Mixed transition metal-manganese oxides as catalysts in MFCs for bioenergy generation from industrial wastewater

Accepted Manuscript Title: Mixed transition metal-manganese oxides as catalysts in MFCs for bioenergy generation from industrial wastewater Authors: V.M. Ortiz-Mart´ınez, K. Touati, M.J. Salar-Garc´ıa, F.J. Hern´andez-Fern´andez, A.P. de los R´ıos PII: DOI: Article Number:

S1369-703X(19)30246-3 https://doi.org/10.1016/j.bej.2019.107310 107310

Reference:

BEJ 107310

To appear in:

Biochemical Engineering Journal

Received date: Revised date: Accepted date:

31 January 2019 14 June 2019 19 July 2019

Please cite this article as: Ortiz-Mart´ınez VM, Touati K, Salar-Garc´ıa MJ, Hern´andezFern´andez FJ, de los R´ıos AP, Mixed transition metal-manganese oxides as catalysts in MFCs for bioenergy generation from industrial wastewater, Biochemical Engineering Journal (2019), https://doi.org/10.1016/j.bej.2019.107310 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mixed transition metal-manganese oxides as catalysts in MFCs for bioenergy generation from industrial wastewater

V. M. Ortiz-Martínez1,*, K. Touati2, M. J. Salar-García 1,*, F.J. Hernández-Fernández1,3, A.P. de

Technical University of Cartagena, Chemical and Environmental Engineering Department, Campus Muralla del Mar, E-30202 Cartagena, Murcia.

(2)

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los Ríos3.

The University of Sciences and Technology of Oran ‘Mohamed Boudiaf’, Department of

University of Murcia, Chemical Engineering Department, Campus de Espinardo, E-30071

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(3)

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Biotechnology, Faculty SNV of Natural and Life Sciences, , 31000 Organ, Argelia.

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Corresponding authors: [email protected], [email protected]

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Graphical Abstract

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*

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Murcia.

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HIGHLIGHTS

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 Mixed manganese oxides as catalysts in microbial fuel cells  Cost-effective and facile synthesis materials for cathode construction

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 NiMn2O4 achieves 80 % out of the power density obtained with platinum

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 Significant COD removal from industrial wastewater using mixed oxide based MFCs

Abstract

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Microbial fuel cell (MFC) technology has attracted growing interest in the past decade for simultaneous bioenergy production and wastewater treatment. These devices require cost-effective cathode catalysts for the oxygen reduction reaction towards their practical implementation. The application of MFC technology for the specific treatment of industrial wastewater, which usually presents persistent pollutants, is also gaining

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growing attention. This work is aimed at studying the use of mixed manganese oxides with copper and nickel synthesized by co-precipitation in MFC devices fed with

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industrial wastewater. The new catalysts were assessed in terms of power performance and chemical oxygen removal. Active phases and constructed cathodes were

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characterized by XRD, EDX and elemental mapping. Among the materials analyzed,

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the oxide with formula NiMn2O4 offered good power performance, achieving 80 % (439

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mW/m3) out of the power density obtained with Pt (549 mW/m3). Ssignificant COD

previous pretreatment applied.

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removal from industrial wastewater after 168 h of operation was achieved with no

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Keywords: microbial fuel cells; bioenergy; wastewater treatment; manganese oxides.

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1. Introduction

Energy crisis and environmental protection are among the pressing issues that

the world is facing nowadays. Among the most alarming human effects on the

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environment is water pollution. Human activities such as industry, agriculture or urbanization usually introduce several types of pollutants including organics, heavy metals and persistent pollutants into water environments. In the last years, fuel cells and bioelectrochemical systems (BESs) have emerged as prominent technology for renewable energy generation and environmental management[1–3]. Particularly, 3

Microbial Fuel Cells (MFCs) attract big interest since these systems are capable of producing bioenergy while treating wastewater[4–8]. Thus, this technology could help to reduce the environmental impact derived of non-renewable energy sources as well as to alleviate problems related to the availability of clean water[9,10]. The performance of MFC devices relies on many factors including set-up design, flow rate, types of

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substrate, separators and electrode materials. Single-chamber MFCs have shown to be

the most promising configuration. In them, the oxidation of organic matter takes place

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at the anode chamber while electrons and protons are generated. Then, they are

externally and internally transferred to an air-exposed cathode, respectively, combining

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with oxygen to form water (oxygen reduction reaction or ORR)[11–15].

