Alkaline direct methanol fuel cell based on a novel anion-exchange composite polymer membrane

The anion-exchange composite polymer membrane based on quaterized poly(vinyl alcohol)/poly(epichlorohydrin) (designated as Q-PVA/PECH) was prepared by a solution casting method and a quaternization process. The characteristic properties of the Q-PVA/PECH anion-exchange composite polymer membranes were investigated by scanning electron microscopy, thermal gravimetric analysis, micro-Raman spectroscopy, and AC impedance method. Alkaline direct methanol fuel cells (ADMFC) comprised Q-PVA/PECH anion-exchange composite polymer membranes were assembled and examined. Experimental results indicate that an alkaline DMFC employing an inexpensive non-perfluorinated Q-PVA/PECH composite polymer membrane shows excellent electrochemical performances. The peak power densities of the DMFC using 4 M KOH + 1 M CH₃OH, 2 M CH₃OH, and 4 M CH₃OH fuels are 17.22, 22.30, and 20.81 mW cm⁻², respectively, under ambient conditions. The Q-PVA/PECH composite polymer membrane appears as a viable candidate for use in an ADMFC.     

Studies of some operating parameters and cyclic voltammetry for a direct ethanol fuel cell

A direct ethanol fuel cell (DEFC) of 5 cm² membrane-electrode area was studied systematically by varying the catalyst loading, ethanol concentration, temperature and different Pt based electro-catalysts (Pt–Ru/C, Pt-black High Surface Area (HSA) and Pt/C). A combination of 2 M ethanol at the anode, pure oxygen at the cathode, 1 mg cm⁻² of Pt–Ru/C (40%:20%) as the anode and 1 mg cm⁻² of Pt-black as the cathode gave a maximum open circuit voltage (OCV) of 0.815 V, a short circuit current density of 27.90 mA cm⁻² and a power density of 10.3 mW cm⁻². The optimum temperatures of the anode and cathode were determined as 90 °C and 60 °C, respectively. The power density increased with increase in ethanol concentration and catalyst loading at the anode and cathode. However, the power density decreased slightly beyond 2 M ethanol concentration and 1 mg cm⁻² catalyst loading at the anode and cathode. These results were validated using cyclic voltammetry at single electrodes under similar conditions to those of the DEFC.     

Direct Biomass Fuel Cell (BMFC) with Anode/Catalyst Comprising a Nanocomposite of a Mesoporous n-Semiconductor Film and a Metal Thin Layer: A New Concept of Catalyst Design

Abstract  

We designed an efficient direct biomass fuel cell (BMFC) anode and prepared a nanocomposite [base electrode/mesoporous n-semiconductor (SC) thin film/metal thin layer]. A Pt thin layer was photodeposited onto a mesoporous 20-μm thick TiO₂ thin film having a roughness factor of 2000, which was coated on an F-doped tin oxide/glass base electrode (FTO). This anode/catalyst nanocomposite was efficient at decomposing aqueous solutions of glucose and other biomass-related compounds in combination with an O₂-reducing cathode the other side of which was exposed to ambient air. The nanocomposite exhibited sharp optimum conditions at the atomic ratio of Pt/Ti = 0.33 in the BMFC, generating high electrical power of 2 mW cm⁻² without any light irradiation or bias potential when using a 1 M glucose aqueous solution. This output power is 20 times as large as that generated by a mesoporous TiO₂ film anode under UV-light (18 mW cm⁻²) irradiation. At this ratio, the coated Pt specifically exhibited metallic luster, and its average Pt thickness on the mesoporous TiO₂ nanostructure was calculated to be 0.40 nm. The high BMFC activity was interpreted by the simultaneous Schottky-junction/Ohmic contact nature of the nanocomposite. Other biomass compounds such as sucrose, ethanol and polysaccharides were also effective as direct fuels for the BMFC. Immediately after soaking this composite anode without a cathode in a glucose aqueous solution, continuous evolution of H₂ bubbles was observed from the anode surface. The electrical power generation and H₂ production are easily changed by connecting and disconnecting a cathode, respectively. Based on a simple design and calculation, the present system with glucose fuel has the potential to construct a module stack of 2 kW m⁻³. Simultaneous material/energy circulation by using the BMFC with biomass and its waste fuel is proposed for application in future social systems.     

