polyaniline-copper hybrid and its

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Fabrication of a carbon paper/polyaniline-copper hybrid and its utilisation as an air cathode for microbial fuel cells Jayesh Sonawane, Deepak Pant, Prakash Chandra Ghosh, and Samuel B. Adeloju ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02017 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Energy Materials

Fabrication of a Carbon Paper/Polyaniline-Copper Hybrid and its Utilisation as an Air Cathode for Microbial Fuel Cells Jayesh M. Sonawanea, b,c, Deepak Pant d, Prakash C. Ghosha,b, Samuel B. Adelojub,c,e* aIITB-Monash

Research Academy,

Indian Institute of Technology Bombay, Mumbai, 400076, India b Fuel

Cell Research Facility,

Department Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai-400076, India *c School

of Chemistry,

Monash University, Clayton Campus, Victoria-3800, Australia d Separation

and Conversion Technology,

Flemish Institute for Technological Research (VITO), Boeretang 200, 2400 Mol, Belgium. *e Faculty

of Science,

Charles Sturt University, Wagga Wagga, NSW 2678, Australia Ph.: +61 2 693 32510 Email: [email protected]

∗ Corresponding author

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Highlights 

Successful integration of a carbon paper with a copper-polyaniline composite



Effective use of carbon paper/copper-polyaniline hybrid as an air cathode for microbial fuel cells



Comparable performance of CP/PANi-Cu hybrid cathode with that of CP/Pt composite cathode



Evidence of less biofouling of Nafion membrane with CP/PANi-Cu hybrid cathode than with CP/Pt cathode

Nomenclature: F

Faraday’s constant [96,485 C mol-1]

I

Current [mA]

i0

Exchange current [mA]

j0

Exchange current density [mA cm-2]

R

Ideal gas constant [8.31 J mol-1 K-1[

Rext

External Resistor [Ω]

T

Absolute temperature, [298.15 K]

V

Voltage [mV]

η

Overpotential [mV]

Abbreviations: EPS

Exopolysaccharide

FePc

Iron phthalocyanine

GDL

Gas diffusion layer

LSV

Linear sweep voltammetry

MEAs

Membrane electrode assemblies

MFC

Microbial fuel cells

MnFe2O4

Manganese ferrite 2 ACS Paragon Plus Environment

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MWNT

Multi-walled carbon nanotube

OCV

Open circuit voltage

ORR

Oxygen reduction reaction

PANi

Polyaniline

PANi/C

Polyaniline/carbon black

PB/PANi

Prussian Blue/polyaniline

PtC

Platinum cathode

SPG

Spectrographic pure graphite

Abstract The use of platinum (Pt) catalysts in air cathode for microbial fuel cells (MFCs) is expensive and is limiting the adoption of MFCs for power generation from wastewaters. In this study, we propose an innovative approach for the fabrication of a carbon paper/copper-polyaniline (CP/PANi-Cu) hybrid which can be employed as a relatively inexpensive and effective air cathode for MFCs. The CP/PANi-Cu hybrid cathode composed of the uniform layer and relatively strong adhesion of PANi-Cu to CP which enables a significant oxygen reduction reaction (ORR). Consequently, the composite cathode demonstrated superior performance than a conventional Pt (CP/Pt) cathode by generating a maximum current of 0.50 ± 0.02 mA cm-2 compared with 0.35 ± 0.03 mA cm-2 obtained with the latter. When employed in MFCs, the power densities obtained with these cathodes were 0.244 ± 0.014 mW cm-2 under a current density of 0.85 ± 0.08 mA cm-2 for Pt cathode, and 0.101 ± 0.01 mW cm-2 at 0.37 ± 0.04 mA cm-2 for CP/PANi-Cu cathode. Moreover, the Nafion membrane attached to the CP/PANi-Cu cathode experienced much less biofouling than with that used with the CP/Pt cathode and, thus, indicates that the new hybrid cathode is superior on the basis of overall performance, usage and stability. A very detailed characterisation of the Nafion membranes by Fourier transform infrared spectroscopy, and 3D profilometry revealed more complex and thicker biofouling of the membrane attached to the CP/Pt cathode than that of the CP/ PANi-Cu cathode. Keywords Platinum, carbon paper, copper-polyaniline composite, air cathode, biofouling, microbial fuel cell 3 ACS Paragon Plus Environment


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1. Introduction The choice of the cathode used in microbial fuel cells (MFCs) has to date been a significant limiting factor which has the most influence on MFC performance and, thus, thas led to considerable interest in recent years in the development of new cathodes

1–5.

The most efficient

cathode to date are air cathodes4, and these often employ platinum (Pt) for their construction, with loadings ranging from 0.3 – 0.7 mg Pt cm-2 on carbon papers 5–7. The use of Pt in these cathodes, therefore, represents a significant contributor to the cost of MFCs and has, to some extent, limited their wider adoption for power generation from wastewaters. In an on-going effort to identify potential cheaper alternatives, the use of various cathode catalyst (non-precious metal) on carbon paper or cloth has been considered for MFCs. These include the use of non-activated and activated biochars prepared from banana 8, iron aminoantipyrine (FeAAPyr)

9,

cobalt-aminoantipyrine (Co-AAPyr)

manganese-aminoantipyrine (Mn-AAPyr) octahedral molecular sieve

10,

9,

9,

nickel aminoantipyrine (Ni-AAPyr)

9,

manganese oxides with a cryptomelane-type

flower-like cobalt(II,III) oxide (Co3O4)

11;

iron-based catalysts

12

and platinum-iron-titanium dioxide (Pt–Fe/TiO2) 13. The methods used in various studies for deposition of catalyst on a gas diffusion layer (GDL) include brush coating 14, inkjet printing 15, ultrasonic spray coating 16, doctor blade coating 17, and sputtering 18. To our knowledge, there has been no direct method for binding the catalyst directly onto GDL. One of the common approaches for making gas diffusion electrodes (GDEs) comprising of an activated carbon incorporated catalyst layer, a gas porous GDL, and the current collector is cold rolling method

19.

