonstrated that the larger specific area of supporting carbon enhances the performance of the catalyst [31,33]. Nano-sized metal oxides are also reported to have ORR activity in MFCs. MnOx is the most widely investigated because it had been utilized in primary or secondary battery and alkaline fuel cells, and it had been proved to have a higher ORR activity than other metal oxides such as Fe2O3. The most exiting results are reported by Liu et al. [35] that the electron transfer number of ORRs by electrochemical deposited MnOx nanorods is 3.5, showing that the four-electron pathway is possibly as follows: MnOx þ Hþ þ e / MnOx1 OH

(1.5.1)

2MnOx1 OH þ O2 / ½ðMnOx1 OHÞOads 2

(1.5.2)

½ðMnOx1 OHÞOads 2 þ e þ Hþ / ðMnOx1 OHÞOads þ H2 O þ MnOx

(1.5.3)

ðMnOx1 OHÞOads þ e þ Hþ / MnOx þ H2 O

(1.5.4)

where proton initially enters the MnOx, followed by the adsorption of oxygen molecules onto two neighboring MnOOH sites. The second electron transfers and completes the reduction of adsorbed oxygen. When the nano-MnOx was applied on a carbon paper as an MEA, the maximum power density was not comparable with that of Pt (773 mW/m3, w46 mW/m2, normalized by the cathode area). Although the maximum power density was further increased to 125 mW/m2 using b-MnO2 (better than a-MnO2 and g-MnO2), it is still 53% lower than that of Pt (268 mW/m2) in the same MFC reactor [36]. MnO2 on the cathode is reported to be biologically reduced and oxidized as the catalyst [37]: MnO2 þ e þ Hþ / MnOOH

(1.5.5)

MnOOH þ e þ Hþ / Mn2þ þ H2 O

(1.5.6)

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1.5. AIR-CATHODES MOB

Mn2þ þ O2 ƒ! MnO2

(1.5.7)

where MOB represents manganese-oxidizing bacteria. MnO2 obtains electrons from the cathode to form Mn2þ, and then the Mn2þ is oxidized by MOB at aerobic conditions. Electrons are finally transferred into oxygen by the intermediation of MnO2 with MOB as the biocatalyst, which provides a possible way to enhance cathode performance of MnOx. However, this has not been confirmed in air-cathode MFCs yet.

1.5.3.3 Carbon-Based Catalysts Carbon materials attract increasing attention as ORR catalyst of air-cathode as they are inexpensive, have no potential of heavy metal leakage, and are easy to be fabricated. On a glassy carbon electrode, the proposed ORR mechanism through a two-electron pathway is [38] O2 / O2ðadsÞ

(1.5.8)

O2ðadsÞ þ e / O2ðadsÞ

(1.5.9)

O2ðadsÞ þ e / O2ðadsÞ 

(1.5.10)

O2ðadsÞ þ H2 O / HO2ðadsÞ þ OH

(1.5.11)

HO2ðadsÞ þ e / HO2ðadsÞ

(1.5.12)

HO2ðadsÞ / HO2

(1.5.13)

O2 / O2ðadsÞ

(1.5.14)

O2ðadsÞ þ e / O2ðadsÞ

(1.5.15)

O2ðadsÞ þ H2 O / O2 þ HO2 þ OH

(1.5.16)

or

to produce hydrogen peroxide as an intermediate. It is then oxidized to hydroxyl as HO2  þ H2 O þ e / OH . However, the ORR is much more complex if the specific area of carbon is significantly increased or the carbon surface is modified. Carbon nanotube (CNT) is an excellent carbon material due to its high specific area and unique electrochemical properties. As the size of bacteria are micron order, the application of nanometer CNTs on the anode cannot provide large enough available areas for the growth of exoelectrogenic bacteria. However, nano-dimensional CNTs are promising as a catalyst for ORR at the cathode.

