Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine

A role of metallophthalocyanine (MPc) as an anti-oxidizing agent of polymer membrane and an accelerating agent of proton conductivity was discussed. The poly(ether sulfone)s bearing MPc (Ni, Co and Fe), PMPc were prepared by two-step reaction from phenolphthalein and fumaronitrile and followed reaction with metal (II) chloride (Ni, Co and Fe) and 1,2-dicyanobenzene in quinoline. The sulfonated polymer was synthesized by condensation polymerization using 1,2-bis(4-hydroxyphenyl)-1,2-diphenyl ethylene, bis(4-fluorophenyl) sulfone and followed by sulfonation reaction with concentrated sulfuric acid. A series of hybrid membranes (H–Ni, H–Co and H–Fe) were prepared from a mixture of the sulfonated copolymer and PMPcs in dimethylacetamide (DMAc). The structural properties of the synthesized polymers were studied by ¹H-NMR spectroscopy and FT-IR. The membrane properties were investigated by measurements of ion exchange capacity (IEC), water uptake, and proton conductivity, chemical degradation test, and atomic force microscopy (AFM) analysis. The cell performance of the membranes was compared with those of normal sulfonated poly(ether sulfone)s and Nafion.     

Block copolymers combining semi-fluorinated poly(arylene ether) and sulfonated poly(arylene ether sulfone) segments for proton exchange membranes

A series of aromatic multiblock copolymers based on alternating segments of hydrophilic sulfonated polysulfone (PSU) and hydrophobic polyfluoroether (PFE) were prepared and characterized as proton exchange membranes. PSU precursor blocks were synthesized by polycondensation of dichlorodiphenylsulfone and resorcinol, and PFE precursor blocks were prepared by combining decafluorobiphenyl and isopropylidenediphenol. After preparation of the multiblock copolymers via a mild coupling reaction of the precursor blocks, the resorcinol units of the PSU blocks were selectively and almost completely sulfonated under mild reaction conditions using trimethylsilylchlorosulfonate. Transparent and robust membranes with different PSU-PFE copolymer compositions and ion-exchange capacities were cast from solution. Atomic force microscopy of the membranes revealed a distinct nanophase separated morphology. At 80 °C, the proton conductivity reached 10 mS cm⁻¹ under 65% relative humidity and 100 mS cm⁻¹ under fully hydrated conditions.     

Synthesis and characterization of crosslinked sulfonated poly(arylene ether sulfone) membranes for high temperature PEMFC applications

Sulfonated poly(arylene ether sulfone) copolymers containing carboxyl groups are prepared by an aromatic substitution polymerization reaction using phenolphthalin, 3,3′-disulfonated-4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfone and 4,4′-bisphenol A as polymer electrolyte membranes for the development of high temperature polymer electrolyte membrane fuel cells. Thin, ductile films are fabricated by the solution casting method, which resulted in membranes with a thickness of approximately 50 μm. Hydroquinone is used to crosslink the prepared copolymer in the presence of the catalyst, sodium hypophosphite. The synthesized copolymers and membranes are characterized by ¹H NMR, FT-IR, TGA, ion exchange capacity, water uptake and proton conductivity measurements. The water uptake and proton conductivity of the membranes are decreased with increasing the degree of crosslinking which is determined by phenolphthalin content in the copolymer (0-15 mol%). The prepared membranes are tested in a 9 cm² commercial single cell at 80 °C and 120 °C in humidified H₂/air under different relative humidity conditions. The uncrosslinked membrane is found to perform better than the crosslinked membranes at 80 °C; however, the crosslinked membranes perform better at 120 °C. The crosslinked membrane containing 10 mol% of phenolphthalin (CPS-PP10) shows the best performance of 600 mA cm⁻² at 0.6 V and better performance than the commercial Nafion^® 112 (540 mA cm⁻² at 0.6 V) at 120 °C and 30 % RH.     

