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Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis
international journal of hydrogen energy xxx (xxxx) xxx
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Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis A. Carbone*, S. Campagna Zignani, I. Gatto, S. Trocino, A.S. Arico Institute for Advanced Energy Technologies "Nicola Giordano", CNR-ITAE, Via S. Lucia Sopra Contesse 5, 98126, Messina, Italy
highlights A commercial FAA3-50 membrane was used as a solid polymer electrolyte in an alkaline water electrolysis. An improved chemical treatment based on alkaline KOH solution was carried out. A specific MEA based on FAA3-50 anionic membrane and NiMn2O4 anode catalyst was developed and tested. Alkaline stability was assessed by a potential cycling-based durability test for 1000 h.
article info
abstract
Article history:
A commercial FAA3-50 membrane was investigated as a solid polymer electrolyte in an
Received 29 October 2019
alkaline water electrolysis. An improved chemical treatment based on alkaline KOH so-
Received in revised form
lution was carried out. A limited degradation of the functional groups was observed
15 January 2020
allowing to maintain a good anion conductivity approaching 55 mS cm-1 at 100 C. Thermal
Accepted 21 January 2020
stability up to 200 C was assessed by thermal analysis.
Available online xxx
A specific membrane-electrodes assembly based on FAA3-50 anionic membrane and NiMn2O4 anode catalyst was developed and investigated in a single cell for water elec-
Keywords: Alkaline water electrolysis
trolysis at a moderate temperature (50 C). Performance stability was assessed by a potential cycling-based durability test for
Anionic exchange membrane
1000 h by varying the cell potential between 1 and 1.8 V for the FAA3-50 and NiMn2O4
NiMn2O4 catalyst
based-MEA.
Durability test
According to this evaluation, both the FAA3-50 membrane and the NiMn2O4 catalyst appear sufficiently stable for electrolysis operation under mild operating temperatures. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The urgent need to move from fossil fuels to renewable sources makes hydrogen very promising as an alternative
energy carrier. In this energy transition period, hydrogen is progressively used as a fuel to produce electrical power in stationary (domestic and industrial) and automotive sector [1e3]. The main advantages of using hydrogen are the highly
* Corresponding author. E-mail address: [email protected] (A. Carbone). https://doi.org/10.1016/j.ijhydene.2020.01.150 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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efficient conversion into electricity and vice versa, and the clean combustion of hydrogen offers solution for grid balancing service to convert the surplus of renewable energy. Among the several methods for “green” hydrogen production, water electrolysis remains the most reliable and efficient. There are two different technologies for low temperature water electrolysis that can be distinguished as a function of the used electrolyte: liquid alkaline using KOH electrolyte and polymer electrolyte membrane using a solid ion exchange membrane. The former technology has as main drawback the recirculation of caustic solutions and an excessive reactivity of hydroxyl groups with carbon dioxide causing the formation of potassium carbonate precipitate. This also reduces the ion exchange and the efficiency of the overall system. On the contrary, the ion exchange polymeric membrane does not require liquid corrosive electrolyte; the functional groups of the membrane are confined in a solid matrix and an easy scale-up of the system is possible [4e8]. The ion exchange membranes used for this purpose can be divided in two classes: proton exchange membrane (PEM) [9,10] and anion exchange membrane (AEM) [11e15]. The PEM based electrolysis shows the advantage of a high hydrogen production rate and energy efficiency but the relevant disadvantage of being limited by the use of platinum group metal catalysts and perfluorinated membranes such as Nafion or Aquivion [16]. This results in a high production cost. On the contrary, AEM based electrolysis presents several advantages since it can operate with cheap non-platinum or non-precious group metal catalysts while concentrated KOH (7 M) can be replaced by distilled water or diluted alkaline solutions. The absence of corrosive solutions prevents the system from leaking. Low cost hydrocarbon membranes can replace Nafion saving costs. Regarding the use of catalysts that meet the requirements of improving the oxygen evolution reaction (OER), the research is moving towards the development of mixed metal oxides spinels, perovskites, organic compounds, etc. [17e27]. Regarding the use of anionic membranes, different typologies of polymers were studied in the recent years. In particular the attention was devoted to the modulation of functional groups and their stability in alkaline environment. These functional groups can be distinguished in nitrogen-based (quaternary ammonium, pyridinum, benzyl imidazolium) and nitrogen-free (phosphonium, sulphonium) [28e32]. The quaternary ammonium is the most used due to the easy synthesis and the possibility to make a crosslinking of the terminal groups to improve the stability. One of the most used commercial membranes for electrochemical devices application is the quaternary ammonium-based Fumatech FAA3 polymer [33e35]. In electrochemical devices, it is fundamental to use the same ionomer of the membrane during electrodes preparation, to minimize the interfacial resistance and avoid delamination. It appears also important to carry out the same chemical treatments on both electrodes and membranes. According to the above-mentioned requirements, in this work, a FAA-3 ionomer and membrane based MEA were developed for water electrolysis application and an improved chemical treatment procedure was adopted in order to limit the degradation of the functional groups in alkaline
environment. This membrane was combined with a NiMn2O4 as promising catalyst for Oxygen evolution in the alkaline environment.
Experimental Catalyst preparation The oxygen evolution catalyst was in-house prepared by using the oxalate method [36]. Ni and Mn nitrates were dissolved in distilled water and mixed with a solution of oxalic acid neutralised at pH 6.5 with NaOH. The molar ratio between the chelating agent and the metals was 10. A complex was formed and then decomposed at 80 C with hydrogen peroxide to obtain a precipitate made of amorphous oxide. This was filtered, washed and dried at 100 C for 24 h. The powder was then calcined at 350 C for 120 min to form the NiMnOx catalyst. The NiMnOx was characterised in terms of structure, properties and crystallite size. For the hydrogen evolution process, a standard Pt/C catalyst was prepared using the sulphite complex -method followed by reduction in H2 at 300 C [37]. The catalayst powder contained Ketjen black EC at a 30 wt % content.