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Precious materials such as platinum offer outstanding catalytic activity against the

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ORR, but its price is very high due to its low abundance in nature. Other noble metals

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such as palladium and silver have also been investigated as an alternative to platinum[16,17]. However, in order to enhance the sustainability of this technology,

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new economically advantageous materials are constantly being investigated for MFC

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systems. Some authors have developed very low cost options that include electrodes based on activated carbon, in the absence of other metallic elements [11]. For example,

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Gajda et al. [18] used activated carbon mixed with PTFE solution supported onto carbon veil as cathode material in cylindrical ceramic-based MFCs. Another option consists of the development of biocathodes, which use microorganisms (biocatalysts)

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such as proteobacteria supported on carbon materials, but whose functioning is subject to stricter operating conditions[19]. In contrast to them, the use of catalysts based on first-row transition metals (e.g. Fe, Co, Ni or Mn) is among the most promising options to completely replace noble MFC catalysts. Some recent materials studied in these systems include metal complexes and metal oxides. Some examples within the first 4

group are macrocyclic complexes of iron and cobalt, respectively, such as iron phthalocyanine (II)[20], iron(III)-EDTA complexes[21,22] and tetramethoxyphenylporphyrin of cobalt (II)[23], and copper (II) and nickel (II) phthalocyanine nanoparticles[24].

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Single metal oxides such cobalt and manganese oxides draw great interest as ORR catalysts since the metal ions present multiple oxidation states (+2, +3 and +4) that

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favor their catalytic activity and represent cost-effective options [25,26]. This type of catalysts is usually supported on carbonaceous materials such as carbon nanotubes (CNTs), carbon black or even graphene to improve its stability and electronic

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conductivity. The substitution of Co and Mn by other metals (e.g. Ni and Cu) giving

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rise to mixed metal oxides has proven to improve their intrinsic electrocatalytic oxygen

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reduction activity[27]. In such cases, the oxides in which partial substitution occurs can

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be formulated as AxB(3-x)O4, where A and B represent different metal cations. This type of substitution enables the creation of a great variety of oxides of different composition

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and crystallographic structure, e.g. spinel (MgAl2O4) and ilmenite (FeTiO3) type

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structures. Thus, mixed oxides open up a wide research field to evaluate their performance in different energy systems.

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Mixed manganese oxides have been studied as cathode catalysts in chemical energy systems such as solid fuel cells or hydrogen fuel cells[28–30] but their investigation in

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BESs remains very scarce. In this work, several metal oxides with different crystal structures were investigated for the first time as cathode catalyst in MFCs, thereby contributing to widening the range of platinum group metal-free materials that might replace noble metal-based catalysts in these devices for their practical implementation. Previous related research works mainly focused on spinel cobalt-manganese oxides or cobalt spinel oxides for their application in MFCs [31,32], however here new materials 5

in the absence of cobalt are tested. The performances of mixed valance oxides with spinel and ilmenite-type structures deposited on carbon cloth were studied mainly in terms of power generation and compared with that achieved by platinum as standard catalyst. Air-cathode systems were employed as this configuration displays higher

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performances in comparison with dual chamber MFCs [33]. 2. Materials and methods

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2.1 Catalyst preparation.

The catalysts analyzed in this work consisted of mixed oxides of manganese and

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another transition metal, nickel or copper, respectively, with formulas NiMn2O4,

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NiMnO3 and Cu1.4Mn1.6O4 and displaying different crystallographic structures, not

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previously investigated in MFC systems. The three phases were synthesized through a

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co-precipitation route involving the formation of the hydroxides of the respective metals and subsequent thermal decomposition. This method was selected since it constitutes a

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relatively simple synthesis route and has been successfully employed in previous works [34]. Metal sulfates were employed as precursors. NiMn2O4 and NiMnO3 oxides were

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synthesized from NiSO4∙6H2O and MnSO4∙4H2O (99.0 and 98.5% purity, respectively,

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Merck) using stoichiometric molar ratios. In the case of the Cu1.4Mn1.6O4 oxide, CuSO4∙5H₂O (purity> 99%, Merck) was used as a precursor along with MnSO4∙4H2O in stoichiometric proportions. The respective precursor mixtures were mixed with a