Recently, the GDEs made by this method were upscaled to

fabricate large air cathodes. The window-pane air cathode composed of fifteen smaller cathodes welded to a single conductive metal sheet and tested in a 85 L MFC 20. A major disadvantage of the existing coating methods is that they often result in an inadequate and ineffective adhesion of the catalyst to the carbon support21. Recently it was shown that embedding the catalyst within the activated carbon/PTFE matrix performed better compared to spraying the catalyst on the surface exposed to the electrolyte 22. Therefore, there is a pressing need for the development of a more effective coating method that will ensure adequate and effective adhesion of the catalyst to the carbon support and, hence, improve the performance and stability of the cathode. A possible approach for accomplishing this goal is by using electrochemically synthesised conducting polymers 23. The use of conductive polymers as composites, including with graphene 4 ACS Paragon Plus Environment

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as catalysts in MFC cathodes for ORR reaction has been widely reported

24–26.

One of these air

cathodes used iron-containing polyaniline prepared by heat treatment and coated with graphite felt 27.

Polyaniline/carbon black (PANi/C) composite-supported iron phthalocyanine (FePc),

collectively termed PANi/C/FePc, was also coated on carbon cloth 28. A Prussian Blue/polyaniline (PB/PANi)-modified electrode was developed by using a three-step method involving both electrochemical and chemical polymerisation on spectrographic pure graphite (SPG) rod

29.

In

another study, fibrous PANi–MnO2 nanocomposites were prepared by using a chemical oxidative polymerisation method on carbon paper

30.

Polyaniline (PANI)/multi-walled carbon nanotubes

(MWCNTs) composites synthesised chemically have also been explored as a catalyst in air cathodes

31.

The development of a manganese ferrite (MnFe2O4)/polyaniline (PANI)-based

cathode catalyst by electroplymerisation has also been reported 32. Furthermore, the synthesis of a polyaniline nanofiber (PANInf)/carbon black composite by interfacial polymerisation has been considered

24.

Higher electrocatalytic activity was obtained with this composite for oxygen

reduction compared to the use of pristine PANInf. The maximum power density (Pmax) of 185 mW m-2 obtained with the pristine PANInf increased to 496 mW m-2 with the composite cathode 24. In comparison, a Pmax of 91.29 mW -m2 was obtained with a RVC cathode coated with PANI+FePc. Reticulated vitreous carbons (RVCs) doped with PANI, PANI+FePc (iron(II) phthalocyanine) and PANI+CuPc (copper phthalocyanine) catalyst were also used for MFCs 33. Despite the various examples of the use of conducting polymers cited above, it is important to note that most of the PANi cathodes that have been considered for MFCs to date are based on PANi composites32. In one study, heat treated PANi-Fe900 and PANi-Fe700 were synthesised at 900 ℃ and 700 ℃, respectively, and subsequently used as MFC cathodes

27.

A PANInf/carbon black

composite was also explored for MFCs24. It is also important to note that these PANi-based cathodes were mostly prepared by chemical polymerisation which involves various steps. Furthermore, the resulting catalyst inks are often applied manually or mechanically onto the cathode surface. These often lead to uneven catalyst distribution and poor surface adhesion 34. The problem of uneven catalyst distribution and poor surface adhesion can be overcome by employing direct electrochemical polymerisation for the formation of PANi or PANi composite. The inherent advantages of this approach include the ability to grow PANi or PANi composite in a single step directly onto any substrate size and dimension, and consequently minimising contact resistant. It has been well established that the interfacial contact resistance between the membrane 5 ACS Paragon Plus Environment


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and the electrodes can influence the performance of MFCs 35. The microscopic contact resistance of the deposited catalyst and sublayers of the catalyst on carbon paper/ cloth has also been reported to influence the MEA performance14. It is therefore anticipated that the adoption of electrochemical synthesis of PANi composites in this study will produce a more superior cathode capable of accomplishing a more efficient interfacial electron transfer on the triple phase boundary of the membrane electrode assembly (MEA) than those achieved by chemical polymerisation in previous studies. Besides its cathodic application, PANi has been used to fabricate high-performance anode for MFCs. The electrogenic bacterial loading capacity and sluggish exoelectron transfer efficiency between electrogenic biofilm to the anode surface often leads to the limitation of the practical application of MFCs. However, several studies have shown that higher performance can be obtained through the interface of three-dimensional PANi fibres with bacterial biofilm, thus facilitating electron transfer, and providing multiplexed and highly conductive pathways.

36

The

electro-deposition of PANI networks onto graphene nanoribbons coated carbon paper has been found to provide a large surface area for the attachment of bacterial cells and high conductivity to facilitate extracellular electron transfer from microbes to the electrode 37–39. In this study, we propose a method for direct synthesis of a PANi-copper composite onto a carbon paper by galvanostatic polymerisation of aniline (ANi). This is intended to provide a direct method for binding the catalyst onto the GDL and, hence, ensuring adequate adhesion and interfacial contact resistance. The inclusion of cupric ions (Cu(II)) in the monomer solution during the electropolymerisation of ANi results in the production of a Cu doped PANi which may further enhance the oxygen reduction kinetics. Verification of the presence of Cu in the resulting CP/PANi-Cu hybrid will be achieved by analysis with a time-of-flight secondary ion mass spectrometry (ToF-SIMS). The use of the resulting CP/PANi-Cu hybrid to form the half MEA will be investigated by hot-pressing with a Nafion membrane and subsequently explored for use in MFCs. The electrochemical performance of the hybrid cathode will be compared with that of a CP/Pt cathode. Also, the extent of the biofouling of the Nafion membrane attached to both cathodes and the loss of catalytic activity will be investigated by 4D X-ray microscopy and 3D profilometry. Time-dependent Tafel plots (linear sweep voltammetry) will also be used to assess the dependent loss of cathodic activity. Furthermore, FTIR analysis of the Nafion will be used to evaluate the nature and composition of the deposited macromolecules on the fouled membrane. 6 ACS Paragon Plus Environment