High efficiency of proton transport by clustering nanochannels in multi-sulfonated propeller-like nonplanar hexaphenylbenzene poly(ether sulfone)s

The propeller-like nonplanar building unit containing poly(aryl ether ketone)s were synthesized from 1,2-bis(4-hydroxyphenyl)-3,4,5,6-tetraphenyl-benzene (BHPTPB) and bis(4-fluorophenyl) sulfone with bis(4-hydroxydiphenyl) sulfone in NMP. New structures and/or morphologies for existing materials need to be considered in case if increased benefits are desired. The sulfonation was taken selectively on hydrophilic block segment as well as para position of the pendent phenyl groups with concentrated sulfuric acid. The stoichiometry mole ratios were changed with hydrophilic blocks of 12, 15, 17, 20, and 23 mol% of BHPTPB monomer to control the ion exchange capacity. The structural properties of polymer membranes (HP-12, 15, 17, 20, and 23) were studied by FT-IR, ¹H NMR spectroscopy, thermogravimetric analysis (TGA), and atomic force microscope (AFM). The water uptakes were 8.1–69.4% at 75 °C with changing the ion exchange capacities. The resulted proton conductivities were 66.58–103.73 mS/cm at 80 °C with 90% relative humidity. The highest power density of a fuel cell using HP-23 and Nafion 211 was 0.47 and 0.45 W/cm², respectively, at 0.6 V.     

Studies of sulfonated polyphenylene membranes containing benzophenone moiety for PEMFC

The sulfonated polyphenylenes containing benzophenone structure (sulfonated Parmax 1200, S-Parmax) were prepared by sulfonation reaction of Parmax 1200 with 30% fuming sulfuric acid, and degree of sulfonation was controlled by sulfonation reaction time. These polymers have all carbon structure without ether linkages that have the possibility of attack by nucleophiles (made by PEMFC operating system). The polyphenylene structure of Parmax provides a stiff and resistant backbone, whereas the pendant benzoyl group enables the solubility of the material, and also provides sites for chemical modifications. The structure properties of the synthesized polymers were investigated by ¹H NMR spectroscopy. The membranes were studied by ion exchange capacity (IEC), water uptake, and proton conductivity. These membranes deterioration test was performed by Fenton reagent, and compared with normal sulfonated poly(ether sulfone)s and Nafion. The power densities of membranes were performed by single cell.     

Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

Sulfonated organosilane functionalized graphene oxides (SSi-GO) synthesized through the grafting of graphene oxide (GO) with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated poly(ether ether ketone) (SPEEK) membranes. The incorporation of SSi-GOs greatly increases the ion-exchange capacity (IEC), water uptake, and proton conductivity of the membrane. With well-controlled contents of SSi-GOs, the composite membranes exhibit higher proton conductivity and lower methanol permeability than Nafion^® 112 and Nafion^® 115, making them particularly attractive as proton exchange membranes (PEMs) for direct methanol fuel cells (DMFC). The composite membrane with optimal SSi-GOs content exhibit over 38 and 17% higher power densities, respectively, than Nafion^® 112 and Nafion^® 115 membranes in DMFCs, offering the possibilities to reduce the DMFC membrane cost significantly while keeping high-performance.     

Portable PEMFC stack using sulfonated poly (fluorenyl ether ketone) ionomer as membrane

Portable polymer electrolyte membrane fuel cells (PEMFCs) stack was assembled with sulfonated poly(fluorenyl ether ketone) (SPFEK) ionomer membranes. The portable PEMFC stack was studied by means of cell performance tests at high temperatures under low relatively humidity (RH). The experimental results showed that the output power of the stack increased from 28.74 W to 37.11 W with increasing operating temperature from 30 to 90 °C under 100% RH. When the operating temperature was over 100 °C, the output power decreased with further increasing temperature from 27.68 W (100 °C, 85% RH) to 19.87 W (120 °C, 50% RH). The output at 120 °C and under 50% RH was 69% output power of the stack at 30 °C and under 100% RH. These results demonstrated that the self-prepared SPFEK ionomer membrane was a promising PEM for the application in high-temperature PEMFC.     