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magnetic stirrer while adding dropwise NaOH 3 M until a pH of 10 was achieved. The stirring was maintained for 7 h at room temperature. The brownish precipitates obtained were filtered and washed with distilled water and dried at 120 °C for 3 h in a conventional oven. The products were ground manually in a mortar to obtain a fine and homogeneous powder. The products were subjected to heat treatment at 590 °C for 48 h

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in a muffle. Finally, the samples were manually ground once again and hereafter heated at 590 °C for 36 h to remove impurities. 2.2. Cathode preparation. Carbon cloth with waterproofing treatment of 5 % and a thickness of 0.5 mm (Fuel Cell

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Earth, USA) was employed to prepare cathode electrodes of 3 cm of diameter for the deposition of the catalyst samples, including a rectangular section (1.5 cm x 14 cm) to

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connect the electrode to the external resistance load of the MFC systems. The carbon

fabric acts as a diffusion layer of oxygen and conductive material (electrode). To fix the catalysts on the carbon cloth cathodes, respective liquid solutions of the samples in

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isopropanol with water and PTFE (60% w/w in water, Sigma-Aldrich) were prepared.

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The catalysts were supported on carbon black for the enhancement of the electronic

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conductivity. In all cases, a dark ink was generated, which in turns was subjected to

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agitation for 15 min in an ultrasonic bath for its homogenization. Subsequently, the inks

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were uniformly sprayed on respective carbon cloth pieces with a compressed air gun in several passes until the desired catalyst loading was achieved. The cathodes were

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allowed to dry for 24 h for water and isopropanol evaporation. Commercial platinum supported on carbon was employed as standard catalyst for comparison (60% w/w

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carbon support, Cymit Quimica, Spain) with a standard loading of 0.5 mg/cm2 (with respect to the cathode area). In the case of manganese oxides with nickel or copper, the

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samples were mixed with carbon black in the same proportion as that of platinum/carbon support as commercially available (60% w/w), with a total catalyst content of 1 mg/cm2 to obtain a uniform distribution. This loading was fixed considering previous research studies [35]. 2.3. Catalyst and cathode characterization.

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The synthesized oxides were firstly analyzed with a powder X-ray diffractometer (Bruker D8 Advance, Kristallofex K760-80F X-ray generator) to determine its crystalline structure and estimate the purity of the phases (semi-quantitative analysis). The semi-quantitative analysis of the identified phases was obtained from the relative integrated areas of the intensity spectra (Reference Intensity Ratio method). The

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cathodes prepared with respective active layers of platinum, NiMn2O4, NiMnO3 and Cu1.4Mn1.6O4 supported on carbon black, respectively, were morphologically

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characterized by scanning electron microscopy (SEM) and by elemental mapping with a Hitachi S-3500-N equipment coupled to an analyzer for X-ray dispersive energies

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(EDX). The cathodes were also characterized by the linear sweep voltammetry (LSV)

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technique being assessed in MFCs fed with wastewater. For this, a configuration of

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three electrodes was used in an air-cathode cell system using 250 mL of phosphate

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buffered saline solution (PBS) of pH=7 (137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4 and 1.8 mM KH2PO4), with the cathodes directly facing the PBS solution. In this three-

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electrode assembly, the manufactured cathodes act as the working cathode, the graphite rod as counter electrode and the Ag/AgCl electrode was used as a reference electrode.

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While the working cathodes were clamped at the end of the glass projection of the stack

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reactor, the counter electrode and the reference electrode were immersed in the PBS solution. In addition, this solution was previously saturated in oxygen by bubbling air (for this test the reactor remains open). The LSV technique was applied in the range of

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0.6 to -0.6 V, with a scanning speed of 5 mV/s. With the LSV technique it is possible to measure the intensity generated when a certain potential is applied on the working electrode, allowing to establish a preliminary comparison of the catalytic activity of the manufactured cathodes. 2.4. MFC performance 8

2.4.1. Configuration and operation mode. Air-cathode single-chamber MFC systems were selected to perform the assessment of the oxides. They were formed by an anodic chamber consisting of a glass reactor (Schott Duran, Germany) with a maximum capacity of 250 mL, which was filled with

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industrial wastewater. The anode contains a bed of porous graphite granules of irregular shape with an average size of 0.3 cm in diameter and an 18 mm long graphite rod (Fuel

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Cell Earth, USA). The amount of graphite granules used was 100 g and the anodic

chamber was filled with 150 mL of wastewater. The graphite rod in contact with the granules allows the conduction of the electrons generated in the digestion of the organic

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matter to the external circuit. The glass reactor was closed with a plastic cap on which

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different holes were made for the introduction of the graphite rods and for taking

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samples of wastewater at different times. These holes were perfectly sealed with

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silicone to maintain anaerobic conditions during the operation time.

