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PBI nanofiber mat-reinforced anion exchange membranes with covalently linkedinterfaces for use in water electrolysers
Najibah, Malikah; Tsoy, Ekaterina; Khalid, Hamza; Chen, Yongfang; Li, Qingfeng; Bae, Chulsung; Hnát,Jaromír; Plevová, Michaela; Bouzek, Karel; Jang, Jong HyunTotal number of authors:12
Published in:Journal of Membrane Science
Link to article, DOI:10.1016/j.memsci.2021.119832
Publication date:2021
Document VersionPeer reviewed version
Link back to DTU Orbit
Citation (APA):Najibah, M., Tsoy, E., Khalid, H., Chen, Y., Li, Q., Bae, C., Hnát, J., Plevová, M., Bouzek, K., Jang, J. H., Park,H. S., & Henkensmeier, D. (2021). PBI nanofiber mat-reinforced anion exchange membranes with covalentlylinked interfaces for use in water electrolysers. Journal of Membrane Science, 640, [119832].https://doi.org/10.1016/j.memsci.2021.119832
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Journal Pre-proof
PBI nanofiber mat-reinforced anion exchange membranes with covalently linkedinterfaces for use in water electrolysers
Malikah Najibah, Ekaterina Tsoy, Hamza Khalid, Yongfang Chen, Qingfeng Li,Chulsung Bae, Jaromír Hnát, Michaela Plevová, Karel Bouzek, Jong Hyun Jang,Hyun S. Park, Dirk Henkensmeier
PII: S0376-7388(21)00776-6
DOI: https://doi.org/10.1016/j.memsci.2021.119832
Reference: MEMSCI 119832
To appear in: Journal of Membrane Science
Received Date: 15 June 2021
Revised Date: 23 August 2021
Accepted Date: 2 September 2021
Please cite this article as: M. Najibah, E. Tsoy, H. Khalid, Y. Chen, Q. Li, C. Bae, Jaromí. Hnát, M.Plevová, K. Bouzek, J.H. Jang, H.S. Park, D. Henkensmeier, PBI nanofiber mat-reinforced anionexchange membranes with covalently linked interfaces for use in water electrolysers, Journal ofMembrane Science (2021), doi: https://doi.org/10.1016/j.memsci.2021.119832.
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© 2021 Published by Elsevier B.V.
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Malikah Najibah Investigation, Writing - Original Draft, methodology, formal analysis,
Visualization
Ekaterina Tsoy Methodology Hamza Khalid Investigation, methodology, formal analysis
Yongfang Chen Investigation, methodology
Qingfeng Li Resources, Writing - Review & Editing, Supervision
Chulsung Bae Resources, Writing - Review & Editing Michaela Plevová Investigation, formal analysis
Jaromír Hnát Investigation, Writing - Original Draft, Visualization
Karel Bouzek Writing - Review & Editing, Project administration, Funding acquisition
Hyun S. Park Writing - Review & Editing
Jong Hyun Jang Writing - Review & Editing
Dirk Henkensmeier Conceptualization, Validation, Writing - Original Draft, Writing - Review
& Editing, Visualization, Supervision, Project administration, Funding
acquisition
Term Definition
Conceptualization Ideas; formulation or evolution of overarching research goals and aims
Methodology Development or design of methodology; creation of models
Software
Programming, software development; designing computer programs;
implementation of the computer code and supporting algorithms; testing
of existing code components
Validation
Verification, whether as a part of the activity or separate, of the overall
replication/ reproducibility of results/experiments and other research
outputs
Formal analysis Application of statistical, mathematical, computational, or other formal
techniques to analyze or synthesize study data
Investigation Conducting a research and investigation process, specifically performing
the experiments, or data/evidence collection
Resources
Provision of study materials, reagents, materials, patients, laboratory
samples, animals, instrumentation, computing resources, or other analysis
tools
Data Curation
Management activities to annotate (produce metadata), scrub data and
maintain research data (including software code, where it is necessary for
interpreting the data itself) for initial use and later reuse
Writing - Original
Draft
Preparation, creation and/or presentation of the published work,
specifically writing the initial draft (including substantive translation)
Writing - Review &
Editing
Preparation, creation and/or presentation of the published work by those
from the original research group, specifically critical review, commentary
or revision – including pre-or postpublication stages
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Term Definition
Visualization Preparation, creation and/or presentation of the published work,
specifically visualization/ data presentation
Supervision Oversight and leadership responsibility for the research activity planning
and execution, including mentorship external to the core team
Project
administration
Management and coordination responsibility for the research activity
planning and execution
Funding acquisition Acquisition of the financial support for the project leading to this
publication
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Graphical Abstract
(CH2)5
R = Br: mTPBrR = N(Me)3
+ Br- : mTPN
PBI/mTPN
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1
PBI nanofiber mat-reinforced anion exchange membranes with covalently linked interfaces for 1
use in water electrolysers 2
3
Malikah Najibah,1,2# Ekaterina Tsoy,1,2 Hamza Khalid,1,2 Yongfang Chen,3 Qingfeng Li,3 Chulsung 4
Bae,4 Jaromír Hnát,5 Michaela Plevová,5 Karel Bouzek,5 Jong Hyun Jang,1,2 Hyun S. Park,1,2 Dirk 5
Henkensmeier1,2* 6
7
1. Center for Hydrogen and Fuel Cell Research, Korea Institute of Science and Technology, 8
Seongbukgu, Seoul 02792, South Korea 9
2. Division of Energy & Environment Technology, KIST School, University of Science and 10
Technology, Seongbukgu, Seoul 02792, South Korea 11
3. Department of Energy Conversion and Storage, Technical University of Denmark, 12
Fysikvej, Building 310, 2800 Kgs. Lyngby, Denmark. 13
4. Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New 14
York 12180, United States 15
5. Department of Inorganic Technology, University of Chemistry and Technology, 16
Prague, Technická 5, 166 28 Praha 6, Czech Republic 17
*email: [email protected] 18
#main name is Malikah 19
20
Abstract: 21
Anion exchange membranes (AEM) are key components in anion exchange membrane water 22
electrolysers. Recently developed materials are less susceptible to the alkaline degradation of 23
the polymer backbone and quaternary ammonium groups. A remaining challenge is the 24
mechanical stability in contact with hot water and dimensional stability when the temperature 25
of the feed solution changes. One solution is to reinforce membranes with a porous support. 26
Since support materials like PEEK or PTFE have a different swelling behavior than the matrix and 27
no strong interactions with the matrix, voids can form, and gas crossover increases. In this work, 28
we approach this issue by pore filling polybenzimidazole nanofiber mats with the 29
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bromomethylated precursor of mTPN, an ultra-stable AEM material. During drying, a covalent 30
interaction between support (PBI amine groups) and matrix (-CH2Br) is established. After 31
quaternization, the optimized PBI/mTPN-50.120 composite membrane still shows a high 32
conductivity of 62 mS cm-1, but 37% reduced length swelling in comparison to the non-reinforced 33
membrane. Tensile strength and Young modulus increase 17% and 56% to 49 MPa and 680 MPa, 34
respectively. In an electrolyser, a stable voltage of 1.98V at 0.25 A cm-2 was achieved, and no 35
change in membrane resistance was observed over the test time of 200 hours (50 °C, 1M KOH, 36
catalysts based on Ni/Fe and Mo). 37
38
Keywords: polybenzmidazole; electrospinning; nanofiber mat; anion exchange membrane; water 39
electrolysis 40
41
1. Introduction 42
Worldwide, societies aim to increase the use of renewable energy, mainly by increasing the use 43
of intermittent energy sources like solar and wind power. To stabilize the electricity grids, to store 44