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107

Three-dimensional electrode network was created by CNTs, resulting in a more than twice the increase of power density from 151 mW/m2 of plain carbon cloth control to 329 mW/m2 [39]. It can be deduced that the enhancement is mainly based on the increase of specific area but probably lack of acceleration of the ORR itself because this power density is much lower than the 1118 mW/m2 obtained from Pt-loaded CNT (1118 and 1071 mW/m2 for CNT-Pt and carbon cloth-Pt). The nitrogen-based modification of CNTs (NCNTs) using pyridine, ethanol, and ferrocene was demonstrated to increase electron transfer number of CNTs over a range from 2.76 to 3.63 with a H2O selectivity of 38.16%e81.82% at 2500 rpm and 0.5 V (Ag/AgCl) in chemical fuel cells [40]. However, the cathode potential in MFCs should not be so negative and therefore the real performance had been tested by Feng et al. that the maximum power density reached 1600  50 mW/m2 compared with 1393  35 mW/m2 of commercialized Pt/C [41]. The ORR electron transfer number of NCNTs is estimated as 3.8 at neutral phosphate buffer solution. Quaternary nitrogen atoms associated with pyridinic-like and pyrrolic-like nitrogen atoms were identified on the CNT, which could contribute one p-electron to p electron system and weaken OeO by bonding oxygen and nitrogen as ORR catalyst. A cycling chargeedischarge test over the potential range of 0.8 to 0.8 V showed that the NCNTs have a better stability than Pt/C catalyst. Similar to CNTs, other carbon-based powders such as carbon black, graphene, and their functionalized catalysts exhibit excellent ORR activities. Compared with metal-based catalysts, they are more biocompatible and applicable without the risk of heavy metal leakage. However, the production and/or preparation of these materials still need substantial labor and are difficult to enlarge the air-cathode for wastewater treatment plants. AC without any modification is initially reported as ORR catalyst of air-cathode in MFCs by pressing a mixture of AC and PTFE onto the top of nickel mesh as described in context (Fig. 1.5.1F) [23]. The most attractive point especially to large-scale application is that the power output of this AC-PTFE air-cathode (ACAC) is the same as or even higher than that of Pt but with a neglectable cost. Using the ACAC with heated CL, the maximum power density reached 802 mW/m2 with an optimized AC/PTFE mass ratio of 6, which was comparable to 766 mW/m2 obtained using Pt/C catalyst and Nafion binder in the same MFC configuration (two-dimensional anode and 4 cm of electrode spacing) [18,24]. This power density was further improved to 1086 mW/m2 due to the ameliorative gas-diffusion condition when the heating of CL was avoided. Based on the analysis with mercury porosimeter, pores with a dominant diameter of 6 mm were believed as the channel for air/oxygen diffusion in both CL and GDL. This air-cathode was demonstrated to work under 3 m of water pressure [42]. The catalytic activity of ACs is recently found to be closely related with their pore structures and three phase interfaces (TPIs) formed by the PTFE fibers and AC particles [25]. Electron transfer numbers of carbon black (XC-72), plain AC, and ultracapacitor AC were 2.1, 2.6, and 3.0, compared with 3.9 of Pt/C. These values are interesting but not so attractive until they were roll-pressed as CLs. As estimated by Tafel plots, these values dramatically increased to 2.2, 3.2, and 3.5 while the value for brushed Nafion-Pt/C reversely decreased to 2.4, indicating that the ORR pathways can be significantly changed by the variation of TPIs around the catalyst. The electron transfer number is preliminarily demonstrated to linearly increase with the pore area of micropores (diameter < 2 nm), indicating that micropores are possibly responsible to the ORR of AC powders in CL [25]. Besides, the uniform

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1.5. AIR-CATHODES

distribution of micropores is also helpful to further increase the power output of MFCs using ACAC, with a maximum power density up to 1355  26 mW/m2. The power density can be easily enhanced to 2503 mW/m2 if the CL was roll-pressed into SSM before GDL. This change made CL invade into SSM, which substantially enhanced the connection of AC catalyst to current collectors. This is very important to decrease charge transfer resistance, and we reported the lowest charge resistance of 1.5 U using this cathode [43]. A simple pretreatment is required to further improve power density of ACAC. We found that a low temperature (85 C) treatment of AC using 3 M KOH can increase power density by 16% [44]. This pretreatment decreased the cathodic ohmic resistance from 20.58 to 19.20 U, providing more micropores for ORR. However, the use of strong acid should be very careful because the residual Hþ adsorbed in AC may incur an unwanted corrosion on current collectors. As mentioned earlier, the main reaction of oxygen reduction at the cathode is hydroxyl production. Therefore, how to remove hydroxyl from pores of CL is another important problem despite cathode structure. As a functional material that is usually used in anion exchange resins, quaternary ammonium had been demonstrated to enhance the performance of ACAC when it was modified onto AC surface. The maximum power density reached 2781  36 mW/m2, which was attributed to the decrease of local pH after quaternary ammonium anchoring [45]. When this cathode was operated in a buffer-free medium, the maximum power densities of MFCs can be increased by 51%, showing that the transport of OH in buffer-free systems such as real wastewaters is more important. In order to maximize cathodic potential, some metal elements were added into AC as cathodic ORR catalysts. For example, a superhigh power density of 4.7 W/m2 at 200 mM PBS (2.6 W/m2 at 50 mM PBS) was reported in an MFC using a FeeNeC cocatalyst modified AC on the air-cathode [46]. However, the longevity of these cathodes should be tested as these modified metals may release to water after long-term operation.