Preparation of platinum-ruthenium onto solid polymer electrolyte membrane and the application to a DMFC anode

The ‘impregnation-reduction method’ has been investigated as a tool for the preparation of a direct methanol fuel cell (DMFC) anode. In this method, PtRu electrocatalysts were directly bonded onto a polymer electrolyte membrane by the chemical reduction of a mixture of Pt and Ru complexes impregnated in the membrane. The deposited PtRu particles were embedded in the 3-4 μm region of the membrane surface to form a porous and hydrophilic layer. The PtRu layers turned out to be applicable to the DMFC anode, despite their small active surface areas compared to PtRu nanoparticles used in the conventional method. Approximately, 3 mg cm⁻² of the PtRu layer exhibited better catalyst utilization and facilitated the release of evolving CO₂. This preparation technique is attractive for the application of various solid polymer electrolyte materials with low heat-resistance or various shapes, etc.     

Nanocomposite hybrid membranes containing polyvinyl alcohol or poly(tetramethylene oxide) for fuel cell applications

New hybrid membranes containing polyvinyl alcohol (PVA) and poly(tetramethylene oxide) (PTMO) with heteropolyacid (HPA) as a hydrophilic inorganic modifier in an organic/inorganic matrix were developed for low-temperature proton exchange membrane fuel cells (PEMFCs). A maximum conductivity of 4.8 × 10⁻³ S cm⁻¹ was obtained at 80 °C and 75% RH for PVA/PWA/PTMO/H₃PO₄ (10/15/70/5 wt%), whereas the PVA/SiWA/MPTS/H₃PO₄ (50/10/10/30 wt%) membrane demonstrated a maximum conductivity of 8.5 × 10⁻³ S cm⁻¹ under identical conditions. These hybrid composite membranes were subsequently tested in a fuel cell. A maximum current density of 240 mA cm⁻² was produced at 70 °C for the PVA/PWA/PTMO/H₃PO₄ membrane, and the corresponding value for the PVA/SiWA/MPTS/H₃PO₄ membrane under identical conditions was 230 mA cm⁻². The small deviations in cell performance can be explained in terms of the variations in thickness of the membranes as well as differences in their conductivities. The fuel cell performances of these membranes decreased drastically when the temperature was increased to 100 °C.     

Mineralization of hydroxyapatite on a polymer substrate in a solution supersaturated by polyelectrolyte

In this article, a new method to construct composites of hydroxyapatite (HAP) and polymer material is introduced. A previously developed method for mineralization of CaCO₃ on a polymer substrate was applied to HAP. A solution that contained Ca²⁺, PO₄^3–, and OH‐ ions was supersaturated with polyacrylic acid (PAA) that, at the same time, formed a polymer complex with the substrate, a polyvinyl alcohol (PVA) film, at the substrate surface. In this thin surface layer, nucleation of HAP took place. Subsequently, the disklike domains of HAP that were generated spread until they covered the PVA film surface. By regulating the pH of the supersaturated solution at around 7.4, the domain size decreased and the quantity of deposited material increased. Approximately 20 mg of HAP coating was obtained on a PVA film of 1 cm radius when the film was soaked in single 200 mL batch of the supersaturated solution for 21 days. The junction between HAP layer and PVA substrate film was found to be very firm. When a crosslinked PVA/PAA was used as the substrate, the film swelled in the supersaturated solution to form a hydrogel. Then mineralization took place within the gel, and a transparent monolithic composite of HAP and the polymer network was obtained. In 13 days, the weight increase was 29 mg, which corresponded to a 71 wt % HAP mineralization ratio of the composite. By changing crosslinking degree and HAP mineralization ratio, the flexibility of composite will be controlled in a wide range. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1465–1470, 2006     