energy for times when demand exceeds primary supply, and to substitute fossil fuels in the 45
transport sector, hydrogen has been proposed to be a clean and efficient energy carrier. 46
Furthermore, hydrogen can provide a practical means to store large amounts of energy, e.g. in 47
salt caverns.[1] In China in 2016, the amount of abandoned electricity from wind, solar and 48
hydroelectric power amounted to 110,000 GWh, which is estimated to be enough to produce 22 49
billion Nm3 hydrogen.[2] Therefore, hydrogen production by water electrolysis based on 50
renewable energies will play a central role in the future clean energy systems. 51
52
Even though alkaline water electrolysis is a well-established industrial process, the current 53
technology is not suitable for applications in association with renewable energies. Industrial 54
alkaline water electrolysis uses a porous diaphragm as a separator of the gas products, which can 55
cause gas crossover and hence a hydrogen purity issue, especially at differential pressure.[3] 56
Another shortcoming in connection to the diaphragm is the low partial load range (operational 57
at 20-40% full load) and poor dynamics of operation (startup/shutdown and load cycling) of the 58
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technology.[4] In addition, the large thickness of the porous diaphragm dictates the inter-59
electrode distance and hence enhances ohmic resistance, which, in turn, limits the operation at 60
low current densities and the operational capacity. 61
62
Proton exchange membrane water electrolysers (PEMWEs) use dense and thin membranes 63
based on perfluorosulfonic acid with lower ohmic resistance. The technology can operate at high 64
current densities with good dynamic performance to match intermittent renewable energy 65
sources. The disadvantage is that the acidic solid electrolyte requires use of precious metals such 66
as platinum and iridium as catalysts. For example, Germany’s Hydrogen Strategy pushes for the 67
installation of 2 GW water electrolysers until 2030.[5] If this is based on a PEMWE technology 68
operating at a voltage of 2V, current density of 2 A cm-2 and an anode catalyst loading of 2 mg 69
iridium cm-2, Germany would need 1000 kg iridium, which is ca 15% of the global yearly supply 70
of the metal.[6] A solution would be the use of an alkaline solid electrolyte based on an anion 71
exchange membrane which combines the advantages of high and dynamic performance with the 72
potential use of non-precious metal catalysts. 73
74
The main problem hindering the wide promotion of alkaline membrane WE is that no commercial 75
membrane fulfills the requirements.[7] For example, membranes from Tokuyama and Fumatech 76
have a relatively low conductivity, whereas highly conductive Sustainion membranes break into 77
pieces in the dry state.[7] Very promising membranes were developed in Chulsung Bae’s group 78
of Rensselaer Polytechnic Institute[8] and are now commercialized as Orion TM1 membranes. In 79
a recent study surveying 50 membranes from 10 organizations, TM1 membranes showed the 80
lowest IEC loss and conductivity loss after 1000 h degradation study at 80 °C in 1M KOH.[9] 81
However, in the alkaline membrane water electrolyser, the fully hydrated material revealed a 82
low mechanical stability.[10] This specific parameter can be improved by reinforcing the 83
membrane with a porous support. 84
85
A learning from the fuel cell field, where PTFE-supported Nafion membranes are used, is that 86
repeated water absorption and desorption delaminates the Nafion matrix from the rigid Teflon 87
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support, resulting in voids along the support’s surface, and thus increasing gas crossover. To 88
tackle this issue, Yu et al.[11] and Deborah Jones and colleagues recently developed new support 89
materials based on electrospun PBI nanofiber mats.[12-14] Because Nafion has strongly acidic 90
groups, it protonates PBI, resulting in strong ionic interactions,[15, 16] which strengthen the 91
interface between PBI support and Nafion matrix. 92
93
Electrospun PBI nanofiber membranes are also investigated as separator in lithium ion 94
batteries,[17] as window air filter[18] and as material for fine dust masks[19]. However, a 95
literature search revealed that only one experimental work reports the combination of 96
“nanofiber membrane” (polysulfone co-extruded with quaternized polyetherethersulfone) and 97
“AEM” (quaternized polyethersulfone).[20] 98
99
In this work, we suggest the use of PBI nanofiber mats as a porous support for anion exchange 100
membranes. While the quaternary ammonium groups of the anion exchange membrane (AEM) 101
can form ionic interactions with PBI, stronger covalent links could be formed when the 102
halomethylated precursor polymer is first filled into the pores of a PBI nanofiber mat, and is then 103
allowed to react with the PBI amine groups. Here, we used the bromoalkylated precursor 104
polymer of TM1 membranes (mTPBr),[8] which was shown to form highly alkaline stable 105
membranes, as mentioned above.[9] As the last step, the remaining bromoalkyl groups are 106
quaternized by immersing the membrane in a solution containing trimethylamine. The effect of 107
the porous support is investigated by measuring swelling behavior, conductivity, mechanical 108
properties and the performance in the water electrolyser. 109
110
2. Experimental part 111
2.1 Materials 112
Polybenzimidazole (PBI, Danish Power Systems, Mw = 58,000 g mol-1), N,N-dimethylacetamide 113
(DMAc, 99.8%, Sigma Aldrich), lithium chloride (LiCl, >99%, Sigma Aldrich), tetrahydrofuran (THF, 114
>99%, Daejung Chemicals), sodium hydride (NaH, 60% dispersion in mineral oil), trimethylamine 115
(TMA, 28 wt% in water, Tokyo Chemical Industry), potassium hydroxide (KOH flakes, Daejung 116
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Chemicals), potassium carbonate (K2CO3, Sigma Aldrich), sodium chloride (NaCl, Daejung 117
Chemicals), potassium nitrate (KNO3, Daejung Chemicals) were all used as received. mTPBr was 118
synthesized according to the literature.[8] Ni-Fe and Mo based compounds were used as oxygen 119
and hydrogen evolution catalysts respectively. 120
121
2.2. Preparation of PBI nanofiber mats 122
10 wt% PBI and 0.5 wt% LiCl (to suppress polymer-polymer interactions) were dissolved in 89.5 123
wt% DMAc by stirring at 70 rpm for 19 hours at 150 °C under argon atmosphere. For 124
electrospinning (eS-robot electrospinning/spray system ESR200R2, NanoNC NNC-30K-2mA High 125
Voltage Power Supply), 6 ml solution was filled into a syringe, and spun onto a plate covered by 126
aluminium foil at a voltage of 21.1 kV and a flowrate of 0.5 ml h-1. The relative humidity was 127
maintained at above 45%. After 6 hours, the process was stopped, and the nanofiber mat was 128
collected and then dried at 160 °C for 12 hours in vacuum. 129
130
131
2.3 Preparation of reinforced membranes 132
1 mmol of mTPBr and 1 mmol of NaH were dissolved in 4.12 mL of THF. The solution was stirred 133
overnight at room temperature until all was dissolved. Then the solution was cast on a glass plate 134
with a doctor blade adjusted to 200 µm. A PBI mat (10 x 10 cm2) was put on the film, and the 135
solution was cast once more on the PBI mat with the blade gap adjusted to 410 µm. Finally, the 136
membrane was dried under vacuum at 80 °C or 120 °C for 48 h. Dry PBI/mTPBr membranes 137
prepared in this way had a thickness of 50 µm. Samples were noted as PBI/mTPBr-X.Y, with X 138
indicating the targeted thickness (±15% was considered to be within specification), and Y 139
indicating the drying temperature. 140
141
For quaternization, PBI/mTPBr membranes were immersed in 30 ml of TMA solution (28 wt% in 142
water) at room temperature. After 7 days, the PBI/mTPN-membranes were moved to deionized 143
