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Environment
This article describes the development of a dense membrane capable of
selective proton transportation, thus offering an alternative to commercial
membranes used in artificial photosynthesis.
By GUILLEM GILABERT ORIOL. Research fellow at Universitat Rovira i Virgili (URV), Master in Chemical Engineering and Processes,
currently working on doctoral thesis
RICARD GARCÍA-VALLS. Research fellow at Universitat Rovira i Virgili (URV), Doctor in Chemistry from the Universitat Autònoma de
Barcelona, currently director of l'Escola Tècnica Superior de Enginyeria Química (ETSEQ), ([email protected]).
MARTA GIAMBERINI. Research fellow at Universitat Rovira i Virgili (URV), Doctor in Chemistry from the Universitá «Federico II» de
Nápoles, professor of the Universitat Rovira i Virgili.
Introduction
Artificial PhotosynthesisArtificial photosynthesis is a process for obtaining hydrogen and oxygen from sun and water and also hydrocarbons from
carbon dioxide, just as plants do[1].
Photosynthesis itself is a very complicated process involving many complex relations with macromolecules. A new
two-stage technology called artificial photosynthesis has been designed to shorten this process and gain more control over
it. In the first stage water, with the input of sunlight, produces oxygen, protons and electrons. In the second stage protons
and electrons are used to store this energy in the form of hydrogen. Other products like methane or methanol can also be
generated. If hydrocarbons are sought some carbon input is needed, such as atmospheric carbon dioxide[2].
This whole process can be described as:
(1)
The half-reactions taking place in the anode and cathode are:
(2)
(3)
This reaction can be produced in a photoelectrochemical cell (PEC) also called a Grätzel cell [1, 3]. This cell uses
photosensitisers attached to the surface of nanocrystalline TiO2 film. The arrival of a photon excites the complex and
oxidises the water-oxidation catalyst. This catalyst, called «blue dimer», is usually a ruthenium complex [4], and is capable
of attracting the four water electrons, splitting it into protons and oxygen. These electrons then flow through the TiO2
semiconductor to the platinum cathode, where a water molecule is formed for each pair of protons arriving through the
proton-exchange membrane (Figure 1)[1]. This part of the project is being carried out in collaboration with the Institut
Català d’Investigació Química (ICIQ).
Figure 1. Photoelectrochemical cell
Artificial photosynthesis cells and their advantages over photovoltaic cellsAn artificial photosynthesis cell has two main advantages over conventional silica photovoltaic cells: lower manufacturing
costs and better system performance. Both these advantages stem from the use of a catalyst, which drastically reduces the
voltage input necessary for water electrolysis [2]. Moreover, this cell can be used not only to produce hydrogen but also to
carry out chemical-reduction reactions [1]. With this technology humankind will have on hand a cheap and clean energy
source that is not only carbon-free but may also reduces the amount of atmospheric carbon dioxide. This would favour a
more sustainable lifestyle and help in the fight against environmental pollution; it could also be a solution to the fossil-fuel
energy crisis we are currently facing.
The role of the membranes and the object of the studyThis research centres on the development of biomimetic proton-exchange membranes to be used in future artificial
photosynthesis cells[4, 5, 6]. The membranes have been produced by means of the phase inversion technique using a
biomimetic liquid-crystal polymer previously synthesised in the laboratory. The various membranes obtained were tested in
an experimental cell to assess their properties and check them against the commercial Nafion® membrane.
The Nafion® membraneNafion® 117 is a perfluorosulfonic acid membrane developed in 1966 and marketed by DuPont. It is 177.8 μm (0.007 in)
thick. This polymer contains sulfonic acid, which drives the proton-hopping form of transport. This membrane has to be
hydrated to be permeable, since the hydrophilic sulfonic acid groups line up in channels ranging in size from 10 to 50 Å.
Through this channel pass the protons and cations but also the water and other undesired molecules like methanol.