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2 Materials and method 2.1 Electrode preparation 2.1.1 Anode fabrication A low-cost stainless-steel wool (SS-W) (Scotch-Brite, India) was coated with polyaniline as previously described38,39 and used as an anode with an apparent surface area of 50 cm2. Before coating with PANi, the SS-W was degreased by ultrasonication for 15 mins in 100% acetone. Reagent grade aniline (Sigma-Aldrich, Mumbai, India) and nitric acid (Sigma-Aldrich, Mumbai, India) were used for electropolymerisation of aniline on SS-W. The polymerisation of ANi was carried out in 0.4 M aniline which contained 0.7 M nitric acid with an applied current density of 2.5 mA cm-2 for 15 mins in a 3-electrode cell. The monomer solution was purged with nitrogen for 10 mins to remove oxygen before the commencement of polymerisation. The coated electrodes were rinsed with distilled water several times to ensure complete removal of untreated aniline and nitric acid. 2.1.2 Preparation of CP/PANi-Cu cathode The deposition of PANi on carbon paper was achieved in a three-electrode cell assembly by galvanostatic polymerisation of ANi. The electrodeposition was performed with a Biologic SP150 potentiostat-galvanostat (BioLogic, Claix, France) equipped with an EC-Lab V-10.44 software for data processing. Reagent grade aniline (Sigma-Aldrich, Mumbai, India) and nitric acid (Sigma-Aldrich, Mumbai, India) were used for this purpose. The monomer solution consists of 0.4 M aniline in 0.7 M nitric acid and 0.6 M copper (II) sulfate. The PANi was synthesised by applying a constant current density of 2.5 mA cm-2 for 25 mins. Copper was then deposited with an applied potential of -0.1 V for 60 secs to form the CP/PANi-Cu. The solution was purged with nitrogen gas to remove oxygen before the commencement of electropolymerisation. The working electrode was a commercial carbon paper of 210 µm thickness (GDS 210, CeTech, Taichung, Taiwan) with a dimension of 3 × 2.5 cm (7.5 cm2). The carbon paper was degreased in an ultrasonication bath, as described for the SS-W. Copper felt was used as a counter electrode. The presence of a PANi-Cu composite was verified by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Scheme 1 describes the various steps involves the CP/PANi-Cu cathode.



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2.1.3 Fabrication of Pt cathode Pt air cathodes with 0.5 mg cm-2 loading were fabricated by following previously described method 40.

Briefly, Vulcan XC-72 (Sigma-Aldrich, Australia), 20% w/w Pt content was coated onto 12 x

10 cm commercial carbon paper (GDS 210, CeTech, Taichung, Taiwan). PtC ink was prepared in 5% Nafion solution as a binder in isopropyl alcohol (Sigma-Aldrich, Australia) and was coated with a spray paint gun. 2.1.4 Hotpressing of Pt and PANi-Cu electrodes All fabricated electrodes were dried at 60 °C for 6 hrs to ensure complete drying. The resulting electrodes were again coated with a 5% Nafion solution and subsequently dried for 30 mins. Coated electrode was hot pressed on one side using Nafion NRE-212 (Sigma-Aldrich, Australia) under 10 kg cm-2 at 140 °C for 3 mins using a Carver hot press (Carver, Inc, Indiana, USA). The fabricated CP/Pt and CP/PANi-Cu MEAs were cut into a size of 3 × 2.5 cm (apparent surface area 7.5 cm2). Scheme 1: Fabrication of CP/PANi-Cu - step 1 synthesis of PANi in 0.4 M aniline in 0.7 M nitric acid with an applied current density of 2.5 mA cm-2 for 15 mins; step 2 deposition of Cu on PANi film at an applied potential of -0.1 V for 1 min; step 3 use of PANi-Cu electrode for MFC in M9 media inoculated with landfill leachate.


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2.2 Configuration of MFC reactor The MFC reactors were made of a transparent acrylic sheet of 5 mm thickness. The sheets were cut using a laser to ensure identical sizes in all reactors. Three sheets with a dimension of 6.8 × 6.8 cm were fixed together to form the base of the MFC reactor. The dimensions of the top and bottom sheets were 3 × 2.5 cm. The inner dimensions of 3 × 3 × 3 cm were formed as an anode compartment. The empty bed volume of the anode was 27 cm3. MFC system explored for this study is shown in Fig. S1a (Supporting information) and the MFC experimental set-up is shown in Fig. S1b (Supporting information). The resulting one-sided MEAs were sandwiched in an acrylic mesh, with an aluminum mesh of similar dimension acting as a current collector in the MFC reactors. This was kept in place with nuts and bolts to ensure leakproof assembly. A miniature Ag/AgCl (sat. KCl) electrode was used (RRPEAGCL, Pine Research Instrumentation, Durham, USA) as a reference electrode for the electrochemical measurements. The SS-W anode modified with PANi was connected with a tantalum wire which acted as an anode current collector. 2.3 Characterisation of Pt and PANi-Cu cathodes 2.3.1 Microscopy The membranes were carefully harvested after the completion of the experiments to identify the extent of fouling experienced during the study. Appropriate care was taken to avoid surface contamination of membranes. The topography of both Nafion membranes attached to the CP/ PANi-Cu and CP/Pt cathodes were examined in triplicates with the Alicona microscope at three magnifications of 10x, 20x, and 50x, visualised by full 3D scans. Same samples were further subjected to the phase contrast tomography using a 4D X-ray microscopy, (Versa XRM-520, Xradia, Pleasanton, CA). The equipment used was a 4D x-ray microscope to perform 3D microscopy on the samples. 2.3.2 Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) The atomic and molecular species concentration of CP/Pt and CP/Cu-PANi cathodes were investigated using TRIFT V NANO TOF, (Physical Electronics, Inc. (Phi), Chanhassen, Minnesota, USA). The data were recorded by a focused ion beam. The primary ions were 30 KV Ga+ by10 min Spectra acquisition using liquid metal ion gun (LMIG) in 500 µ x 500 µ sq. area. The subsequent secondary ions were augmented into the mass spectrometer, and the ions were