water and kept overnight to remove residual TMA. Cooling the TMA solution in a refrigerator 144
before opening the storage vessel significantly reduced the smell. 145
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146
2.4 Membrane characterization 147
Membranes (1 x 4 cm2) were put into vials filled with 1M NaCl solution to exchange the 148
membranes into their chloride form. After 10 hours, solutions were replaced with DI water and 149
kept for another 24 hours at room temperature. Samples were swollen in water at room 150
temperature, and the wet dimensions were measured. Then the samples were dried 1 day at 151
60 °C in the vacuum for 24 hours to get the dry dimensions. To compare values at room 152
temperature and 60 °C, the same samples were swollen again in water at 60 °C, and wet 153
dimensions were measured. From these wet and dry values, the swelling values were calculated. 154
155
Swelling (length)(%) = 𝐿𝑤𝑒𝑡− 𝐿𝑑𝑟𝑦
𝐿𝑑𝑟𝑦 x 100% 156
157
L is the length of the wet and dry state, as indicated. 158
159
The through-plane conductivity was measured by clamping membrane 1 or several stacked 160
samples between two electrode chambers filled with KOH solution. Gold plated metal discs 161
(1.767 cm2 area) were used as electrodes. Resistances were obtained by impedance spectroscopy 162
with a Zahner IM6 potentiostat. Resistances were plotted against the thickness of the stacked 163
membranes, and the slope S of the linear trend was calculated. The conductivity was calculated 164
by the following equation: 165
166
Through-plane conductivity (mS cm-1) = 1
𝑆· 𝐴 167
168
in which A represents the active membrane area. 169
170
The in-plane conductivity was measured in DI water using a Bekktech BT-110 clamp. To exclude 171
carbonates, nitrogen was constantly bubbled through the water, and a voltage of 2V was applied 172
until the resistance reached a constant value.[21, 22] Resistances were measured by impedance 173
spectroscopy (Zahner IM6), and the conductivity was calculated by the following equation: 174
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175
In-plane conductivity (mS cm-1) = 𝑑
𝑅𝑚𝑒𝑚 𝑤 𝑡𝑤
176
177
Where d represents distance between the voltage sensing electrodes, w is the width of 178
membrane sample, and tw is membrane thickness after immersing in DI water. 179
180
Mechanical properties were measured with a Cometech QC-508E universal testing machine. 181
Membrane samples were cut into 4x1 cm2 stripes, then immersed in 1M NaCl solution for 10 182
hours to exchange the membranes into their chloride form. After drying at 60 °C in the vacuum 183
oven for 24 hours, samples were clamped between the fixed and moveable grip, and then 184
stretched at 10 mm min-1. 185
186
Scanning electron micrographs of PBI nanofiber mats and reinforced membranes were obtained 187
by FE-SEM Inspect F, 15 kV, after the samples were sputtered with Pt/Pd. Cross-sectional images 188
of PBI nanofiber mats were obtained by ion milling using Ar blade 5000 (Hitachi High-Tech.) with 189
the accelerating voltage of 0.1~8 kV, while reinforced membranes were prepared by freeze-190
breaking after cooling with liquid nitrogen. 191
192
IR spectra were obtained with a Lambda Scientific FTIR 7600 spectrometer equipped with an ATR 193
sample holder. 194
195
IEC values were obtained by UV-VIS spectroscopy of exchanged nitrate ions.[23] Membranes 196
were first immersed overnight in DI water at 80 oC, then moved to 1 M KNO3. Solutions were 197
changed every hour for 5 times. Then samples were immersed in the same concentration of KNO3 198
for another 24 hours to complete the ion exchange. Membranes were rinsed with DI water for 199
48 hours, then subsequently immersed in 20 mL of 0.1 mol L-1 NaCl for 1 hour, another 20 mL of 200
0.1 mol L-1 NaCl for 1 hour, and in 60 mL of 0.1 mol L-1 NaCl for 24 hours. At the last step, the NaCl 201
solutions were combined and analyzed by UV-vis spectroscopy. 202
203
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2.5 Catalyst coated membrane electrode assembly preparation 204
Membrane electrode assemblies (MEA) were prepared as catalyst coated membranes (CCM). 205
Non-platinum group metals (non-PGM) based on Ni-Fe and Mo compounds were used as anode 206
and cathode catalyst, respectively (catalyst load 2.5 mg cm-2). A chloromethylated block 207
copolymer of styrene-ethylene-butylene-styrene (PSEBS-CM) dissolved in chloroform was used 208
as a polymer binder of the catalyst layer. After the catalyst layer was deposited, PSEBS-CM was 209
functionalised by 1,4-diazabicyclo[2.2.2]octane (DABCO) functional groups.[24] 210
Prior to the catalyst layer deposition, the membrane was conditioned in the following way: 211
- immersion in 0.1 M NaOH solution for 2 hours in order to swell the membrane and to 212
transform it to the OH- form; 213
- carefully rinsing with deionized water; 214
- immersion into 0.1 M HCl solution in order to change the membrane to the Cl- form and 215
thus to improve its chemical stability prior to the CCM-MEA preparation. 216
To prepare a catalytic ink, 30 mg of catalysts were dispersed in a 5wt.% solution of PSEBS-CM-217
DABCO in chloroform. Volume of solution was calculated as to obtain a catalyst to binder weight 218
ratio (CBR) of 93/7 (45 mg of the 5wt.% solution). Subsequently, additional 9 ml of chloroform 219
was added in order to adjust the ink properties, and the ink was sonicated for 30 minutes in a 220
Bandelin Sonorex Digitec ultrasound bath. 221
222
A CNC (computerized numerical control) unit was used to disperse the ink by means of ultrasound 223
over the membrane surface and thus to produce the CCM-MEA. The membrane was heated to 224
50 °C during the catalyst deposition to accelerate the dispersant evaporation. The catalyst was 225
deposited layer by layer until the mass increase of the dry CCM-MEA corresponded to the desired 226
catalyst load. The prepared CCM-MEA was immersed into 10wt.% solution of DABCO in ethanol 227
for 12 hours in order to transfer the PSEBS-CM polymer binder into the PSEBS-CM-DABCO 228
ionomer binder. After washing the prepared CCM-MEA by deionized water, it was immersed into 229
1 M NaOH for 4 hours prior to the water electrolysis experiment. 230
231
2.6 Water electrolysis 232
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The water electrolysis cell consisted of a polyether ether ketone (PEEK) cell body and gold plated 233
Nickel (Ni) current feeders. The sealing was made of expanded PTFE (Fornaxa s.r.o., Czech 234
Republic). Ni foam (INCO Advanced Technology materials Co., Ltd, Dalian) with a geometrical area 235
of 4 cm2 was used as electrode on both sides of the cell. 236
237
Water electrolysis was controlled by an Autolab potentiostat M204 (Metrohm Autolab B. V., 238
Netherland) equipped with FRA module allowing electrochemical impedance measurement (EIS). 239
Short-term characteristics of the prepared CCM-MEA was obtained by recording the load curve 240
of the cell in a potentiostatic regime for the cell voltage range of 1.5 to 2.0 V with a voltage step 241
of 0.05 V. For each step, the cell current was recorded for 60 seconds. The value averaged over 242
the second half of the interval, i.e. from time 31 to 60 seconds, was taken as the cell current load 243
corresponding to the particular cell voltage. Between recording the individual current load steps, 244
the cell voltage of 1.0 V was applied for 60 seconds. This period served to allow the bubbles of 245
the produced gasses to be removed from the electrodes and from the cell as such. Every load 246
curve measurement was repeated three times to verify data reproducibility. Three 247
concentrations of KOH water solutions were used as liquid electrolytes, 0.01, 0.1 and 1.0 M KOH. 248
This allowed to investigate primarily the effect of the liquid electrolyte conductivity on the anion 249
exchange membrane water electrolysis (AEMWE) performance. Operational temperature of 250