Furthermore Nafion® swells by water absorption, worsening its mechanical properties. The transport mechanism of Nafion®
is therefore due to three factors: the active transport of protons, also called electroosmotic resistance; pore diffusion and
conventional diffusion [7]. This membrane has poor selectivity, allowing through not only protons but also other undesired
substances that might damage the electrodes or provoke undesired reactions (Figure 2[8]). These reactions might
drastically reduce cell performance or produce unforeseeable effects on the cell, such as corrosion, overheating,
explosions. But perhaps the biggest drawback is its price; the Nafion® is under patent and costs from 500 to 1000 dollars
per m2 [8].
Biomimetic membranes involve atotally different concept from
the commercial proton exchangemembranes like the Nafion®
membrane
Figure 2. Chemical structure and water-filled channels of Nafion®.
Nonetheless the Nafion® membrane is the only commercial proton exchange membrane used in large-scale industrial
electrochemical systems [3]. It is therefore the worldwide benchmark for proton-exchange membrane fuel cells (PEMFC). It
has also been thoroughly studied in the literature. For this reason the Nafion® membrane has been used as the crosscheck
for gauging the performance of the new synthesised biomimetic polymers.
Biomimetic MembraneA quick glance at the end bibliography shows that very little research has been carried out to develop new polymers that do
not use sulfonic acid as proton carrier. As already noted, use of the sulfonic group is the main reason for the membrane’s
poor selectivity. This is so because the membrane’s proton-transporting channels are big enough to allow the passage also
of water, methanol and other species.
A new approach has therefore been mooted, avoiding the use of sulfonic groups
for proton transport and channel formation. With this new approach a different
transport mechanism has to be used to increase proton selectivity; a new type of
material therefore has to be designed to form the membrane.
Ideally, fluorine should be avoided due to its high price and the difficulty of
eliminating the membrane at the end of its life cycle due to its high toxicity.
Also the new material must not absorb water since this leads to poor mechanical
properties and low proton selectivity. This is important because Nafion® is useful for hydrogen fuel cells when used in
humid gas phase but performs poorly in direct methanol fuel cells (DMFC) and in artificial photosynthesis cells, with liquid
phase working. A new biomimetic membrane has therefore been developed using oxygen’s coordination capacity rather
than cations to transport the protons through the dative bond along a polyether chain.
This membrane is based on observation of the self-assembly of the tobacco mosaic virus (TMV)[9].
Thus the tapered hydrophilic groups self assemble into columns, giving rise to ionic channels[10], which are then
responsible for mechanical resistance and protection of the membrane from water absorption due to their hydrophobia. The
centre of the column is thus formed by a continuous polar polyether chain acting as the proton carrier[5]. This combination
should favour a much higher proton selectivity against other substances like water and methanol. The self-assembly can be
seen in figure 3.
The biomimetic membranes aremade up by hydrophobic
macromolecules that protect thenanochannels, which transport
the protons by establishinghydrogen bridges with the ether
groups inside
Figur3 3. Chemical structure and self-assembly of the biomimetic polymers forming ionic channels.
To ensure sufficient membrane permeability these channels should run perpendicular to the membrane surface. The
membrane therefore needs to be as thin as possible to reduce the number of non-aligned ionic channels compressed
between the two membrane surfaces.
Materials and Methodology
The biomimetic membranes are built up from a liquid crystal polymer obtained by chemical modification of the commercial
polymer poly(epichlorhydrin) (PECH) with a tapered group comprising potassium 3,4,5-tris[4-(n-dodecan-1-yloxy)
bencyloxy]benzoate [11].
A five-stage chemical synthesis process was used to obtain the polymer.
In the first stage p-(ndodecan-1-yloxy)benzyl alcohol is obtained from a solution of f 4-hydroxybenzyl alcohol (10 g, 0.081
mol), 1-bromododecane (20.0 g, 0.081 mol), anhydrous potassium carbonate (11.0 g, 0.081 mol) and 250 ml de acetone,
using the catalyst 18-crown-6 ether.
