Assessing the ageing process of cation exchange membranes in bioelectrochemical systems

Elsevier Editorial System(tm) for International Journal of Hydrogen Energy Manuscript Draft Manuscript Number: Title: Assessing the ageing process of cation exchange membranes in bioelectrochemical systems Article Type: Full Length Article Section/Category: Electrolysis / Electrolyzers Keywords: Bioelectrochemical system; Cation exchange membranes; Ageing of membranes; Microbial electrolysis cell. Corresponding Author: Miss María Isabel San Martín, Corresponding Author's Institution: Universidad de León First Author: María Isabel San Martín Order of Authors: María Isabel San Martín; Francisco Javier Carmona; Pedro Prádanos; Antonio Morán, Proffesor; Adrián Escapa Abstract: Bioelectrochemical systems (BES) encompass a group of biobased technologies capable of directly converting organic matter into electricity. In these systems, which are derived from conventional electrochemical technologies, the ion exchange membrane represents a key element because of its influence on the economic feasibility and on the performance of BES. This study examines the impact of long-term operation of a BES on the mechanical, chemical and electrochemical properties of five different kind of cation exchange membranes (Nafion-117, CMI-7001, Zirfon UTP 500, FKE and FKB) through several techniques: (i) scanning electron microscopy (SEM) and atomic force microscopy (AFM) to assess the changes on the membranes surface, (ii) thermogravimetric analysis (TGA) to evaluate the structural stability of the membranes, and (iii) ion exchange capacity (IEC) to monitor any change in their electrochemical properties. Results confirmed that there is not an ideal membrane for BES. While Nafion and CMI-7000 exhibited the strongest chemical structure, they also underwent the highest fouling as revealed by a fast increase in surface roughness. Assessing the ageing process of cation 1 exchange membranes in bioelectrochemical 2 systems 3 M. Isabel San-Martín1, Francisco Javier Carmona2, Pedro Prádanos3, Antonio Morán1, 4 Adrián Escapa1,4,*. 5 6 1 Chemical and Environmental Bioprocess Engineering Group, Natural Resources Institute 7 (IRENA), Universidad de León, Avda. de Portugal 41, 24009, León, Spain. 8 2 Dpto. de Física Aplicada. Escuela Politécnica, Universidad de Extremadura, 10004 Cáceres, 9 Spain. 10 3 Grupo de Superficies y Materiales Porosos, Dpto. Física Aplicada, Facultad de Ciencias, 11 Universidad de Valladolid, 47071, Valladolid, Spain. 12 4 Department of Electrical Engineering and Automatic Systems, Universidad de León, Campus de 13 Vegazana s/n, 24071 León, Spain 14 15 * Corresponding author: [email protected] [email protected]; Tel.: +34 987295394 16 17 Abstract: Bioelectrochemical systems (BES) encompass a group of biobased 18 technologies capable of directly converting organic matter into electricity. In these systems, 19 which are derived from conventional electrochemical technologies, the ion exchange 20 membrane represents a key element because of its influence on the economic feasibility 21 and on the performance of BES. This study examines the impact of long-term operation of 22 a BES on the mechanical, chemical and electrochemical properties of five different kind of 23 cation exchange membranes (Nafion-117, CMI-7001, Zirfon UTP 500, FKE and FKB) 24 through several techniques: (i) scanning electron microscopy (SEM) and atomic force 25 microscopy (AFM) to assess the changes on the membranes surface, (ii) 26 thermogravimetric analysis (TGA) to evaluate the structural stability of the membranes, and 27 (iii) ion exchange capacity (IEC) to monitor any change in their electrochemical properties. 28 Results confirmed that there is not an ideal membrane for BES. While Nafion and 29 CMI-7000 exhibited the strongest chemical structure, they also underwent the highest 30 fouling as revealed by a fast increase in surface roughness. 31 Keywords: Bioelectrochemical system; Cation exchange membranes; Ageing of 32 membranes; Microbial electrolysis cell. 33 *Manuscript (Revised, clean, unmarked, FINAL version) Highlights 34 • No significant fouling, breakings or deformities in membrane surfaces were 35 observed. 36 • Nafion and CMI showed the most robust chemical structure and roughness 37 increase. 38 • The ion exchange capacity showed a moderate decay in all membranes. 39 • After the four-month membrane operation, the current density kept almost 40 constant (~1 A∙m-2). 41 42 1. Introduction 43 For the last 15 years, bioelectrochemical systems (BES) have experienced an 44 intense phase of research and development, proving to be a versatile group of 45 technologies with a wide range of potential applications (e.g.: energy recovery from 46 organic matter, chemical production from carbon dioxide, nutrient recovery from 47 waste streams, etc.) [1]. BES are also capable of utilising a broad range of organic 48 and inorganic substrates [2] which gives them a great operational flexibility. Despite 49 that, and despite the numerous scale-up endeavours reported in the literature [3], 50 BES are far from being a mature technology [4]. The still low current densities, 51 relatively large capital cost and the difficulties associated with energy 52 harvesting/management are often reported as major challenges in their way towards 53 practical application. 