50 °C and electrolyte flow rate of 25 ml min-1 were used. EIS was recorded to obtain additional 251
characteristics of the AEMWE cell. The EIS spectra were recorded in the frequency range of 100 252
kHz to 0.01 Hz at cell voltage of 1.8 V. The amplitude of the perturbing voltage signal was set at 253
10 mV. 254
255
A cell stability test was performed at a constant current load of 0.25 A cm-2 at 50 °C using 1.0 M 256
KOH as a liquid electrolyte. The current load was regularly interrupted in order to record the EIS 257
spectra at the cell voltage of 1.8 V, using the same EIS parameters. 258
259
EIS spectra were evaluated using the equivalent electrical circuit approach. The circuit shown in 260
Figure1 was used for this purpose. Here L stands for inductance (Henry, H) corresponding 261
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predominantly to the cell electric connections, Rs represents ohmic resistance of the cell (ohm, 262
Ω), including the resistance of the electric connections, current feeders, Ni foam electrodes and 263
electrolyte, Rp corresponds to the charge transfer resistance (ohm, Ω) related to the kinetics of 264
the anode and cathode reactions and CPE indicates the so called constant phase element, i.e. 265
modified capacitance of the electric double layer formed at the interfaces of the electronically 266
and ionically conductive phases (S sn, n is a number between 0 and 1). The constant phase 267
element provides a model representation of the capacitance element with spatially distributed 268
capacitance value. In some cases, the cathode reaction kinetics are significantly faster than the 269
anode one. EIS spectrum then does not show second time constant. In such a case, a simplified 270
equivalent electrical circuit consisting just of the L, Rs, Rp (anode) and CPE (anode) is used to 271
evaluate experimental data. 272
Figure 1: Equivalent electrical circuit used for EIS spectra evaluation. L – inductance (Henry,
H); Rs – resistance of the cell (ohm, Ω); Rp – charge transfer resistance (ohm, Ω); CPE –
constant phase element describing the electrical double layer capacitance (S sn).
273
2.7 Post-mortem analysis 274
A scanning electron microscope (SEM) Hitachi S4700 (Hitachi, Japan) equipped with a silicon 275
drifted detector for energy dispersive spectroscopy (EDS) (ThermoFisher Scientific, USA) was 276
used to evaluate the morphology and composition of the MEA-CCM after the electrolysis stability 277
test. Samples were cut by a scalpel. The sample for cross-section analysis was placed into a holder 278
equipped with a moving part, which pressed the sample against a fixed wall. Samples for surface 279
analysis were fixed to the plane holder by double-sided carbon tape. Prior to SEM-EDS analysis, 280
a 5 nm layer of Au/Pd was sputtered on the samples to increase the surface conductivity. The 281
accelerating voltage was 15 kV. Elemental mapping was used in order to determine the spatial 282
distribution of the selected elements. 283
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284
3. Results and Discussion 285
3.1 Membrane Fabrication 286
Membranes were prepared in 4 steps (Figure 2). In the first step, PBI nanofiber mats were 287
prepared by electrospinning. In the second step, mTPBr is casted on a glass plate, a PBI nanofiber 288
mat is deposited on the wet film, and after a short drying time, a second mTPBr layer is cast on 289
top. Then, by heating the membrane, a covalently bound interface between PBI and the mTPBr 290
matrix is formed by reacting PBI amine groups on the nanofiber surface with mTPBr in a 291
nucleophilic substitution reaction. In the last step, the brominated membranes are immersed in 292
trimethylamine solution to transfer all remaining bromide groups to trimethylammonium groups. 293
294
Figure 2: Schematic representation of the PBI/mTPN membrane fabrication and composition. 295
296
The first part of this work was to optimize the preparation of PBI nanofiber mats. Even though 297
such materials are very well known,[25-27] it was necessary to adjust the process parameters to 298
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the spinning machine. In the electrospinning process, a polymer solution is pumped through a 299
syringe, which acts as one terminal for the applied voltage. At high voltages, charges move to the 300
surface of the emerging polymer solution droplet. When the electrostatic forces reach a similar 301
magnitude as the surface tension, they deform the droplets at the needle tip into “Taylor cones”. 302
From the tip of the cones, fibers are emitted.[28-30] During their passage through air, repulsive 303
surface charges elongate the fibers, until solvent evaporation (or precipitation of polymer) 304
reaches a point at which the fiber dimensions are fixed. Finally, the fibers are deposited onto the 305
collector. The quality of the nanofibers is influenced by the voltage value. In this work, the voltage 306
was maintained in the range of 19-25 kV. When the value is adjusted below the specified range, 307
the electrostatic force is not strong enough to form the Taylor cone, and large polymer solution 308
droplets appear instead of the desired nanofiber. Otherwise, when the voltage is too high, the 309
Taylor cone is unstable [30] due to the rapid acceleration of charge. 310
311
Other factors influencing the electrospinning process are the distance between the tip and the 312
receiving plate, and temperature and humidity of the surrounding gas phase. For some systems 313
(e.g. cellulose acetate in acetone/DMAc), the fiber diameter was reported to increase with 314
increasing humidity, for other systems, it was reported to decrease (e.g. poly(vinylpyrrolidone) 315
in ethanol).[31, 32] To lower the risk of fire hazards, the machine used in this work was provided 316
with a constant nitrogen stream. The resulting low humidity led to yellow (e.g. dense PBI) areas. 317
This problem was solved by increasing the humidity in the spinning chamber to above 45%, by 318
putting a small ultrasonic humidifier in the chamber and positioning wet tissues around the 319
collector. Presumably, evaporation of DMAc is faster in humid air, because the specific heat 320
capacity of air increases with the relative humidity.[33] Another factor could be that DMAc 321
absorbs water from air, because it is hygroscopic. This should decrease the solubility of PBI. For 322
example, films cast from PBI solutions in DMAC form transparent membranes in dry atmosphere, 323
and opaque membranes in humid atmosphere.[34] 324
325
The size of the obtained nanofiber mats was in the range of 20 cm x 25 cm. SEM analysis of the 326
mats revealed that the fiber mats consisted of individual, not merged fibers (Figure 3). The fiber 327
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diameter was below 1 µm (0.51 ± 0.26 µm). The thickness of the prepared PBI nanofiber mats 328
was in the range of 10 µm by thickness gauge (some compression of the mat is expected during 329
the measurement) and 22 µm by SEM. It appears that handling and storage of the prepared mats 330
needs care, to avoid unwanted compression. Alternatively, the mats could also be compressed 331
or calendered to render the mats more robust, however, at the cost of porosity.[35, 36] As shown 332
in Table 1, the density of the fiber mat can be calculated from its weight and dimensions, and [1 333
- (density of porous membrane/density of non-porous PBI)] gives the porosity: 77% by thickness 334
gauge, and 90% by using the thickness obtained from SEM images. Mercury porosimetry gave a 335
porosity in a similar range (84%), and allowed to determine the average pore diameter as 2.4 µm. 336
337
338
339
Figure 3: Photograph (a) and SEM images of electrospun 20 cm x 25 cm PBI nanofiber mats. (b) 340
air side, (c) cross-section, (d) aluminium side. 341
342
a b
c dJo
urnal
Pre-pro
of
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14