NiCoFe/C cathode electrocatalysts for direct ethanol fuel cells

A direct ethanol fuel cell (DEFC), which is less prone to ethanol crossover, is reported. The cell consists of PtRu/C catalyst as the anode, Nafion^® 117 membrane, and Ni–Co–Fe (NCF) composite catalyst as the cathode. The NCF catalyst was synthesized by mixing Ni, Co, and Fe complexes into a polymer matrix (melamine-formaldehyde resins), followed by heating the mixture at 800 °C under inert atmosphere. TEM and EDX experiments suggest that the NCF catalyst has alloy structures of Ni, Co and Fe. The catalytic activity of the NCF catalyst for the oxygen reduction reaction (ORR) was compared with that of commercially available Pt/C (CAP) catalyst at different ethanol concentrations. The decrease in open circuit voltage (V_oc) of the DEFC equipped with the NCF catalysts was less than that of CAP catalyst at higher ethanol concentrations. The NCF catalyst was less prone to ethanol oxidation at cathode even when ethanol crossover occurred through the Nafion^®117 film, which prevents voltage drop at the cathode. However, the CAP catalyst did oxidize ethanol at the cathode and caused a decrease in voltage at higher ethanol concentrations.     

Microfabricated polymer electrolyte membrane fuel cells with low catalyst loadings

Miniaturized fuel cells as compact power sources fabricated in Pyrex glass using standard polymer electrolyte membrane (PEM) and electrode materials are presented. Photolithographic patterned and wet chemically etched serpentine flow channels of 1 mm in width and 250  m in depth transport the fuels to the cell of 1.44 cm² active electrode area. Feeding H₂/O₂ a maximum power density of 149 mW cm⁻² is attained at a very low Pt loading of 0.054 mg cm⁻², ambient pressure, and room temperature. Operated with methanol and oxygen about 9 mW cm⁻² are achieved at ambient pressure, 60 °C, and 1 mg cm⁻² PtRu/Pt (anode/cathode) loading. A planar two-cell stack to demonstrate and investigate the assembly of a fuel cell system on Pyrex wafers has successfully been fabricated.     

Alkaline blend polymer electrolytes based on polyvinyl alcohol (PVA)/tetraethyl ammonium chloride (TEAC)

Alkaline blend polymer electrolytes based on PVA/TEAC were obtained by a solution casting technique. Tetraethyl ammonium chloride (TEAC) was added to PVA polymer matrix to form an alkaline blend polymer electrolyte exhibiting excellent ionic transport and mechanical properties. The ionic conductivity of the alkaline PVA/TEAC blend polymer electrolyte was found to be of the order of 10⁻² S cm⁻¹ at ambient temperature when the blend ratio of PVA:TEAC varied from 1:0.2 to 1:2. The characteristic properties of alkaline PVA/TEAC blend polymer electrolytes were examined using DSC, TGA, XRD, SEM, EA, stress–strain tests and AC impedance spectroscopy. The ionic transport properties for the blend polymer electrolytes were measured using Hittorf’s method. It was found that the anionic transport numbers (t ⁻) were between 0.82 and 0.99; the membranes are highly dependent on the types of alkali metal salts and the chemical composition of the polymer blend. The ionic transport and mechanical properties were greatly improved at the expense of the ionic conductivity. In this work we demonstrate that alkaline blend polymer electrolyte can be tailored with a blend technique to achieve specific characteristic properties for battery applications.     

La0.5Sr0.5Fe0.9Mo0.1O3-δ-CeO2 anode catalyst for Co-Producing electricity and ethylene from ethane in proton-conducting solid oxide fuel cells

A La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ-CeO₂ (LSFM-CeO₂) composite was prepared by impregnating CeO₂ into porous La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ perovskite and was used as an anode material for proton-conducting solid oxide fuel cells (SOFCs). The maximum power densities of the BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) electrolyte-supported single cell with LSFM-CeO₂ as the anode reached 291 mW cm^?2 and 190 mW cm^?2 in hydrogen and ethane fuel at 750 °C, respectively, which are significantly higher than those of a single cell with only LSFM as the anode. Additionally, the ethylene selectivity and ethylene yield from ethane for the fuel cell at 750 °C were as high as 93.4% and 37.1%, respectively. The single cell also showed negligible degradation in performance and no carbon deposition during continuous operation for 22 h under an ethane fuel atmosphere. The improved electrochemical performance due to the impregnation of CeO₂ can be a result of enhanced electronic and ionic conductivity, abundant active sites, and a broad three-phase interface in the resultant composite anode. The LSFM-CeO₂ composite is believed to be a promising anode material for proton-conducting SOFCs for co-producing electricity and high-value chemicals from hydrocarbon fuels.     