Membrane FAA3-50 membrane (FumaTech), based on a brominated polysulfone backbone with quaternary ammonium side chain groups provided in bromide form [38], was subjected to anion exchange process and characterised for this application.
Ionomer preparation The ionomer dispersion was prepared by solubilizing the received solid ionomer powder (FAA3-shredded film) in a mixture of solvents. An alcoholic solution of n-propanol and ethanol (1:1 wt) was used for this purpose and the FAA3 ionomer was solubilised at room temperature under stirring in order to have ~5 wt% dispersion.
Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) of the polymer electrolyte was performed using a thermo balance Netzsch (mod. STA 409) in static air by monitoring the percentage mass loss change in the temperature range between 20 and 1200 C with a temperature variation rate of 5 C min1
Determination of ion exchange capacity (IEC) To determine the total amount of ionic groups contained in the membrane a back-titration based on the Volhard method was carried out on FAA3-50 samples using Cl as a counterion, after an exchange in NaCl 1 M solution (72 h). The dried membrane was immersed in NaNO3 0.1 M for 48 h at room temperature, then it was removed. The remaining solution was treated with 5 ml of AgNO3 0.1 M and 5 drops of Fe(NO3)3 (11 wt%). A back-titration of the solution with KSCN 0.1 M was performed. The amount of volume of the KSCN needed to
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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achieve the so-called equivalent point (VKSCN determined. The IEC was calculated as follows:
e.p.)
was
IEC ¼ VAgNO3 VKSCNe:p: ½KSCN mdry
3
A is OH or Cl anion and the corresponding IEC (meq/g) is calculated by acid-base or Volhard method; mdry is the dry mass of the sample expressed in g; Vwet is the wet volume, expressed in cm3.
where: VAgNO3 is the initial volume, expressed in ml VKSCNe.p is the volume of the titrant at the equivalent point expressed in ml [KSCN] is the concentration of titrant, expressed in molarity mdry is the dry mass of the sample determined at 50 C for 2 h under vacuum (1000 mbar), expressed in grams. Moreover, to calculate the amount of halides converted into OH ions, an acid-base back-titration was carried out. In this case, the membrane was exchanged for 24 h in KOH 1 M solution at room temperature, then the dried sample was immersed in HCl 0.01 M solution for 24 h at room temperature and removed. The solution was back-titrated with NaOH 0.01 M, using an automatic titrator (Metrohm Mod.751GPD Titrino). The IEC was calculated as follows: IEC ¼ VHCl VNaOHe:p: ½NaOH mdry where: VHCl is the initial volume, expressed in ml VNaOHe.p is the volume of the titrant at the equivalent point expressed in ml [NaOH] is the concentration of titrant, expressed in molarity mdry is the dry mass of the sample determined at 50 C for 2 h under vacuum (1000 mbar), expressed in grams. The final IEC values were calculated from the average of three different measurements.
Water and hydroxide uptake, swelling, l, ion concentration The water and KOH uptakes were calculated by the difference in weight before and after the immersion of the dried samples in water or KOH 1 M solution at 30 C for 24 h. To reach the dry state, the samples were maintained in an oven under vacuum (1000 mbar) for 2 h at 50 C. The dimensional variation was calculated in the same operative conditions of water uptake on a rectangular sample, measuring the length (L) of the longest side. The swelling was calculated as the ratio DL/L. The l, which is the number of water/KOH molecules per functional group, is calculated as the ratio between the uptake and the IEC. The reported values are an average of three different measurements. The anion concentration, expressed in M, is calculated as follows: ½A ¼ IECA mdry Vwet where:
In-plane anion conductivity, effective mobility, diffusion coefficient The in-plane anion conductivity of samples with different counterions (Cl, OH) was carried out by a four electrodes method by Electrochemical Impedance Spectroscopy (EIS). The EIS parameters for the measurement were: 100 KHz - 1 Hz of frequency range and a 50 mV of amplitude. The measurements were carried out in the range of temperature 30e120 C, after a conditioning day at 30 C flowing humidified N2 (100% RH) at room pressure. The activation energy was calculated by the Arrhenius law from conductivity results. The effective anion mobility was calculated as follows: m ¼ s = ½A F where: m is the mobility, expressed in cm2/Vs s is the anion conductivity, expressed in S/cm [A] is the anion concentration F is the Faraday constant (96485C/mol) The anion diffusion coefficient (cm2/s) in water is calculated as follows: Ds ¼ sRT = ½A F2 where. s is the anion conductivity of the hydroxide or chloride form measured in humidified Nitrogen; R is the universal gas constant (8.31 J mol1 K1) T is the absolute temperature (K) [A] is the anion concentration F is the Faraday constant
Membrane-electrodes assembly Membrane-electrode assemblies based on NiMnOx spinel electrocatalyst [39] and Pt/C catalyst [40] at the anode and cathode, respectively, were prepared by spray coating the catalyst over the membrane. The catalysts were mixed with an ionomer dispersion of the same polymer membrane in ethanol (~5 wt%) for preparing catalytic inks. The loadings were 0.5 and 3 mg cm2 for the cathode and the anode, respectively. Ni felt from Bekaert were used as diffusing layers and current collectors. A FAA3-50 anion exchange membrane in the OH form was used as a polymer electrolyte separator between anode and cathode compartments. Membrane-electrode assemblies (MEAs) with a 5 cm2 geometrical area were prepared by a coldassembly procedure. This was adopted to avoid membrane degradation during hot pressing.