343
Table 1: Properties of PBI nanofiber mats. 344
Weig
ht
Are
a
Thickne
ss
(gauge)
Thickne
ss
(SEM)
ρ
(gau
ge
base
d)
ρ
(SEM
base
d)
ρ
(den
se
PBI)
Porosi
ty
(gauge
based)
Porosi
ty
(SEM
based)
Porosity
(mercury
porosimet
ry)
Average
pore
diameter
(mercury
porosimet
ry)
0.3
mg
100
m
m2
10 µm 22 µm 0.3 g
cm-3
0.13
3 g
cm-3
1.3 g
cm-3
[37]
77% 90 % 84 % 2.4 µm
345
Membranes were prepared by casting a solution of the brominated precursor polymer, 346
embedding a PBI nanofiber mat, and casting a second layer of the precursor polymer. For the 347
bromoalkyl groups to react with surface amine groups of PBI, the membranes were heated up. 348
The resulting precursor membranes are denoted as PBI/mTPBr-X.Y, with X describing the 349
thickness in µm and y describing the reaction temperature in °C. Finally, the membranes were 350
transferred into AEM by immersion in trimethylamine solution. As can be seen in Figure 4, the 351
color of the nearly white fiber mat changed into the yellow-brown color known for PBI 352
membranes, indicating that the pores were successfully filled. SEM images proved that the two 353
membrane surfaces were dense and smooth. A cross-sectional image revealed a high density of 354
small bright dots in the center of the membrane. These dots show the location of PBI nanofibers. 355
356
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357
Figure 4: Photograph (a) and surface (b, d) and cross-sectional (c) SEM images of a nanofiber mat-358
supported anion exchange membranes (PBI/mTPN-50.80). 359
360
Compared to the overall concentration of PBI amine groups and mTPN ammonium groups, the 361
number of PBI-mTPN connecting points is very small. Therefore, the bonding points themselves 362
cannot be well analyzed. As an indirect prove for the successful reaction between the two 363
polymers, PBI-mTPBr membranes were immersed in THF to leach out all mTPBr which did not 364
react with the PBI surface. Such treated membranes again showed the nearly white, opaque 365
appearance of the pristine PBI nanofiber mat. This indicates that no crosslinking side-reactions 366
occurred during the heating step (e.g. by Friedel-Crafts alkylation of phenyl groups in mTPBr by 367
bromoalkyl groups). Analysis by ATR-FTIR (Figure 5, and additional information in the supporting 368
information) proved that not all mTPBr was leached out, indicating that the reaction between 369
a b
c d
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mTPBr and PBI was successful: All strong bands of mTPBr are also found in the spectra of PBI-370
mTPBr after leaching with THF. Qualitatively, the spectra also indicate that membranes reacted 371
at 120 °C (PBI/mTPN-50.120) have a higher mTPBr-content after leaching with THF than those 372
reacted at 80 °C. Therefore, the PBI/mTPN-50.120 membranes appear to have a stronger 373
supportmatrix interface than PBI/mTPN-50.80 membranes. 374
375
376
Figure 5: ATR-FTIR spectra of PBI, mTPBr and the composite membrane after leaching mTPBr out 377
with THF (24 h, room temperature); (a) PBI/mTPN-50.80, (b) PBI/mTPN-50.120. Arrows indicate 378
the bands from mTPBr which are still seen after leaching. 379
380
Because the sampling depth in XPS is just about 10 nm, spectra of PBI/mTPBr-50.80 and -50.120 381
after leaching with THF should not show nitrogen signals from PBI, but only signals related to 382
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mTPBr. A uniform coating PBI nanofibers with a layer of mTPBr was confirmed by XPS. 383
(Supporting Information, Figure S2). 384
385
3.2 Swelling and conductivity 386
The most important membrane properties are swelling in the area (which can delaminate 387
membrane and electrode and thus reduce the lifetime of the electrolyser) and ion conductivity. 388
Figure 6 shows that the conductivity of PBI-mTPN is always higher than that of dense PBI 389
membranes, which are practically insulating in pure water and in low concentrated KOH solutions. 390
At higher KOH concentrations, PBI gets deprotonated, and potassium polybenzimidazolide is 391
formed. Because of their ionic nature, such membranes easily absorb additional water molecules 392
and potassium and hydroxide ions, rendering them conductive, and the conductivity increases 393
with increasing KOH concentration, up to about 20 wt% (around 4M).[38, 39] 394
395
For ion exchange membranes, two opposing factors need to be considered. (a) With increasing 396
KOH concentration, the osmotic pressure difference between the ionic membrane and the 397
surrounding water phase decreases, reducing the water uptake and thus the conductivity. (b) 398
Although AEMs are generally considered as single ion conductors, which selectively conduct 399
anions, Donnan exclusion is only strictly observed for membranes in very diluted solutions.[40, 400
41] When the KOH concentration increases, also the concentration of potassium ions and (for 401
electroneutrality) hydroxide ions in the membrane increases, which results in increasing 402
conductivity. This means that the conductivity should show an initial drop when the KOH 403
concentration increases, and then an increasing conductivity, because both PBI[39] and mTPN 404
absorb additional potassium and hydroxide ions. This expected trend is observed for all 405
membranes. For PBI/mTPN composite membranes, the resistance should further decrease when 406
the KOH concentration is high enough to render the PBI support conductive, and the 407
conductivities of reinforced and non-reinforced membranes indeed reach similar values in 4M 408
KOH. In comparison to commercial Fumatech FAA3-50 and FAA3-PK-75, the mPBI/mTPN-50.120 409
membrane shows superior conductivity at all tested KOH concentrations. 410
411
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Interestingly, the conductivity of PBI/mTPN-50.80 is lower than that of PBI/mTPN-50.120. 412
Hypothetically, this is because evaporation of THF during membrane formation cools down the 413
polymer solution and thus promotes absorption of water from air. At 80 °C, this water is not fully 414
removed and can degrade bromoalkyl groups to alcohol groups. At 120 °C, water traces are 415
efficiently evaporated, and have no time to react with bromoalkyl groups. This should result in 416
lower IEC values for mTPN-50.80, and indeed the IEC values of mTPN-50.80 and mTPN-50.120 417
were 1.69 ± 0.04 and 1.94 ± 0.02 mmol chloride g-1, respectively (Table 2). The IEC of PBI/mTPN-418
50.120 was 1.57 mmol chloride g-1. Assuming a density of 1.2 g cm-3 for mTPN, and a thickness of 419
50 µm, 1 cm2 membrane contains 6 mg mTPN and 0.3 mg PBI. Thus, the expected IEC is 5% lower 420
than 1.94 if no reaction with PBI occurs, and additional 16% lower if all PBI NH groups react with 421
mTPN, i.e. 1.54 mmol chloride g-1. Therefore, the experimental IEC value is in a reasonable range, 422
and supports that some of the bromomethyl groups reacted with amine groups on the PBI surface, 423
possibly accompanied by hydrolysis, because PBI in contact with ambient air contains some water. 424
425
Table 2: IEC values of used materials and membranes 426
Membrane or material IEC
[mmol Cl (g dry membrane)-1]
Comment
mTPN 2.17 Based on chemical structure
PBI 0 Based on chemical structure
mTPN-50.80 1.69 Based on analysis
mTPN-50.120 1.94 Based on analysis
PBI/mTPN-50.120 1.57 Based on analysis
427
428
The conductivity of mTPN-50.120 in pure water (in-plane) was 72 mS cm-1. This is significantly 429
higher than the 54 mS cm-1 reported by Lee et al.[8], presumably because the samples in that 430
work were not purged electrochemically[21,22] and therefore may still have contained traces of 431
(bi)carbonate. A similar observation was also made for other AEM types.[21] 432
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433
Figure 6: Conductivity of membranes (in hydroxide form, a) and length swelling of the best 434