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then separated by determining time-of-flight. A 2D image was obtained by focussed ion beam on the specimen surface. 2.3.3 Attenuated Total Reflectance -Fourier Transform Infrared Spectroscopy A Vertex 80 FTIR spectrometer (Bruker Corporation, Massachusetts, USA) was used to analyse the FTIR properties of the biofouled Nafion membranes attached to the cathodes at the completion of the experiments. FTIR bands were obtained with the attenuated total reflectance (ATR) method with a resolution of 4 cm–1. The wavenumbers were in the range of 4000–550 cm–1 for Nafion attached to the CP/Pt and CP/ PANi-Cu cathodes. The baseline rectifications were made carefully, and the peaks were resolved with an Opus software (Bruker Corporation, Massachusetts, USA). 2.4 MFC Operation The MFCs were fed in triplicate with M9 media to which 4% landfill leachate was added as a bacterial inoculum. We previously proposed the use of landfill leachate as a promising substrate for MFCs41 and, thus, it served as the basis for selecting a bacterial source in this study. The ingredients of the M9 media per L of deionised water are as follows - NH4Cl: 0.31 g, KCl: 0.13 g, NaH2PO4.H2O: 2.69 g, Na2HPO4: 4.33 g, and 12.5 mL of each trace metal and vitamin solutions 42,43.

Additionally, 2% sodium acetate was added as the sole carbon source in the M9 media. The

landfill leachate was sampled from a landfill site in Turbhe, Mumbai. The feed and reactor was sparged with nitrogen gas to remove oxygen and minimise aerobic oxidation during electrogenesis. All experimentations were carried at an ambient temperature of 27 ± 3 ℃. The experiments were conducted in a fed-batch mode. First, two cycles operated at open circuit voltage (OCV) mode for 6 days per batch and subsequently with an applied constant external resistance of 100 Ω for 5 days per batch. After each successive batch, 90% of the spent medium was replaced with fresh medium. In the first phase, the MFCs ran initially in an OCV mode for two cycles to enrich biofilm on the anode. In the second phase, a 100 Ω external resistor was applied to evaluate the magnitude of the current generation and was continued until a stable performance was achieved. The control set of each cathode materials were run only in sterile M9 media.


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2.5 Analytics and calculations MFCs equipped separately with CP/Pt cathode, and CP/ PANi-Cu cathode were run initially in an OCV mode to establish biofilm formation without any external load. After two succeeding cycles, a 100 Ω resistor was coupled to each MFC set-up to evaluate current generation. Each reading was taken at 30 min intervals using a data logger (Graphtec GL820, Graphtec corporation, Shinanocho, Japan). From the recorded voltage (V), the corresponding current (I) was calculated by Ohm`s law: I=V/R; where R is the Rext and V is the voltage between the electrodes. The electrochemical performance and data were standardised with the apparent surface area (7.5 cm-2) of the cathode. All electrochemical experiments and Tafel plot analysis were conducted with a Biologic SP-150 potentiostat (BioLogic, Claix, France) equipped with EC-Lab V 11.10 software. The polarisation curves were obtained using linear sweep voltammetry (LSV) recorded at a scan rate of 1 mV s-1 in a potential window from OCV to 0 V. These were carried out during the 6th batch cycle running across constant resistance mode when the observed current was at stationary phase. The cathodic Tafel plots were carried out in both systems in the middle of every batch cycles using LSV (scanning from OCV to 150 mV, at a scan rate of 1 mV s-1) After the application of overpotential (η), positive deviation from OCV to 150 mV shows an initial log of η and further Tafel lines become linear. With the Tafel equation, the X-axis intercept gives the log of current exchange densities (ln j0). Cathodic Tafel plot, as derived from the equation below, was employed to measure the reaction kinetics.

() ( )

𝑙𝑛

𝑖 𝐹𝜂 = 𝛽 𝑖0 𝑅𝑇

where i0 is exchanged current, i is the electrode current density (mA cm-2), β is the electron transfer coefficient, R is the ideal gas constant (8.31 J mol-1 K-1), F is the Faraday’s constant (96485 C mol1),

T is the absolute temperature, K (298.15 K) and η is the activation overpotential. The purpose

of using the Tafel plot is to calculate the i0 value, an essential parameter in the rate of electrooxidation or electro-reduction of a chemical species at an electrode in equilibrium. The exchange current densities were calculated as previously described 23,38.



