The role of membranes in the use of natural salinity gradients for reverse electrodialysis

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(1)The role of membranes in the use of natural salinity gradients for reverse electrodialysis. Ca2+ Na+. Mg2+. Timon Rijnaarts.

(2) The role of membranes in the use of natural salinity gradients for reverse electrodialysis. Timon Rijnaarts.

(3) This work was performed in the cooperation framework of Wetsus, European Centre of Excellence for Sustainable Water Technology (www.wetsus.eu). Wetsus is cofunded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the Province of Fryslân, and the Northern Netherlands Provinces. The authors like to sincerely thank the participants of the research theme “Blue Energy” for fruitful discussions and financial support.. ISBN: 978-90-365-4540-2 DOI: 10.3990/1.9789036545402. © 2018, Timon Rijnaarts All rights reserved. PhD thesis, University of Twente, The Netherlands With references, with summaries in English and Dutch 195 pages. Cover design: Timon Rijnaarts Printed by: Gildeprint, Enschede.

(4) THE ROLE OF MEMBRANES IN THE USE OF NATURAL SALINITY GRADIENTS FOR REVERSE ELECTRODIALYSIS. PROEFSCHRIFT. ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, prof. dr. T.T.M. Palstra volgens besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 3 mei 2018 om 14.45 uur. door. Timon Rijnaarts geboren op 12 april 1989 te Nijmegen, Nederland.

(5) Dit proefschrift is goedgekeurd door:. Promotor: Prof. dr. ir. D.C. Nijmeijer Co-promotor: Dr. ir. W.M. de Vos. Promotiecommissie: prof. dr. ir. J.W.M. Hilgenkamp (voorzitter). University of Twente. prof. dr. ir. D.C. Nijmeijer (promotor). University of Twente. dr. ir. W.M. de Vos (co-promotor). University of Twente. prof. dr. ir. H.D.W. Roesink. University of Twente. prof. dr. ir. J. Huskens. University of Twente. prof. J. Crespo. Universidade Nova de Lisboa. prof. dr. M.C.M. van Loosdrecht. Delft University of Technology. dr. L.C.P.M. de Smet. Wageningen University & Research.

(6) Contents 1. Summary Chapter 1. Introduction. 10. Chapter 2. Effect of divalent cations on reverse electrodialysis 26 performance and cation exchange membrane selection to enhance power densities. Chapter 3. Divalent cation removal by Donnan dialysis for 60 improved reverse electrodialysis. Chapter 4. Layer-by-layer coatings on ion exchange membranes: 90 effect of multilayer charge and swelling on monovalent ion selectivities. Chapter 5. Improved fluid mixing and power density in reverse 118 electrodialysis stacks with chevron-profiled membranes. Chapter 6. Role of anion exchange membrane fouling in reverse 152 electrodialysis using natural feed waters. Chapter 7. General discussion and outlook. 174.

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(8) Summary There is an urgent need to limit our CO2 emissions to keep global warming within acceptable levels. To achieve this, the use of renewable energies needs to show a major increase. One of these renewable energies is reverse electrodialysis (RED), where energy is harvested from a salinity gradient. For RED, two streams with a difference in salinity are used in combination with ion exchange membranes. These charge-selective membranes allow either the transport of cations (for cation exchange membranes, CEMs) or anions (for anion exchange membranes, AEMs). Using these membranes, the salinity gradient can be used to facilitate directional charge transport that can be converted into electrical energy. Anticipated feed streams for the salinity gradient can be natural waters, for example, river and seawater. However, the contents of these waters pose challenges to the process. Divalent ions, such as Mg2+ and Ca2+, have several negative effects (which will be discussed in Chapter 2 – 4). Moreover, dissolved organic matter such as humic acids, is expected to foul AEMs (Chapter 6). Furthermore, the use of spacers in these stacks has drawbacks (such as partially blocking the membrane surface for ion transport) and can be replaced by using profiled membranes. Their design is discussed in Chapter 5. In previous work, it has been shown that divalent ions have two negative effects in RED. Firstly, when present in the river water, these divalent ions exchange with monovalent ions in the seawater, causing a lower salinity gradient. Secondly, the resistance of the stack increases when divalent ions are present. In Chapter 2, these effects of divalent cations in RED are investigated using CEMs with different divalent transport properties. Monovalentselective CEMs are used to block divalent cations, while multivalent-permeable CEMs are selected to allow the transport of divalent cations. These membranes are characterized on their monovalent-ion selectivity using membrane resistance in NaCl and MgCl2, and show large differences in the transport of divalent cations. In RED, monovalent-selective CEMs are able to block divalent cations transport and are able to operate without voltage losses due to a lack of uphill transport. Alternatively, multivalent-permeable CEMs allow divalent cation transport and do not experience a membrane resistance increase. Both these specific. 1.

(9) types of CEMs improve gross power densities by at least 30% compared to standard-grade CEMs when divalent cations are present in the feed water. In Chapter 3, uphill transport by divalent cations from the river water to the seawater is discussed. This uphill transport causes a voltage loss, and is still present with the improvements gained in Chapter 2 using multivalent-permeable CEMs. Here, a pretreatment using Donnan Dialysis (DD) is proposed to exchange divalent cations from the river water with monovalent cations in the seawater to avoid uphill transport. Surprisingly, we do not find an improvement in the obtained voltage in RED for DD-pretreated river water with divalent cations. This is caused by the exchange of divalent for monovalent cations in DD prior to the RED process, where the effective salinity gradient is lowered because divalent cations in the river water are exchanged by Na+ in the seawater. However, this exchange of Na+ to the river water decreases the resistance of the river water – which comprises over 80% of the total stack resistance. This leads to an overall improvement of 6.3% in net power density. In Chapter 4, tunable monovalent-selective membranes are made using polyelectrolyte multilayer coatings on commercial ion exchange membranes. After the promising results for monovalent-selective membranes discussed in Chapter 2, it is important to devise a tunable and simple method to prepare such membranes. We find that for these multilayer coatings a net positive charge and a low swelling ratio, in combination with a defect-free multilayer are important properties for monovalent-selective coatings. The coated membranes have high monovalent cation selectivity (of Na+/Mg2+) up to 7.8 compared to 3.5 for the uncoated CEM, for experiments on single membranes. In large-scale electrodialysis stack experiments, this leads to an improved monovalent cation selectivity of 5.4 compared to 1.4 for uncoated CEMs. However, these multilayers coating did not enhance monovalent-selectivity for AEMs, likely due to the positive charge of the multilayer.These tunable coatings could be used to counteract uphill transport in RED as well. In Chapter 5, profiled membranes are compared with using spacers in RED. These profiled membranes do not block part of the membrane surface nor the channel for ion transport, which is the case for spacers (the so-called spacer shadow effect). In this work, pillar-profiled and chevron-profiled membranes are compared to RED with spacers. It is found that profiled2.

(10) membranes indeed have an improved stack resistance due to absence of the spacer shadow effect. Chevron-profiled membranes improve the mixing as well, leading to even lower resistances at the cost, however, of additional pressure drops in the channels. In the end, chevron-profiled membranes are found to have the highest net power densities of all studied configurations. However, alignment of the chevron profiles during assembly of the RED stack proves difficult and, therefore, an improved design without alignment issues is proposed based on crossed chevrons. In Chapter 6, the effect of natural water fouling in RED is discussed, with a specific focus on the role of AEMs. Therefore, six different AEMs were tested in RED stacks using natural waters at the Afsluitdijk and these AEMs were separately characterized on the membrane level before and after the RED experiments. It is found that the change of properties of the AEMs (permselectivity and membrane resistance) due to fouling in natural waters depends on the chemistry and water content of the membranes. For the RED experiments, a decrease of 15 – 20% in power density is observed for all AEMs. However, the change in AEM properties due to fouling can only explain 2 – 4% decreases in power density. The largest effect of fouling losses in power density are assumed to be caused by spacer fouling. In Chapter 7, a discussion on the findings in this thesis is given. Based on these results, three main challenges for future research on RED are proposed. Firstly, the role of divalent anions is not studied separately as has been done for divalent cations. When present in the river water, these are expected to cause uphill transport, just like divalent cations do. Secondly, natural fouling should be studied on different profiled membranes, to discuss the influence of profile shape on fouling tendency. Finally, to enable investigation of different profile shapes, rapid prototyping using 3D printing can be used. Alternative uses of RED are discussed as well, where the salinity gradient is used to drive other electrochemical reactions than those required to produce electricity.. 3.