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Electrochemical characterisations The single cell was tested at different temperatures and under atmospheric pressure. Humidified N2 was fed to the cathode compartment at a flow rate of 50 ml/min and at atmospheric pressure. Initially, pure water and thereafter KOH (1 M and 6 M) solutions were supplied by a peristaltic pump to the anode compartment, at a flow rate of 4 ml/min. To study the effect of hydroxide concentration chronoamperometric curves were carried out with a Keithley power supply system (Tektronic). Polarization curves (cell potential as a function of current density) were performed at a scan rate of 5 mV/s. A durability test was carried out for 1000 h, using potential cycles of 1 min between 1.0 and 1.8 V. Ac-impedance analysis was used to determine the cell resistance (Rs). Series resistance (Rs) was measured from the high frequency intercept on the real axis of the Nyquist plot. Measurements at different temperatures were carried out on the same cell.
Results and discussion The IEC values of membrane samples, measured with acidbase (1st row) and Volhard (2nd row) back-titration methods, are reported in Table 1. It is evident that the IEC of the membrane sample in OH form is lower than the sample exchanged with Cl as counterion. This was due to an uncomplete conversion or to some degradation during the alkaline treatment needed to achieve the final ion exchange [1]. Only 86% of the initial groups were active in hydroxide form. The Volhard method allows to measure all the halides present in the membrane. The nominal value reported in the data sheet of the commercial membrane was confirmed. This behaviour was analysed by thermogravimetric analysis. As reported in Fig. 1, the membranes with different counterions show different thermal profiles. According to the IEC data, different exchange processes produced different polymer stability characteristics. A mass loss of about 15 wt% up to 100 C was found for both samples. This was due to the water loss. In the temperature range 100e200 C, the degradation of the functional groups took place whereas above 200 C the main chain degradation occurred. For the Cl exchanged sample, a better thermal stability was found than the OH one. The Cl neutralised quaternary ammonium started to give rise to a degradation at about 150 C instead of 100 C as recorded for the OH sample. The difference was clearly due to the different ion exchange that affects the thermal and mechanical properties of the membrane, in particular the stability of the functional groups. The physico-chemical parameters measured at 30 C are reported in Table 2 for the FAA3 membrane in OH form immersed in liquid water and KOH 1 M solution.
Table 1 e FAA3-50 IEC data. Membrane
IECexp.
IECnom.
% active groups
FAA3-50 (OH) FAA3-50 (Cl)
1.59 1.85
1.85 1.85
86 100
Fig. 1 e Thermal profile of Q041 membrane with different counter-ions.
The water uptake of the OH sample was in accordance with the mass loss of the TGA, in fact a similar value was found. The samples were analysed in the thermo-balance in a wet form. Regarding the KOH uptake (2nd raw), this was largely higher than the water uptake. This effect was due to the strong affinity of functional groups of the membrane for the alkaline solution. Such property is useful for the alkaline electrolyser application because the membrane acts as a reservoir for the OH electrolyte. Accordingly, the swelling is 10% higher for the KOH uptake than water, and consequently the l value was higher than in pure water. The l value, calculated from water uptake, corresponds to 6 water molecules per quaternary ammonium group. This value is typically occurring for the hydroxide diffusion and transport and it corresponds to what reported in the literature for this kind of polymer membrane. It results from a hyper-coordination of hydroxide ions [41]. The l value that was calculated according to the KOH uptake was obviously the highest amongst those observed. The value of ionic concentration is also reported in Table 2. Since the ionic concentration is calculated considering the wet volume of the sample that is directly related to the liquid uptake, the ionic concentration after the treatment in KOH solution is lower than in water, due to a dilution effect. The OH concentration in KOH solution was 1.1 M, i. e. close to the concentration of the solution in which it was immersed (1 M). It can be hypothesised that the ions confined in the hydrophilic portion of the membrane, where the ion conductivity takes place, have the same concentration of the solution and probably the same diffusion coefficient. The effective ion mobility was calculated for each solution investigated and it increased with the decrease of the ion concentration; however all these values are of the same order of magnitude. These results are in accordance with those reported in literature for similar polymer membranes [42]. Regarding the diffusion coefficient, this takes into account the ratio between the ion conductivity measured in humidified nitrogen and the ion concentration. The observed value of 7.17 106 cm2/s for the membrane sample after KOH uptake is only one order of magnitude lower than the value 5.3 105 cm2/s reported in numerous papers for OH solutions
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Table 2 e FAA3-50 physico-chemical parameters. Membrane FAA3-50 (OH) (H2O) FAA3-50 (OH) (KOH 1M)
Wup, %
DL/L, %
l
[ion], М
meff, cm2/Vs
1.63 106 7.17 106
17
9
6
1.5
6.27 10
70
19
25
1.1
8.83 105
under infinite dilution [31,41,43]. This means that the diffusion of hydroxide ions confined in the solid environment of the membrane is not so different from the diffusion in a liquid infinite solution, leading to an improved conductivity path. This behaviour is in accordance to what has been reported by Chen et al. [44] for AEM based on poly(vinyl benzyltrimethylammonium) (PVBTMA). In that study, it is reported that the coordination shells of quaternary ammonium cations overlap forming a continuous surface region for hydroxide transport with a mixed Grotthus and vehicular conduction mechanisms. The results of anion conductivity measurements, carried out on samples with hydroxides as counter-ions and fully hydrated gas, are reported is Fig. 2. As expected, the conductivity increased with the increase of temperature. The maximum value of 55 mS/cm was reached at 100 C but above this temperature the conductivity decreased due to the degradation of the membrane which suffered by an excessive swelling at that temperature and humidification levels. Since the OH conductivity, in the temperature range 30e100 C, shows Arrhenius behaviour, the activation energy can be roughly calculated and it corresponds to 27.4 kJ/mol. Various studies have reported on different typology of AEMs in which the activation energy for the mixed Grotthus and vehicular mechanism, with predominance of vehicular one, is calculated in the range 10e15 kJ/mol [30]. A slightly higher value of activation energy is here determined than that reported for the mixed conduction mechanism. In this case, a predominance of Grotthus mechanism over the vehicular one can be envisaged. This behaviour could be explained by considering the values reported in Table 2, showing that the hydroxide solvation and dissociation is less
Fig. 2 e Conductivity as a function of temperature.