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3 Results and discussion 3.1 Morphological examination of CP/Pt and CP/PANi-Cu cathodes The morphologies of the uncoated CP, CP/PANi-Cu, and CP/Pt(0.5 mg cm-2) were examined with an Alicona profilometer, as shown in Figure 1. The micrographs show distinct variances in the surface profiles of the cathode materials at mm/µm scale. The surface profile of CP/Pt and CP/PANi-Cu cathode is given in Table S1. The mean roughness of pristine CP, CP/ PANi-Cu, and CP/Pt were 8.4, 1.2, and 6.0 µm, respectively. The surface of the CP/Pt cathode was obviously the roughest due to the variation of coating layers (than CP/ PANi-Cu cathode) caused by pressure from spray coating. In comparison, the CP/ PANi-Cu cathode showed a more uniform surface morphology with electrodeposition of PANi on the CP. Also, the electrodeposition was much better controlled and uniform than with spray coating. During the electropolymerisation, the growth of PANi-Cu starts on the core of the GDL and results in the formation of PANi-Cu nanofibers of 40.8 ± 7.2 nm diameter, as evident in Figure 1d. Due to the in-situ polymerisation, the bonding of PANi-Cu nanofiber network to the surface of the carbon paper lowers the interfacial contact resistance by its strong adhesion.

Fig. 1 Micrographs of the different cathodes. Profilometry images of (a) pristine CP, (b) CP/ PANiCu cathode and (c) CP/Pt cathode; and (d) scanning electron micrograph of the CP/ PANi-Cu cathode (magnification 100,000X).


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3.2 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) The composition of atoms in the coating was evaluated by ToF-SIMS. The 2D images in Fig. 2 show the total ion counts and densities of CP/Pt and CP/ PANi-Cu cathodes, respectively. The carbon ion count obtained for the CP/Pt cathode was almost four times greater than obtained for the CP/ PANi-Cu cathode. This was due to the pure carbon content relative to the minuscule amount of Pt nanoparticles present. The images obtained for the CP/PANi-Cu cathode revealed the presence of Cu in the sample. The distribution of Cu ions in 500 µ x 500 µ sq. area is shown in Fig. 2 (b). Notably, the counts for hydrogen and nitrogen ions are significantly higher in PANi cathode than in Pt cathode The high nitrogen moieties in PANi coating, particularly pyridinic nitrogen, are responsible for the excellent electrocatalytic effect of Cu on ORR 44,45.

Fig. 2 2D image of ion counts obtained by ToF SIMS. (a) CP/Pt cathode and (b) CP/ PANi-Cu cathode. 3.3 Bioelectrocatalytic performance of CP/Pt and CP/PANi-Cu cathodes The cathodes were employed in reactors operated in batch modes. The first cycle was operated in OCV mode without any external load. The MFCs with CP/Pt, and CP/PANi-Cu cathodes gave high OCVs which may be due to an aerobic degradation of the organic substrate. Figure 3a shows the initial OCVs obtained for the MFCs which employed both CP/Pt and CP/PANi-Cu cathodes. In the first cycle of the MFCs with both cathodes, the OCV declined slightly over time. This phenomenon is attributed to a log phase due to the adjustment of the bacterial consortia at the electrode surface. After the first cycle and with the addition of 25 mM acetate, the OCVs obtained with both cathodes increased exponentially with time. This represents the lag phase of both MFCs, and the highest 13 ACS Paragon Plus Environment


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OCV was observed on the 6th day and 4th day of the second cycle of the operation of CP/PANi-Cu and CP/Pt cathodes, respectively. The MFC with the CP/PANi-Cu cathode gave stable readings from the 4th to 6th day of operation, followed by a decline in the OCV. The average OCV obtained for the MFC with the CP/Pt cathode increased from 137 ± 22 mV to 477 ± 43 mV in the second cycle. Likewise, the OCV recorded for the MFC with the CP/PANi-Cu cathode rose from 107 ± 18 mV to 290 ± 29 mV. In both cases, the MFC with the CP/Pt cathode gave a high OCV than that with the CP/PANi-Cu cathode. However, the MFC with CP/PANi-Cu cathode gave a maximum OCV on the 9th day of the operation and remained stable for the next three days, followed by a decline in OCV. In contrast, the MFC with the CP/Pt cathode remained in the exponential phase up to the 12th day of operation. These results indicate that the decrease in the OCVs in both systems may be due to their high start-up times in the M9 media.

Fig. 3 Performance of CP/Pt and CP/PANi-Cu cathodes in MFCs. (a) Open circuit variation with time, and (b) current generation with time. Both MFCs were subsequently ran in a constant resistance (CR) mode with a Rext of 100 Ω for seven batch cycles, with each cycle lasting around 5 days (Fig. 3b). Overall, the MFC with the CP/Pt cathode again gave higher current density than with the CP/PANi-Cu cathode. Also, the startup time for the current generation was quick with the CP/Pt cathode than with the CP/PANiCu cathode. Both CP/Pt and CP/PANi-Cu cathodes produced maximum current density (jmax) during the 8th and 9th batch cycles (based on the two OCV cycles). The jmax were 0.50 ± 0.02 and 0.35 ± 0.03 mA cm-2 for the CP/Pt and CP/PANi-Cu cathodes, respectively. The difference in the achieved jmax may be due to the higher oxygen reduction kinetics of the CP/Pt cathode than with 14 ACS Paragon Plus Environment

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the CP/PANi-Cu cathode. Both MFCs enabled immediate current generation from the 3rd cycle, indicating that there was no delay in the startup of the bioelectrocatalyst due to the operation of the two prior OCV cycles to establish the growth of electroactive biofilm on the anode. This phenomenon can be ascribed to their 3D available areas that offer more surface area to the anode respiring bacterial communities. 3.4 Current-voltage characteristics The polarisation behaviour was studied by using LSV, as shown in Fig. 4. The scan rate was kept at 1 mV s-1. The area-specific power densities for the CP/Pt and CP/PANi-Cu cathodes were 0.244 ± 0.014 mW cm-2 under a current density of 0.85 ± 0.08 mA cm-2, and 0.101 ± 0.01 mW cm-2 at 0.37 ± 0.04 mA cm-2, respectively. It has been reported that the Pt loading on a CP/Pt cathode significantly affects power generation in MFCs

46.