performing membrane type (in chloride form, b); data for dense PBI in DI water is from 435
literature[42]; conductivity data for water were measured in-plane, all other data were measured 436
through-plane; (c,d) uptake and swelling in KOH solutions. *The negative thickness swelling 437
values and the related volume swelling values seem to stem from increased waviness in the dry 438
state. 439
440
The swelling behavior of the chloride form membranes was only analyzed for membranes heated 441
at 120 °C, because (a) they showed highest conductivity and (b) FTIR data indicated better 442
interaction between PBI and mTPBr. PBI/mTPN-50.120 swelled 37% less at room temperature 443
and 25% less at 60 °C than mTPN-50.120 (Figure 6b). If membranes are assembled in the wet 444
state and never allowed to dry, the main issue is temperature induced swelling (wet, RT 60 °C), 445
a b
c
in-plane through-plane conductivity
mTPN-50.120
PBI/mTPN-50.120FAA3-50
FAA3-PK-75
0
2
4
6
8
10
12
14
16
18
Le
ng
th s
we
llin
g (
%)
@RT
@60 oC
d
FAA3-50 0.1M 1M KOH
FAA3-PK-75 0.1M 1M KOH
Uptake SR length SR thickness SR volume-10
0
10
20
30
40
50
60
(%)
Uptake SR length SR thickness SR volume-10
0
10
20
30
40
50
60
(%)
mTPN-50.120 0.1M 1M KOH
PBI/mTPN-50.120 0.1M 1M KOH
mTPN-50.120
PBI/mTPN-50.120
mTPN-50.80
PBI/mTPN-50.80
FAA3-50
FAA3-PK-75
dense PBI
0 1 2 3 4
0
20
40
60
80co
nd
uctivity (
mS
cm
-1)
KOH concentration (M)
**
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which was 2% for mTPN-50.120 and 2.7% for PBI/mTPN-50.120. In summary, while mTPN-50-120 446
may be considered to be slightly better under optimal conditions, introduction of the PBI 447
nanofiber mats renders the membrane more robust, e.g. if electrolysis cells are not assembled 448
under fully humidified conditions, or if the cells run dry during maintenance work. 449
450
Swelling in KOH solution transfers the membrane into the hydroxide form. In general, the length 451
swelling of increases in the order Cl form (water) < OH form, 0.1M KOH < OH form, 1M KOH 452
(Figure 6c). For mTPN and PBI/mTPN, length swelling is very similar at room temperature in 0.1 453
M KOH and 1M KOH and at 60 °C in the Cl form, indicating that length swelling is restricted to 454
about 10 %. Because the PBI fiber mat does not cover the whole membrane volume, but only a 455
layer in the center of the membrane, it is no surprise that length swelling is restricted, whereas 456
thickness swelling is not much affected – it is practically same for reinforced and non-reinforced 457
membranes in 1M KOH. While thickness swelling can have the unwanted effect that the catalyst 458
layer is pressed into the porous transport layer, strong length swelling and shrinking can be 459
detrimental for the membrane, because expansion leads to increased waviness and shrinking 460
leads to mechanical stresses, the result being cracks, pinholes, and catalyst-membrane 461
delamination. Therefore, the low length and thickness swelling of PBI/mTPN-50.120 is beneficial. 462
463
In comparison with the commercial materials FAA3-50 and FAA3-PK-75 (Figure 6d), the effect of 464
the PBI support on the restriction of swelling is weaker than the effect of the PK support, which 465
is a strong fiber mash. The negative thickness swelling and therefore also very low volume 466
swelling of FAA3-PK-75 was observed by two co-authors and for different samples, and also in 467
the chloride for. Most probably, some waviness in the dry state increased the apparent thickness. 468
469
3.3. Mechanical properties 470
Membranes experience mechanical stresses when they swell and shrink due to changes in 471
humidity and temperature, when the cell is operated under differential pressure (conditions 472
under which the membrane and catalyst layer are pushed into the pores of the porous transport 473
layer) and during handling. Commercial membranes show tensile strength values of 30 MPa 474
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(Orion TM1, i.e. mTPN),[8, 43] 25-40 MPa (Fumatech FAA3), 45 MPa (reinforced FAA3), 60 MPa 475
(Aemion membranes) and 96 MPa (Tokuyama’s A201, which is not produced anymore); for 476
Sustainion membranes, values are not reported, because they get very brittle in the dry state and 477
have to be handled in the wet state.[7] 478
479
Figure 7: (a) Tensile strength, Young’s Modulus and elongation at break at ambient and wet (30 °C) 480
conditions; chloride form, measured at 28 °C and 22 %relative humidity, FAA3-50: 26 °C and 481
74 %relative humidity. (b) Stress-strain curves for PBI/mTPN-50.120 and commercial FAA3-PK-75 482
FAA3-PK-75 (old, degraded batch)
b
c
a
Young Modulus (MPa)
Elongation at Break (%)
Tensile Strength (MPa)
mTPN-50.120(wet)
PBI/mTPN-50.120
(wet)
FAA3-50(wet)
FAA3-PK-75(wet)
0
50
100
200
400
600
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bought in 2021 (new batch, 74 %rh) and 2017 (old batch, 22 %rh), and (c) photograph of FAA3-483
PK-75 after testing. 484
485
At 28 °C and 22 % relative humidity, mTPN-50.120 showed a tensile strength of 42 MPa (Figure 486
7a). Reinforcement with a PBI nanofiber mat increased the tensile strength of mTPN by 17%. A 487
larger effect was seen for the Young Modulus, which increased 56% from 436 MPa to 680 MPa 488
for PBI/mTPN-50.120. For comparison, Nafion 212 has a Young Modulus of just 160 MPa.[44] The 489
elongation at break decreased from 16% for mTPN to 11% for PBI/mTPN-50.120. FAA3 showed a 490
lower tensile strength than the value found in the literature, possibly because the humidity was 491
above 70% during the measurement. An even stronger effect was observed for all membranes, 492
when the wet samples were analyzed. In general, tensile strength and Young modulus decreased 493
in the wet state, and elongation at break increased. A slightly different behavior is seen for FAA3-494
PK-75, for which the PK support strongly influences the properties, nearly irrespective of the 495
humidity. While the tensile strength of wet FAA3 was 8.6 MPa and that of wet mTPN was 4.9 496
MPa, it was 17.5 MPa for wet PBI/mTPN, and 23 - 26 MPa for FAA3-PK-75 in both wet and dry 497
state. A clear improvement was seen for the Young Modulus in the wet state. The reinforcement 498
increased it from 75 MPa for mTPN (wet) to 240 MPa for reinforced PBI/mTPN-50.120 (wet). 499
500
Figure 7b shows the stress-strain curves of a slightly thinner PBI/mTPN-50.120 membrane (43 501
µm for this sample) and commercial FAA3-PK-75, which is a 75 µm thick AEM reinforced with a 502
porous poly(ether ether ketone). PBI/mTPN has a 2-3 times higher tensile strength than FAA3-503
PK-75. While PBI/mTPN and new FAA3-PK-75 show smooth curves and the membrane has the 504
same appearance before and after testing, an old batch of FAA3-PK-75 which was stored for over 505
4 years cracked in several places, before the sample broke. The reason for this could be 506
degradation of the poly(ether ketone) fiber mash support. But Fumatech also advises that drying 507
of wet membranes may lead to micro-cracks. Therefore, minute repeated swelling and shrinking 508
due to changes in ambient humidity over the years may also have induced micro-cracks. 509
Comparing the photograph of the broken sample (Figure 7c) and the number of spikes in the 510
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stress-strain curve reveals that each spike represents a cracking event (see also videos in 511
supporting information). 512
513
In summary, it can be stated that the reinforced membranes show a sufficient tensile strength 514
and Young Modulus. In future work, further improvements seem to be possible by using thicker 515
fiber mats or embedding more than one fiber mat, so that the ratio of support to matrix increases. 516
We expect that this would increase the tensile strength and Young Modulus, and further 517
decrease the length swelling. Due to the high porosity of the PBI nanofiber mat, the negative 518