In the second stage p-(n-dodecan-1-yloxy)benzyl chloride is obtained by reaction of the first-stage product with Tionyl
chloride (6.93g, 0.058 mol) dissolved in p-(n-dodecan-1-yloxy)benzyl alcohol (17.02 g, 0.058 mol) and in 119 ml of
dichloromethane (CH2Cl2).
In the third stage methyl 3,4,5-Tris[4-(n-dodecan-1-yloxy)benzyloxy]benzoate is obtained by reaction of the second-stage
product with methyl gallate (methyl 3,4,5-hydoxy benzoate) (2.8 g, 0.015 mol), dry dimethylformamide (DMF 40 ml),
potassium carbonate (6.5 g, 0.045 mol) and tetrabutylammonium hydrogen sulphate (0.1 g) dissolved in 4-(n-dodecan-
1-yloxy)benzyl chloride (15.5 g, 0.05 mol) and 60 ml of dry DMF.
In the fourth stage the potassium salt Potassium 3,4,5-Tris[4-(n-dodecan-
1-yloxy)benzyloxy]benzoate is obtained by reaction of the third-stage product t
with a solution of KOH in C2H5OH (potassium hydroxide 34.3 g in ethyl alcohol
102 ml) dissolved in methyl 3,4,5-Tris[4-(n-dodecan-1-yloxy) benzyloxy]benzoate
(10.2 g, 0.01 mol).
Finally, in the fifth stage the biomimetic liquid crystal polymer with a 67%
degree of modification is obtained by reaction of the fourth-stage product with
the poly(epichlorhydrin) (PECH) dissolved in THF. Its structure is shown in Figure
3.
Membrane production by castingThe first membranes were made with the polymer obtained from the former synthesis by means of an evaporation process
called casting. The biomimetic polymer obtained by modification of the PECH was dissolved in chloroform and the polymer
solution was placed in water to align ionic channels, at a ratio of 3 g of water to 5 g of polymer solution. The water aligns
the columns, which are hydrophobic in their outer part and therefore try to minimise their contact with water. In the first
membrane obtained, 6% in weight of the polymer was mixed in chloroform. In the second membrane obtained, 5% in weight
of the polymer was mixed in chloroform. The two immiscible phases formed in the mixture are then placed in a container in
Artificial photosynthesis is aprocess for obtaining energy
from water and sunlight, using acatalyst of ruthenium and
titanium oxide, and then storingthis energy in the form of
hydrogen
an oven at 40 ºC. This evaporates off the water and solvent from the polymer solution, thus precipitating the membrane.
Membrane production by phase inversion and glass modificationPhase inversion is a simple membrane-production process whereby the liquid crystal polymer is made to precipitate in an
organic solvent, submerged in a bath of water. The precipitation occurs because the organic solvent leaves the polymer
solution to dissolve in the bath water. The membrane thus precipitates out because the polymer is insoluble in water.
The membrane preparation method by phase inversion has clear advantages over the traditional casting technique. With
the casting method there is a long wait until the water evaporates off and then another wait until the solvent evaporates
off. With phase inversion, on the other hand, the membrane is formed instantly. It is also an easily controllable process
widely used in industry and can also be used for forming a tailor-made membrane. This means that, depending on the
precipitation conditions, two clearly differentiated zones can be obtained. The first is a dense zone or non-porous zone,
which is what confers on the membrane the selective ion-carrying property through the biomimetic nanochannels. The
second zone is porous, giving the membrane the necessary mechanical properties for its actual industrial application.
To prepare the membrane the liquid crystal polymer was dissolved at 25% weight
in the THF solution.
To ensure better alignment of the biomimetic channels and a solid surface as
polar as possible, two glass supports were modified with the ions NH3+ (totally
modified glass) and OH- (partially modified glass) according to the technique
described in [12]. A more polar surface facilitates better alignment of the
membranes, since the glass surface is hydrophilic (polar) whereas the coating of
the biomimetic channel is hydrophobic (non-polar). A repulsion is therefore set
up between them, enabling the channels to line up perpendicular to the glass,
thus minimising their energy interaction with the surface.