54 The ion exchange membrane (IEM), which is a core element in BESs, has a 55 significant impact on both the performance [5] (e.g.: coulombic efficiencies and 56 internal electrical resistance) and the capital costs [4], being responsible for up to 57 40% of the overall cost of BES [6]. Thus, it is not surprising that, in an effort to 58 improve the commercial perspectives of BES, many researcher groups have come 59 up with alternative designs that completely dispense with the membrane. Although 60 many of them have proved to be successful in several fields of application such as 61 organic and inorganic contamination removal [7,8] or energy production [9], the IEM 62 becomes a critical element when the aim is to optimize energy efficiency (by 63 avoiding oxygen and hydrogen crossover [4,6] ) or accomplish processes of 64 industrial interest (nutrients recovery, microbial electrosynthesis or seawater 65 desalination [10][11][12]). The application of several types of IEM such as anion 66 exchange membranes (AEM), cation exchange membranes (CEM), microfiltration 67 membrane, bipolar membrane and ultrafiltration membranes to BES has been 68 extensively covered in the literature [13]. Nafion has remained the most popular 69 because of its good mechanical stability and high proton selectivity [14], although its 70 high price has motivated the search for other cheaper alternatives [11], such as 71 Fumasep, Ultrex, Zirfon or Hyflon. 72 Ideally, a membrane apt for BES use should have low electrical resistance (to 73 facilitate the ion transport), should prevent gas and substrates diffusion, have a high 74 biofouling resistance and at a relatively with low-cost [15]. Membrane degradation, 75 commonly known as ‘membrane ageing’, is another important aspect as it is directly 76 related to their durability. Membrane ageing is usually characterised in terms of 77 physical, chemical and electrochemical stability and to this end, a vast array of 78 methods is available including microscopy, impedance spectroscopy or titration 79 technics. Previous studies have made use of these techniques to investigate the 80 ageing of individual membranes [16], but there is a lack of knowledge on how these 81 membranes compare in terms of their ageing patters a stability. In this study, we aim 82 at filling this gap by assessing how the mechanical, chemical and electrochemical 83 properties of five commercially available cation exchange membranes develop 84 during four months of continuous operation within the environment of a microbial 85 electrolysis cell (MEC). All the membranes were subjected to identical operational 86 conditions by being fitted within the same MEC reactor. Membranes samples were 87 collected on a monthly basis. Sscanning electron microscopy (SEM) and atomic 88 force microscopy (AFM) were used to assess the changes on the membranes 89 surface; thermogravimetric analysis (TGA) to evaluate the structural stability of the 90 membranes, and ion exchange capacity (IEC) to monitor any change in their 91 electrochemical properties. 92 2. Materials and Methods 93 2.1. Microbial electrolysis cell design and operation 94 This work assesses the ageing process of five commercially available CEM (see 95 section 2.2 for a complete description of them). To make sure that all membranes 96 were subjected to the same operational conditions (i.e.: same pH, temperature, 97 hydraulic retention time, etc) they were arranged within the same MEC reactor by 98 using the frame shown in Figure 1. This frame consisted of two identical 99 methacrylate plates with 30 windows (30 mm x 12 mm) opened in the plates to 100 accommodate the membranes samples (6 for each type of CEM tested). The two 101 plates were joined by small screws to facilitate the sampling process which was 102 carried out on a monthly basis (see section 2.2). 103 The anodic and the cathodic chambers were constructed with methacrylate 104 plates too and were separated by the frame above described. Each chamber 105 retained 230 mL of liquid and had a headspace of 20 mL. The anode was made of a 106 5 mm-thick graphite felt (Sigratherm®, Germany), and the cathode consisted of a 107 stainless steel mesh, both measuring 24.5 cm x 9.5 cm. The carbon felt was 108 pre-treated with a method listed elsewhere [17] and the stainless steel was cleaned 109 with distilled water. A titanium wire was used as current collector to connect the 110 electrodes to a power supply PS2000B (Elektro-Automatik, Germany). The applied 111 voltage was set at 1 V between anode and cathode and the electrical current was 112 recorded every 10 minutes across an 8 Ω resistor by means of a 2700 Keithley 113 multimeter (Keithley Instruments, USA). 