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(12) Samenvatting Het is urgent om onze CO2 emissies te beperking zodanig dat de opwarming van de aarde binnen acceptable grenzen blijft. Om dit te kunnen bereiken, moet de productie van hernieuwbare energie flink worden verhoogd. Eén van de methodes om hernieuwbare energie te creëren is omgekeerde electrodialyse (RED), waarbij energie uit een zout gradient wordt gewonnen. Voor RED zijn in ieder geval twee stromen nodig met een verschil in ionsterkte nodig en ion uitwissel membranen. Deze ladingsselectieve membranen laten het transport van of kationen (voor zogenaamde kation uitwissel membranen, CEMs) of anionen (voor zogenaamde anion uitwissel membranen, AEMs). Door gebruik te maken van deze membranen, kan de zoutgradiënt omgezet worden in ladingstransport met een richting, die weer kan worden omgezet naar elektrische energie. Het gebruik van natuurlijke zoutgradiënten kan voor deze technologie zeer geschikt zijn, zoals bijvoorbeeld rivier- en zeewater. Echter, de inhoud van deze natuurlijke wateren levert de nodige uitdagingen op voor dit proces. Divalente kationen, zoals Mg2+ en Ca2+, hebben een aantal negatieve effecten (welke worden besproken in Hoofdstuk 2 – 4). Daarnaast wordt verwacht dat opgeloste organische materie, zoals humus zuren, AEMs vervuilt (Hoofdstuk 6). Het gebruik van spacers in deze systemen heeft negatieve effecten (zoals deels blokkeren van het membraanoppervlak voor ion transport) maar deze kan vermeden worden door geprofileerde membranen te gebruiken. Het ontwerp van deze geprofileerde membranen wordt besproken in Hoofdstuk 5. In werk voorafgaand aan dit proefschrift, is er aangetoond dat divalente ionen twee negatieve effecten hebben in RED. Ten eerste kunnen deze divalent ionen, als ze aanwezig zijn in het rivier water, uitwisselen met monovalente ionen in het zeewater. Dit zorgt voor een verlaagde zoutgradiënt en daarmee beschikbare voltage. Ten tweede neemt de weerstand van het systeem toe als er divalent ionen aanwezig zijn. In Hoofdstuk 2 zijn deze effecten van divalente kationen voor RED onderzocht door gebruik te maken van CEMs met verschillende transport eigenschappen wat betreft divalente kationen. Monovalent-selectieve CEMs worden gebruikt om divalente kationen te blokeren, en multivalent-permeabele CEMs zijn gekozen om het transport van divalente kationen toe te staan. Deze membranen worden 5.

(13) gekarakteriseerd op hun monovalente ion selectiviteit door membraanweerstanden in NaCl en MgCl2 oplossingen te meten. Deze membranen laten grote verschillen zien in het transport van divalente kationen. In RED, monovalent-selectieve CEMs zijn in staan om het transport van divalente kationen te transporteren en daardoor zonder verliezen in voltage te opereren doordat transport tegen de richting in voorkomen wordt. Als alternatief laten multivalentpermeabele CEMs het transport van divalente kationen toe en daardoor hebben deze membranen geen verhoogde membraanweerstand. Beide specifieke typen CEMs verbeteren de bruto vermogensdichtheid met tenminste 30% ten opzichte van standaard CEMs, als divalente kationen aanwezig zijn in het voedingswater. In Hoofdstuk 3 wordt gekeken naar transport tegen de gradiënt in door divalente kationen van het rivierwater naar het zeewater. Dit opwaartse transport zorgt voor een voltage verlies, en is nog steeds aanwezig met de verbeteringen gehaald in Hoofdstuk 2 door multivalentpermeabele CEMs te gebruiken. In dit hoofdstuk wordt een voorbehandelding voorgesteld op basis van Donnan dialyse (DD) om divalente kationen in het rivier water uit te wisselen met monovalente kationen in het zeewater en zo opwaarts transport te vermijden. Verrassend genoeg vinden we geen verbetering in het voltage voor RED als we DD-voorbehandeld rivier water met divalente kationen gebruiken. Dit wordt veroorzaakt door de uitwisseling van divalente voor monovalente kationen in DD voor het RED proces, waardoor de effectieve zoutgradiënt vermindert wordt doordat divalente kationen uitgewisseld worden door Na+ in het zeewater. Maar, deze uitwisseling voor Na+ in het rivierwater zorgt wel voor een verlaging van de weerstand van het rivier water – wat in de praktijk meer dan 80% van de weerstand van het totale systeem is. Samengevat leidt deze voorbehandeling tot een verbetering van 6.3% in netto vermogensdichtheid voor RED. In Hoofdstuk 4 zijn afstembare monovalent-selectieve membranen gemaakt door gebruik te maken van polyelektroliet multilaag coatings op commercieel-beschikbare ion uitwissel membranen. Na de veelbelovende resultaten voor de monovalent-selectieve membranen, zoals besproken in hoofdstuk 2, is het belangrijk om een simpele en variabele methode te ontwikkelen om zulke membranen te kunnen maken. We vinden, in dit hoofdstuk, dat de positieve netto lading en een lage zwellingsratio in combinatie met defect-vrije multilagen cruciale eigenschappen zijn voor coatings met monovalent-selectieve eigenschappen. De 6.

(14) gecoate membranen hebben, op membraanniveau, een hogere monovalente kation selectivity (van Na+/Mg2+) tot 7.8 vergelijken met 3.8 voor niet-gecoate membranen. In grote schaal electrodialyse experimenten leidt dit tot een verbeterde monovalente kation selectiviteit van 5.4 ten opzichte van 1.4 voor ungecoate membranen. Echter, deze multilaag coatings verbeteren de monovalente selectiviteit niet voor AEMs, waarschijnlijk doordat er een positieve lading in de multilaag aanwezig is. Deze afstelbare coatings kunnen worden gebruikt om opwaarts transport in RED ook tegen te gaan. In Hoofdstuk 5 zijn geprofileerde membranen vergeleken met systemen die gebruik maken van spacers in RED. Deze geprofileerde membranen blokkeren geen deel van het membraanoppervlak noch het waterkanaal voor iontransport, wat wel het geval is voor spacers (dit heet het zogenoemde spacer schaduw effect). In dit werk worden pillaar- en visgraat-geprofileerde membranen vergelijken met RED met spacers. Er is gevonden dat geprofileerde membranen inderdaad een verlaagde systeemweerstand hebben, door afwezigheid van het spacer schaduw effect. Visgraat-geprofileerde membranen verbeteren ook nog het mengen, wat leidt tot nog lagere weerstanden, maar wel ten koste van een extra drukval in de waterkanalen. Uiteindelijk presteren de visgraat-geprofileerde membranen met de hoogste netto vermogensdichtheid van alle configuraties. Echter, het uitlijnen van deze visgraat profielen tijdens assemblage van de RED stacks was complex en daarom is er een verbeterd ontwerp zonder deze uitlijningsproblemen voorgesteld, op basis van gekruiste visgraten. In Hoofdstuk 6 is het vervuilingseffect van natuurlijk water in RED bespreken, met daarbij een specifieke focus op de rol van de AEMs. Daarom zijn zes verschillende AEMs getest met natuurlijk water in RED systemen op de Afsluitdijk. Deze AEMs zijn apart gekarakteriseerd op membraan-niveau voor en na de RED experiments. De verandering van AEM eigenschappen (permselectiviteit en membraanweerstand) als gevolg van vervuiling door natuurlijk water hangt af van de chemie en waterinhoud van de membranen. Voor de RED experimenten wordt een verlaging van 15 – 20% in vermogensdichtheid gemeten voor alle AEMs. Echter, de verandering in AEM eigenschappen als gevolg van vervuiling verklaart maar 2 – 4% verlies in vermogensdichtheid. Het grootste effect van vervuilingsverliezen in vermogensdichtheid wordt hoogstwaarschijnlijk veroorzaakt door vervuiling van de spacers. 7.

(15) In Hoofdstuk 7 volgt een discussie over de bevindingen in deze thesis. Op basis van deze resultaten worden drie voorname uitdagingen voor toekomstig onderzoek voor RED voorgesteld. Als eerste is de rol van divalente anionen nog niet specifiek, net als voor kationen, bestudeerd. Als deze aanwezig zijn in het rivierwater, veroorzaken ze waarschijnlijk opwaarts transport, net als divalente kationen. Ten tweede moet natuurlijke vervuiling bestudeerd worden in combinatie met geprofileerde membranen om de vorm het profiel in relatie tot de vervuilingsgraad te kunnen onderzoeken. Als laatste moet er gebruik worden gemaakt van snelle prototypen met 3D print technieken om de vorm en chemie van het profiel te bestuderen. Alternatieve gebruiken voor RED passeren ook de revue, waarbij de zoutgradiënt wordt gebruikt om andere elektrochemische reacties te bedrijven dan diegene die stroom te maken.. 8.