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The reactors were equipped with a straight glass tube (1.5 cm inner diameter) at which the separator is placed. Nafion®-117 membranes (Fuel Cell Earth, USA) were employed

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as high proton conductivity separators placed between the anode chamber and the carbon cloth cathodes by using rubber gaskets and 30 mm rounded metal joint clips (J.P

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Selecta, Spain).When the cathode and separator are assembled, the active face of the cathode (area in which the catalyst is deposited) remains in contact with it. The

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membranes were cut in a circular shape (2.5 cm in diameter). Prior to use, membranes were activated by firstly being immersed in a H2O2 solution (3% v/v) for 1 h at 80 ° C, washed with distilled water and immersed again in a 1 M sulfuric acid solution for 1 h at 80 °C. Afterwards, they were washed again and immersed in hot distilled water for 1 h at 80 °C. Finally, the membranes were rinsed before being assembled. The systems were externally load with an external resistance load of 1 kΩ. 9

2.4.2. Voltage and power measurements The fuel cell systems were operated in batch mode and the voltage measurements under the fixed load was made with a Picolog multichannel data acquisition system for 168 h (Pico Technology, UK). The continuous acquisition mode enables the study of the

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fluctuations in terms of potential and the determination of the moment when the systems approach to the steady state. The external resistance loading (Rext) of the MFC systems

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was varied from 1.1 MΩ of 0.9 Ω to perform polarization tests. For each point, voltage output (E) was measured after stabilization (typically a few minutes), including initial measurement of open circuit voltage (OCV). Current was (I) calculated as I = E / Rext

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while power was obtained as P = I2 ∙ Rext or alternatively as P=E2 / Rext. The values of

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current and power were normalized to available volume of wastewater employed as

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fuel. Polarization curves (E) and power curves (P vs I) were plot for each configuration

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to respectively analyze how MFCs maintain voltage output as a function of the current density and to calculate the maximum power density attained by MFCs. The internal

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resistance (Rint) of the MFCs can be calculated from the maximum power density, when

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the external load is equivalent to the internal cell resistance [36]. Since the same anode conditions were maintained in all MFC systems equipped with different cathode

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catalysts, the voltage differences observed can be attributed to the different cathode materials employed. Therefore, the power generated by an MFC depends on the choice of the cathode and can be used for comparison purposes in terms of cathode

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performance [36].

2.4.3. COD removal and Coulombic Efficiency.

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MFCs were fueled with wastewater from an industry devoted to the manufacture of white mineral oils and natural petroleum sulfonates equipped with a small plant for the treatment before discharge to the general network. The selected water sample was collected from the entrance to a secondary treatment stage with an initial COD load of 1750 mg∙L-1 and a pH of 6.8. This pH value is suitable for the development of bacterial

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activity. Wastewater was employed without any additional treatment or previous

conditioning for the inoculation of the MFCs. Wastewater treatment was monitored by

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measuring the soluble chemical oxygen demand (COD). For a certain time, the

percentage of COD removal (CODe) can be calculated from the initial value (COD0)

CODo

x 100

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CODo −COD𝑡

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CODe (%) =

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and the one measured at that instant (CODt) by:

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To compare the evolution of this parameter in the MFC systems against conventional anaerobic digestion, a reactor with the same volume of wastewater as that used in the

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MFCs was monitored (150 mL) under anaerobic conditions, in the absence of any other element (baseline). Water samples were taken on days 2, 4 and 7 (final time). For COD

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measurements, a photometric method based on COD tests was used (Merck Millipore).