Sulfonated poly(arylene ether sulfone)/disulfonated silsesquioxane hybrid proton conductors for proton exchange membrane fuel cell application

Fuel cell operating at high temperature and low humidity conditions is in urgent demand. Low glass transition temperature, high cost, and high humidity dependence of commercial membranes such as Nafion, however, are major obstacles to commercialization. Sulfonated poly (arylene ether sulfone) is a promising polymer that may show a breakthrough in this respect as it shows high thermal stability and mechanical strength while maintaining performance and cost competitiveness. Its relatively high dependence on humidity levels, however, is still an obstacle that needs to be tackled. The incorporation of silsesquioxane particles with disulfonated naphthol (NSi) functionalization is designed to increase the number of proton conducting moieties in the polymer matrix thus aiding proton transport. The incorporation of NSi has drastically improved performance especially at lower humidity conditions. Although current density of 5 wt.% NSi hybrid membrane shows a 2.0% increase in performance at 80°C/100 R.H.% that at 120 °C/30 R.H.% shows a 200% rise in current density at 0.7 V compared to that of pristine membranes. In addition, the evenly distributed silsesquioxane particles physically reduce fuel crossover values by 33.4%.     

Phosphoric acid doped sulfonated poly(tetra phenyl isoquinoline ether sulfone) copolymers for high temperature proton exchange membrane potential application

Phosphoric acid-doped sulfonated poly(tetra phenyl isoquinoline ether sulfone)s (PA-SPTPIESs) were successfully synthesized for high temperature proton exchange membrane. Poly(tetra phenyl ether ketone sulfone)s (PTPEKS) were prepared from 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenyl benzene (BFBTPB) and bis(4-fluorohenyl) sulfone with bis(4-hydroxyphenyl) sulfone. The synthesis of the poly(tetra phenyl isoquinoline ether sulfone)s (PTPIESs), was carried out via an intramolecular ring-closure reaction of dibenzoylbenzene of PTPEKS with benzylamine. The sulfonated poly(tetra phenyl isoquinoline ether sulfone)s (SPTPIESs) were obtained by following sulfonation with concentrated sulfuric acid and doped by phosphoric acid. Different contents of sulfonated unit on PTPIESs (8, 12, 16 mol% of BFBTPB) and PA-SPTPIESs were studied by FT-IR, ¹H NMR spectroscopy, and thermogravimetric analysis (TGA). Strong acid–base interaction effect between poly benzisoquinoline (PBI) and sulfonic acid groups formed ionic crosslinking network between polymer chains. The ion exchange capacity (IEC) and proton conductivity of PA-SPTPIESs were evaluated with degree of sulfonation and doping of phosphoric acid.     

Confinement of functionalized graphene oxide in sulfonated poly (ether ether ketone) nanofibers by coaxial electrospinning for polymer electrolyte membranes

A novel cambiform-like core-shell nanofiber containing sulfonated graphene oxide core is fabricated through coaxial electrospinning method to improve proton conductivity, fuel blocking and inorganics/polymer compatibility for fuel cell applications. As induced by the strong electrostatic force, the sulfonated organosilane functionalized graphene oxide nanosheet is axially elongated to form a unique cambiform-like and highly wrinkled morphology in the core of the sulfonated poly (ether ether ketone) nanofiber, which is evidenced by the transmission electron microscopy images. It provides a forced contact and good dispersion of graphene oxide in the polymer to improve the tensile strength (approximately 2.6 and 1.8 folds of that of the blend monoaxial electrospun and cast membranes, respectively). The wrinkled graphene oxide core contains the sulfonated functional groups and micro-voids which are favorable for water uptake, making the co-spinning membrane exhibit approximately 43.2% and 33.0% increase of water uptake compared with that of the blend monoaxial electrospun and cast membranes, respectively, and thus facilitate the formation of hydrogen bond networks for proton hopping but tortuous pathways for fuel permeation. Accordingly, both lower hydrogen permeation and much higher methanol selectivity (11 folds of that of Nafion 115) are achieved in the co-spinning membrane.     