In comparison, the air cathode reportedly

generated a Pmax of 0.054 mW cm-2 in a MFC 31. It has also been observed that the Pmax obtained for different cathodes are proportional to the voltage output of the MFC. Similarly, the attainment of higher OCVs has been linked to the achievement of a higher Pmax 31. It is important to note that the Pmax of 0.101 ± 0.01 mW cm-2 obtained for CP/PANi-Cu cathode is superior to those obtained with other PANi-based catalysts for MFCs reported to date. These include the Pmax of 0.082 mW cm-2 achieved for nanostructured PANi cathode catalyst Nanofiber/carbon black composite cathode catalyst

24,

47,

0.0496 mW cm-2 for PANi

0.0248 mW cm-2 for PANi/β-MnO2

nanocomposites 48 and 0.0124 mW cm-2 for graphene PANi 49 . Apart from being conductive, the CP/PANi-Cu cathode has also been shown to serve as a catalyst with high redox activity 50. The catalytic activity of the CP/PANi-Cu cathode was enhanced by the presence of the cupric ions from copper sulfate and generated from the exposure of the copper counter electrode to nitric acid during CP/PANi-Cu deposition. The available cupric ions in the acidic aniline monomer solution were reduced at an applied potential of -0.1 V for 60 secs to form a CP/PANi-Cu composite. Also, the synergistic ORR activity from oxygen transport from the cathode interphase to the triple phase boundary was facilitated by the resulting 40 nm PANi-Cu nanofiber networks (determined to be 40.8 ± 7.2 nm, as shown in Fig. 1d).



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Fig. 4 Polarisation curves of the MFCs operated with the c. (a) CP/Pt cathode, and (b) CP/PANiCu cathode. 3.5 Longevity of cathodic activity After completion of the experiment, both cathodes were removed from the aluminium current collectors. Interestingly, deposition of aluminum oxide (Al2O3) was found on the interface of the cathodes. Notably, more Al2O3 deposition was observed on the CP/Pt cathode current collector than on that of the CP/PANi-Cu cathode, as shown in Figure 5. This observation indicates that the CP/Pt cathode current collector corroded more severely with the passage of current through the collector. This may be due to the much higher potential difference between the CP/Pt cathode and the Al current collector and the higher current density generated by CP/Pt cathode MFC. It suggests that the use of a more stable current collector is required when running the MFCs for extended periods with the CP/Pt cathode.


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Fig. 5 Evidence of the formation of Al2O3 on current collectors used with the CP/Pt and CP/PANiCu cathodes after exposure to the M9 media. The longevity of the cathodic activity of the cathodes was evaluated electrochemically by linear sweep voltammetry (LSV) after each successful batch cycle. The Tafel plots were derived from the LSV to gain an understanding of the kinetics of the cathodes. From the Tafel-type linear equation obtained from the LSV plots, the corresponding slope is F/RT, and the y-axis intercept is the logarithm of the exchange current. For both MFCs, the Tafel plots were recorded at the beginning of the experiments. The exchange current density (j0) was calculated by extrapolation to η = 0 with a linear regression (R2 > 0.99) between η = 150 mV. By application of 150 mV overpotential, the j0 values depend on the charge limited electrochemical process where mass transfer is limited during the measurements. The j0 value signifies the rate of electron exchange from the anodic biofilm to the cathode at equilibrium. The average j0 values based on triplicate measurements were used to plot the Tafel curves. The Tafel plots obtained for CP/Pt and CP/PANiCu cathodes are shown in Fig. 6 (a,b). Also, the j0(apparent) values obtained for these cathodes (calculated from the Tafel plots) are shown in Fig. 6 (c,d). The Tafel plots were analysed, as previously described 23,38. The initial j0(apparent) values obtained for CP/Pt and CP/PANi-Cu cathodes were 0.0199 ± 0.0019 and 0.0193 ± 0.0004 mA cm-2, respectively. Although slightly lower, the values obtained with the CP/PANi-Cu cathodes were more reproducible. The j0(apparent) values obtained after the first OCV cycle for both MFCs decreased to 0.0181 ± 0.0013, and 0.0154 ± 0.0005 mA cm-2, respectively. The changes in the j0(apparent) were 2.6 % and 20.2 % for the CP/Pt and CP/PANi-Cu cathodes, respectively. When operated under a constant current mode, the j0(apparent) at the end of the first 17 ACS Paragon Plus Environment


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current cycle were 0.0164 ± 0.0007 mA cm-2 and 0.0114 ± 0.0006 mA cm-2 for the CP/Pt and CP/PANi-Cu cathodes, respectively. In this case, the cathodic loss observed in both MFCs were 11.8% and 41% for CP/Pt and CP/PANi-Cu cathodes, respectively. The cathodic loss is the sum of the catalyst deterioration and the current collector corrosion which ultimately increases contact resistant of the MFC. The j0(apparent) of the CP/Pt and CP/PANi-Cu cathodes at the end of the experiment were 0.0100 ± 0.0001 mA cm-2 and 0.0056 ± 0.0003 mA cm-2, respectively.

Fig. 6 Electrochemical analysis of the longevity of the cathodes. (a) Tafel plots of CP/Pt cathode, (b) Tafel plots of CP/PANi-Cu cathode. (c) j0apparent of CP/Pt cathode after every successive cycle. (i) is for the initial cycle, (ii) for cycle 2, (iii) for cycle 3, and so on to cycle 6 (vi) for cycle 6 and (d) j0apparent of CP/PANi-Cu cathode after every successive cycle.(i) is for the initial cycle, (ii) for cycle 2, (iii) for cycle 3, and so on to cycle 6.


