Ignition behavior of pure and blended methyl octanoate,n-nonane,and methylcyclohexane

Excited-state species profiles and ignition delay times were obtained under dilute conditions (99% Ar) using a heated shock tube for methyl octanoate (C₉H₁₈O₂), n-nonane (n-C₉H₂₀), and methylcyclohexane (MCH) over a broad range of temperature and equivalence ratio (? = 0.5, 1.0, 2.0) at pressures near 1 and 10 atm. Measurements were then extended to include two ternary blends of the fuels using 5% and 20% (vol.) of the methyl ester under stoichiometric conditions. Using three independently validated chemical kinetics mechanisms, a model was compiled to assess the influence of methyl ester concentration on ignition delay times of the ternary blends. Under near-atmospheric pressure, ignition delay times were of the following order for the pure fuels: methyl octanoate < n-nonane < methylcyclohexane. Experimental results indicate that the ignition behavior of the higher-order methyl ester approaches that of the higher-order linear alkane with increased pressure regardless of equivalence ratio. Methyl octanoate also displayed significantly lower pressure dependence relative to the linear alkane and the cycloalkane species. Both of these results are supported by model calculations. Blending of methyl octanoate with n-nonane and methylcyclohexane impacted ignition delay time results more strongly at 1.5 atm, yet had only a small effect near 10 atm for temperatures above 1400 K. The study provides the first shock-tube data for a ternary blend of a linear alkane, a cycloalkane, and a methyl ester. Ignition delay time measurements for the C₉:0 methyl ester were also measured for the first time.     

High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels

Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p₅ = 10, 20, and 40 bar) for equivalence ratios ? = 0.5 and 1.0 and over a wide temperature range 690 K < T₅ < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, ? = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated.     

Ignition delay times of methyl oleate and methyl linoleate behind reflected shock waves

Ignition delay times for methyl oleate (C₁₉H₃₆O₂, CAS: 112-62-9) and methyl linoleate (C₁₉H₃₄O₂, CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. (2011) [10], which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements.     

Experimental and modeling study on the pyrolysis and oxidation of iso-octane

High pressure iso-octane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, iso-octane represents the branched paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on iso-octane was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on iso-octane oxidation and pyrolysis were obtained for temperatures from 835 to 1757 K, pressures from 21 to 65 atm, reactions times from 1.11 to 3.66 ms, and equivalence ratios from 0.52 to 1.68, and ∞. Iso-octane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at conditions studied. This observation prompted an experimental and modeling study of iso-octane pyrolysis using an iso-octane sub-model taken from a recently published n-decane/iso-octane/toluene surrogate model. The revised iso-octane sub-model showed improvements in predicting intermediate species profiles from pyrolytic experiments and oxidation experiments. The modifications to the iso-octane sub-model also contributed to better agreement in predicting the formation of carbon monoxide and carbon dioxide when compared to the recently published 1st Generation Surrogate model and a recently published iso-octane oxidation model. Model improvements were also seen in predicting species profiles from flow reactor oxidation experiments and ignition delay times at temperatures above 1000 K at both 10 and 50 atm.     

Experimental and modeling study on the pyrolysis and oxidation of n-decane and n-dodecane

High pressure n-decane and n-dodecane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, n-decane or n-dodecane represent the normal paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on both n-alkanes was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on both n-decane and n-dodecane oxidation and pyrolysis were obtained for temperatures from 867 to 1739 K, pressures from 19 to 74 atm, reaction times from 1.15 to 3.47 ms, and equivalence ratios from 0.46 to 2.05, and ∞. Both n-decane and n-dodecane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at the conditions studied. This observation prompted an experimental and modeling study of n-decane and n-dodecane pyrolysis using a recently submitted revised n-decane/iso-octane/toluene surrogate model. The surrogate model was extended to n-dodecane in order to facilitate the study of the species and the 1-olefin species quantified during the pyrolysis of n-dodecane and n-decane were revised with additional reactions and reaction rate constants modified with rate constants taken from literature. When compared against a recently published generalized n-alkane model and the original and revised surrogate models, the revised (based on our experimental work) and extended surrogate model showed improvements in predicting 1-olefin species profiles from pyrolytic and oxidative n-decane and n-dodecane experiments. The revised and extended model when compared to the published generalized n-alkane and surrogate models also showed improvements in predicting species profiles from flow reactor n-decane oxidation experiments, but similarly predicted n-decane and n-dodecane ignition delay times.     