Ds, cm2/s
5
pronounced in water than in KOH solution, leading to a high energy barrier for anion conduction. It is expected that in KOH solution, the conductivity increases and the activation energy can be reduced. To better understand the perspective applications of this AEM and its specific pre-treatment for alkaline electrolyzers, single cell tests were carried out in the temperature range 30e80 C by recirculating KOH 1 M solution at the cathode. Fig. 3 shows the polarization curves recorded at different temperatures and at ambient pressure. The combination of the FAA3-50 alkaline membrane and ionomer with a NiMn2O4 anode catalyst showed excellent performance as a function of temperature up to 80 C. This is comparable to the best AEM electrolysis performance reported in literature [21]. A current density of 530 mA/cm2 was obtained at 2 V at 80 C. In order to understand the contribution of the membrane to the electrolysis performance, the raw polarization curves were corrected for the ohmic drop (Fig. 4) (IR-free polarization curves) . By comparing the raw and IR-free curves at 50 C(Fig. 5), at about 0.4 A/cm2, the voltage loss was about 120 mV, corresponding to a voltage efficiency loss of about 5% from 79% to 74% vs. HHV. Accordingly, the contribution of the membrane to the electrolysis performance at low temperature appears good with minimal loss of efficiency at practical operating current density. Alkaline stability was assessed by a potential cycling-based durability test for 1000 h by varying the potential between 1 and 1.8 V for the FAA3-50 and NiMn2O4 based-MEA. Fig. 6 shows an increase of performance in the first 1000 h. Probably, the activation of the catalyst occurs after some potential
Fig. 3 e Water electrolysis polarization curves for the FAA3-50 membrane based-cell fed with 1 M KOH and containing NiMn2O4 anode catalyst and Pt/KB 40% cathode.
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Fig. 4 e IR-Free electrolysis polarization curves for the FAA3-50 membrane based-cell fed with 1 M KOH and equipped with NiMn2O4 catalyst at the anode and Pt/KB 40% at the cathode.
Fig. 5 e Comparison of IR-Free and raw polarization curves at 50 C for the electrolysis cell based on the FAA3 membrane.
cycles. A moderate performance decay was observed after 1000 h. The potential cycling experiment represents an accelerated stress test to evaluate the load cycling behaviour that
Fig. 6 e Cycling behaviour (1e1.8 V) for the electrolysis cell at 50 C.
Fig. 7 e Polarization curves before and after the 1000-hrs durability test for the NiMn2O4 KB 70:30 catalyst on the anode and Pt/KB 40% on the cathode with the FAA3-50 membrane fed with 1 M KOH.
needs to be addressed by the electrolysis devices in grid balancing service to convert the surplus of renewable energy in hydrogen. The results of these tests would suggest that some improvement is required to enhance the MEA stability under cycled operation. However, it is not easy to distinguish in these experiments the recoverable losses from irreversible losses. Polarizations curves have thus been compared before and after the 1000-hrs potential cycling test at 50 C(see Fig. 7). The polarization behaviour was essentially similar indicating that the recorded potential losses in the durability test were essentially recoverable losses caused by mass transport constrains These can be addressed by improving cell design, catalyst layers and diffusive layers. According to this evaluation, both the FAA3-50 membrane and the NiMn2O4 catalyst appear sufficiently stable for electrolysis operation under mild (50 C) operating temperatures.
Conclusions Commercial FAA3-50 membrane with an improved chemical treatment was used for water electrolysis. After the treatment in KOH solution, the membrane presented about 86% of the initial groups active in hydroxide form. A thermal stability of the polymer main chain and functional groups was determined up to 200 C. The OH concentration in KOH solution is close to the concentration of the solution in which it was immersed. It can be hypothesised that the ions confined in the hydrophilic portion of the membrane, where the ion conductivity takes place, have the same concentration of the solution. The diffusion coefficient value of 7.17 106 cm2/s for the membrane sample after KOH uptake is only one order of magnitude lower than the value 5.3 105 cm2/s reported for OH solutions under infinite dilution. The OH conductivity of 55 mS/cm was reached at 100 C and activation energy of 27.4 kJ/mol was calculated, indicating
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
international journal of hydrogen energy xxx (xxxx) xxx
that a predominance of the Grotthus mechanism over the vehicular one can be envisaged. The combination of the FAA3-50 alkaline membrane and ionomer with a NiMn2O4 anode catalyst showed excellent performance as a function of temperature up to 80 C and a current density of 530 mA/cm2 was obtained at 2 V at 80 C. By comparing the raw and IR-free curves at 50 C, the voltage loss at 0.4 A/cm2 was about 120 mV, corresponding to a voltage efficiency loss of about 5% from 79% to 74% vs. HHV. A moderate performance decay was observed after 1000 h. However, polarization curves carried out after the 1000 h durability experiments have clearly shown that this performance decay was due to recoverable losses.
[10]
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Acknowledgements [13]
The authors acknowledge the financial support from the EU H2020 LOTERCO2M project “CRM-free Low Temperature Electrochemical Reduction of CO2 to Methanol” Grant Agreement number: 761093.