114 The anode chamber was inoculated with a mixture activated sludge and effluent 115 from another MEC operated for 4 months. The anolyte stock solution was composed 116 0.55 g·L−1 sodium acetate, 6 g·L−1 K2HPO4, 3 g·L−1 KH2PO4, 1.5 g·L−1 NH4Cl, 1 g·L−1 117 NaHCO3, 0.5 g·L−1 NaCl, 0.2 mg·L−1 CaCl2, 0.15 g·L−1 MgSO4·7H2O and 1 mL·L−1 of 118 a mixed trace element solution and vitamins [18]. The catholyte stock solution 119 consisted of a 0.1 M phosphate buffer. Both were sparged with nitrogen gas for 20 120 minutes to ensure anaerobic conditions and stored at 4ºC until use. Anolyte and 121 catholyte were feed in continuous mode by mean of two peristaltic pumps at a flow 122 rate of 30 mL·h−1, thus providing a 7 h retention time. 123 124 125 Figure 1. The methacrylate frame housing the 6x5 CEM membrane’s samples 126 2.2. Membranes and membranes characterisation 127 Five commercially available CEM were used: Nafion 117 (DuPont, USA), 128 CMI-7000 (Membranes International, USA), Zirfon Perl UTP 500 (Agfa, Belgium), 129 Fumasep FKE and FKB (Fumatech, Germany). Table 1 shows some of their 130 characteristics as provided by the manufacturer. After being placed in the frame 131 (Figure 1), the 5x6 membrane samples were soaked in demineralised water for at 132 least 24 h. 133 Table 1. Physical and chemical properties of the cation exchange membranes used in this research Property Units FKE FKB Nafion 117 Zirfon UTP 500 CMI-70 00 Selectivity % > 0.98 > 0.98 - - > 0.97 Electric resistance Ω·cm2 < 3 < 4 1.5 < 0.3 < 30 Stability pH 1-14 1-14 - 6 M KOH 1-10 Thickness mm 0.05-0.07 0.08-0. 1 0.18 0.5 0.45 Ion exchange capacity meq/q > 1 0.9-1.0 0.95-1.01 - 1.6 Cost €/m2 195 320 400 45 170 The ageing process of the five membranes was assessed on samples taken after 1, 134 2, 3 and 4 months of operation (Figure 1). After sampling, a fresh membrane was 135 placed in the vacant window, the MEC was reassembled and both chambers were 136 purged with nitrogen gas. Although the initial plan for the experiment was to continue 137 the sampling process for 6 moths (hence the 6 samples for each type of membrane 138 in the frame), it was stopped at month 4 because the ageing tended to stabilize after 139 the first month of operation of the MEC. The ageing of the membranes was 140 evaluated according to the observed changes in their (i) superficial morphology 141 (analysed by scanning electron microscopy and atomic force microscopy), (ii) 142 structural stability (measured by thermogravimetric analysis), (iii) number of fixed 143 charges inside the cation exchange membrane (measured according to the ion 144 exchange capacity) and (iv) electrical resistance (calculated by electrical impedance 145 spectroscopy). All these techniques are described in the following sections. 146 2.3. Scanning electron microscopy (SEM) 147 A JSM-6480 LV (JEOL, Japan) scanning electron microscope was used. The 148 samples were fixed in a 2.5% glutaraldehyde solution in phosphate buffered saline 149 (PBS) solution for 2 hours at 4° C. After drying the membranes with ethanol, they 150 were transferred into the chamber of a critical point dryer CPD 030 (Bal-Tec, 151 Germany). Subsequently, prior to observation of the microstructure by SEM, the 152 membranes were coated with a thin layer of gold in a sputter coater equipment EM 153 ACE200 (Leica Microsystems, Switzerland). 154 2.4. Atomic force microscopy (AFM) 155 A Nanoscope Multimode IIIa scanning probe microscope from Digital 156 Instruments (Veeco Metrology Inc., USA) was used, following the method described 157 elsewhere [19]. The calculated values of surface roughness were averaged over 5 158 different profiles for each membrane sample using a scanning area of 1×1 μm2. The 159 AFM allows to monitor changes on the membrane surface and get a quantitative 160 measurement of roughness as Sq [20]. Sq refers to the root mean square roughness 161 and represents the statistical measure of the magnitude of the height distribution. It 162 was calculated according to Eq. 1: 163 (1) 164 where m and n are the number of the pixels in the x and y directions 165 respectively (512×512 in this case), and Z is the height of a pixel. 166 2.5. Thermogravimetric analysis (TGA) 167 The thermogravimetric analyses were carried out with a Thermogravimetric 168 Analyzer SDT Q600 (TA Instruments, USA), in a temperature range from 30 to 750 169 °C, at the constant rate heating of 10 °C·min-1, under nitrogen saturated 170 environment. 171 2.6. Ion exchange capacity (IEC) 172 The ion exchange capacity was determined by acid-base titration. Initially the 173 membranes samples were immersed in distilled water and soaked in a large volume 174 of 1 M HCl solution for at least 24 hours (the solution was replaced three times to 175 complete the ion exchange). After that, they were washed again with distilled water 176 to remove the excess of HCl and soaked in a 2 M NaCl solution for at least 24 hours 177 to replace the protons by sodium ions (similary, the solution was replaced three 178 times to ensure a complete exchange). The different NaCl solutions were collected 179 and then titrated with 0.1 M NaOH. The ion exchange capacity titration of the 180 membrane was calculated as the ratio between the total charge (meq) and the dry 181 weight of the measured membrane (g). 