A Study on Process Parameters of Direct Ethanol Fuel Cell

Ethanol is one of the promising future fuels of Direct Alcohol Fuel Cells (DAFC). The electro‐oxidation of ethanol fuel on anode made of carbon‐supported Pt‐Ru electrode catalysts was carried out in a lab scale direct ethanol fuel cell (DEFC). Cathode used was Pt‐black high surface area. The membrane electrode assembly (MEA) was prepared by sandwiching the solid polymer electrolyte membrane, prepared from Nafion^® (SE‐5112, DuPont USA) dispersion, between the anode and cathode. The DEFC was fabricated using the MEA and tested at different catalyst loadings at the electrodes, temperatures and ethanol concentrations. The maximum power density of DEFC for optimized value of ethanol concentration, catalyst loading and temperature were determined. The maximum open circuit voltage (OCV) of 0.815 V, short circuit current density (SCCD) of 27.90 mA/cm² and power density of 10.30 mW/cm² were obtained for anode (Pt‐Ru/C) and cathode (Pt‐black) loading of 1 mg/cm² at a temperature of 90°C anode and 60°C cathode for 2M ethanol.     

A parametric study of a platinum ruthenium anode in a direct borohydride fuel cell

Data on the performance of a direct borohydride fuel cell (DBFC) equipped with an anion exchange membrane, a Pt–Ru/C anode and a Pt/C cathode are reported. The effect of oxidant (air or oxygen), borohydride and electrolyte concentrations, temperature and anode solution flow rate is described. The DBFC gives power densities of 200 and 145 mW cm⁻² using ambient oxygen and air cathodes respectively at medium temperatures (60 °C). The performance of the DBFC is very good at low temperatures (ca. 30 °C) using modest catalyst loadings of 1 mg cm⁻² for anode and cathode. Preliminary data indicate that the cell will be stable over significant operating times.     

Power from marine sediment fuel cells: the influence of anode material

The effect of anode material on the performance of microbial fuel cells (MFC), which utilise oxidisable carbon compounds and other components present in sediments on ocean floors, estuaries and other similar environments is reported. The MFC anode materials were carbon sponge, carbon cloth, carbon fibre, and reticulated vitreous carbon (RVC). Power was produced through the microbial activity at the anode in conjunction with, principally, oxygen reduction at a graphite cloth cathode. After a period of stabilisation, open circuit voltages up to 700 mV were observed for most cells. Steady state polarisations gave maximum power densities of 55 mW m⁻² using carbon sponge as the anode; which was nearly twice that achieved with carbon cloth. The latter material typically gave power densities of around 20 mW m⁻². The performance of the cell was reduced by operation at a low temperature of 5 °C. Generally, for cells which were capable of generating power at current densities of 100 mA m⁻² and greater, mass transport was found to limit both the anode and the cathode performance, due primarily to the low concentrations of electro-active species present or generated in cells.     

Performance of internal reforming methanol fuel cell under various methanol/water concentrations

Copper-based reforming catalyst was placed adjacent to ADVENT Technologies high-Temperature polymer electrolyte membrane/electrode assembly in a novel internal reforming methanol fuel cell (IRMFC) and tested for their electrochemical properties and chemical stability under various methanol/water anode feedstreams. Polarization measurements and AC impedance spectroscopy combined with measurements of reactor outlet composition were carried out. Methanol is being reformed inside the anode compartment of the fuel cell at 200 °C producing H₂, which is readily oxidized at the anode to produce electricity. The reformer provides enough hydrogen supply for efficient fuel cell operation at 600 mV with 0.2 A cm⁻² and a hydrogen stoichiometric ratio of 1.2 (λ_H2 = H₂ fed/H₂ reacted = 1.2). However, unreacted MeOH (~5 %) in combination with low H₂ content poisons the anode electrode and the cell performance rapidly decreases. Gradual recovery of the initial performance under pure H₂ is observed after switching to pure H₂. A slight improvement of the cell’s design by the introduction of a pre-reforming step significantly improves the electrocatalytic behavior.     