1.5.4 THE BINDER The commonly used expensive sulfonated tetrafluoroethylene (Nafion) binder is a sulfonated tetrafluoroethyleneebased fluoropolymer-copolymer with the ability of adsorbing and holding water in its abundant hydrophilic sulfonic channels (Fig. 1.5.4A). Different from some chemical fuel cells, Nafion is not the “ideal” binder for MFCs. When Nafion is utilized as the binder and submerged in wastewater, polymer-enclosed catalyst particles are therefore flooded, limiting the diffusion of oxygen in air to active catalytic sites as the soluble oxygen concentration in electrolyte (w7.6 mg/L at 30 C) is far below that in the air (w20%). The ammonia in wastewater will bind with the sulfonic group in Nafion and incur a serious contamination. Furthermore, it was demonstrated that in ionic polymers, such as Nafion and poly(phenylsulfone) (Radel), the performance of MFCs decreased with the increase of ion exchange capacities from 0 to 2.54 meq/g, and the maximum power density was obtained using unsulfonated Radel (nonionic binder) [47]. Very recently, Popat et al. showed that the transport of OH from CL to bulk solution was one of the dominant losses, with a potential drop up to >0.3 V at a current density of 10 A/m2 [48]. The concentration of proton on TPIs was nearly zero, so that the real ORR taken place should be the last reaction in Table 1.5.1. The generated OH, not the Hþ, needs appropriate channels to export. However,

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1.5.4 THE BINDER

(A)

F

F

F

C C F

F

(B)

F

C C m

F n

O F F

CF3

C

F

F

CH3

CH3 Si O

F O

F

F

n

C O C C S OH F

(C)

F

C C

F

CH3 Si

F O

CH3 O

Si

CH3

n CH3

CH3

CH3

FIGURE 1.5.4 The schematic diagram of molecular structures of sulfonated tetrafluoroethylene (Nafion) (A), polytetrafluoroethylene (B), and poly(dimethylsiloxane) (C).

Nafion and sulfonated Radel only provide channels for cation transfer. This is the reason why a fresh air-cathode with Nafion binder has a temporary high potential followed by a sharp decrease. Therefore, new inexpensive binders for the CL here should be found or developed with enough channels for gas diffusion and OH transport. Inexpensive nonionic hydrophobic polymers are good choices. PTFE was applied to be a substitute of Nafion as it simply consists of hydrophobic CeF without the ability of ionic selectivity (Fig. 1.5.4B) and is 500 times cheaper than Nafion. When PTFE instead of Nafion was mixed with Pt/C and brushed as the CL, a 31% decline was initially observed in power output [27]. However, the difference in power densities was reduced to 10% due to the relative lower power decay of PTFE (from 360 to 331 mW/m2) than that of Nafion air-cathodes (from 480 to 400 mW/m2) after 35 cycles (31 days). In this CL, the way of binding by brushing and heating brings not enough channels for gas and hydroxyl diffusion toward and out of TPIs. Therefore, the performance was not increased until the PTFE was distributed by AC particles to form a more porous CL. As illustrated by rolling ACAC, PTFE fibers were first well mixed and extended among AC particles in the process of repeated roll-pressing, provided there is enough channels for gas diffusion, and therefore resisted the flooding. On the other hand, the heating played a key role in the performance of ACAC. It was found that the heating (sintering) had reverse effects on porous structures of the CL and the GDL [49]. The pore area in CL was substantially decreased after heating because of the contract of PTFE fibers between AC aggregations (Fig. 1.5.5). However, pores with a dominant diameter of 6 mm were generated for air diffusion by the heating of the GDL as PTFE is the main content of GDL. Therefore, to avoid heating, the CL produces more gas diffusion channels and enhances the oxygen supplement to TPIs. Furthermore, the avoidance of heating increases the hydrophilicity of CL, which also accelerated the transport of hydroxyl out of TPIs and increased the performance of air-cathodes. Poly(dimethylsiloxane) (PDMS) is an alternative hydrophobic binder that consists a flexible SieO backbone with a very low rotation barrier (Fig. 1.5.4C). The substantially low solidification temperature of PDMS (80 C) than PTFE (340 C) makes it more promising as both a binder and water proofing material for air-cathodes in terms of energy saving. Using