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Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis A. Carbone*, S. Campagna Zignani, I. Gatto, S. Trocino, A.S. Arico Institute for Advanced Energy Technologies "Nicola Giordano", CNR-ITAE, Via S. Lucia Sopra Contesse 5, 98126, Messina, Italy
highlights A commercial FAA3-50 membrane was used as a solid polymer electrolyte in an alkaline water electrolysis. An improved chemical treatment based on alkaline KOH solution was carried out. A specific MEA based on FAA3-50 anionic membrane and NiMn2O4 anode catalyst was developed and tested. Alkaline stability was assessed by a potential cycling-based durability test for 1000 h.
article info
abstract
Article history:
A commercial FAA3-50 membrane was investigated as a solid polymer electrolyte in an
Received 29 October 2019
alkaline water electrolysis. An improved chemical treatment based on alkaline KOH so-
Received in revised form
lution was carried out. A limited degradation of the functional groups was observed
15 January 2020
allowing to maintain a good anion conductivity approaching 55 mS cm-1 at 100 C. Thermal
Accepted 21 January 2020
stability up to 200 C was assessed by thermal analysis.
Available online xxx
A specific membrane-electrodes assembly based on FAA3-50 anionic membrane and NiMn2O4 anode catalyst was developed and investigated in a single cell for water elec-
Keywords: Alkaline water electrolysis
trolysis at a moderate temperature (50 C). Performance stability was assessed by a potential cycling-based durability test for
Anionic exchange membrane
1000 h by varying the cell potential between 1 and 1.8 V for the FAA3-50 and NiMn2O4
NiMn2O4 catalyst
based-MEA.
Durability test
According to this evaluation, both the FAA3-50 membrane and the NiMn2O4 catalyst appear sufficiently stable for electrolysis operation under mild operating temperatures. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The urgent need to move from fossil fuels to renewable sources makes hydrogen very promising as an alternative
energy carrier. In this energy transition period, hydrogen is progressively used as a fuel to produce electrical power in stationary (domestic and industrial) and automotive sector [1e3]. The main advantages of using hydrogen are the highly
* Corresponding author. E-mail address: [email protected] (A. Carbone). https://doi.org/10.1016/j.ijhydene.2020.01.150 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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efficient conversion into electricity and vice versa, and the clean combustion of hydrogen offers solution for grid balancing service to convert the surplus of renewable energy. Among the several methods for “green” hydrogen production, water electrolysis remains the most reliable and efficient. There are two different technologies for low temperature water electrolysis that can be distinguished as a function of the used electrolyte: liquid alkaline using KOH electrolyte and polymer electrolyte membrane using a solid ion exchange membrane. The former technology has as main drawback the recirculation of caustic solutions and an excessive reactivity of hydroxyl groups with carbon dioxide causing the formation of potassium carbonate precipitate. This also reduces the ion exchange and the efficiency of the overall system. On the contrary, the ion exchange polymeric membrane does not require liquid corrosive electrolyte; the functional groups of the membrane are confined in a solid matrix and an easy scale-up of the system is possible [4e8]. The ion exchange membranes used for this purpose can be divided in two classes: proton exchange membrane (PEM) [9,10] and anion exchange membrane (AEM) [11e15]. The PEM based electrolysis shows the advantage of a high hydrogen production rate and energy efficiency but the relevant disadvantage of being limited by the use of platinum group metal catalysts and perfluorinated membranes such as Nafion or Aquivion [16]. This results in a high production cost. On the contrary, AEM based electrolysis presents several advantages since it can operate with cheap non-platinum or non-precious group metal catalysts while concentrated KOH (7 M) can be replaced by distilled water or diluted alkaline solutions. The absence of corrosive solutions prevents the system from leaking. Low cost hydrocarbon membranes can replace Nafion saving costs. Regarding the use of catalysts that meet the requirements of improving the oxygen evolution reaction (OER), the research is moving towards the development of mixed metal oxides spinels, perovskites, organic compounds, etc. [17e27]. Regarding the use of anionic membranes, different typologies of polymers were studied in the recent years. In particular the attention was devoted to the modulation of functional groups and their stability in alkaline environment. These functional groups can be distinguished in nitrogen-based (quaternary ammonium, pyridinum, benzyl imidazolium) and nitrogen-free (phosphonium, sulphonium) [28e32]. The quaternary ammonium is the most used due to the easy synthesis and the possibility to make a crosslinking of the terminal groups to improve the stability. One of the most used commercial membranes for electrochemical devices application is the quaternary ammonium-based Fumatech FAA3 polymer [33e35]. In electrochemical devices, it is fundamental to use the same ionomer of the membrane during electrodes preparation, to minimize the interfacial resistance and avoid delamination. It appears also important to carry out the same chemical treatments on both electrodes and membranes. According to the above-mentioned requirements, in this work, a FAA-3 ionomer and membrane based MEA were developed for water electrolysis application and an improved chemical treatment procedure was adopted in order to limit the degradation of the functional groups in alkaline
environment. This membrane was combined with a NiMn2O4 as promising catalyst for Oxygen evolution in the alkaline environment.
Experimental Catalyst preparation The oxygen evolution catalyst was in-house prepared by using the oxalate method [36]. Ni and Mn nitrates were dissolved in distilled water and mixed with a solution of oxalic acid neutralised at pH 6.5 with NaOH. The molar ratio between the chelating agent and the metals was 10. A complex was formed and then decomposed at 80 C with hydrogen peroxide to obtain a precipitate made of amorphous oxide. This was filtered, washed and dried at 100 C for 24 h. The powder was then calcined at 350 C for 120 min to form the NiMnOx catalyst. The NiMnOx was characterised in terms of structure, properties and crystallite size. For the hydrogen evolution process, a standard Pt/C catalyst was prepared using the sulphite complex -method followed by reduction in H2 at 300 C [37]. The catalayst powder contained Ketjen black EC at a 30 wt % content.