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(18) Chapter 1 Introduction Nowadays, we use vast amounts of energy (500,000,000 Terajoules per year globally) and this keeps increasing with every year [1]. This large energy demand requires energy sources, and historically we have used fossil fuels because of their natural abundance and high energy densities. However, the use of these fossil fuels comes at a significant cost. To extract energy from these fossil fuels, one needs to combust them, which causes emission of greenhouse gasses such as CO2. From 1750 until 2011, humans have emitted approximately 2000 Gigatonne (Gt) CO2 in the atmosphere. This has already caused global warming of approximately 0.8 oC (relative to the year 1880) and has resulted in an increase of 26% in ocean acidity (relative to 1990) and a sea level rise of 0.19 m (relative to 1901) [2]. Predictions due to these changes forecast more extreme weather (such as more frequent floods and drought) as well as degradation of coral reefs. Governments and companies are aware that we should limit emitting greenhouse gasses. To ensure that the temperature of the earth remains stable (i.e. well below the 2.0 oC increase as decided in the Paris Agreement [3]), the IPCC calculated a ‘carbon budget’ for the world from 2012 to 2100 of ~1000 Gt CO2 (see Fig. 1.1) [4]. If the world continues as we do today - emitting 34 Gt CO2 per year (as of 2011) - this carbon budget is spent by 2042 and by the year 2100 we would have emitted over 3000 Gt CO2. This would cause a temperature increase of ~3 oC with drastic and possibly irreversible change to our environment. It is therefore essential to lower our CO2 emissions and to eventually aim for no net CO2 emissions [2, 4].. 11.

(19) Temperature change relative to 1870 (oC). 4 Predictions. 3. 2. 1 0. 2000 1990 1940 1970 1880. 1000 2000. .. CO2 cap: 3000 Gt CO2. 3000 4000 5000. 6000. CO2 emissions from 1870 (Gt CO2). Figure 1.1 Global mean surface temperature increase (relative to the average of 1861 - 1880) as a function of cumulative total global CO2 emissions from 1870, redrawn from [2]. The Paris agreement, which limits global warming below 2 oC, requires a total limit ~3000 Gt CO2 of which ~2000 Gt CO2 was already emitted up to 2012. To reduce these CO2 emissions and move towards a zero emission society, we need to develop and use renewable zero emission methods of power generation or carbon capture for existing techniques. For transportation, especially aviation, the change to renewables is more challenging as liquid hydrocarbon fuels are not easily replaced. However, in the stationary electricity sector renewable alternatives are present. Nowadays, approximately 25% of the global greenhouse emissions come from the electricity sector, running on mostly coal (40%) and natural gas (22%). The share of electricity production with renewable energy is increasing and in 2015, about 28 and 34% of electricity was produced with renewable energy in North America and Europe respectively [5]. To enable the production of electricity without emissions, these renewables energies should be able to compete with current technologies. At the moment, renewable energies are in the same price range as energy generated by fossil fuel. However, in 2040 it is expected that they can produce electricity at a lower price than fossil-based electricity (see Table 1.1) due to further developments and maturity of renewable energies. Here, the added costs for CO2 emissions or CO2 capture and storage (CCS) are not even taken into account. In the near 12.

(20) future, it is anticipated that emitting CO2 will be taxed (so-called carbon pricing), however, it is difficult to forecast this carbon pricing and, hence, this is not taken into account in most estimates. By 2040, the lowest cost of fossil fuel electricity generation (using natural gas power plants) is expected to reach 40 $/MWh [6], which is more expensive than the predicted cost for most renewable energies, such as onshore wind and photovoltaic solar generation. Last year (2017), record prices for unsubsidized renewable energies were already as low as 24, 30 and 49 $/MWh for solar (photovoltaic), onshore and offshore wind, respectively [6]. In addition to price, the global demand of electricity is expected to increase in 2040 by ~100% compared to 2015 [6]. It is therefore anticipated that wind, hydroelectric and especially solar energy generation will increase tremendously in the near future. Table 1.1 Overview of current (2016) cost (in US) and global share of electricity capacity, and predictions for 2040 [6, 7]. Source. Plant type. Coal Natural gas Nuclear Renewables (total) Wind Onshore Offshore Solar PV* Hydroelectric Total Capacity *. Cost in 2016 ($/MWh) 65 42. Cost in 2040 ($/MWh) 58 40. 50. 30. 66. 20. Current share of capacity in 2016 30% 24% 5% 35% 12%. Estimated share of capacity in 2040 13% 14% 3% 68% 15%. 5%. 32%. 17% 6,719 GW. 12% 13,919 GW. PV = photovoltaic cells.. Wind and solar energy, however, have one major drawback: their intermittency. The power generation using these techniques fluctuates because of the weather and day and night cycles, as it is dependent on availability of wind and solar irradiation. The power generation of solar energy peaks during the middle of the day, whereas the electricity demand is usually the highest in the early morning and at the end of the afternoon [8]. To avoid this mismatch between generation and demand, storage of electrical energy is essential. Currently, 13.

(21) researchers are investigating large-scale energy storage systems and predictions are that we can store energy for 0.50 €/kWh (600 $/MWh) and 0.10 €/kWh (120 $/MWh) for long- and short-term respectively [8]. The use of electric cars could play a role in energy storage, since in 2040 there will be ~500 million electric cars in use – all of which are equipped with a large battery [6].. 1.1. Salinity gradient energy Another option is to develop renewable energies without intermittency, such as geothermal or hydroelectric energy. One of these potential technologies is salinity gradient energy, where a difference in salinity between two aqueous streams is used to harvest [9] or store [10] energy of mixing. The energy of mixing 1 m³ river water (with 0.01 M NaCl) and 1 m³ of seawater (with 0.5 M NaCl) is ~ 0.4 kWh, depending on the salinity [11]. This difference in salinity can generate a potential as the system is driven to reduce the salinity gradient. If one allows the controlled mixing of either water or ions, one can use this drive to equilibrium to generate electricity [12]. The first option, where water is transported to the concentrated solution (osmosis), can utilize the pressure generated by the water to harvest power. This process is called pressure retarded osmosis (PRO). The second option, where ions are transported from the concentrated to the dilute solution, can utilize the charge transport of ions to generate power. This process is the topic of this thesis, and is named reverse electrodialysis (RED). The proposed feed streams to harvest electricity using RED are natural waters with salinity gradients, since those will mix eventually (such as rivers flowing in the sea). Natural waters move in a hydrological cycle, where water evaporates from the oceans without salts and condenses in lakes and rivers. These rivers flow back to the sea at estuaries, where the fresh rivers are joining the salty sea (see Fig. 1.2). At these locations, there are two types of waters in close proximity with a large salinity gradient.. 14.

(22) Figure 1.2. The water cycle, where seawater evaporates and is precipitated in fresh water reservoirs. These flow into the sea over time and at these locations, natural salinity gradients are in close proximity. In a RED system, river and seawater are fed alternatingly in a stack creating a salinity gradient, see Figure 1.3. Between these water streams charge-selective membranes are placed, the so-called ion exchange membranes. These allow transport of only positive ions (for cation exchange membranes; CEMs) or only negative ions (for anion exchange membranes; AEMs). In this way, the ions can move from the concentrated to the dilute solution only through the charge-selective ion exchange membranes – in such a way that there is net charge transport. This ionic transport can drive electrical charge transport if the ionic system is connected with an electrical system through redox reactions at electrodes. The redox reactions at the electrodes transfer the ionic charge to electrical charge by reducing or oxidizing molecules, typically using a reversible redox couple such as ferro/ferricyanide, or water electrolysis [13]. The flow channels are made using spacers to allow for water distribution.. 15.

(23) Figure 1.3. Schematic build-up of a RED stack, with alternating river and seawater being fed to the system. CEMs and AEMs separate the water streams and allow for charge-selective ionic transport, with cations moving in one direction and anions in the other. Reduction (Red) and oxidation (Ox) reaction convert the ionic charge into electrical charge (electrons, e-) at the electrodes.. 1.2. Ion exchange membranes At the heart of the RED technology are so-called ion exchange membranes. They facilitate the charge-selective ionic transport that allows harvesting the salinity gradient. These membranes are thin (~ 100 µm) polymer films that contain fixed charged groups, such as ~SO3 groups for CEMs and ~NR3 groups for AEMs, on the polymer backbone (see Fig. 1.4). Next to differences between anion and cation exchange membranes (the fixed charged groups), there are also differences with respect to the polymer backbone chemistries. In general, there are two types of chemistries available for ion exchange membranes. These are aliphatic, often based on acrylamide monomers, and aromatic, which are based on styrenelike monomers. Typically, a charged monomer (e.g. styrene-sulfonate) is used to introduce fixed charges and a crosslinker (e.g. divinylbenzene) prevents excessive swelling of the polymer film. These membranes can be made using a single (crosslinked) charged polymer, 16.