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The percent change of the electrokinetics in both MFCs from the initiation to the end of the experiment was 46% for the CP/Pt cathode and 71% for CP/PANi-Cu cathode. The large change in the electrokinetics of the cathodes suggests that the CP/PANi-Cu cathode deteriorated faster compared to the CP/Pt cathode. However, the deposition of aluminum oxide was more pronounced on the CP/Pt cathode which may have contributed significantly to the cathodic activity loss because aluminum oxide is a poor conductor of electricity and, thus, directly increases the contact resistance of the cathode. The deterioration of the aluminium current collector in this study was due to the permeation of the M9 media from the Nafion membrane51–53 through the carbon paper. However, it is also possible to protect the aluminium current collector by coating with PANi, as previously reported 54. 3.6 Biofouling of Nafion membrane Bacterial communities tend to grow on hydrophilic surfaces 55. The biofilm slowly forms on the membrane surface when one of its sides is exposed to the liquid organic material on the anode side. It has been reported that the biofouling of membranes leads to an unwanted distortion of its function and structure 56,57. It is recognised as a major obstacle to the long-term operation of MFCs 56,57.

It has also been observed that the formation of slime on Nafion leads to resistance to mass

transfer from the anode chamber to Nafion 58. The electrogenic bacterial communities produced extrapolymeric substances consisting of proteins, nucleic acids, lipids, and polysaccharides. These macromolecules promote mechanical adhesion to the Nafion surface to form a three-dimensional polymeric network which immobilises the biofilm communities

58.

The changes due to biofilm

formation on the Nafion membrane exposed to M9 media for nine batch cycles was examined. The membrane was carefully harvested and detached from the carbon paper. The photographs were taken before and after the experiment. As obvious in Fig. 7a, the Nafion membrane before the experiment was black from the platinised carbon coated on the carbon paper, but after exposure to the M9 media, the membrane revealed a significant deposition of bacterial debris, exopolysaccharides (EPS) and a biofouled wrinkled surface. In Fig 7b, the Nafion membrane before the experiment was slightly green from the PANi-Cu deposition, but after the experiment, slight deposition of bacterial debris and EPS was obvious. Less deposition of EPS or biofouling was observed on the Nafion membrane attached to CP/PANiCu cathode. This was due to the presence of copper particles close to the Nafion membrane in this 19 ACS Paragon Plus Environment


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case. It has been established that Cu nanoparticles have antimicrobial properties 59. More recently, Myung et al.57 reported that the use of a copper current collector reduced long-term biofouling of air cathodes in MFCs. The investigation ran for 27 weeks to compare cathodic fouling between copper and SS current collectors. It was noted that the copper current collector showed less biofilm formation compared with the SS current collector 60. In another study, the use of copper hollow fibers was demonstrated to be effective as an anti-biofouling material in electrochemical membrane bioreactors 61

Fig. 7 Observed changes in Nafion membranes before and after exposure to M9 media. (a) membrane attached to the CP/Pt cathode before and after the experiment, and (b) membrane attached to the CP/PANi-Cu cathode before and after the experiments. 3D-profilogram of the used Nafion membrane attached to (c) CP/Pt and (d) CP/PANi-Cu cathodes (50X). 3.6.1 Four-dimensional X-ray microscopy The 4D X-ray microscopy was performed by scanning both fouled membranes at 360 degrees. Triplicate scans were carried out at 4x and 20x magnifications. Fig. S2a shows the membrane attached to the CP/Pt cathode and, from the lateral view, the presence of bacterial debris along with macromolecule deposition can be seen. The front view shows the uneven deposition of bacterial debris and EPS. The clear black embossed impressions are from current collector material that did not come in contact with the M9 media. Fig. S2b shows the Nafion membrane attached to 20 ACS Paragon Plus Environment

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the CP/PANi-Cu cathode and, in this case, the lateral view shows a smoother deposition in comparison to the Nafion attached to the CP/Pt cathode. A smooth deposition was also observed in the front view. In the auxiliary view, the EPS deposition was denser than observed for the Nafion membrane of the CP/Pt cathode. These observations clearly demonstrate that membrane fouling was more pronounced in the MFC which employed the CP/Pt cathode than with that which employed the CP/PANi-Cu cathode. However, the current generation was higher with the CP/Pt cathode and, thus, suggests that the proton transfer across the membrane is not the rate-limiting process in the energy conversion. 3.6.2 Attenuated total reflectance Fourier transformed infrared spectroscopy Attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FITR) was employed to analyse the biofilm formation, based on the fingerprints of biomolecules, such as proteins, nucleic acids, and polysaccharides. The FTIR spectra for CP/Pt and CP/PANi-Cu cathodes are shown in Fig. 8 (a) and (b), respectively. Evidently, this approach is useful for the detection of accumulated biomass 62–64. The FTIR spectra of both membranes from the CP/Pt and CP/PANi-Cu cathodes revealed the presence of polysaccharides. The peaks at 2961 cm-1 assigned to CH3, 2925 cm-1 to CH2, 2897 cm-1 to CH tertiary, 2874 cm-1 to CH3, and 2854 cm-1 to CH2 are all principal compounds of cell membranes’ fatty chains 63. The peak at 1631 cm-1 in the spectrum for the Pt membrane and 1638 cm-1 for the PANi-Cu membrane are assigned to Amide I (νC

O

coupled with δN–H), δH2O. These are markers of proteins and water molecules which is the major component of the membrane, flagella, pili, and cytoplasm

63.