effect on conductivity should be tolerable. 519
520
3.4 Anion exchange membrane water electrolysis 521
Figure 8 shows the AEMWE performance of PBI/mTPN-50.120 and the evaluated EIS parameters 522
for three different KOH concentrations (0.01, 0.1, 1 M). The comparison of the PBI/mTPN-50.120 523
and mTPN membrane is shown in Figure SX. It confirms the only small effect of the PBI presence 524
on the AEMWE performance. The load curves in Figure 8a indicate strong dependence of the cell 525
performance on the KOH concentration. The highest current density of 0.172 A cm-2 was achieved 526
for the highest concentration of the liquid electrolyte, i.e. for 1M KOH. Direct comparison of the 527
obtained data with the literature is difficult due to the different experimental conditions used by 528
different authors. Nevertheless, according to the review published recently by Hamish et al.[45] 529
current densities ranging from 0.007 to 0.98 A cm-2 at the cell voltage of 1.8 V are reported by 530
the authors for KOH concentrations < 1 M. In the case of the present CCM-MEA based on the 531
PBI/mTPN-50.120 membrane, the maximum current density at cell voltage of 1.8 V is about 0.045 532
A cm-2 with 1 M KOH as liquid electrolyte. However, as pointed out by Hamish et al.,[45] the high 533
current densities reported are typically achieved using PGM catalysts.[46-49] When considering 534
non-PGM catalysts exclusively, the performance of the PBI/mTPN-50.120 membrane is 535
comparable with the literature data.[50-53] Despite the fact that PBI membranes typically need 536
KOH concentrations higher than 1 M KOH,[39, 54-58] Liu at al.[59] applied a Celazole® PBI 537
membrane in their comparative study and achieved a current density of 0.035 A cm-2 at a cell 538
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voltage of 1.8 V with 1 M KOH liquid electrolyte at 60 °C. It is thus possible that the PBI support 539
is not fully inert and partially contributes to the AEMWE performance. 540
541
The impact of the increased KOH concentration on the MEA performance was analysed by EIS 542
(Figure 8b). As clearly evident from the figure, increasing KOH concentrations improves both the 543
cell (Rs) and polarisation (Rp) resistances. Rs includes the contributions from several parts of the 544
electrolysis cell, but is primarily dominated by the resistances of the liquid and solid electrolytes. 545
Therefore, changes in Rs do not strictly follow the trend given by the conductivity of the KOH 546
feed solution. While the conductivity of the KOH solution increased by two orders of magnitude 547
from 0.0032 to 0.29 S cm-1 (at 50 °C) when the KOH concentration increased from 0.01 to 1 M, 548
Rs values decreased just by a factor of 16. This is connected with the solid polymer electrolyte, 549
i.e. the anion exchange membrane and ionomer. As demonstrated in Figure 6, the ionic 550
conductivity of the membrane is only partially influenced by the composition of the surrounding 551
liquid electrolyte. In sum, at lower KOH concentrations, Rs is dominated by the liquid electrolyte; 552
at higher KOH concentrations, Rs is limited by the conductivity of the solid electrolyte, and only 553
negligible changes of Rs are observed when the KOH concentration is further increased in this 554
regime.[50,60] 555
It is, however, Rp, which clearly determines the AEMWE performance. When less concentrated 556
KOH solutions are used as a liquid electrolyte, one has Rp >> Rs. This is primarily caused by the 557
limited ionic conductivity of the polymer binder ensuring ionic contact between the three 558
dimensional structure of the catalyst layer and the membrane. Liquid electrolyte of sufficient 559
conductivity can improve this ionic contact and thus improve catalyst utilisation and performance. 560
At lower concentration, the reduced catalyst utilisation increases the local current density in the 561
active regions of the catalyst layer. Consequently, Rp increases. In the case of the 1 M KOH liquid 562
electrolyte, fitting revealed that only one time constant contributed to the EIS spectrum. The 563
oxygen evolution reaction (OER) is generally characterised by the sluggish kinetics when 564
compared to hydrogen evolution reaction (HER). This is also documented by the values of Rp 565
shown in Figure 8b, where HER in general exhibits lower values of polarisation resistance. As the 566
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extent of the catalyst layer utilisation increases with increasing KOH concentration, HER kinetics 567
improve to such an extent that they become overlapped in the EIS spectrum by the OER. 568
569
570 571
572
Figure 8: Characteristics of the AEMWE in terms of (A) load curves and (B) system (Rs) and 573
polarisation (Rp) resistances. Operating conditions: PBI/mTPN-50.120 membrane; PSEBS-CM-574
DABCO polymer binder; 2.5 mg cm-2 Ni-Fe and 2.5 mg cm-2 Mo based compounds were used as 575
anode and cathode catalysts respectively; CBR 93/7; Ni foam electrodes of geometrical area 4 576
cm2; 50 °C; liquid KOH electrolyte of concentrations given in the figures (A, B). EIS measured at a 577
cell voltage of 1.8 V. 578
579
Finally, the durability test of the PBI/mTPN-based CCM-MEA was investigated by its galvanostatic 580
polarisation at 0.25 A cm-2. The results are summarised in Figure 9. Figure 9a documents good 581
stability of the cell voltage. The average value of the cell voltage over the 200 hours experiment 582
falls into the interval of (1.98 ± 0.02) V. This corresponds to a fluctuation of ± 1.1 %. At the average 583
value of the cell voltage during stability test short term load curves showed current density of 584
only 0.17 A cm-2. The difference is due to the different regimes of the electrode polarisation, i.e. 585
potentiostatic vs. galvanostatic regime. Observable periodic voltage oscillations are mainly due 586
to changes in the KOH concentration caused by the continuous water depletion by electrode 587
reactions together with evaporation and its regular replenishment to its original concentration. 588
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When water is consumed, the KOH concentration increases. This reduces the cell resistance and 589
improves access of hydroxide ions to the anodic catalyst sites. As a result, cell voltage decreases. 590
When the KOH concentration is re-adjusted to 1 M KOH by adding appropriate amount of water, 591
the voltage returns to the original value. 592
593
Further characteristics of the AEMWE cell were again obtained by EIS. Since 1 M KOH solution 594
was used as a liquid electrolyte, the EIS spectrum showed only one time constant and thus, only 595
OER Rp was determined. Figure 9b shows the evolution of Rs and Rp values (fitted to the 596
experimental data) over the time of electrolysis. While the variation of Rs values over time is 597
practically negligible, (0.08 ± 0.01) Ω, Rp was only stable for the first 80 hours. After exceeding 598
this time, it started to rise. The polarisation resistance increase agrees well with the rise of the 599
average cell voltage observed for the same time. Variation of both Rp and cell voltage correspond 600
well to each other confirming the strong impact of the Rp on the cell voltage and at the same 601
time stability of the membrane. The observed Rp fluctuations can be explained by local KOH 602
concentration changes and/or by partial blockage of the catalyst surface by evolving gas bubbles. 603
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Figure 9: Stability of the PBI/mTPN-based CCM-MEA in terms of (A) cell voltage and (B) EIS
results. Operating conditions: PBI/mTPN-50.120 membrane; PSEBS-CM-DABCO polymer
binder; 2.5 mg cm-2 Ni-Fe and 2.5 mg cm-2 Mo based compounds as anode and cathode
catalysts respectively; CBR 93/7; Ni foam electrodes of geometrical area 4 cm2; 50 °C; 1 M
KOH liquid electrolyte; 0.25 A cm-2 current density. EIS measured at 1.8 V.