Membrane FAA3-50 membrane (FumaTech), based on a brominated polysulfone backbone with quaternary ammonium side chain groups provided in bromide form [38], was subjected to anion exchange process and characterised for this application.
Ionomer preparation The ionomer dispersion was prepared by solubilizing the received solid ionomer powder (FAA3-shredded film) in a mixture of solvents. An alcoholic solution of n-propanol and ethanol (1:1 wt) was used for this purpose and the FAA3 ionomer was solubilised at room temperature under stirring in order to have ~5 wt% dispersion.
Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) of the polymer electrolyte was performed using a thermo balance Netzsch (mod. STA 409) in static air by monitoring the percentage mass loss change in the temperature range between 20 and 1200 C with a temperature variation rate of 5 C min1
Determination of ion exchange capacity (IEC) To determine the total amount of ionic groups contained in the membrane a back-titration based on the Volhard method was carried out on FAA3-50 samples using Cl as a counterion, after an exchange in NaCl 1 M solution (72 h). The dried membrane was immersed in NaNO3 0.1 M for 48 h at room temperature, then it was removed. The remaining solution was treated with 5 ml of AgNO3 0.1 M and 5 drops of Fe(NO3)3 (11 wt%). A back-titration of the solution with KSCN 0.1 M was performed. The amount of volume of the KSCN needed to
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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achieve the so-called equivalent point (VKSCN determined. The IEC was calculated as follows:
e.p.)
was
IEC ¼ VAgNO3 VKSCNe:p: ½KSCN mdry
3
A is OH or Cl anion and the corresponding IEC (meq/g) is calculated by acid-base or Volhard method; mdry is the dry mass of the sample expressed in g; Vwet is the wet volume, expressed in cm3.
where: VAgNO3 is the initial volume, expressed in ml VKSCNe.p is the volume of the titrant at the equivalent point expressed in ml [KSCN] is the concentration of titrant, expressed in molarity mdry is the dry mass of the sample determined at 50 C for 2 h under vacuum (1000 mbar), expressed in grams. Moreover, to calculate the amount of halides converted into OH ions, an acid-base back-titration was carried out. In this case, the membrane was exchanged for 24 h in KOH 1 M solution at room temperature, then the dried sample was immersed in HCl 0.01 M solution for 24 h at room temperature and removed. The solution was back-titrated with NaOH 0.01 M, using an automatic titrator (Metrohm Mod.751GPD Titrino). The IEC was calculated as follows: IEC ¼ VHCl VNaOHe:p: ½NaOH mdry where: VHCl is the initial volume, expressed in ml VNaOHe.p is the volume of the titrant at the equivalent point expressed in ml [NaOH] is the concentration of titrant, expressed in molarity mdry is the dry mass of the sample determined at 50 C for 2 h under vacuum (1000 mbar), expressed in grams. The final IEC values were calculated from the average of three different measurements.
Water and hydroxide uptake, swelling, l, ion concentration The water and KOH uptakes were calculated by the difference in weight before and after the immersion of the dried samples in water or KOH 1 M solution at 30 C for 24 h. To reach the dry state, the samples were maintained in an oven under vacuum (1000 mbar) for 2 h at 50 C. The dimensional variation was calculated in the same operative conditions of water uptake on a rectangular sample, measuring the length (L) of the longest side. The swelling was calculated as the ratio DL/L. The l, which is the number of water/KOH molecules per functional group, is calculated as the ratio between the uptake and the IEC. The reported values are an average of three different measurements. The anion concentration, expressed in M, is calculated as follows: ½A ¼ IECA mdry Vwet where:
In-plane anion conductivity, effective mobility, diffusion coefficient The in-plane anion conductivity of samples with different counterions (Cl, OH) was carried out by a four electrodes method by Electrochemical Impedance Spectroscopy (EIS). The EIS parameters for the measurement were: 100 KHz - 1 Hz of frequency range and a 50 mV of amplitude. The measurements were carried out in the range of temperature 30e120 C, after a conditioning day at 30 C flowing humidified N2 (100% RH) at room pressure. The activation energy was calculated by the Arrhenius law from conductivity results. The effective anion mobility was calculated as follows: m ¼ s = ½A F where: m is the mobility, expressed in cm2/Vs s is the anion conductivity, expressed in S/cm [A] is the anion concentration F is the Faraday constant (96485C/mol) The anion diffusion coefficient (cm2/s) in water is calculated as follows: Ds ¼ sRT = ½A F2 where. s is the anion conductivity of the hydroxide or chloride form measured in humidified Nitrogen; R is the universal gas constant (8.31 J mol1 K1) T is the absolute temperature (K) [A] is the anion concentration F is the Faraday constant
Membrane-electrodes assembly Membrane-electrode assemblies based on NiMnOx spinel electrocatalyst [39] and Pt/C catalyst [40] at the anode and cathode, respectively, were prepared by spray coating the catalyst over the membrane. The catalysts were mixed with an ionomer dispersion of the same polymer membrane in ethanol (~5 wt%) for preparing catalytic inks. The loadings were 0.5 and 3 mg cm2 for the cathode and the anode, respectively. Ni felt from Bekaert were used as diffusing layers and current collectors. A FAA3-50 anion exchange membrane in the OH form was used as a polymer electrolyte separator between anode and cathode compartments. Membrane-electrode assemblies (MEAs) with a 5 cm2 geometrical area were prepared by a coldassembly procedure. This was adopted to avoid membrane degradation during hot pressing.