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Coulombic efficiency (EC) expresses the fraction of electrical energy that can be generated in a fuel cell from a given substrate and was calculated as follows:

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EC (%)

=

t

M .∫0 I dt

F∙b∙Va ∙ΔCOD

x 100

(2)

Where M is the molecular mass of oxygen (32 g/mol), I is the current intensity (C/s), F is the Faraday constant (96485 C/mol e-), b is the number of electrons produced per mole of oxygen (b = 4) according to the theoretical oxygen reduction reaction that takes

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place at the cathode, Va is the volume of substrate (wastewater) at the anode chamber (150 mL) and ΔCOD is the variation of the chemical oxygen demand of the water used for a time t. The integral of the numerator is equivalent to the accumulated charge for a given time.

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3. Results and discussion 3.1. XRD analysis.

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The XRD diffractograms obtained for the synthesized samples, namely NiMn2O4,

NiMnO3 and Cu1.4Mn1.6O4, are presented in Figure 1 and were compared with those

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stored in Powder Diffraction File (PDF) databases for their identification and detection

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of secondary phases. Figure 1.A shows the XRD pattern for the main phase NiMn2O4

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oxide, which corresponds to PDF#04-002-3144. The analysis reveals oxygen face-

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centered cubic lattice with spatial group Fd-3m (227) and presenting largely inverse spinel structure [37]. In the diffractogram, a small percentage of 2.5 % (semiquantitative

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analysis) of another undesirable phase of MnNiO3 was formed. The XRD pattern (Figure 2.B) for the NiMnO3 oxide reveals a rhombohedral structure with hexagonal

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packing and spatial group R-3 (148), according to the PDF file # 00-048-1330. This

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structure is typical of ilmenite[38]. In this case, another two different phases apart from NiMnO3 appear identified as Mn2O3 and NiO, being the first most abundant with a percentage of up to 9.4 %. The NiO phase is not clearly visible in the diffractogram, but

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the analysis confirm its presence with a percentage of 0.5 %. As shown in Table 1, among the three oxides synthesized, the phase NiMnO3 presents the highest percentage of total secondary phases. Finally, Figure 1.C shows the XRD pattern for the oxide Cu1.4Mn1.6O4, with face centered packing and spatial group Fd-3m (227) according to the diffraction file PDF#00-035-1030, corresponding to a normal spinel type structure

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[39]. Secondary phases also appear in this case (CuO), although in a lower percentage in comparison with NiMnO3 and similar to the percentage of undesirable phases for NiMn2O4. Table 2 summarizes the XRD results for the three catalysts synthesized, including the values of the semi-quantitative analysis. According to these values, the synthesis of the phases used is generally satisfactory, although the NiMnO3 phase has a

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higher content of secondary phases.

Figure 1. XRD patterns of metal oxides Table 1. Semi-quantitive (SQ) analysis of synthesized oxides (XRD analysis). 13

NiMnO3

Cu1.4Mn1.6O4

Identified phases NiMn2O4 NiMnO3 NiMnO3 Mn2O3 NiO Cu1.4Mn1.6O4 CuO

SQ analysis (%) 97.5 2.5 90.1 9.4 0.5 97.4 2.6

PDF of main phase PDF#04-002-3144 PDF#00-048-1330

PDF#00-035-1030

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Targeted oxide NiMn2O4

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3.2. Characterization of cathodes.

Carbon cathode electrodes were characterized once loaded with the active layer formed by the respective catalysts (including Pt) and supported with carbon black in the case of

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the oxides. They were first analyzed by scanning electron microscopy (SEM). Figure 2

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shows representative SEM images of each type of the prepared cathodes (first column).

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The carbon cloth fibers are cleary visiable in all SEM images except for the cathode

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loaded with Cu1.4Mn1.6O4. In this case, oxide particles appear more agglomerated after being deposited over the difussion layer in comparison with the rest of catalyts. Besides,

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in the SEM images for NiMn2O4 and NiMnO3, some particles appear to be less finely

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divided in comparision to commercial Pt. The elementary mapping images obtained with X-ray dispersive energy analysis (Figure

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2) offer the distribution of the metallic elements over the cathode catalysts. Figure 2 offers representative imIn cases for which more than one metal is present (mixed oxides

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of manganese with copper or nickel, respectively), the overlapping of individual colors can offer a different set color (for example, for the oxide Cu1.4Mn1.6 O4, copper and and manganese are identified in green and red, respectively, with a final yellowish color due to overlapping). In any case, in general, a homogeneous distribution of the different catalysts (including platinum) is observed when they are printed on the carbon cathodes. Moreover, EDX spectra revealed the presence of the characteristic elements of the 14

active phases as well as fluorine from the PTFE solution used as binder and carbon from

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carbon cloth and the carbon back support of the catalysts (see suplementary material).