After the stability test, the morphology of the mPBI/mTPN-50.120-based CCM-MEA and the 604
distribution of the catalyst metals over the examined cross-sectional area was analysed by SEM-605
EDS (Figure 10 and Figure S3). The minor presence of the catalyst elements in the membrane is 606
most probably due to the sample preparation method by scalpel cutting. The thicknesses of the 607
catalyst layers are 10 and 4 µm for cathode and anode, respectively. Presumably, some of the 608
catalyst sticked to the porous transport layer when the cell was disassembled (see SEM-EDS 609
analysis of the fresh sample in Figure SX1). But it is nicely seen that neither cathode nor anode 610
side show signs of membrane/electrode delamination even after 200 hours of electrolysis 611
operation and sample preparation by mechanically cutting the MEA. 612
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Figure 10: SEM-EDS-analysis of a cross-section of the PBI/mTPN-50.120-based CCM-MEA
after 200 hours electrolysis operation. Acceleration voltage was 15 kV; 600x magnification.
Even though Rp started to increase after 80 hours of electrolysis operation, the cell voltage 613
showed a stable value with only 1.1 % fluctuation around its average value during the 200 hours 614
electrolysis stability test. This proves good stability of the prepared CCM-MEA. In addition, the 615
performance of the water electrolysis during the stability test is promising: The cell voltage 616
reached 1.98 V at 0.25 A cm-2 in 1 M KOH at 50 °C. For comparison, Park et al. reported that a 617
water electrolyser operating with a non-reinforced mTPN membrane (0.5 M NaOH, 50 °C, each 618
electrode 2 mg cm-2 PGM catalyst, AS-4 ionomer binder) required a voltage of 2.2 V to operate 619
at 0.25 A cm-2 (iV curve) and 0.2 A cm-2 at steady-state operation.[10] In comparison to other 620
reported systems using commercial membranes, the performance of 1.98 V at 0.25 A cm-2 does 621
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not exceed the state of art materials (see supporting information, Table S1), current densities in 622
the range of up to 500 mA cm-2 can be obtained even without noble metal catalysts.[7] However, 623
the commercially available AEM FAA-3-50 measured under the same conditions achieved the 624
comparable performance of 0.275 A cm-2 at cell voltage 2 V, see Figure S5. It should be stressed 625
that this work was not focused on the cell performance optimization, but on the proof of concept 626
that membranes with an engineered covalently bonded matrix/support interface can be 627
fabricated. Better performance can be reached by further optimization of the MEA setup 628
(matching of membrane and binder) and cell operational conditions targeted to reduce the 629
polarization resistance contributing mainly to the performance losses. 630
631
For future work, we intend to further optimize the PBI nanofiber mats’ robustness, for example 632
by calendering, solvent welding to increase the number of connection points between the fibers, 633
or crosslinking. Second, we plan to test other ion conducting matrices – the reinforcement, 634
especially after increasing its robustness, could well allow the use of materials which are cheaper 635
to produce and have higher IEC and thus conductivity. 636
637
4. Conclusions 638
In this work, polybenzimidazole nanofiber mats were prepared by electrospinning. In a second 639
step, the bromomethylated precursor of highly alkaline-stable mTPN was filled into the pores of 640
PBI nanofiber mats. During drying, covalent bonds were established between the PBI support and 641
the mTPBr matrix. A leaching test in THF supported the formation of covalent bonds. After 642
quaternisation to PBI/mTPN, the reinforced membranes showed the expected behavior: 37% 643
reduced length swelling, 17% increased tensile strength, and 56% increased Young’s Modulus. 644
During the test, PBI/mTPN-50.120 did not change its appearance. This indicated that the PBI 645
nanofiber mat efficiently transfers forces into all directions, and that the interaction between 646
support and matrix is strong. The hydroxide conductivity in DI water decreased only from 72 mS 647
cm-1 to 62 mS cm-1. Finally, a MEA prepared with Ni/Fe and Mo-based catalysts (no PGM) and a 648
SEBS-based binder showed a stable voltage of 1.98V at 250 mA cm-2 in an anion exchange 649
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membrane water electrolysis test (1M KOH, 50 °C.), which lasted 200 hours. Furthermore, EIS 650
showed no significant change in the membrane resistance. 651
652
In sum, the PBI/mTPN-50.120-based CCM-MEA utilising PSEBS-CM-DABCO as ionomer binder in 653
the catalyst layer and non-PGM Ni-Fe and Mo based catalysts represents a promising candidate 654
for AEMWE utilising diluted KOH as a liquid electrolyte. 655
656
Acknowledgements 657
This work was done as part of the NEWELY project, and has received funding from NRF (Korea) 658
and from the Fuel Cells and Hydrogen 2 Joint Undertaking under Grant No. 875118. This Joint 659
Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation 660
programme, Hydrogen Europe and Hydrogen Europe research. CB thanks the US Department of 661
Energy ARPA-E (IONICS DE-Ar0000769) for financial support. 662
663
Competing Interests 664
The authors declare no competing financial interest. 665
666
Supporting Information: 667
Videos of stress-strain tests for aged FAA3-PK-75 and PBI/mTPN-50.120. 668
Additional SEM images, XPS data and information on the ATR-FTIR measurements, literature 669
comparison of commercial membranes, and electrolysis data for mTPN and FAA3-50. 670
671
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Highlights
A bromomethylated polymer was filled into the pores of PBI nanofiber mats
Curing led to covalent bonds between porous support and ion conducting matrix
In stress-strain tests, no cracks appeared before failure
Water electrolyzer: [email protected] A/cm-2 (50 °C, 1M KOH, catalysts based on Ni,Fe,Mo)
200 hours stability test: voltage and membrane resistance were stable
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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