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After Heating

Before Heating

GDL

TPIs

TPIs

CL

Carbon Particle

PTFE

Pore

FIGURE 1.5.5 Variation of pore structures in catalyst layer (CL) and gas diffusion layer (GDL) before and after heating. PTFE, polytetrafluoroethylene, TPIs, three phase interfaces. Reprinted with permission from H. Dong, H. Yu, H. Yu, N. Gao, X. Wang, Enhanced performance of activated carbon- polytetrafluoroethylene air-cathode by avoidance of sintering on catalyst layer in microbial fuel cells, J. Power Sources 232 (2013) 132e138. Copyright 2017 Elsevier B. V.

the brushing method to apply Pt on metal mesh matrix as showed in Fig. 1.5.1E, Zhang et al. found that MFCs using PDMS exhibited a comparable performance as those using Nafion after six cycles of operation [50]. Electrochemical impedance spectroscopy analysis showed that the use of PDMS decreased the diffusion resistance probably due to its higher affinity for oxygen than Nafion. However, because of the less mechanical strength of PDMS, we failed to roll-press PDMS and AC as the CLs. Thus, for PDMS, to enhance the mechanical strength and replace the toxic solvent of toluene are two points requiring improvements in the future. A one-step method called “phase inversion” was conducted on cathode fabrication using poly(vinylidene fluoride) (PVDF) [51]. PVDF was dissolved in N,N-dimethylacetamide (DMAc) for 8 h at room temperature and then mixed with AC and carbon black. The mixture was then spread on SSM before immerging into deionized water. This one-step AC cathode has no diffusion layer and can produce a 1470 mW/m2 power density with a cost of w15 $/m2.

1.5.5 BIOFOULING Biofouling is formed due to the aerobic growth of biofilm on the surface of CLs, and the thickness of the biofilm is partly dependent on the amount of excess oxygen diffused through the air-cathode. Biofouling is one of the biggest problems that limit the performance, either