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Figure 2. SEM images and elemental mapping analysis of loaded cathodes with: a)

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platinum, b) NiMn2O4, c) NiMnO3 and d) Cu1.4Mn1.6O4.

3.3. LSV characterization. The respective cathodes with different types of catalysts were analyzed with the LSV technique facing a PBS solution saturated with oxygen as described in section 2.3. The cathodes were analyzed in the same conditions (with the oxides supported on carbon 15

black) as they were set up in the MFC systems from -0.6 to 0.6 V at a scan rate 5 mV.s-1 according to the method described by Li et al.[40]. The results obtained with this technique (intensity vs. potential) are shown in Figure 3. The measured intensity shows a similar behavior of the curve (in shape) for the three oxides synthesized, with a flatter zone between 0.2 and -0.2 V, which differs from the intensity curve of the Pt-loaded

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cathode, which displays a more pronounced drop and reaches the highest intensity current (in absolute value) at the end of the interval, -3.01 mA at -0.6 V. For this

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potential value, in the rest of cathodes, maximum intensities of -2.64 mA (NiMn2O4), 2.47 mA (Cu1.4Mn1.6 O3) and -2.19 mA (NiMnO3) were obtained. The onset potential

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also follows this trend. This first test of the loaded cathodes with respective catalysts

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enables it to establish a preliminar tendency of the catalytic activity of the phases for

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their subsequent application in MFCs. While the Pt cathode offered the greatest

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response, the oxide NiMn2O4 outperfomed the rest of catalysts. These values will be compared with the performance of the catalyst-loaded cathoses once they are set-up in

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MFCs and fed with wastewater.

Figure 3. LSV analysis. 3.4. Power performance in MFCs. 16

Figure 4 displays the voltage output measured over time for each configuration under an external resistance load of 1 kΩ. In all cases, there is a decreasing trend from the initial time to the stabilization of the voltage response. After 3 days, the polarization were performed (time indicated by the dotted line in Figure 4). Since Figure 4 offers the voltage reached by the MFC systems externally load with 1 kΩ, the levels of voltage

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does not have to be necessarily the optimal according to what was explained in the

materials and methods section, but the measurements ennable the determination of the

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moment from which the systems reaches the steady state for the performance of the

polarization tests. On the other hand, from day 5 a general tendency of decrease in the

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voltage response is observed.

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Figure 4. Voltage output for external fixed loading (1 kΩ).

Figure 5 shows the results obtained from the polarization tests, including power curves (P vs. I) and polarization curves (E vs. I) for the replicates of the MFC systems working with the different catalyst-loaded cathodes. Power measurements were fitted to

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polynomal curves with a coefficient of determination r2 over 0.99 for all catalysts except for the MFC systems working with NiMnO3, in which case the coeficient was 0.98. Table 5 summarizes the most relevant parameters related to MFC perforamce. As can be seen, the configurations working commercial Pt sprayed cathodes offered the highest power levels, with a maximum power density of 549.57 mW/m3 (and an associated IP

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current of 3.01 mA/ dm3). The widest current density ranges are also reached by these

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systems, with a maximum intensity of 4.97 mA/dm3.

Among the oxides analyzed, the systems loaded with NiMn2O4 offered the highest power density output with 439.43 mW/m3, which represents 79.9 % out of the power

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density value obtained in comparison with platinum. In the case of Cu1.4 Mn1.6O4

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approximately half the power density is reached with respect to this noble metal (48.4

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%). Although this value is lower compared to NiMn2O4, the power output can still be

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considered significant. On the contrary, MFCs working with NiMnO3 sprayed cathodes achieved the lowest powest density with 153.21 mW/m3, representing only 27.8 % out

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of the power generated by using Pt. This trend is repeated for the intensity currents

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associated with the maximum values of the power density (Ip), with NiMn2O4 reaching 2.69 mA/dm3.