The peak at 1545 cm-1 for the Pt

membrane and at1535 cm-1 for the PANi-Cu membrane are assigned to Amide II (δN–H coupled with νC–N) which is a principal compound of proteins. The peak at 1401 cm-1 for both membranes are assigned to νsCOO− which is a signature of amino acids, fatty acid chains and mostly composed of peptidoglycan. The peaks at 1232 and 1260 cm-1 are assigned to phosphodiester, phospholipids, lipopolysaccharides (LPS), and nucleic acids which are the major component of membranes and nucleoid 63. The FTIR spectra of the membranes attached to CP/Pt and CP/PANi-Cu cathodes revealed the presence of polysaccharides and nucleic acids (1,600 to 900 cm−1), as well as the presence of proteins, lipids, and polysaccharides (3700 to 3000 cm−1). The spectra have variances both in shape and in absorbance intensity, indicating that there was variation in the composition and quantity of



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each component. The peaks obtained for both protein and polysaccharides are substantially higher for membranes attached to the CP/Pt cathode than with the CP/PANi-Cu cathode. These were deposited from the organic substrate as well as from the bacterial biomass. Interestingly, a low level of lipids was also observed in the Nafion membrane attached to the CP/Pt cathode, whereas the lipid peaks were slightly intense in the membrane attached to CP/PANi-Cu cathode. Water permeation and water intake in Nafion membrane have been reported in previous studies 51–53.

Fig. 8 ATR-FTIR spectra of Nafion membrane attached to (a) CP/Pt and (b) CP/PANi-Cu cathodes. It has been elucidated that the Nafion NRE - 212 has water membrane permeabilities (kw) of 4.09 at room temperature (23 ℃ used in the experiment) 65. Moreover, the water flux was higher in the Nafion NRE - 212 (50.8 µ), than Nafion NE – 1035 (89 µ), Nafion – 115 (125 µ), and Nafion – 1110 (254 µ) 65. However, another study suggested that the permeation coefficients may not be dependent on membrane thickness as long as the feed side is in contact with liquid water 53. In MFCs where proton transfer from the anode to cathode is facilitated via PEM (Nafion), it was observed that the interfacial proton transport enhances the water permeation through the hydrophilic phase of the Nafion membrane 53. The M9 media interacted readily with the interface of the Nafion membrane, and PtC nanoparticles from the Pt cathode since one side of the GDL is exposed to the anode. In contrast, the MFC with the CP/PANi-Cu cathode does not react with a metal catalyst. It is clear from the 3D profilometric images, and FTIR of the Nafion attached to both cathodes that they show variable biofouling. The CP/Pt cathode exhibited higher biofouling than the CP/PANi-Cu 22 ACS Paragon Plus Environment

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cathode. Reith et al66 reported that a biofilm which contained abundant platinum group elements nanoparticles was mostly dominated by proteobacteria. Intestinal electrogenic bacterial communities predominantly belong to α-, β-, γ- or δ-proteobacteria 67. From the FTIR data, it was clear that the CP/Pt cathode displayed higher and more complex macromolecule signatures than the CP/PANi-Cu cathode. The lower degree of biofouling on the CP/PANi-Cu cathode suggests that the use of the CP/Pt air cathode enhances the membrane biofouling. Ultimately, biofouling deteriorates the performance of the MFC by increasing the mass transfer resistance over time 68,69. Therefore, our study suggests that the Pt catalyst may not be suitable for long-term and practical MFC operation if used with an Al current collector. It is possible that the use of a more robust, but cheap current collector may give a better outcome, and this will be considered in future studies. 4. Conclusion Direct electrochemical synthesis of PANi-Cu composite on a carbon paper has been demonstrated to provide a cost-effective alternative to the use of expensive Pt-based cathodes for MFCs. Notably, we have demonstrated for the first time the in-situ synthesis of a Cu catalyst on top of a highly conducting polyaniline layer. The advantages of our proposed method over traditional methods include the direct incorporation of the catalyst on the substrate, attainment of desired coating thickness, and ease of tuning the loading by varying electrodepositing time, monomer concentration and/or increasing applied current density. Furthermore, the CP/PANi-Cu cathode is relatively cheap and easy to prepare for about 1% of the cost of a CP/Pt catalyst. Although the power density is relatively lower than for Pt catalyst, its ease of synthesis and much lower cost makes PANi-Cu an excellent alternative to Pt catalyst for large-scale MFC fabrication. The loss of cathodic activity with both cathodes after nine cycles of the MFC operation were 46% and 71% for CP/Pt and CP/PANi-Cu cathode, respectively. Most of the cathodic performance loss is contributed by the corrosion of the aluminum current collector which hinders the interfacial charge transfer from the cathode to an external circuit. The loss of cathodic activity can be suppressed by coating the current collector with PANi-Cu. The CP/Pt cathode was surprisingly found to be more prone to the biofouling of the Nafion membrane and may lead to the deterioration of performance in long-term MFC operations. The FTIR analysis of fouled membranes revealed that there were more complex macromolecules on the membrane attached to the CP/Pt cathodes supporting the biofouling of the Nafion membrane over a period of time. Biofouling may hinder the proton 23 ACS Paragon Plus Environment


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transfer from the anode compartment to the triple phase boundary of MEA for the cathodic reaction. Further studies are needed to improve the efficacy of the CP/PANi-Cu cathode and gain a good understanding of the membrane biofouling. We are conducting additional investigations into these two critical aspects.

Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors Conflicts of Interest There are no conflicts of interest to declare. Supporting Information Experimental setup of microbial fuel cells; X-Ray micrograph of Nafion membranes attached to CP/Pt and CP/PANi-Cu cathode; Surface profile of CP/Pt and CP/PANi-Cu cathode Acknowledgment: Jayesh M. Sonawane acknowledges the scholarship, research facility and travel grant provided by IITB-Monash Research Academy and Monash University. Thank you to Patrick Diep for the helpful suggestions to improve the quality of the manuscript. References (1)

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polyaniline-copper hybrid and its

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