Finally, using a Kpaint applicator, a 250 μm sieve was passed over the glass surface to create an even film of uniform
thickness. After phase-inversion precipitation of the membrane, it was extracted and dried to remove any impurities. The
characteristics of the membranes thus obtained are shown in Table 1.
Table 1. Characteristics of the synthesised membranes
Name Modification (%) Polymer (%) Glass
e1 67 25 Completely modified with NH3+
f1 67 25 Partially modified with OH-
Determination of proton permeabilityThe membranes’ coefficient of proton permeability, and also their permeability, were measured using the technique
described in [13]. In this way membrane permeability was gauged by means of a testing cell. The feed cell A contains the
HCl solution, while the stripping cell B contains the NaCl solution (Figure 4). By measuring the change of pH in the stripping
cell, membrane permeability can be determined, keeping matter balanced in the cell. For the permeability experiments a
0.05 M NaCl solution and a 0.05 M HCl solution were used.
Figure 4. Permeability Cell.
Thus, to determine the coefficient of permeability p (cm/s), the following equation has been used:
(4)
The flow of protons through the membrane J (mols/m2s) can be determined from the equation:
(5)
The permeability of membrane P (cm2/s) can be obtained from the following equation, where l is the membrane thickness.
(6)
Results and Discussion
Membrane production by castingA membrane has been successfully obtained following this method. This method was dropped, however, on the grounds of
irreproducible results. This method is also much more time-consuming than phase inversion, so it is difficult to control and
cannot produce thin membranes. Even so, a membrane produced by this method has been used to compare its permeability
with the phase-inversion membranes.
Membrane production by phase inversionMembranes produced by this technique gave different permeability readings. The membrane obtained with the completely
modified glass matched the permeability of the casting-produced membrane while the permeability of the membrane
synthesised with partially modified glass was lower.
This method was also found to produce thin membranes with a dense non-porous layer. This is a very positive factor in
terms of their application in artificial photosynthesis fuel cells, preventing the gaseous hydrogen from leaking out through
the membrane, while also endowing the membrane with a certain degree of flexibility since the membrane is not
completely dense.
The development of this membrane preparation method has enabled us to establish a standardised, rapid and efficient
method for producing membranes with reproducible proton-transporting properties.
Proton permeabilityThe permeability assessment results obtained in the experimental cell are shown in Table 2. The membrane’s time to
proton conduction is also given. The coefficient of permeability readings p (cm/s) are shown in Figure 5, while the
permeability data P (cm2/s) are shown in Figure 6, excluding the experiments made with non-woven membranes since their
thickness is uneven.
Table 2. Proton permeability readings.
Membrane Experiment p (cm/s) P (cm2/s) J (mols/m2s) Time to proton conduction
Perforated 1 4,3E-2 n/a 2,2E-3 -
Non woven 1 7,0E-4 n/a 3,5E-5 70 min
Non woven 2 9,1E-4 n/a 4,6E-5 220 sec
Nafion® 1 6,2E-4 1,2E-5 3,1E-5 40 sec
44-4 1 1,9E-4 3,8E-6 9,5E-6 140 sec
e1 1 4,7E-5 9,3E-8 2,3E-6 2,92 days
e1 2 4,5E-6 9,0E-9 2,3E-7 1,78 days
e1 3 (inv) 6,9E-7 1,4E-9 3,4E-8 5,30 days
e1 4 (inv) 2,0E-5 4,0E-8 1,0E-6 5,70 days
f1 1 1,6E-3 3,3E-6 8,1E-5 5,29 h
f1 2 1,5E-4 2,9E-7 7,3E-6 33 min
f1 3 3,3E-4 6,6E-7 1,7E-5 11,5 min
The membranes e1 obtained with totally modified glass show irregular conductivity and long times to proton conduction.