(24) often impregnated in a support material. These membranes are considered homogeneous, at least concerning their charged group distribution. If one blends charged polymer with uncharged polymers (e.g. polyethersulfone) and a membrane is produced from this blend, then these membranes are classified as heterogeneous [14]. Aliphatic anion exchange membrane Aromatic anion exchange membrane. Cl. Cl. -. N. HN. +. O. O. N. -. +. HN. N. +. Cl. -. HN NH. O N. +. O. Cl. -. Aliphatic cation exchange membrane Aromatic cation exchange membrane. Na. +. Na. O O S O. O HN. O HN HN. NH O. +. O O S O O S O O + Na. O O. S O O. Na. +. Figure 1.4. Aliphatic and aromatic anion and cation exchange membrane structures. Aliphatic structures are often made with acrylamide or acrylate monomers, whereas the aromatic structures consist of styrene-based monomers. The backbones are shown to illustrate the network structure (formed by crosslinkers) and do not necessarily consist of only carbonbased backbones (as depicted here). These ion exchange membranes are characterized by their charge selectivity and their ability to permeate ions (see Fig. 1.5). The charge selective transport in this field of research is described by the permselectivity of the membrane. In general, permselectivity of a cation exchange membrane describes its ability to transport the desired cations (counter ion) and retain anions (co ion). The same applies for anion exchange membranes, except that the desired ion is the anion. For most applications, it is beneficial to use membranes with high permselectivities to allow for efficient charge transport [15]. The transport rate through these 17.

(25) membranes is described by the membrane area resistance. This is essentially the current (or quantity of ions per second) that can pass through a membrane area at an applied driving force (voltage), see Fig. 1.5. A lower membrane area resistance means that with the same driving force more ions will be transported through the membrane. The membrane resistance depends on the ion that it is transporting as well as its concentration in solution [14].. CEM. -. Na + ClPermselectivity. Resistance. -. Figure 1.5. Cation exchange membrane (with negative fixed charges) where Na+ can permeate through the CEM and Cl- is being rejected. The charge selectivity (or Cl- rejection) can be characterized by the permselectivity. The flux of counter ions (Na+ in this case) at a given driven voltage is determined by the membrane (area) resistance. These ion exchange membranes were not specifically designed for RED and are used mostly in other processes (see Fig. 1.6), such as in electrodialysis (ED). This process is used on commercial scale, for example, to desalinate aqueous streams for drinking water or to concentrate salt for table salt production [16]. In ED, as the opposite of RED, feed water (often at brackish water salt concentrations) is fed to all compartments in the stack. By applying an electrical potential on the electrodes, ions are selectively transported to a concentrated solution. The result is a desalinated feed stream (the diluate) and an enriched feed stream (the concentrate).. 18.

(26) M2+. RED. DD. ED M2+. Figure 1.6. Processes with ion exchange membranes studied in this thesis. Their respective purpose, and in- and output are given schematically. Another process with ion exchange membranes that is studied in this thesis is Donnan dialysis (DD). This process can be used to exchange undesired ions from a feed with other ions in a draw solution, for example to remove divalent ions (so-called softening) of feed water. In this process, concentration differences are used to drive exchange. As such, there are no electrodes in these systems. In practice, DD can be used to remove troublesome ions, such as uranyl and ammonium [17].. 1.3. Current state and challenges for RED At the moment, the highest gross and net power density (expressed as W per m² of total membrane area) are 2.9 and 1.3 – 1.5 W/m² for RED with artificial river and seawater using a stack with breathing cells [18]. For electricity cost prices, estimates use net power densities with natural water of 2.0 – 2.7. W/m². An assessment of the state of RED in 2014, showed that the price is estimated at 710 €/MWh [19]. When the improved net power densities are used at a membrane price of 2.0 – 4.3 €/m², the price is estimated at 80 up to 180 €/MWh [19, 20]. Several pilot-scale installations are tested on RED with natural waters, where necessary pioneering work is being performed on a 50 kW pilot on the Afsluitdijk using natural river and seawater [21] and a 1 kW pilot in Italy using brine and seawater [22]. Moreover, small-scale studies using natural waters are performed in other locations as well [23]. Recently, the efforts in the Netherlands have been increased with the announcement of a 1 MW pilot plant in Katwijk [24]. 19.

(27) Several effects play a role if the feed water changes from simple NaCl solutions to complex natural waters, where divalent ions [25, 26], organic molecules and inorganic particles [27] are present. These components of natural waters should be studied specifically to assess their effect on power densities and, if their negative effect is significant, to mitigate these performance decreases. Therefore, the main challenges for RED with natural waters are divalent ions, high stack resistances and fouling of spacers and membranes. Divalent ions have several negative effects on RED [25, 26]. Firstly, there is uphill transport from divalent ions in the river water to the seawater, which effectively lowers the salinity gradient. Secondly, membrane resistance is increased, especially in the case of CEMs. These effects have been studied in solutions containing NaCl where MgSO4 is added. Losses of 0.2 – 0.3 W/m² (30 - 40%) compared to NaCl have been measured with 10 % MgSO4 [26]. However, it is unclear what the separate effect of anions or cations is and how much the losses with concentrations found in natural waters are. The resistance of a RED stack determines the current at a given voltage, and a lower resistance leads to a higher current and thus power density in the RED system. This resistance is composed of membrane resistances as well as water compartment resistances. Especially the river water compartment has a high resistance due to its low salt concentration, and is usually responsible for the majority of the total stack resistance. Spacers in these feed water compartments determine the compartment thickness, and thus their resistance. An alternative is a dynamic stack – the breathing cell – that has reduced intermembrane distances for the river water during intervals [18]. This stack can generate with artificial NaCl solutions a net power density of 1.3 W/m². However, as spacers are not able to transport ions, part of the channel is unavailable for transport. Hence, profiled membranes are suggested to decrease compartment thickness and use the complete channel volume for ion transport [28]. Finally, the effects of natural fouling needs to better understood. Spacers can clog because of matter and particles in natural waters, which leads to increased pressure drops in feed water channels [21, 27]. Ridge-profiled membranes foul less compared to spacers in these systems. In addition, the role of organic components in the feed water, which are often negatively charged, is not fully understood in RED application.. 20.

(28) 1.4. Aim and outline The aim of this thesis is to advance the understanding of the effects of natural waters in RED. The RED energy cost predictions (as discussed in section 1.3) can be reached if the power densities obtained with artificial water can be obtained with natural waters as well. In this thesis one will find studies investigating specific aspects of natural water for RED (see Fig. 1.7). This is of crucial importance if we are to use this technology with natural water feed streams and compare them with existing renewable energies. Moreover, by understanding the negative effects that stem from using natural waters we can propose strategies to mitigate these effects.. Stack resistance. Ch. 6 AEM natural fouling M2+. Membrane development for RED. Fouling. M2+. Ch. 3 Donnan dialysis. -. -. Divalent ions. -. -. M. 2+. M2+. Ch. 4 Monovalent-selective CEMs by LbL. -. Ch. 2 Multivalent-permeable CEMs. Figure 1.7. Schematic overview of this thesis’ content. Three main focus areas are studied: the role of divalent cations (Ch. 2, 3 and 4), the decrease of stack resistance (Ch. 3 and Ch. 5) and the effect of fouling on AEMs using natural waters (Ch. 6). 21.

(29) In the first three chapters, the effect of divalent cations on ion exchange membranes and in the RED process will be studied. Moreover, mitigations for these losses are proposed and experimentally evaluated as well. In Chapter 2, the effect of divalent cations on RED performance is investigated. Monovalent-selective CEMs are able to block Mg2+ and could mitigate reductions in stack voltage caused by uphill transport. Multivalent-permeable CEMs, on the other hand, could transport these divalent cations without major increases in membrane resistance. Both types of membranes are compared on their power densities with standard-grade membranes with divalent cations in feed streams. In Chapter 3, the work on divalent cations in RED is continued. In Chapter 2, multivalentpermeable membranes were evaluated with feed streams containing divalent cations. However, they still suffer from a lower voltage due to the presence of divalent cations in the river water. In Chapter 3, DD is proposed as a pretreatment for RED to exchange these divalent cations. Seawater will be tested as a draw solution to exchange divalent cations from the river water. For the DD process, different membrane properties can influence the process and, for this reason, three different CEMs will be tested. In the end, DD is evaluated as a pretreatment to improve RED by mitigation of effects caused by divalent cations in river water. In Chapter 4, alternative cheap monovalent-selective CEMs are developed, which could be used to mitigate uphill transport (as discussed in Chapter 2). Polyelectrolyte multilayer coatings on ion exchange membranes are studied for their ability to make standard membranes monovalent-selective. Detailed studies are performed on the multilayer using optical techniques, which can investigate the excess of one of the polyelectrolytes in these multilayers, to evaluate the charge of the layers. Multilayer-coated CEMs are compared with the commercial monovalent-selective CSO membrane (6.9). Experiments in a large-scale ED stack are performed as well, where uncoated and multilayer-coated CEMs are compared on their monovalent-selectivity. In Chapter 5, chevron-profiled membranes in RED stacks are compared to stacks with spacers as well as pillar-profiled membranes to study reductions of stack resistance and improvements in power densities. The use of profiled membranes enabled a spacer-less design of RED stacks, which does not have partial blockage of the membrane surface and 22.