Passive micro tubular direct formic acid fuel cells (DFAFCs) with chemically assembled Pd anode nano-catalysts on polymer electrolytes

Wet-chemical assembling process has been developed for the formation of the anode electrocatalyst layers of a micro tubular direct formic acid fuel cell. By using this method, a porous layer of Pd nano-catalyst was bonded onto the inner surface of a tubular polymer electrolyte membrane by chemical reduction of Pd complex impregnated in the membrane. The performance characteristics as a function of parameters such as catalyst loading amount of Pd or the cell temperature were evaluated by using a half-cell testing method. The micro tubular DFAFC with a 2.5 mg-Pd cm⁻² anode and 6 mg-Pt cm⁻² cathode fabricated by wholly wet-chemical assembling process exhibited a peak power density over 4 mW cm⁻² under passive and air breathing conditions at ambient temperature and pressure.     

Electrical properties of poly(vinyl alcohol) (PVA) based on LiFePO<Subscript>4</Subscript> complex polymer electrolyte films

Li ion conducting polymer electrolyte films were prepared based on poly(vinyl alcohol) (PVA) with 5, 10, 15, 20, 25 and 30 wt% lithium iron phosphate (LiFePO₄) salt using a solution-casting technique. X-ray diffraction (XRD) was used to determine the complexation of the polymer with LiFePO₄ salt. Differential scanning (DSC) calorimetry was used to determine the melting temperatures of the pure PVA and complexed films. The maximum ionic conductivity was found to be 1.18 × 10⁻⁵ S cm⁻¹ for (PVA:LiFePO₄) (75:25) film, which increased to 3.12 × 10⁻⁵ S cm⁻¹ upon the addition of propylene carbonate (PC) plasticizer at ambient temperature. The Li⁺ ion transport number was found to be 0.40 for (PVA: LiFePO₄) (75:25) film using AC impedance and DC polarization methods. Dielectric studies were performed for these polymer electrolyte films in the frequency range of 10 Hz to 10 MHz at different temperatures. The activation energies of the complexed films were calculated from the dielectric loss tangent spectra and were found to be 0.35, 0.30, 0.27 and 0.28 eV. The cyclic voltammogram (CV) curves of (PVA: LiFePO₄) (75:25)+PC film exhibited higher specific capacities than those for other films.     

Fabrication of PANI@Ti3C2Tx/PVA hydrogel composite as flexible supercapacitor electrode with good electrochemical performance

Developing a new strategy to effectively prevent the restacking of MXene nanosheets will have significant impacts on designing flexible supercapacitor electrodes. Herein, a novel Ti₃C₂T_x/polyvinyl alcohol (PVA) porous sponge with 3D interconnected structures is prepared by sol-gel and freeze-dried methods. This Ti₃C₂T_x/PVA porous sponge is used as the template of in-situ polyaniline (PANI) polymerization, and the fabricated PANI@Ti₃C₂T_x/PVA hydrogel composite is applied as flexible supercapacitors electrodes. 1D conductive polymer chains PVA could increase the interlayer spacing of Ti₃C₂T_x nanosheets, which is beneficial to expose more electrochemical active sites. The supercapacitor based on PANI@Ti₃C₂T_x/PVA hydrogel composite exhibits the coexistence of double-layer capacitance and pseudocapacitance behavior. This supercapacitor shows a maximum areal specific capacitance of 103.8 mF cm^?2 at 2 A m^?2, and it also exhibits a maximum energy density of 9.2 μWh·cm^?2 and an optimum power density of 800 μW cm^?2. The capacitance of this supercapacitor is almost not change under different bending angles. Moreover, 99% capacitance retention is achieved after 10 000 charge/discharge cycles of the supercapacitor. The synergistic effect between PANI and Ti₃C₂T_x/PVA composite may improve the number of reactive sites and provide efficient channels for ion diffusion/electron transport.     