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for membraneless MFCs or MFCs with separators. As shown previously, the removal of the cathodic biofilm increased the cathode potential (at 1000 U of external resistance) by 100% from w50 to w100 mV (vs. Ag/AgCl, þ195 mV vs. standard hydrogen electrode) after 40 cycles (w42 days) [17]. As two sides of a coin, the growth of cathodic biofilm is considered to have both positive and negative effects. Positively, it consumes the excess oxygen to maintain the anaerobic niche for exoelectrogenic bacteria. The thick biofilm functions as a separator to limit oxygen diffusion and increase the CE. Negatively, it limits the diffusion of proton/hydroxyl toward the CL/electrolyte and increases internal resistance, especially for porous ACACs. The average size of bacillus is 0.3e1.0 mm by diameter and 2e5 mm by length. Therefore, bacteria is possible to enter macropores (w6 mm) of the CL but impossible to enter nanosized micropores. With the growth of the biofilm, bacteria living in pores will be dead due to the lack of substrate. These dead bacteria and their extracellular polymer substances (EPSs) clog pores and increase diffusion resistance [52] as macropores are considered to be main channels for the diffusion of gas and hydroxyl. It was reported that the maximum power density of porous ACAC decreased by 40% from 1214  123 to 734  18 mW/m2 after 1 year of operation [52]. This value can be partly recovered by 12%e822  29 mW/m2 when the biofilm was removed compared with the full recovery of power by refreshing the ACAC, indicating that the biofouling had an irreversible degradation on the performance of ACACs. As shown in Fig. 1.5.6, the pink part (outside pores) of the biofilm is removable and the gray part (in pores) is hardly removable. By using separators, although the distance between the anode and cathode is remarkably reduced to increase the power output, the biofouling cannot be eliminated because the cathodic biofilm moved from the surface of CL to the separator [17]. Materials capable of antibacteria can be added to restrict the growth of biofilm. For example, despite the cost, silver nanoparticles had been demonstrated a good performance as both an ORR catalyst and bacteriostat on the surface of submerged graphite cathodes [53]. Very recently, it was found that the ORR of the air-cathode can be catalyzed by autotrophic bacteria (called biocathode) [54,55]. However, a compact separator (normally an ion exchange membrane) was needed in this system to keep an oligotrophic environment. If the excess substrate is absolutely consumed by heterotrophic bacteria with large amounts of CO2 simultaneously accumulated at the inner layer of the biofilm, it provides an adapt circumstance for the growth of those autotrophic bacteria (the green part in Fig. 1.5.6) as the bacteria have a higher oxygen affinity than chemical catalysts [56], which might enhance the performance of the cathode performance after a long time of operation. This hypothesis needs to be demonstrated in the future. The addition of antibacterial materials during CL fabrication is another choice to avoid cathodic biofilm. Silver nanoparticles were applied on the cathode as a bifunctional material for both ORR and inhibiting bacterial growth [53]. Organic antibacterial functional groups such as quaternary ammonium had also been tested [57]. When quaternary ammonium was modified on AC, the protein content on cathode decreased by a factor of 26, while the power density increased by 33%. Other researches even added antibiotics such as enrofloxacin into CL to inhibit the growth of cathodic biofilm [58]. It should be noted that the decay of cathodic performance after long-term operation cannot be fully attributed to the biofouling. Salt precipitation into CL is another problem, and this chemical contamination can be removed by weak hydrochloric acid [59].

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EPS

EPS

Bulk

Bulk

Bulk

Solution

Solution

Solution

J Substrate

Hytheterotrophic bacteria (living)

CL

Cs

Cs

Cs Distance to the matrix

J CO2

CL

C CO2

CL

Distance to the matrix

Dead bacteria

Distance to the matrix

Autotrophic bacteria (living)

FIGURE 1.5.6 Schematic of the formation of biofouling. Cs and CCO2 represent concentrations of the substrate and soluble CO2, J represents flux and EPS represents extracellular polymer substances.

1.5.6 SUMMARY AND FUTURE PERSPECTIVES The past decade has witnessed the fast development and significant achievements in aircathodes of MFCs. As one of the most critical parameters, the maximum power density of mediator-less whole cell increased by 4.4 times from w260 to w1400 mW/m2 with the cathodic material cost decreased by 98% from 1400 to 30 $/m2 using the same defined culture, plane anode, and configuration of the reactor, which can be mainly attributed to the improvement in the CL of air-cathodes. In order to equip MFC into wastewater treatment plants as the main structure for pollutant removal and electrical energy recovery, AC is the most promising and ideal catalyst due to its high performance and low cost. A binder that is capable to transfer hydroxyl and oxygen is needed to optimize the TPIs and maximize the performance of AC. Efforts on exploiting antibacterial catalysts and binders are important to resist the biofouling on the CL. For the large-scale application in the future, the production and the test of meter-scaled aircathodes are necessary as the bottleneck is various at different physical dimensions. For example, the current collector, a component usually neglected in laboratory-scale test, plays a more important role when the system is enlarged. As calculated according to the market price, the cost of the matrix takes the largest part of air-cathodes (over 50%) in both carbon cloth with Pt/C (Fig. 1.5.1D) and SSM with AC (Fig. 1.5.1F). Therefore, development of inexpensive and highly conductive matrix with additional advantages of antibiocorrosion and high mechanical strength is also urgently needed. One of the most important parts usually missed when reporting a new air-cathode is the sustainability of water pressure. The performance of all air-cathodes under a water pressure up to several meters should be tested. The ACAC made by rolling press method can sustain 3 m of water without any leakage (data not

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published). This characteristic determines the height of large MFCs. Besides, the structure of air-cathodes should be further improved. As AC is not a good conductor, the conductivity of the CL made by AC needs to be improved by pretreatments of AC powders or adding ancillary materials to further decrease the ohmic resistance of the CL.

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