An experimental and modeling study of the autoignition of 3-methylheptane

An experimental and kinetic modeling study of the autoignition of 3-methylheptane, a compound representative of the high molecular weight lightly branched alkanes found in large quantities in conventional and synthetic aviation kerosene and diesel fuels, is reported. Shock tube and rapid compression machine ignition delay time measurements are reported over a wide range of conditions of relevance to combustion engine applications: temperatures from 678 to 1356 K; pressures of 6.5, 10, 20, and 50 atm; and equivalence ratios of 0.5, 1.0, and 2.0. The wide range of temperatures examined provides observation of autoignition in three reactivity regimes, including the negative temperature coefficient (NTC) regime characteristic of paraffinic fuels. Comparisons made between the current ignition delay measurements for 3-methylheptane and previous results for n-octane and 2-methylheptane quantifies the influence of a single methyl substitution and its location on the reactivity of alkanes. It is found that the three C₈ alkane isomers have indistinguishable high-temperature ignition delay but their ignition delay times deviate in the NTC and low-temperature regimes in correlation with their research octane numbers. The experimental results are compared with the predictions of a proposed kinetic model that includes both high- and low-temperature oxidation chemistry. The model mechanistically explains the differences in reactivity for n-octane, 2-methylheptane, and 3-methylheptane in the NTC through the influence of the methyl substitution on the rates of isomerization reactions in the low-temperature chain branching pathway, that ultimately leads to ketohydroperoxide species, and the competition between low-temperature chain branching and the formation of cyclic ethers, in a chain propagating pathway.     

Autoignition of gasoline and its surrogates in a rapid compression machine

The analysis and interpretation of the combustion chemistry is greatly simplified by using simple mixtures of pure components, referred to as surrogates, in lieu of fully-blended transportation fuels, such as gasoline. Recognizing that the ability to model autoignition chemistry is critical to understanding the operation of Homogeneous Charged Compression Ignition engines, this work is an attempt to experimentally and computationally assess the autoignition responses of research grade gasoline and two of its proposed surrogates reported in the literature using a rapid compression machine (RCM), for the low-to-intermediate temperature range and at high pressures. The first surrogate studied is a three-component mixture of iso-octane, n-heptane, and toluene. The second is a four-component mixture that includes an olefin (2-pentene), in addition to the ones noted above. Ignition delay times of stoichiometric mixtures, for gasoline and the two surrogates in air, are measured using an RCM for pressures of 20 and 40 bar, and in the temperature range of 650–900 K. The four-component surrogate is found to emulate the ignition delay times of gasoline more closely when compared to the three-component surrogate. Additionally, the experimental data are compared against the computed results from a recently developed surrogate model for gasoline combustion. A good agreement between the experimental and computed results is observed, while discrepancies are also identified and discussed.     

Experiments and modeling of ignition delay times, flame structure and intermediate species of EHN-doped stoichiometric n-heptane/air combustion

The combustion of stoichiometric Ethyl-hexyl-nitrate (EHN)-doped n-heptane/oxygen/argon and (EHN)-doped n-heptane/air mixtures, respectively, was investigated in a low-pressure burner with a molecular-beam mass spectrometer and ignition delay-time (τ_ign) measurements were performed in a high-pressure shock tube. The experiments with the low-pressure flame were used for the determination of the flame structure including concentration profiles of reactants, products and important intermediates in the flame. The shock tube experiments provided τ_ign for a temperature range of 690 K ? T ? 1275 K at a pressure of 40 ± 2 bar for stoichiometric and lean mixtures under engine relevant conditions. A chemical mechanism for n-heptane/EHN mixtures was developed from an automatically generated mechanism for n-heptane by manually adding reactions to describe the influence of EHN. This mechanism was validated against the shock-tube data for various temperatures, levels of EHN-doping and equivalence ratios by homogeneous reactor calculations.The ignition delay times predicted by the model agree well with the shock tube results for a large range of temperatures, equivalence ratios and EHN concentrations. The influence of EHN onto ignition delay was largest in the low-temperature regime (770-1000 K).Numerical analysis suggests that the prevalent reason for the ignition-enhancing effect of EHN is the formation of highly reactive heptyl radicals by thermal decomposition of EHN. Due to this comparatively simple and generic mechanism, EHN is expected to have a similar ignition-enhancing effect also for other hydrocarbon fuels.     