Synthesis, Characterization, Conductivity, Band Gap, and Thermal Analysis of Poly-[(2-mercaptophenyl)iminomethyl]-2-naphthol and Its Polymer–Metal Complexes

Oxidative polycondensation reaction conditions of [(2-mercaptophenyl)iminomethyl]-2-naphthol (2-MPIM-2N) were studied using oxidants such as air and NaOCl in an aqueous alkaline medium between 40 °C and 90 °C. The structure of poly-[(2-mercaptophenyl)iminomethyl]-2-naphthol (P-2-MPIM-2N) was characterized by ¹H- ¹³C NMR, FT-IR, and UV–Vis spectroscopy, size exclusion chromatography (SEC), and elemental analysis. At optimum reaction conditions, the yield of P-2-MPIM-2N was found to be 78 and 82% for air and NaOCl oxidants, respectively. From SEC measurements, the number-average molecular weight (M _n ), weight-average molecular weight (M _w ) and polydispersity index (PDI) of P-2-MPIM-2N are 2900, 3500 g mol⁻¹ and 1.207; 2200, 2500 g mol⁻¹ and 1.136, for air and NaOCl oxidants, respectively. Polymer–metal complexes were synthesized by the reaction of P-2-MPIM-2N with Co²⁺, Cu²⁺, Zn²⁺, Pb²⁺ and Cd²⁺ ions. The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and electrochemical band gaps (E^￠_g E^{\prime}_{g} ) of 2-MPIM-2N and P-2-MPIM-2N were −5.97, −2.66 and 3.31 eV and −5.82, −2.68 and 3.14 eV, respectively. The conductivity of polymer and polymer–metal complexes were determined in the solid state. Conductivity measurements of doped and undoped Schiff base polymer and polymer–metal complexes were carried out at room temperature and atmospheric pressure by the four-point probe technique using an electrometer. The conductivities of the polymer and polymer–metal complexes increased when iodine was used as doping agent.     

Modeling and experimental validation of overpotentials of a direct ethanol fuel cell

Direct ethanol fuel cell (DEFC) is a promising power source for future use in portable electronic equipments. In general, the power density obtained in DEFC is lower than that of direct methanol fuel cell. In the present study, various losses in DEFC are estimated by performing experiments with the prepared membrane electrode (MEA) to obtain current–voltage characteristics and comparing it with the prediction of mathematical model. MEA for the DEFC is prepared using Pt–Ru (40:20 by wt.%)/C as anode catalyst, Pt–black as cathode catalyst with 1 mg/cm² of loadings and cast Nafion^® (SE5112, DuPont) ionomer as proton exchange membrane. The mathematical model for DEFC is developed considering different overpotentials. The activation overpotential term is formulated considering ethanol electrooxidation mechanism proposed in literature and Butler–Volmer equation. The ohmic overpotential is modeled based on proton conductivity of Nafion^® membrane and ohmic losses at the electrodes, current collectors and electrode–current collector interfaces. The concentration overpotential is formulated using Fick's law, modified Butler–Volmer equation and transport process through electrodes and electrocatalyst layers. The experiment data on current–voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is captured by the model.     

The Construction and Application of Dual-Modified Carbon Nanotubes in Proton Exchange Membranes with Enhanced Performances

In this study, the carbon nanotubes (CNTs) are successively coated via sol-gel method with SiO₂ (SiO₂@CNTs), followed by grafting with 3-merraptnpropyltrimethnxysilane and oxidation with hydrogen peroxide to yield dual-modified CNTs (SSiO₂@CNTs). The SSiO₂@CNTs material is applied to prepared chitosan (CS) based composite proton exchange membranes by the incorporation of various content of SSiO₂@CNTs, the structure and properties of as-prepared composite membranes are fully investigated. Compared to pristine CS membrane, the SSiO₂@CNTs-filled composite membranes show improved thermal stability, mechanical stability, and methanol resistance, owing to the effective interface interaction and good compatibility between SSiO₂@CNTs and CS matrix. Additionally, the doping of SSiO₂@CNTs also generates a positive effect on the electrochemistry performance, due to the construction of abundant transport channel and providing more proton sources or proton sites. Particularly, the CS/SSiO₂@CNTs-7 membrane exhibits tensile strength of about 40.1 MPa and proton conductivity of 35.8 mS cm⁻¹ at 80 °C, which is almost 1.6 and 2.0 times higher than pure CS membrane, and lower methanol permeability of 0.9 × 10⁻⁶ cm² s⁻¹. The direct methanol fuel cell performance (DMFC) of CS/SSiO₂@CNTs-7 membrane is also improved with open circuit voltage of 0.67 V and maximum power density of 60.7 mW cm⁻² at 70 °C.