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Electrochemical characterisations The single cell was tested at different temperatures and under atmospheric pressure. Humidified N2 was fed to the cathode compartment at a flow rate of 50 ml/min and at atmospheric pressure. Initially, pure water and thereafter KOH (1 M and 6 M) solutions were supplied by a peristaltic pump to the anode compartment, at a flow rate of 4 ml/min. To study the effect of hydroxide concentration chronoamperometric curves were carried out with a Keithley power supply system (Tektronic). Polarization curves (cell potential as a function of current density) were performed at a scan rate of 5 mV/s. A durability test was carried out for 1000 h, using potential cycles of 1 min between 1.0 and 1.8 V. Ac-impedance analysis was used to determine the cell resistance (Rs). Series resistance (Rs) was measured from the high frequency intercept on the real axis of the Nyquist plot. Measurements at different temperatures were carried out on the same cell.
Results and discussion The IEC values of membrane samples, measured with acidbase (1st row) and Volhard (2nd row) back-titration methods, are reported in Table 1. It is evident that the IEC of the membrane sample in OH form is lower than the sample exchanged with Cl as counterion. This was due to an uncomplete conversion or to some degradation during the alkaline treatment needed to achieve the final ion exchange [1]. Only 86% of the initial groups were active in hydroxide form. The Volhard method allows to measure all the halides present in the membrane. The nominal value reported in the data sheet of the commercial membrane was confirmed. This behaviour was analysed by thermogravimetric analysis. As reported in Fig. 1, the membranes with different counterions show different thermal profiles. According to the IEC data, different exchange processes produced different polymer stability characteristics. A mass loss of about 15 wt% up to 100 C was found for both samples. This was due to the water loss. In the temperature range 100e200 C, the degradation of the functional groups took place whereas above 200 C the main chain degradation occurred. For the Cl exchanged sample, a better thermal stability was found than the OH one. The Cl neutralised quaternary ammonium started to give rise to a degradation at about 150 C instead of 100 C as recorded for the OH sample. The difference was clearly due to the different ion exchange that affects the thermal and mechanical properties of the membrane, in particular the stability of the functional groups. The physico-chemical parameters measured at 30 C are reported in Table 2 for the FAA3 membrane in OH form immersed in liquid water and KOH 1 M solution.
Table 1 e FAA3-50 IEC data. Membrane
IECexp.
IECnom.
% active groups
FAA3-50 (OH) FAA3-50 (Cl)
1.59 1.85
1.85 1.85
86 100
Fig. 1 e Thermal profile of Q041 membrane with different counter-ions.
The water uptake of the OH sample was in accordance with the mass loss of the TGA, in fact a similar value was found. The samples were analysed in the thermo-balance in a wet form. Regarding the KOH uptake (2nd raw), this was largely higher than the water uptake. This effect was due to the strong affinity of functional groups of the membrane for the alkaline solution. Such property is useful for the alkaline electrolyser application because the membrane acts as a reservoir for the OH electrolyte. Accordingly, the swelling is 10% higher for the KOH uptake than water, and consequently the l value was higher than in pure water. The l value, calculated from water uptake, corresponds to 6 water molecules per quaternary ammonium group. This value is typically occurring for the hydroxide diffusion and transport and it corresponds to what reported in the literature for this kind of polymer membrane. It results from a hyper-coordination of hydroxide ions [41]. The l value that was calculated according to the KOH uptake was obviously the highest amongst those observed. The value of ionic concentration is also reported in Table 2. Since the ionic concentration is calculated considering the wet volume of the sample that is directly related to the liquid uptake, the ionic concentration after the treatment in KOH solution is lower than in water, due to a dilution effect. The OH concentration in KOH solution was 1.1 M, i. e. close to the concentration of the solution in which it was immersed (1 M). It can be hypothesised that the ions confined in the hydrophilic portion of the membrane, where the ion conductivity takes place, have the same concentration of the solution and probably the same diffusion coefficient. The effective ion mobility was calculated for each solution investigated and it increased with the decrease of the ion concentration; however all these values are of the same order of magnitude. These results are in accordance with those reported in literature for similar polymer membranes [42]. Regarding the diffusion coefficient, this takes into account the ratio between the ion conductivity measured in humidified nitrogen and the ion concentration. The observed value of 7.17 106 cm2/s for the membrane sample after KOH uptake is only one order of magnitude lower than the value 5.3 105 cm2/s reported in numerous papers for OH solutions
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Table 2 e FAA3-50 physico-chemical parameters. Membrane FAA3-50 (OH) (H2O) FAA3-50 (OH) (KOH 1M)
Wup, %
DL/L, %
l
[ion], М
meff, cm2/Vs
1.63 106 7.17 106
17
9
6
1.5
6.27 10
70
19
25
1.1
8.83 105
under infinite dilution [31,41,43]. This means that the diffusion of hydroxide ions confined in the solid environment of the membrane is not so different from the diffusion in a liquid infinite solution, leading to an improved conductivity path. This behaviour is in accordance to what has been reported by Chen et al. [44] for AEM based on poly(vinyl benzyltrimethylammonium) (PVBTMA). In that study, it is reported that the coordination shells of quaternary ammonium cations overlap forming a continuous surface region for hydroxide transport with a mixed Grotthus and vehicular conduction mechanisms. The results of anion conductivity measurements, carried out on samples with hydroxides as counter-ions and fully hydrated gas, are reported is Fig. 2. As expected, the conductivity increased with the increase of temperature. The maximum value of 55 mS/cm was reached at 100 C but above this temperature the conductivity decreased due to the degradation of the membrane which suffered by an excessive swelling at that temperature and humidification levels. Since the OH conductivity, in the temperature range 30e100 C, shows Arrhenius behaviour, the activation energy can be roughly calculated and it corresponds to 27.4 kJ/mol. Various studies have reported on different typology of AEMs in which the activation energy for the mixed Grotthus and vehicular mechanism, with predominance of vehicular one, is calculated in the range 10e15 kJ/mol [30]. A slightly higher value of activation energy is here determined than that reported for the mixed conduction mechanism. In this case, a predominance of Grotthus mechanism over the vehicular one can be envisaged. This behaviour could be explained by considering the values reported in Table 2, showing that the hydroxide solvation and dissociation is less
Fig. 2 e Conductivity as a function of temperature.