Sulfonated poly(ether sulfone) (SPES)/boron phosphate (BPO4) composite membranes for high-temperature proton-exchange membrane fuel cells

A new series of sulfonated poly(ether sulfone) (SPES)/boron phosphate (BPO₄) composite membranes for proton-exchange membrane fuel cells (PEMFCs) applications, with a BPO₄ content up to 40 wt%, were prepared by a sol–gel method using tripropylborate and phosphoric acid as precursors. Compared to a pure SPES membrane, BPO₄ doping in the membranes led to a higher thermal stability and glass-transition temperature (T_g) as revealed by TGA–FTIR, DSC and DMTA. Water uptake and oxidative stability were significantly increased by increasing the content of BPO₄. At both operating temperature conditions, namely 20 °C and 100 °C, the tensile strength of all the composite membranes were lower than that of the SPES membrane. However, even when the content of BPO₄ was as high as 30%, the composite membrane still possessed strength similar to the Nafion 112 membrane. SEM–EDX indicated that the BPO₄ particles were uniformly embedded throughout the SPES matrix, which may facilitate proton transport. Proton conductivities increased from 0.0065 to 0.022 S cm⁻¹ at room temperature as BPO₄ increased from 0 to 40%. The conductivities also increased with the temperature. The SPES/BPO₄ composite membrane is a promising candidate for PEMFCs applications, especially at higher temperatures.     

Alkyl chain modified sulfonated poly(ether sulfone) for fuel cell applications

A new alkyl chain modified sulfonated poly(ether sulfone) (mPES) was synthesized and formed into membranes. The MEAs were tested in the PEMFC and evaluated systematically in the DMFC by varying the methanol concentration from 0.5 to 5.0 M at 60 °C and 70 °C. The synthesized mPES copolymer has been characterized by nuclear magnetic resonance spectroscopy, fourier transform infrared spectroscopy, thermogravimetric analysis, and gel permeation chromatography. The proton conductivity of the resulting membrane is higher than the threshold value of 10⁻² S cm⁻¹ at room temperature for practical PEM fuel cells. The membrane is insoluble in boiling water, thermally stable until 250 °C and shows low methanol permeability. In the H₂/air PEMFC at 70 °C, a current density of 600 mA cm⁻² leads to a potential of 637 mV and 658 mV for 50 μm thick mPES 60 and Nafion NRE 212, respectively. In the DMFC, mPES 60's methanol crossover current density is 4 times lower than that for Nafion NRE 212, leading to higher OCV values and peak power densities. Among all investigated conditions and materials, the highest peak power density of 120 mW cm⁻² was obtained with an mPES 60 based MEA at 70 °C and a methanol feed of 2 M.     

Sulfonated poly(ether sulfone)-based silica nanocomposite membranes for high temperature polymer electrolyte fuel cell applications

Fuel Cell operation at high temperature (e.g. 120 °C) and low relative humidity (e.g. 50%) remains challenging due to creep (in the case of Nafion^®) and membrane dehydration. We approached this problem by filling PES 70, a sulfonated poly(ether sulfone) with a T_g of 235 ± 5 °C and a theoretical IEC of 1.68 mmol g⁻¹, with 5-20% silica nano particles of 7 nm diameter and 390 ± 40 m² g⁻¹ surface area. While simple stirring of particles and polymer solutions led to hazy, strongly anisotropic (air/glass side) and sometimes irregular shaped membranes, good membranes were obtained by ball milling. SEM analysis showed reduced anisotropy and TEM analysis proved that the nanoparticles are well embedded in the polymer matrix. The separation length between the ion-rich domains was determined by SAXS to be 2.8, 2.9 and 3.0 nm for PES 70, PES 70-S05 and Nafion^® NRE 212, respectively. Tensile strength and Young’s modulus increase with the amount of silica. Ex-situ in-plane proton conductivity showed a maximum for PES 70-S05 (2 mS cm⁻¹). In the fuel cell (H₂/air, 120 °C, <50%), it showed a current density of 173 mA cm⁻² at 0.7 V, which is 3.4 times higher than for PES 70.     

Novel high-performance nanocomposite proton exchange membranes based on poly (ether sulfone)