Partially modified glass achieved better membrane alignment. This prompted development of the f1 membranes, showing a
coefficient of permeability to match Nafion® and membrane 44-4. Nonetheless, since membrane f1 is thinner, its
permeability is slightly lower than the latter two.
The biomimetic membranes havebeen produced by the phase
inversion technique, obtaining adense proton-transporting layer
that prevents hydrogen leaks
Figure 5. Coefficient of permeability (cm/s) and membrane’s time to proton conduction.
Figure 6. Permeability (cm2/s) and membrane’s time to proton conduction
SwellingOne of the main drawbacks of the Nafion® membrane, apart from its high fluorine content, is the need of being moistened
to conduct protons. This produces a swelling effect in the membrane, impairing its mechanical properties. It also increases
the difficulty of operating with fuel cells, since the relative humidity on both sides of the membrane needs to be controlled
to induce proton conductivity.
Unlike the Nafion® 117 membranes, therefore, which swell up, retaining 15% of
water, the biomimetic membranes do not vary their weight or thickness under
the same conditions. This behaviour can be explained by the biomimetic
membranes’ low water interaction and different transport mechanism.
Biomimetic membranes transport the protons through an ionic channel formed by
ether groups with which the proton establishes hydrogen bonds; the Nafion®
membrane, being a strong acid, works by dissociating the protons from the
sulfonic acid and reacting strongly with the water to enable proton passage.
Contact angle of glass support and membrane
The membranes obtained canwork for a minimum of 335
hours without stopping, offeringa good proton conductivity
The contact angle is the angle at which a water droplet interface meets a solid surface. Determination of the contact angle
between the water and glass serves to assess the glass’s hydrophilicity, i.e., finding out which glass is most polar and hence
has the best membrane alignment properties. The results are shown in Figure 7, and the contact angle readings in Table 3.
Figure 7. Contact angle of the partially modified glass (left) and the completely modified glass (right).
Table 3. Contact angle of membranes e1 and f1 with the membrane surface facing up (water) and down (glass).
Glass Contact angle
Partially modified with OH- 38,1º
Completely modified with NH3+ 82,5º
The figure and table show that the partially modified glass has a lower contact angle than the totally modified glass. This is
so because the water has more affinity for the surface and therefore spreads out rather than shrinking; this makes the
surface much more polar. A more polar solid surface has thus been developed to facilitate membrane alignment during the
phase inversion process. This explains why the conductivity of membrane f1, obtained on the more polar support, is higher
than e1 obtained on a less polar support.
The contact angle with the two sides of the membrane has also been determined, both that which is in contact with the
glass during membrane precipitation and that which is in contact with the water. The membranes assessed were e1, formed
with the totally modified glass, and f1, obtained with the partially modified glass. The results are shown in table 4.
Table 4. Contact angle of membranes e1 and f1 with the sides in contact with the water and the glass.
Membrane Water-contact side Glass-contact side
Membrane e1 76,0º 88,8º
Membrane f1 74,2º 86,2º
This table shows that membrane f1, obtained with the partially modified glass, has a slightly lower contact angle than e1,
although the difference is negligible. This would mean that membrane f1 might be more hydrophilic than e1. The better
alignment of membrane f1 thus ensures that there are more ionic channels on the membrane face, whose lower part is
hydrophile (polar) due to the ether groups making it up. At the same time there are fewer tapered non-polar groups, thus
increasing the membrane’s hydrophilicity.
Thickness and morphologyTo find out the membrane’s alignment each one was analysed under polarised light, a regular alignment showing up.
A scanning electronic microscope (SEM) was used to determine the membranes’
thickness and morphology. The results reflect asymmetric membranes with a
porous part and a dense part. This gives them good mechanical properties,
enabling them to bend without breaking, while the dense part prevents large
hydrogen gas molecules from passing freely through the membrane. This
without suffering structuraldamage or warping since no
water is retained inside
requirement is essential for any device that needs to be selective, like the
artificial photosynthesis cell or fuel cells.