(30) allows for improved fluid mixing. Chevron-profiles are designed, such that they improve the fluid mixing in the water compartment. The net power densities for chevron- and pillarprofiled membranes will be compared with stacks using spacers, to evaluate the use of profiled membranes and discuss the role of profile shape. In Chapter 6, fouling of RED is studied with natural waters specifically for AEMs. Six AEMs, with different backbone chemistries and water contents, are investigated in natural waters. Power densities are evaluated for each AEM at the same time and the effect of natural water on the AEM properties is studied separately as well. The relevance of the chemistry and water content of the AEMs is related to the effect of fouling on the membrane level (i.e. change in AEM properties). In the end, the effect of changing AEM properties is compared to the measured power density decrease to discuss the role of AEMs in fouling of RED with natural waters. Finally, Chapter 7 contains a general discussion and outlook with respect to the themes studied in this work. Here a summary of our major findings in this thesis is given, and some of the key areas are highlighted where this technology can improve.. References [1] United Nations, 2015 Energy Statistics Yearbook, 2017. [2] IPCC, Summary for Policymakers, in: T.F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, P.M. Midgley (Eds.) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013, pp. 1–30. [3] United Nations, The Paris Agreement, in, 2017. [4] T.F. Stocker, D. Qin, G.-K. Plattner, L.V. Alexander, S.K. Allen, N.L. Bindoff, F.-M. Bréon, J.A. Church, U. Cubasch, S. Emori, P. Forster, P. Friedlingstein, N. Gillett, J.M. Gregory, D.L. Hartmann, E. Jansen, B. Kirtman, R. Knutti, K. Krishna Kumar, P. Lemke, J. Marotzke, V. Masson-Delmotte, G.A. Meehl, I.I. Mokhov, S. Piao, V. Ramaswamy, D. Randall, M. Rhein, M. Rojas, C. Sabine, D. Shindell, L.D. Talley, D.G. Vaughan, S.-P. Xie, Technical Summary, in: T.F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, P.M. Midgley (Eds.) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013, pp. 33–115. 23.

(31) [5] World Energy Council, World Energy resources | 2016, in, World Energy Council, 2016, pp. 1028. [6] M. Liebreich, Breaking Clean, in: EMEA Summit 2017, Bloomberg New Energy Finance, London, 2017. [7] Bloomberg New Energy Finance, New Energy Outlook 2017, in, 2017. [8] V. Jülch, Comparison of electricity storage options using levelized cost of storage (LCOS) method, Applied Energy, 183 (2016) 1594-1606. [9] R.E. Pattle, Production of Electric Power by mixing Fresh and Salt Water in the Hydroelectric Pile, Nature, 174 (1954) 660-660. [10] W.J. van Egmond, M. Saakes, S. Porada, T. Meuwissen, C.J.N. Buisman, H.V.M. Hamelers, The concentration gradient flow battery as electricity storage system: Technology potential and energy dissipation, Journal of Power Sources, 325 (2016) 129-139. [11] J.W. Post, H.V.M. Hamelers, C.J.N. Buisman, Energy Recovery from Controlled Mixing Salt and Fresh Water with a Reverse Electrodialysis System, Environmental Science & Technology, 42 (2008) 5785-5790. [12] J.W. Post, J. Veerman, H.V.M. Hamelers, G.J.W. Euverink, S.J. Metz, K. Nymeijer, C.J.N. Buisman, Salinity-gradient power: Evaluation of pressure-retarded osmosis and reverse electrodialysis, Journal of Membrane Science, 288 (2007) 218-230. [13] J. Veerman, M. Saakes, S.J. Metz, G.J. Harmsen, Reverse electrodialysis: evaluation of suitable electrode systems, Journal of Applied Electrochemistry, 40 (2010) 1461-1474. [14] T. Sata, Ion Exchange Membranes: Preparation, Characterization, Modification and Application, Royal Society of Chemistry, 2004. [15] P. Długołęcki, K. Nymeijer, S. Metz, M. Wessling, Current status of ion exchange membranes for power generation from salinity gradients, Journal of Membrane Science, 319 (2008) 214-222. [16] H. Strathmann, Electrodialysis, a mature technology with a multitude of new applications, Desalination, 264 (2010) 268-288. [17] Y. Tanaka, Chapter 7 Donnan Dialysis, in: T. Yoshinobu (Ed.) Membrane Science and Technology, Elsevier, 2007, pp. 495-503. [18] J. Moreno, E. Slouwerhof, D.A. Vermaas, M. Saakes, K. Nijmeijer, The Breathing Cell: Cyclic Intermembrane Distance Variation in Reverse Electrodialysis, Environmental Science & Technology, 50 (2016) 11386-11393. [19] A. Daniilidis, R. Herber, D.A. Vermaas, Upscale potential and financial feasibility of a reverse electrodialysis power plant, Applied Energy, 119 (2014) 257-265. [20] J.W. Post, C.H. Goeting, J. Valk, S. Goinga, J. Veerman, H.V.M. Hamelers, P.J.F.M. Hack, Towards implementation of reverse electrodialysis for power generation from salinity gradients, Desalination and Water Treatment, 16 (2010) 182-193. [21] J. Moreno, N. de Hart, M. Saakes, K. Nijmeijer, CO2 saturated water as two-phase flow for fouling control in reverse electrodialysis, Water Research, 125 (2017) 23-31. 24.

(32) [22] M. Tedesco, A. Cipollina, A. Tamburini, G. Micale, Towards 1kW power production in a reverse electrodialysis pilot plant with saline waters and concentrated brines, Journal of Membrane Science, 522 (2017) 226-236. [23] R.S. Kingsbury, F. Liu, S. Zhu, C. Boggs, M.D. Armstrong, D.F. Call, O. Coronell, Impact of natural organic matter and inorganic solutes on energy recovery from five real salinity gradients using reverse electrodialysis, Journal of Membrane Science, (2017). [24] H2O water netwerk, Blue Energy krijgt tweede locatie in Nederland, in: H2O Nieuws, 2017. [25] J.W. Post, H.V.M. Hamelers, C.J.N. Buisman, Influence of multivalent ions on power production from mixing salt and fresh water with a reverse electrodialysis system, Journal of Membrane Science, 330 (2009) 65-72. [26] D.A. Vermaas, J. Veerman, M. Saakes, K. Nijmeijer, Influence of multivalent ions on renewable energy generation in reverse electrodialysis, Energy & Environmental Science, 7 (2014) 1434-1445. [27] D.A. Vermaas, D. Kunteng, M. Saakes, K. Nijmeijer, Fouling in reverse electrodialysis under natural conditions, Water Research, 47 (2013) 1289-1298. [28] D.A. Vermaas, M. Saakes, K. Nijmeijer, Power generation using profiled membranes in reverse electrodialysis, Journal of Membrane Science, 385-386 (2011) 234-242.. 25.

(33) This chapter has been published as T. Rijnaarts, E. Huerta, W. van Baak, K. Nijmeijer, Effect of divalent cations on RED performance and cation exchange membrane selection to enhance power densities, Environmental Science & Technology, 2017.. 26.

(34) Chapter 2 Effect of divalent cations on reverse electrodialysis performance and cation exchange membrane selection to enhance power densities Abstract Reverse Electrodialysis (RED) is a membrane-based renewable energy technology that can harvest energy from salinity gradients. The anticipated feed streams are natural river and seawater, both of which contain - next to monovalent ions - also divalent ions. However, RED using feed streams containing divalent ions experiences lower power densities due to both uphill transport and increased membrane resistance. In this study, we investigate the effects of divalent cations (Mg2+ and Ca2+) on RED and demonstrate the mitigation of those effects using both novel and existing commercial cation exchange membranes (CEMs). Monovalent-selective Neosepta CMS is known to block divalent cations transport and can therefore mitigate reductions in stack voltage. The new multivalent-permeable Fuji T1 is able to transport divalent cations without a major increase in resistance. Both strategies significantly improve power densities compared to standard-grade CEMs when performing RED using streams containing divalent cations.. 27.