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On the other hand, open circuit voltage (OCV) was measured in each system about 30 min after the external resistance were removed to performe polarization tests. In

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observed that the systems that offer higher levels of power density, also offered higher OCV values (MFC working with Pt and NiMn2O4), although this relationship is inveted when comparing the NiMnO3 and Cu1.4Mn1.6O4. In any case, the voltage response decreases with increasing intensity level (increasing the external resistance value). As indicated in section 2, a more pronounced drop of OCV value is related to internal ohmic losses. This fact is revealed precisely when comparing the behavior of the 18

NiMn2O4 and Cu1.4 Mn1.6O4 oxides. Although the OCV values in both configurations are not very different, the Rint value is cleary higher for NiMnO3 (859.24 versus 430.80 Ω).

OCV (mV)

Pmax (mW/m3)

Ip (mA)

Rint

Pt

508.73

549.57

3.01

NiMn2O4

457.75

439.43

2.68

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Table 2. Relevant performance parameters of MFC working with different cathodes

NiMnO3

397.37

153.71

Cu1.4Mn1.6O4

333.34

265.83

405.48

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410.28 859.24

2.04

430.80

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1.10

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According to the parameters related to electrical performance, the following favorable

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trend can be established for the synthezied oxides when used as catalysts in MFCs fed

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with wastewater: Pt > NiMn2O4 >> Cu1.4 Mn1.6O4> NiMnO3. As expected, platinum

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offered the best performance as catalyst due to its noble nature. However, the oxide with spinel structure NiMn2O4 offered close values, and the oxide Cu1.4Mn1.6 O4, also with

PT

spinel structure, still allowed significant power density levels to be obtained. Manganese oxide with nickel at a 1:1 substitution ratio and ilmenite type structure

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achieved rather modest results.

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Figure 5. Power (blue color) and polarization (red color) curves for MFC systems.

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On the other hand, it is worth observing that the trends stablished for the different phases when assessed in MFC systems is in accordance with the results obtained from

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the LSV test although in this case the differences are not so significant when the oxides NiMn2O4 and Cu1.4Mn1.6O4 are compared. In spite of this, the linear sweep voltammetry

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test allows preliminary determination of the catalytic activity tendencies of the oxides synthesized against platinum. It must be taken into that the medium with which the loaded cathodes are in contact are not the same in the three-electrode system as when

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they are assembled in the MFC systems. In the first case they are in direct contact with an O2-saturated PBS solution, while in the second they are in contact with the Nafion® membrane that acts as a separator.

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Regarding the Cu1.4Mn1.6O4 oxide, the greater agglomeration of this solid on the carbon cathodes may have influenced by limiting the level of performance achieved, since the lower surface dispersion could increase the cathodic resistivity. Likewise, the oxide with the worst performance, NiMnO3, is also the oxide that, according to the XRD analysis, offered a higher percentage of unwanted phases. These results suggest that

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other synthesis and spraying methods may be tested in future works to optimize their

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performance in MFC systems.

The analyzed manganese oxides offered satisfactory performance when displaying spinel structure (NiMn2O4 and Cu1.4 Mn1.6 O4). In comparision with other research

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studies, there are works that analyze mixed oxides that include cobalt together with

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manganese and a third metal (for example, copper). Given MFC configurations and the

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materials used for their construction can vary greatly, an adequate parameter to establish

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this type of comparisons is the percentage of yield of power performance with respect to that achieved by the platinum. For example, it has been reported that cathodes based on

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the mixed oxide Mn0.6 Cu0.4Co2O4 have reached up to 86% of the power density offered

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by platinum, in a dual-chamber fuel cell configuration. The composition of this type of oxide can significantly influence the performance, since for catalysts with formulas

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Mn0.4Cu0.6Co2O4 and Mn0.2Cu0.8Co2O4 the percentages of performance against platinum fall around 56 % [31]. In the case of manganese and cobalt oxides such as CoMn2O4, without a third type of metal cation, the percentage of performance against platinum is

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76 % in a simple chamber configuration similar to that used in this work but fed witth synthetic wastewater. This value falls to 52 % when the atomic ratio Mn:Co decresases up to 1 and 0.5, respectively [32]. In this work, a maximum percentage of 78.6 % has been reached with the catalyst NiMn2O4 when the systems are fed with real wastewater, and a percetange of 50 % against platinum was achieved with Cu1.4Mn1.6O4 oxide. 21

Although the yield obtained with the manganese oxides synthesed is lower than that of platinum, the cost of the lab-prepared catalysts remains well below. While the cost of the commercial platinum used in this study amounts to 250 €/g, the cost of the precursors used for the synthesis of the oxides (manganese, copper and nickel sulphates, respectively) are between 0.1 and 0.38 €/g . The lower cost of the mixed oxides

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provides a clear advantage for up-scaled systems.