Analysis of the SEM images shows a mean membrane thickness of 20 μm (Figure
8). This figure is crucial because, working with such thin membranes, compound
membranes can be built up merely by joining and adding on more layers.
The membranes were also analysed using an atomic force microscope (AFM) to delve into more detail of the membrane
surface structure. These images show the same column aggregates before and after the experiment. (Figure 9). These
aggregates have a mean size of 40 nm and could correspond to biomimetic column aggregates, thereby confirming the good
chemical resistance of the membrane.
Figure 8. Biomimetic membrane cross section.
También se han analizado las membranas utilizando el Microscopio de Fuerza Atómica (AFM), con objeto de conocer más
detalles sobre la estructura de la superficie de la membrana. En estas imágenes se pueden ver las mismas agregaciones
columnares antes y después del experimento (Figura 9). Estas agregaciones tienen un tamaño medio de 40 nm y podrían
corresponder a agregaciones de columnas biomiméticas, lo que confirmaría la buena resistencia química de la membrana.
Figure 9. AFM phase diagram of membrane e1 before and after the experiment.
Mechanical and chemical strengthThe proton permeability experiments showed that synthesised membrane e1 can work for 335 hours on the trot without
suffering any structural damage, even with strong constant agitation and in the presence of acids at pH 1.30 without
suffering any sort of deterioration. A noteworthy feature here is the membrane thickness of 20 μm.
Transport Mechanism
Sodium and pH analysis hasshown the proton transportmechanism to be counter-
transport
Since there are no references to this transport mechanism in the scientific literature, a study was made of the proton
transport mechanism through the membrane. To do so samples of the feed and stripping solutions were taken at the start
and end of the e1 membrane permeability experiment. A reading was hence taken of the cation’s sodium concentration
(Table 5) and pH concentration (Table 6).
By converting pH readings to molar concentration, a calculation can then be made of the change of proton concentration in
the stripping solution. The calculation is made in the stripping solution because it is where the pH variation can be
determined most progressively. The variation in the sodium ion concentration is measured in the feed solution, where the
best analytical precision is to be obtained. The results are shown in Table 7.
Table 5. Sodium analysis before and after the experiment.
Feed Stripping
Start Na+ 1,118 mg/l Na+ 1150 mg/l Na+
End Na+ 4,24 mg/l Na+ 1275 mg/l Na+
Table 6. pH readings before and after the experiment
Feed Stripping
Start pH 1,36 5,55
End pH 1,28 3,87
Table 7. Difference between the start and end concentration of sodium protons
Feed Stripping
ΔH+ 1.33x10-4 M
ΔNa+ 1,36x10-4 M
This table shows that the change in molar concentration is the same for protons and for sodium ions. This means that for
each proton passing from the feed to stripping solution a sodium ion passes from the stripping to the feed solution; this
shows the proton transport mechanism to be counter-transport (Figure 4).
Conclusions
This research has proven the feasibility of producing an alternative to such a well-established commercial membrane as
Nafion®. This study merges liquid crystal polymer materials science with nanotechnology and chemical engineering.
A methodology has been developed to obtain an aligned biomimetic membrane
with a similar proton permeability to its commercial alternative. A methodology
has also been established for ensuring reproducible and rapid membrane
manufacture. All this has been achieved by chemically modifying the glass
support where the membrane is precipitated to make it more polar and thus
ensure better aligned membranes. The membranes obtained were also thin and
uniform, with a dense layer and a porous layer. The membranes withstood mechanical stress and chemical agents for a long
time. The hydrophilicity of the support and membrane has also been ascertained by measuring the contact angle.
Furthermore, by means of the swelling experiment, a membrane has been obtained that, unlike Nafion®, absorbs
absolutely no water.
This study lays down the foundations for a more in-depth study of the transport mechanism. A comparison of the variation
over time of sodium-ion and proton concentration in the permeability cell shows this to be a counter-transport mechanism.
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