In the present research, proton exchange membranes based on partially sulfonated poly (ether sulfone) (S-PES) with various degrees of sulfonation were synthesized. It was found that the increasing of sulfonation degree up to 40% results in the enhancement of water uptake, ion exchange capacity and proton conductivity properties of the prepared membranes to 28.1%, 1.59 meq g⁻¹, and 0.145 S cm⁻¹, respectively. Afterwards, nanocomposite membranes based on S-PES (at the predetermined optimum sulfonation degree) containing various loading weights of organically treated montmorillonite (OMMT) were prepared via the solution intercalation technique. X-ray diffraction patterns revealed the exfoliated structure of OMMT in the macromolecular matrices. The S-PES nanocomposite membrane with 3.0 wt% of OMMT content showed the maximum selectivity parameter of about 520,000 S s cm⁻³ which is related to the high conductivity of 0.051 S cm⁻¹ and low methanol permeability of 9.8 × 10⁻⁸ cm² s⁻¹. Furthermore, single cell DMFC fuel cell performance test with 4 molar methanol concentration showed a high power density (131 mW cm⁻²) of the nanocomposite membrane at the optimum composition (40% of sulfonation and 3.0 wt% of OMMT loading) compared to the Nafion^®117 membrane (114 mW cm⁻²). Manufactured nanocomposite membranes thanks to their high selectivity, ease of preparation and low cost could be suggested as the ideal candidate for the direct methanol fuel cell applications.     

Poly(arylene ether ketone) with pendant pyridinium groups for alkaline fuel cell membranes

Using the step-growth polycondensation reaction, poly(arylene ether ketone) (PAEK) and activated poly(arylene ether ketone)-NHS intermediates (PAEK-N) were synthesized. PAEK-NHS intermediates with pyridinium groups (PAEK-PYR) were obtained by adding different amounts of PYR groups. The successful syntheses of PAEK, PAEK-N, and PAEK-PYR were confirmed by nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy. Several important membrane properties (e.g., ionic exchange capacity (IEC), water uptake, anion conductivity, and thermal and mechanical stability) were investigated for their applications in alkaline fuel cells. Water uptake, swelling ratio, anion conductivity, and IEC increased with increasing PYR contents, while the mechanical properties decreased. Among a series of prepared membranes, the PAEK-PYR100 and PAEK-PYR125 membranes showed IEC and anion conductivity values that were higher than those of a commercial AHA membrane. Also, all of the prepared membranes were thermally stable up to 255 °C and show excellent chemical stability in alkaline conditions.     

Sulfonated poly(arylene ether sulfone) membranes blended with hydrophobic polymers for direct methanol fuel cell applications

In this paper, proton exchange membranes for direct methanol fuel cells were prepared by blending sulfonated poly(arylene ether sulfone) with poly (vinylidene fluoride-co-hecafluoropropylene)(PVdF-HFP) and polyethersulfone (PES) to decrease methanol permeability while maintaining high proton conductivity. The content of the second polymer, such as PES and PVdF, in the blend membranes was controlled at 10–40 wt% based on SPAES. In order to investigate the effects of the second polymer content in the blended membranes, parameters of the prepared membranes related to fuel cell performance were characterized, including their morphology, mechanical properties, methanol permeability, and proton conductivity. Surface roughness of the blend membrane was increased by the introduction of a hydrophobic polymer. Mechanical properties of the PES/SPAES blend membrane were improved owing to interaction between the sulfonic acid groups in SPAES and PES. However, the tensile strength of the PVdF/SPAES blend membrane was decreased by due to the poor compatibility of SPAES and PVdF. The methanol permeability in the blended membranes decreased with increasing content of PES and PVdF. The SPAES/PES blend membranes exhibited good proton conductivity and lower methanol permeability than the SPAES membrane. The SVdF15 blend membrane showed the highest selectivity due to the absence of methanol crossover and a small decrease of proton conductivity. These blend membranes are suitable for DMFC applications.     

Semi-interpenetrating polymer network electrolyte membranes composed of sulfonated poly(ether ether ketone) and organosiloxane-based hybrid network

A semi-interpenetrating polymer network (semi-IPN) proton exchange membrane is prepared from the sulfonated poly(ether ether ketone) (sPEEK) and organosiloxane-based organic/inorganic hybrid network (organosiloxane network). The organosiloxane network is synthesized from 3-glycidyloxypropyltrimethoxysiane and 1-hydroxyethane-1,1-diphosphonic acid. The semi-IPN membranes prepared were stable up to 300 °C without any degradation. The methanol permeability is much lower than Nafion^® 117 under addition of the organosiloxane network. The proton conductivity of semi-IPN membranes increases with an increase the organosiloxane network content; the membrane containing the 20-24 wt% organosiloxane network shows higher conductivity than Nafion^® 117. The power density of the MEA fabricated with the semi-IPN membrane with 24 wt% organosiloxane network is 135 mW cm⁻², much better than that of the pristine sPEEK membrane, 85 mW cm⁻². Chemical synthesis of the semi-IPN membranes is identified using FTIR, and its ion cluster dimension examined using SAXS. The dimensional stability associated with water swelling and dissolution is investigated at different temperatures, and the semi IPN membranes dimensionally stable in water at elevated temperature.     