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3.5. Wastewater treatment.

The treatment of the wastewater used as fuel in the anodic chambers was assessed in terms of COD removal. Figure 6 displays the evolution of this parameter over time for

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the MFC working with the different cathodes. This parameter was also monitored in a

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reactor in which a conventional anaerobic digestion is performed (glass reactor loaded

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with the same amount of wastewater as the rest of the devices but in the absence of

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typical MFC components). As a first result, it can be highlighted that COD elimination

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values are generally higher in the MFC reactors compared with conventional anaerobic digestion at the end of the cycle. In the case of the systems working with NiMnO3, this

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parameter remains below those achieved in MFC conditions and close to the performance provided by anaerobic digestion, being also the system that offerred the

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lowest electrical performance. In the most favorable cases, the percentage of COD removal reaches 58.1 % and 56.0 % for the cathodes of platinum and NiMn2O4,

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respectively, followed by 49.3 % for Cu1.4Mn1.6O4 and 34.1 % when NiMnO3 is used. COD removal in anaerobic digestion conditions remains only around 31.6 %. On the other hand, the pH value in the anode at the final time remains around the initial pH value (6.8).

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Figure 6. COD removal in MFC systems

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While the evolution of the percentage of COD offers information about the treatment of

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waterwater in the devices, the coulombic efficiency (EC) allows it to study the

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relationship between the number of electrons transferred from the substrate present in the wastewater to the anode (current output) and the theoretical number of electrons that

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could be produced. According to the values shown in Figure 7, it is observed that the highest coulombic efficiency (10.5 %) is achieved by the fuel cell that works with Pt-

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sprayed cathode. Among the oxides analyzed, the most efficient MFC system is that

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loaded with NiMn2O4, followed by the Cu1.4Mn1.6O4 oxide. The system working with NiMnO3 cathode only offered the half the EC of the Pt-based system. The relative order of the EC values is in line with the levels of catalytic activity of the

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different materials in terms of maximum power density. The values of coulombic efficiency obtained in this work are relatively low in all cases, including Pt. MFCs can reach low EC values when wastewater is used without previous conditioning. Previous works have reported CE below 15% when using wastewater without additional substrate in MFC systems similar to the devices used in the present work [41]. However, from a 23

practical point of view, the direct use of waste effluents is of great interest for practical

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Figure 7. Coulombic efficiency (CE) in MFC systems.

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implementation.

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Conclusions

In this work, the assessment of several mixed oxides of manganese with nickel and

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copper, respectively, was performed as alternative catalysts to platinum for the construction of cathodes in microbial fuel cells in real wastewater environments. The

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yield of these oxides was quantified in terms of maximum power density and compared

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with that achieved by platinum as standard noble metal. In addition, the evolution of COD in the anodic chamber was evaluated and the results were compared to the performance of conventional anaerobic conditions with the same type of feed. Among

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the active phases synthesized, the oxide with spinel structure NiMn2O4 offered the highest performance with a power level of 439.43 mW/m3, which represents almost 80 % out of the value obtained with platinum. This oxide also showed the lowest internal resistance value and the highest coulombic efficiency (again surpassed only by platinum). The second oxide with the best results is the manganese-copper oxide with 24

formula Cu1.4Mn1.6O4 and spinel structure, obtaining over 50 % of power output in comparison with Pt. Besides, wastewater treatment rates were higher in all the devices compared to anaerobic digestion, although higher purification was also observed in the systems that offered higher power density levels (NiMn2O4). Mixed oxides of transition metals can be formulated to different compositions and represent a wide field of

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research. They offer a real and cheaper alternative for the scaling of this technology. The performance of the oxides analyzed may be further improved by using other

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advanced supporting materials such as carbon nanoparticles or alternative deposition

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techniques in future research attempts.

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