(35) 1. Introduction There is an increasing need for sustainable and clean energy sources, because the use of fossil fuels has significant environmental drawbacks such as greenhouse gas emissions. One of the emerging clean energy approaches is to harness salinity gradient energy, in which a difference in salinity between two solutions is used to harvest energy [1,2]. At locations where rivers flow into the sea, there are two distinct water reservoirs with a difference in salinity that can be used to generate salinity gradient energy. These bodies of water mix anyway and therefore the environmental impact of such a process is expected to be nil. The salinity gradient can be harvested through a reverse electrodialysis (RED) process [2]. RED uses ion exchange membranes to harvest energy from a salinity difference between two aqueous streams. Cation exchange membranes (CEMs) and anion exchange membranes (AEMs) are stacked in an alternating way with spacers in between to allow the flow of river and seawater. At both ends of the stack, electrodes and an electrolyte solution are used to transfer the ionic current from the salinity gradient into an electrical current. Under lab-scale conditions, gross power densities in the range of 2.2 - 2.9 W/m² of membrane area have been achieved using artificial river and seawater streams containing only NaCl [3-5]. However, the natural and abundant sources for RED power harvesting are natural river and seawater streams. As well as NaCl, these also contain divalent ions (such as Ca2+, Mg2+ and SO42-) [68]. Previous work has shown that the presence of these divalent ions leads to a decrease in RED power densities [7,8]. Such decrease in power densities in RED in the presence of divalent cations can be due to: (1) the transport of divalent cations against the overall concentration gradient (uphill transport); and/or (2) an increase in membrane electrical resistance [7,8]. Uphill transport is well known from both diffusion dialysis [9,10] and RED [7,8], and results in divalent cations being transported from the low concentration side to the high concentration side when a large excess of monovalent cations is present on the concentrated side of the membrane. This is a purely entropic process (i.e. mixing) in which the entropy gained by moving two monovalent ions from the concentrated to the diluted side outweighs the entropy lost by the single divalent ion moving from the dilute to the concentrated side.. 28.

(36) The second negative effect of the presence of divalent ions in RED stems from an increase in the membrane resistance due to interactions between the divalent ions and the fixed charged groups in the membrane. The electrical resistance is a measure of the required driving force (voltage) to transport charge (ionic current) through a membrane. If the driving force needed to transport ions increases, then the resistance of the membrane for these ions will also increases. The membrane resistance increases if, in addition to monovalent salts, there are divalent cations present [11], and Badessa et al. [12] hypothesize that a chelating effect of divalent cations is the cause of this resistance increase. In other words, the increase in membrane resistance is due to a single divalent cation binding to two fixed charged groups in the membrane. Moreover, those authors indicate that there is a correlation between activation energies to transport ions through membranes and the corresponding membrane resistances, such that monovalent ions such as Na+ that have low transport activation energies result in a low membrane resistance. However, anion exchange membranes do not exhibit the significant increase in resistance for divalent anions, which are common in natural waters, as shown by Krivcik et al. [13]. Also in porous membrane applications, cations show a high variability in properties as compared to anions. Anions do not differ as much as cations in terms of hydrated radius and hydration free energy, which are measures of the interaction of the ion with the surrounding water [14]. For these reasons, cations are assumed to affect the performance most. Both effects resulting of divalent cations, namely uphill transport and membrane resistance increase, are shown schematically in Figure 1A and B. Mitigation strategies - as presented graphically in Figure 1C and D - will be discussed in the next sub-section.. 29.

(37) A. Uphill transport Na +. B. Membrane resistance. -. Na+. Na +. Na. -. Mg2+. C. Monovalent selective Na+. Mg2+ Mg2+. -. D. Multivalent permeable. -. Na+. Na+. -. Na+. Improved Voltage. -. +. Mg2+. Mg2+. -. Decreased Resistance. Figure 1. Effect of divalent cation on cation exchange membranes in RED: A) Principle of uphill transport from divalent cations from the river water (right) to the seawater (left); B) Membrane resistance increases due to divalent cation binding inside the membrane. Mitigation strategies are: C) monovalent-selective membranes to prevent uphill transport; and D) multivalent-permeable membranes to prevent resistance increase. In this study, we investigate the negative effects of the presence of divalent cations in RED. Moreover, we present strategies to mitigate these effects using various types of cation exchange membranes (CEMs), as shown in Figure 1. Two strategies are considered. In the first approach, the use of monovalent-selective membranes (figure 1C) is expected to reduce the transport of divalent cations such as Mg2+ against the concentration gradient (Figure 1A). In the second approach, multivalent-permeable membranes (Figure 1D), recently developed specifically for RED in natural water streams, are expected not to suffer from significant resistance increase in the presence of divalent cations (Figure 1B). In these multivalentpermeable membrane type the negative charges are structured, providing pathways for ion 30.

(38) transport. This construct is assumed to decrease the strong multiple binding of divalent cations, hence lowering the membrane resistance for these specific ions. We perform a detailed RED stack analysis on obtainable voltage, stack resistances and power densities for divalent cations at concentrations found in natural waters using these two types of cation exchange membranes. These special membranes are compared with existing standard-grade membranes, and the stack results are related to the specific membrane properties.. 2. Theory. 2.1. Open circuit voltage. The open circuit voltage (OCV) equals the voltage of a RED cell that is not subject to a load. A high OCV results in a high power density. The OCV (V) can be calculated according to the modified Nernst equation [15]. The voltage is given for one cell pair which consists of one AEM and one CEM with river water flowing on one side and seawater flowing on the other side: EOCV =. RT. z. α F CEM Na+. ln. C C. γ Na+ ,c Na+ ,c. γ Na+ ,d Na+ ,d. +. RT α zCl- F AEM. ln. CCl- ,c γCl- ,c CCl- ,d γCl- ,d. (1). where EOCV is the open circuit voltage (V), R is the gas constant (J/K∙mol), T is the temperature (K), z is the valence of the ion (-) and F is the Faraday constant (s∙A/mol). α is the permselectivity of the membrane (with α = 1 representing a perfect charge-selective membrane) and Cc and Cd are the Na+ or Cl- concentrations (M) in the concentrated stream (seawater) and diluted stream (river water) respectively. γ (-) is the activity coefficient of the respective ion in solution at a known concentration as determined from the CRC handbook [16]. In the case of ideal membranes (α = 1) and solutions containing only NaCl (z = 1) with typical river and seawater concentrations (0.017 M and 0.5 M)7 at room temperature (T = 293 K), the OCV is 0.143 V/cell, implying a stack voltage of 1.43 V for 10 cell pairs.. 31.

(39) 2.1.1 Uphill transport The OCV and uphill transport have the same fundamental origin, namely the electromotive force (EMF) generated by a concentration gradient across charge-selective membranes. Uphill transport - or the exchange of ions against a concentration gradient - can be explained by looking at the Donnan potential as expressed in eq 2. EDon =. RT z+ F i. ln. C + γ𝑖𝑖+ ,c i ,c C+ γ+. (2). i ,d i ,d. For a CEM with typical cations found in RED, the separate Donnan potentials for Na+ and Mg2+ can be determined by inserting the appropriate concentrations, charge (valence) and activity coefficients for each ion i in eq 2. Salt concentrations of 0.5 M and 0.017 M for sea and river water respectively are typical in RED feed waters.7 In natural waters, about 10 mol% of Na+ in both streams is replaced by Mg2+. Calculating the Donnan potentials of Na+ and Mg2+ across a CEM allows the prediction of the transport direction.7 The Donnan potential over a perfect CEM (α = 1) with 10% Mg2+ in the feed streams is 0.079 V for Na+ and 0.039 V for Mg2+. These cations influence each other, so Na+ and Mg2+ will start moving until the overall Donnan potential is balanced, achieving equilibrium (EDon,Na+ = EDon,Mg2+) and maintaining charge neutrality (two Na+ exchange for one Mg2+). From the initial starting condition, the Na+ driving force is higher than that of the Mg2+ and, through ion exchange, the Donnan potential for Na+ will decrease while that for Mg2+ will increase. In practice, this creates a reduced concentration gradient for Na+ and an increased one for Mg2+. The calculated Donnan potentials and equilibrium concentrations are provided in the Supplementary Information (SI) 1.. 2.2 Resistance In RED, the total stack resistance is an important parameter, because the higher the resistance, the lower the power density. The total stack resistance consists of the Ohmic resistances of the membranes, the electrodes and the feed water compartments, and the non-Ohmic resistances, which are amongst others the resistances of the diffusion boundary layers.. 32.