182 3. Results and discussion 183 As described in section 2.2, the ageing of the five membranes considered in this 184 study was assessed according to modification in their surfaces, structural stability 185 and ionic exchange capacity. In the following paragraphs, these properties are 186 described and discussed in detail. 187 3.1. Assessment of membrane surface modifications through SEM and AFM 188 Cation exchange membranes are liable to undergo biological and chemical 189 fouling when operated in BESs [21]. The interest in tracking this phenomenon lays in 190 the fact that fouling represents a physical blockage to charge transport through the 191 membrane, which can result in a decay of current [22]. Although there is no specific 192 procedure to measure its formation rates, SEM can provide a qualitative estimation 193 of its physical extent [6]. As it can be observed in Figure 2, SEM analyses did not 194 reveal any visible difference between fresh (unused) membranes and those samples 195 taken after 1 and 4 months of operation. No biofouling was perceptible on either the 196 anode nor on the cathode side and no breakings or deformities in the internal 197 structure of the membranes were detected at a 90x magnification. 198 Still, these results need to be interpreted with caution precisely because of their 199 qualitative nature. The apparent lack of any of surface deterioration might be hiding 200 inorganic salts precipitations, biomass and extracellular polymers that are not easily 201 perceptible on the SEM images and that may be depositing on the membranes 202 surfaces. Here, monitoring the evolution of surface roughness through AFM 203 analyses and computed by the root mean square roughness (Sq, see section 2.4) 204 can provide an indirect and quantitative evidence of fouling (any change on Sq in 205 membranes exposed to bioreactors indicates that foulants are deposited [23]). 206 Figure 3 shows how Sq increases more visibly in CMI and Nafion membranes 207 (especially after the first month of operation). The Zirfon membrane, despite 208 displaying the highest surface roughness on the fresh sample (probably because of 209 the presence of ZrO2), developed the least relative increase over the four months of 210 312 Figure 6. Current density profile over the 120 days periods of operation (Arrows indicate 313 membrane sampling at 1, 2, 3 and 4 months). 314 315 Here, it is important to bring to mind that the membranes investigated in this study 316 are typically used in conventional electrochemical systems where current densities 317 can be as high as 10,000 A·m-2 [33]. This contrasts with the ~1 A·m-2 (Figure 6) 318 measured in our cell, which is an usual current density for BES [34] but highlights 319 the low electrical stress to which conventional membranes are subjected in BES 320 applications. In other words, existing commercial membranes are somehow 321 “oversized” when used in BES. This, of course, would not be problem if it were not for 322 the relatively large cost that these membranes have (Table 1). The challenge is thus 323 to develop “tailor-made” membranes, that can show a fair long-term stability and 324 resilience, with an economic cost that does no threat the commercial development of 325 BES. In this regard, new developments that can provide similar or even better results 326 than commercial membranes at a cheaper cost are already underway [24]. These 327 advances are also helping to expand the range of applications of BES [35,36], 328 opening new exciting possibilities and paving their way towards practical 329 applicability. 330 4. Conclusions 331 The ion exchange membrane explains much of the cost of BES and its durability 332 has a marked impact on their economic feasibility. In this study, we compared the 333 ageing process of 5 commercially available cation exchange membranes. Nafion 334 and CMI-7000 showed a remarkably robust chemical structure, although they suffer 335 the greatest surface modification. This revealed a tendency to promote fouling and 336 biofouling that was more pronounced than in FKB and, specially, than in FKE. The 337 ion exchange capacity, which is related to the electric conductivity of the 338 membranes, showed a moderate decay in all membranes. Despite these apparent 339 signs of deterioration, the current density kept almost constant (~1 A m-2) which 340 suggest that the studied membranes can maintain their electrochemical 341 performance within the demanding environment of BES. 342 343 Acknowledgments 344 This research was possible thanks to the financial support of the ‘Ministerio de 345 Economía y Competitividad’ (ref: CTQ2015-68925-R), project cofinanced by FEDER 346 funds. M.I. 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Bioresour Technol 2015;195:202–9. doi:10.1016/J.BIORTECH.2015.05.058. 457 458 459 Figure 1 Click here to download high resolution image Figure 2 Click here to download high resolution image Figure 3 Click here to download high resolution image Figure 4 Click here to download high resolution image