Sulfonated poly(arylene ether sulfone) nanocomposite electrolyte membrane for fuel cell applications: A review

Polymer electrolyte membrane (PEM) fuel cells are considered a promising technology for generating power with water as a byproduct. Recently, sulfonated poly(arylene ether sulfone) (SPAES) has emerged as a most suitable alternative for PEM applications because of its high proton conductivity, high CO tolerance, and low fuel crossover. However, the existing SPAES polymeric membrane materials have poor chemical reactivity, mechanical processability, and thermal usability. Thus, the effects of mixing inorganic nanomaterials with SPAES polymers on proton conductivity, power density, fuel crossover, thermal and chemical stability, and durability are discussed in this review. Further, the progress in preparation methods and fuel cell characteristics by the addition of silica, clay, heteropolyacids (HPA), and carbon nanotubes (CNTs) in polymer membrane materials for PEM applications is also discussed.     

Crosslinked sulfonated poly(arylene ether sulfone) membranes for fuel cell application

A series of sulfonated poly(arylene ether sulfone) with photocrosslinkable moieties is successfully synthesized by direct copolymerization of 3,3′-disulfonated 4,4′-difluorodiphenyl sulfone (SDFDPS) and 4,4′-difluorodiphenyl sulfone (DFDPS) with 4,4′-biphenol (BP) and 1,3-bis-(4-hydroxyphenyl) propenone (BHPP). The content of crosslinkable moieties in the polymer repeat unit is controlled from 0 to 10 mol% by changing the monomer feed ratio of BHPP to BP. The polymer membranes can be crosslinked by irradiating UV with a wavelength of 365 nm. From FT-IR analysis, it can be identified that UV crosslinking mainly occurs due to the combination reaction of radicals that occurs in conjunction with the breaking of the carbon–carbon double bonds (–CH = CH-) of the chalcone moieties in the backbone. Consequently, a new bond is created to form cyclobutane. The crosslinked membranes show less water uptake, a lower level of methanol permeability, and good thermal and mechanical properties compared to pristine (non-crosslinked) membranes while maintaining a reasonable level of proton conductivity. Finally, the fuel cell performance of the crosslinked membranes is comparable to that of the Nafion 115 membrane, demonstrating that these membranes are promising candidates for use as polymer electrolyte membranes in DMFCs.     

Sulfonated poly(fluorenyl ether ketone nitrile) electrolyte membrane with high proton conductivity and low water uptake

High molecular weight sulfonated poly(fluorenyl ether ketone nitrile)s with different equivalent weight (EW) from 681 to 369 g mequiv.⁻¹ are synthesized by the nucleophilic substitution polycondensation of various amounts of sulfonated difluorobenzophenone (SDFBP) and 2,6-difluorobenzonitrile (DFBN) with bisphenol fluorene (BPF). The synthesized copolymers are characterized by ¹H NMR, FT-IR, TGA, and DSC techniques. The membranes cast from the corresponding copolymers exhibit superior thermal stability, good oxidative stability and high proton conductivity, but low water uptake due to the strong nitrile dipole interchain interactions that combine to limit swelling. Among all the membranes, the membrane with EW of 441 g mequiv.⁻¹ shows optimum properties of both high proton conductivity of 41.9 mS cm⁻¹ and low water uptake of 42.6%. Accordingly, That membrane is fabricated into a membrane electrode assembly (MEA) and evaluated in a single proton exchange membrane fuel cell (PEMFC). The experimental results indicate its similar cell performance as that of Nafion^® 117 at 70 °C, but much better cell performance at higher temperatures. At the potential of 0.6 V, the current density of fuel cell using the prepared membrane and Nafion^® 117 is 0.46 and 0.25 A cm⁻², respectively. The highest current density of the former reaches as high as 1.25 A cm⁻².