(40) The Ohmic resistances act in series, so they can be summed up as shown in eq 3. Typically, CEMs have higher resistances than AEMs in RED, and the river water compartment has a very high resistance compared to the other resistances due to its low salt concentration and is often the dominant resistance [3]. Rstack,ohmic =RCEM +RAEM +RRW +RSW. (3). In eq 3, the resistances are area resistances (R) expressed in Ω·m², and RW and SW are river water and seawater respectively. Non-Ohmic resistances are challenging to study in RED due to their transient nature, as they consist of diffusion boundary layers and double layers in channels [3].. 2.3 Gross and net power density The power density expresses the power that can be generated per m² of membrane area. The gross power density (Pgross) depends directly on the OCV (EOCV,stack) and stack resistance (Rstack) and it is calculated as follows [17]: Pgross =. EOCV,stack ∙j-Rstack ∙j2 Nm. (4). where EOCV,stack is in V, j is current density in A/m² and Rstack is in Ω·m². Nm is the total number of both anion and cation membranes (rather than cell pairs). The net power density (Pnet in W/m² membrane) can be calculated by subtracting the pumping losses from the gross power density. The (normalized) pumping losses can be calculated as follows: Ppump =. ∆p∙Φ Nm ∙A. (5). Where Δp is the average pressure drop over the river and seawater compartment (Pa), Φ is the average flow rate of river and seawater in m³/s and A is the total membrane area in m².. 33.

(41) 3. Materials and Methods. 3.1 Membranes and chemicals The following ion exchange membranes were used in this study: heterogeneous Ralex CMHPES (MEGA, Czech Republic), homogeneous monovalent-selective Neosepta CMS (Astom Corp. Ltd, Japan), homogeneous multivalent-permeable Fuji T1, homogeneous Type I CEM, homogeneous T0 CEM and homogeneous Type I AEM (FUJIFILM, The Netherlands). MgCl2 ∙ 6 H2O, CaCl2∙2 H2O, K3Fe(CN)6 and K4Fe(CN)6 ∙ 3 H2O were purchased from Sigma-Aldrich. NaCl (Emprove Ph. Eur. Grade) was obtained from Merck.. 3.2 Membrane characterization Before the measurements, membranes were soaked in 0.5 M NaCl for 48 h to exchange them to Na+-form for the CEMs and Cl--form for the AEMs. The CEMs were soaked for 48 h in 0.5 M MgCl2, a mixture of 0.45 M NaCl & 0.05 M MgCl2 and 0.5M NaCl respectively, for the resistance measurements in pure Mg2+, a mixture of 90% Na+ & 10% Mg2+ and pure Na+. In the same solutions, the membrane resistances are measured, to compare with RED data and determine the selectivity of Na/Mg. The membrane thickness was measured by a digital screw micrometer (Mitutoyo 293-240, Mitutoyo Co., Japan). Membrane area and specific resistance measurements were performed in a six-compartment cell, as described in previous work [18,19]. When measuring the membrane in MgCl2, AEMs rather than CEMs are used as auxiliary membranes to prevent mixing with the NaCl shielding solution. Both AC and DC resistances were measured, as AC allows for ohmic resistance analysis. DC resistance measurements included non-ohmic resistances and allowed measuring the repulsion of divalent cations by monovalent-selective membranes. For all these measurements, a potentiostat (PGSTAT302N) equipped with a frequency response analyzer (FRA) was controlled by NOVA software (Metrohm Autolab, The Netherlands).. 34.

(42) Membranes were evaluated according to their resistances in NaCl and MgCl2, and the ratio of these resistances defines their transport selectivity, as shown in eq 6 [13]. Here, selectivity is based on electrical resistance measurements for different ions and not as a ratio of ions transported [20]. As in the case of RED operation, resistance is a more accurate predictor than specific ion fluxes. Rmg and Rna are membrane resistances (Ω∙cm²) in Mg2+ and Na+ form respectively and SNaMg (-) is selectivity. SNa Mg =. RMg RNa. (6). 3.3 Stack performance evaluation A cross-flow stack (REDstack B.V., The Netherlands) of active area 6.5 × 6.5 cm (42.25 cm2) and 10 cell pairs was equipped with Ti/Ru-Ir Electrodes (MAGNETO Special Anodes B.V., The Netherlands). Masterflex peristaltic pumps (Cole-Parmer) were used to pump the feed and the electrolyte solutions. Solution concentrations and conductivities of the feed waters are given in the supplementary information (SI 2). The feed streams were switched in the following order: from pure NaCl, to 10% Mg2+ in only seawater, to both streams with 10% Mg2+, to 10% Mg2+ in only river water and back to pure NaCl. In-house built pulsation dampeners were used for both feed streams. The stack was assembled with membranes presoaked in 0.5 M NaCl. Polyamide woven spacers were obtained from Deukum (Deukum GmbH, Germany) and had a thickness of 200 µm with a void fraction of 0.726 and a free surface fraction of 0.476. A torque of 2 Nm was applied on the stack in a cross-wise fashion. The outer membranes were FUJFILM T0 CEMs as these are able to retain the electrode rinse solution. The resistances of the extra outer CEM, the electrode rinse solution and the electrodes were subtracted from the measured stack resistances. Since in this study CEMs are compared rather than stack hydrodynamics, the stack was in all cases equipped with the same spacers and operated at the same flow rate of 53 ml/min (linear flow velocity of 0.92 cm/s), and the resulting average pressure drop was found to be 0.13 bar at an electrolyte pressure of 0.22 bar. This results in a small overpressure on the electrolyte of 0.09 bar to ensure packing of the membranes. The flow speed is chosen such that it is close to the value needed to obtain the optimal net power density [3]. 35.

(43) Before electrochemical analysis of a stack, current is applied (20 A/m²) for 20 minutes to ensure equilibration of the ion exchange membranes with the ionic feed solutions. Subsequently, the OCV and the AC and DC resistances were measured. For AC resistance measurements, three measurements are performed, at 10, 5 and 1 kHz and an amplitude of 0.01 A (2.4 A/m²) with 0.125 s integration time for the frequency response analyzer in the potentiostat. For the DC resistance, ten current steps from 0 to 50 A/m² and ten steps back to 0 A/m² are applied to calculate the resistance and to assess the stability (hysteresis) of the system (see SI 3). The effective OCV is determined from the IV curve used for DC resistance at zero current density, as this gives a realistic value of the OCV for the obtainable power density. In theory, a linear IV curve should be obtained. However, in practice the curve is not completely linear because of changing water compartment resistance among other reasons (see SI 3 for the experimental IV curves). To improve the accuracy of the power density data, measurements at multiple current densities were performed to find the optimum current density for power production. Each current density was applied for 10 seconds (1.4 times the stack residence time) before the voltage over the electrodes was measured. If the two measurements did not overlap, which is an indication that the cations are not yet in equilibrium within the membranes, the same procedure was repeated until overlap was achieved. Finally, reported power densities are maximum gross power densities which are calculated using the OCV and Rstack - based on the DC resistance - at the optimal experimental current density.. 4. Results & Discussion. 4.1 Membrane characterization The properties of ion exchange membranes determine the stack performance, especially in the presence of divalent cations. To understand stack effects, we first studied individual cation exchange membranes for their ability to conduct the various cations. The area resistance is a membrane property and can be a predictor of the performance, whereas the specific resistance is the Ohmic area resistance normalized over the thickness of the membrane (the specific resistances are shown in SI 4). In Figure 2, the area resistances 36.

(44) of the membranes in solutions with NaCl, a mixture of 90% NaCl and 10% MgCl2, and MgCl2 are all shown. The numbers next to the bars are the SNaMg values calculated using eq 6. In this study, all membranes had area resistances between 2.6 and 11.3 Ω∙cm². Both Fuji and Neosepta membranes are thin (125-150 µm), so they have low area resistances in the case of NaCl, in contrast to the Ralex membrane, which is thick (680 µm) and therefore has a high area resistance.. 30. 158 Ω·cm². NaCl. Area resistance (Ω·cm²). 90% Na⁺ + 10% Mg²⁺ MgCl₂ 20 2.3 34 10 2.9. 2.0. 0 Ralex CMH-PES. Fuji Type 1. Fuji T1. Neosepta CMS. Figure 2. Measured area resistance (determined by direct current) of CEMs in 0.5M NaCl, a mixture of 90% NaCl and 10% MgCl2, and MgCl2 respectively. Values next to bars are transport selectivities calculated by eq 6. The MgCl2 resistance for CMS was very high due to the tailored transport properties for monovalent cations [21]. In addition to thickness effects, effects for monovalent over divalent cations are investigated. In MgCl2, the area resistance of the monovalent-selective CMS membrane (158 Ω∙cm²) showed a remarkable difference of over 10 times greater compared to the other CEMs. In mixtures of 90% NaCl and 10% MgCl2, similar trends were found, although differences between membranes were smaller due to the lower Mg2+ concentration. These resistances are of interest for the RED stack experiments on this mixture with 10% MgCl2. These results give a clear picture of the (different) cation transport behavior of these CEMs, since T1 37.