Ds, cm2/s
5
pronounced in water than in KOH solution, leading to a high energy barrier for anion conduction. It is expected that in KOH solution, the conductivity increases and the activation energy can be reduced. To better understand the perspective applications of this AEM and its specific pre-treatment for alkaline electrolyzers, single cell tests were carried out in the temperature range 30e80 C by recirculating KOH 1 M solution at the cathode. Fig. 3 shows the polarization curves recorded at different temperatures and at ambient pressure. The combination of the FAA3-50 alkaline membrane and ionomer with a NiMn2O4 anode catalyst showed excellent performance as a function of temperature up to 80 C. This is comparable to the best AEM electrolysis performance reported in literature [21]. A current density of 530 mA/cm2 was obtained at 2 V at 80 C. In order to understand the contribution of the membrane to the electrolysis performance, the raw polarization curves were corrected for the ohmic drop (Fig. 4) (IR-free polarization curves) . By comparing the raw and IR-free curves at 50 C(Fig. 5), at about 0.4 A/cm2, the voltage loss was about 120 mV, corresponding to a voltage efficiency loss of about 5% from 79% to 74% vs. HHV. Accordingly, the contribution of the membrane to the electrolysis performance at low temperature appears good with minimal loss of efficiency at practical operating current density. Alkaline stability was assessed by a potential cycling-based durability test for 1000 h by varying the potential between 1 and 1.8 V for the FAA3-50 and NiMn2O4 based-MEA. Fig. 6 shows an increase of performance in the first 1000 h. Probably, the activation of the catalyst occurs after some potential
Fig. 3 e Water electrolysis polarization curves for the FAA3-50 membrane based-cell fed with 1 M KOH and containing NiMn2O4 anode catalyst and Pt/KB 40% cathode.
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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Fig. 4 e IR-Free electrolysis polarization curves for the FAA3-50 membrane based-cell fed with 1 M KOH and equipped with NiMn2O4 catalyst at the anode and Pt/KB 40% at the cathode.
Fig. 5 e Comparison of IR-Free and raw polarization curves at 50 C for the electrolysis cell based on the FAA3 membrane.
cycles. A moderate performance decay was observed after 1000 h. The potential cycling experiment represents an accelerated stress test to evaluate the load cycling behaviour that
Fig. 6 e Cycling behaviour (1e1.8 V) for the electrolysis cell at 50 C.
Fig. 7 e Polarization curves before and after the 1000-hrs durability test for the NiMn2O4 KB 70:30 catalyst on the anode and Pt/KB 40% on the cathode with the FAA3-50 membrane fed with 1 M KOH.
needs to be addressed by the electrolysis devices in grid balancing service to convert the surplus of renewable energy in hydrogen. The results of these tests would suggest that some improvement is required to enhance the MEA stability under cycled operation. However, it is not easy to distinguish in these experiments the recoverable losses from irreversible losses. Polarizations curves have thus been compared before and after the 1000-hrs potential cycling test at 50 C(see Fig. 7). The polarization behaviour was essentially similar indicating that the recorded potential losses in the durability test were essentially recoverable losses caused by mass transport constrains These can be addressed by improving cell design, catalyst layers and diffusive layers. According to this evaluation, both the FAA3-50 membrane and the NiMn2O4 catalyst appear sufficiently stable for electrolysis operation under mild (50 C) operating temperatures.
Conclusions Commercial FAA3-50 membrane with an improved chemical treatment was used for water electrolysis. After the treatment in KOH solution, the membrane presented about 86% of the initial groups active in hydroxide form. A thermal stability of the polymer main chain and functional groups was determined up to 200 C. The OH concentration in KOH solution is close to the concentration of the solution in which it was immersed. It can be hypothesised that the ions confined in the hydrophilic portion of the membrane, where the ion conductivity takes place, have the same concentration of the solution. The diffusion coefficient value of 7.17 106 cm2/s for the membrane sample after KOH uptake is only one order of magnitude lower than the value 5.3 105 cm2/s reported for OH solutions under infinite dilution. The OH conductivity of 55 mS/cm was reached at 100 C and activation energy of 27.4 kJ/mol was calculated, indicating
Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150
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that a predominance of the Grotthus mechanism over the vehicular one can be envisaged. The combination of the FAA3-50 alkaline membrane and ionomer with a NiMn2O4 anode catalyst showed excellent performance as a function of temperature up to 80 C and a current density of 530 mA/cm2 was obtained at 2 V at 80 C. By comparing the raw and IR-free curves at 50 C, the voltage loss at 0.4 A/cm2 was about 120 mV, corresponding to a voltage efficiency loss of about 5% from 79% to 74% vs. HHV. A moderate performance decay was observed after 1000 h. However, polarization curves carried out after the 1000 h durability experiments have clearly shown that this performance decay was due to recoverable losses.
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The authors acknowledge the financial support from the EU H2020 LOTERCO2M project “CRM-free Low Temperature Electrochemical Reduction of CO2 to Methanol” Grant Agreement number: 761093.
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Please cite this article as: Carbone A et al., Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.01.150