(45) allowed Mg2+ transport (low resistance Mg2+) while CMS blocked it (high resistance Mg2+). Further detailed membrane characterization is given in SI 4 (IEC and water content) as well as in SI 5 (ion exchange isotherms).. 4.2 RED stack performance. 4.2.1. OCV Once we had a clear view of the membrane transport properties we investigated the performance in a RED stack and compared the results to calculations based on the previously described theory. RED stack performance measurements were performed using simulated river and seawaters (aqueous NaCl solutions). Natural compositions fluctuate over time and location, hence we chose the divalent cation composition comparable to that used in previous research: replacing 10% of NaCl by MgCl2 in river and seawater. In Table 1, in general a decrease of OCV was observed after introducing divalent cations. As the Nernst potential is reduced by a factor of 2 for pure solutions of only divalent ions (see eq 1), partially replacing sodium for divalent cations lowers the potential. However, this is a very small effect on the OCV.. 38.

(46) Table 1. Solutions used in studies and the relative OCV (experimental value divided by theoretical, calculated value) OCV per cell [V]. Errors in the relative OCV values are ± 0.010.. Na+. 10% Mg2+. SW & RW RW. SW. SW. RW SW & RW. CMH-PES. Type I. T1. CMS. Standard. Standard. Multivalentpermeable. Monovalentselective. uphill. Rel. theor. OCV [-]. Rel. exp. OCV [-]. Rel. exp. OCV [-]. Rel. exp. OCV [-]. Rel. exp. OCV [-]. N. 1.000. 1.000. 1.000. 1.000. 1.000. N. 0.998. 1.011. 0.963. 0.984. 0.989. N. 1.002. Y. 0.969. 0.880. 0.966. 0.960. 0.998. N. 1.000. Y. 0.966. 0.901. 0.925. 0.923. 1.003. 39.

(47) In Table 1, the experimental OCV values were calculated relative to the values obtained in NaCl. By doing this, only effects of divalent cations on membranes are taken into account and stack effects such as co-ion leakage are excluded. The calculated (theoretical) values were based on only the EMF of the monovalent-species present (Na+ and Cl-), with or without uphill transport. If uphill transport is taken into account, the exchange of Mg2+ in the river water to the seawater by Na+ is considered and the OCV is calculated using the concentrations obtained at equilibrium. For this equilibrium, in all cases the exchange of Mg2+ with Na+ was nearly complete (see SI 1 Figure S1.2, at equilibrium there was only 0.11 mM of Mg2+ left in the river water). All membranes showed a drop in OCV when Mg2+ is present in either feed stream. In this sub-section the various contributions to this OCV drop are discussed. The CMH-PES showed a drop in relative OCV (to 0.88); however, the calculated change for uphill transport (to 0.97) does not seem to justify this drop. Possibly this disagreement is due to CMH-PES’s heterogeneous nature [8]. The monovalent-selective CMS showed a near constant relative OCV for all the feed streams, even with divalent cations. Clearly, the effect of Mg2+ on the OCV through uphill transport was mitigated as even with only Mg2+ in the river water (implying a high driving force for uphill transport), there is no significant decrease in relative OCV. As for the multivalent-permeable T1 and standard-grade Type I, the drop in relative OCV was very similar in all cases. For both these membranes, the presence of Mg2+ in the river water decreased the experimental relative OCV by 3.4 - 4.0%. These values, when error margins are considered, agreed with the relative theoretical OCV (3.1%). It seems that by correcting for the equilibrium concentrations obtained by equalizing EMFs, one can predict the relative OCV decrease when divalent cations are present. In addition to uphill transport, another effect was decreasing the OCV: when there is Mg2+ in the seawater, relative decreases of 3.7, 1.7 and 1.1 % for Type 1, T1 and CMS, respectively, were observed. This OCV decrease was different for each membrane and cation, and can be caused by cation and charged-group interactions. This decrease has been observed in single-membrane permselectivity measurements for different cations [22,23], which can explain the OCV decrease we observed in this study. Although these single-membrane experiments [22,23] were performed in pure solutions, these findings are expected to be applicable to the mixed systems studied here. 40.

(48) Finally, if we consider Mg2+ in both river and seawater, both effects (of uphill transport and permselectivity decrease) played a role in OCV loss. For standard-grade Type I and multivalent-permeable T1, there was a relative drop of 7.3 - 7.5%, which seems to suggest that both uphill transport (3.4% loss) and permselectivity decreases (1.7 - 3.7% loss) were causing this OCV reduction. In summary, we showed and decoupled the effects on OCV when divalent cations are present: uphill transport, permselectivity decrease or a combination thereof.. 4.2.2. Resistance Stack resistances for the various CEMs in a RED system both with and without divalent cations are evaluated. The total (ohmic and non-ohmic) resistance was generally 110-125% of the ohmic resistance (see SI 6 and SI 7 for Mg2+ and Ca2+ data respectively). The nonohmic resistance was the highest with divalent cations present but no clear trends between different membranes were observed. In this section, the ohmic stack resistance was calculated and compared with experimental results (shown in Figure 3).. 41.

(49) 50. Data SW AEM. 40 30 20. CEM. 10 0. RW. 50 40 30 20 10 0 NaCl. RW 10% RW & SW Mg²⁺ 10% Mg²⁺. SW 10% Mg²⁺. RW 10% RW & SW Mg²⁺ 10% Mg²⁺. C. multivalent-permeable T1. D. monovalent-selective CMS Ohmic resistance per cell (Ω∙cm²). SW 10% Mg²⁺. Ohmic resistance per cell (Ω∙cm²). NaCl. B. standard-grade Type I Ohmic resistance per cell (Ω∙cm²). Ohmic resistance per cell (Ω∙cm²). A. standard-grade CMH-PES. 50 40 30 20 10 0 NaCl. SW 10% Mg²⁺. RW 10% RW & SW Mg²⁺ 10% Mg²⁺. 50 40 30 20 10 0 NaCl. SW 10% Mg²⁺. RW 10% RW & SW 10% Mg²⁺ Mg²⁺. Figure 3. Ohmic area resistance for NaCl and 10% Mg2+ in river and seawater feed streams for Ralex CMH-PES, Fuji Type I, Fuji T1 and Neosepta CMS. Diamonds show the measured total stack ohmic area resistances and bars show the calculated ohmic area resistances for the individual stack components. Ohmic resistances of all components were calculated, as described in section 2, and compared with the experimental total ohmic resistance of a cell. Resistances for the AEM and the seawater compartment have a low relative contribution due to a low anion membrane area resistance (1.0 Ω∙cm2) and a high concentration of salt, respectively. However, the river water – due to its low conductivity resulting from the low concentration of salt – and the CEM – resulting from a high membrane area resistance – account for most of the cell resistance. In this study, the feed compositions were next changed from NaCl to 10% Mg2+ (and 90% Na+). 42.

(50) When Mg2+ was introduced in the river water, the resistance of the river water will be lower due to a 10% higher concentration of Cl-. When Mg2+ was introduced in seawater, its resistance will also decrease slightly. These solution conductivity changes are shown in SI 7. In addition, introducing Mg2+ in seawater will increase the CEM resistance, as seen in Figure 2. The CEM resistance was assumed to be the membrane resistance measured in 10% Mg2+. However, this assumption is not fully valid, as in a RED stack one side of the membrane faces a high concentration solution and the other side a low concentration [18]. This assumption gave a reasonable approximation when using the current experimental set-up. For CMH-PES, a standard-grade heterogeneous membrane, the measured cell resistance did not change dramatically after introducing Mg2+ in either feed stream. This was expected as a low selectivity of 2.3 (shown in SI 2) implies a low relative change in resistance. However, the resistance of this membrane was high and this led to the highest cell resistances of all membranes. The overestimation of the calculated resistances can be caused by co-ion diffusion as a result of low permselectivity, which was also shown by the low OCV. For Type I, a standard-grade homogeneous membrane, a large change in resistance was observed after introducing Mg2+ in the seawater stream. A large change in resistance was expected from the selectivity of 2.9. However, in the experiments an even larger change was observed though the trend is as predicted. For the multivalent-permeable T1, there was no significant change in experimental cell resistance, as can be expected from the low selectivity of 2.0. The resistance of this membrane was hardly affected by Mg2+, which clearly demonstrates its ability to permeate divalent cations, and this resulted in the lowest absolute cell resistance of all stacks, especially with divalent cations. Finally, for the monovalent-selective CMS, a large change in resistance was calculated due to the high selectivity; however, hardly any change in resistance was observed in experiments. The monovalent-selective properties are the cause for Mg2+ being hindered in exchanging with this membrane. Our hypothesis is therefore, that over time the resistance of CMS will increase. Future studies involving long-term experiments could verify this hypothesis. 43.

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