Autoignition of gasoline surrogate mixtures at intermediate temperatures and high pressures: Experimental and numerical approaches

Ignition-delay times were measured in shock-heated gases for a surrogate gasoline fuel comprised of ethanol/iso-octane/n-heptane/toluene at a composition of 40%/37.8%/10.2%/12% by liquid volume with a calculated octane number of 98.8. The experiments were carried out in stoichiometric mixtures in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: Temperatures of 690-1200 K, and pressures of 10, 30 and 50 bar, respectively. Ignition delay times were determined from CH^∗ chemiluminescence at 431.5 nm measured at a sidewall location. The experimental results are compared to simulated ignition delay times based on detailed chemical kinetic mechanisms. The main mechanism is based on the primary reference fuels (PRF) model, and sub-mechanisms were incorporated to account for the effect of ethanol and/or toluene. The simulations are also compared to experimental ignition-delay data from the literature for ethanol/iso-octane/n-heptane (20%/62%/18% by liquid volume) and iso-octane/n-heptane/toluene (69%/17%/14% by liquid volume) surrogate fuels. The relative behavior of the ignition delay times of the different surrogates was well predicted, but the simulations overestimate the ignition delay, mostly at low temperatures.     

Effects of NO2 addition on hydrogen ignition behind reflected shock waves

Ignition delay time measurements of H₂/O₂/NO₂ mixtures diluted in Ar have been measured in a shock tube behind reflected shock waves. Three different NO₂ concentrations have been studied (100, 400 and 1600 ppm) at three pressure conditions (around 1.5, 13, and 30 atm) and for various H₂–O₂ equivalence ratios for the 100 ppm NO₂ case. Results were compared to some recent ignition delay time measurements of H₂/O₂ mixtures. A strong dependence of the ignition delay time on the pressure and the NO₂ concentration was observed, whereas the variation in the equivalence ratio did not exhibit any appreciable effect on the delay time. A mechanism combining recent H₂/O₂ chemistry and a recent high-pressure NOx sub-mechanism with an updated reaction rate for H₂ + NO₂ ? HONO + H was found to represent correctly the experimental trends over the entire range of conditions. A chemical analysis was conducted using this mechanism to interpret the experimental results. Ignition delay time data with NO₂ and other NOx species as additives or impurities are rare, and the present study provides such data over a relatively wide pressure range.     

Effects of fuel properties on self-ignition and flame-holding performances in SCRAM jet combustor for n-alkane fuels

Combustion characteristics of liquid hydrocarbon fuels are studied in a model combustor of SCRAM jet engines. The Mach number and total pressure of main flow in the combustor are 2.0 and 0.38 MPa, respectively, and the total temperature is varied from 1800 to 2400 K. Five kinds of n-alkane fuels such as n-heptane, n-octane, n-decane, n-tridecane and n-hexadecane are employed in experiments. Fuels are injected with a carrier nitrogen gas perpendicular to the mail flow in the combustor and the self-ignition behavior is investigated. The results show that the liquid fuels with lower carbon number have better self-ignition performance. This suggests that physical properties of liquid fuels such as volatility have a dominant effect on the self-ignition. The flame-holding behavior is investigated with the addition of pilot hydrogen to carrier nitrogen gas. The critical equivalence ratio at which the stable combustion keeps after cut-off of the pilot hydrogen is obtained. The relationship between the critical equivalence ratio and carbon number of fuel shows that fuels with the carbon numbers from 8 to 10 have the best flame-holding performance among the tested fuels. These experimental results can be expressed qualitatively by the simplified analysis with the concept of physical and chemical induction times.     

Ignition of non-premixed cyclohexane and mono-alkylated cyclohexane flames

Ignition temperatures of non-premixed cyclohexane, methylcyclohexane, ethylcyclohexane, n-propylcyclohexane, and n-butylcyclohexane flames were measured in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 373 K, a local strain rate of 120 s^?1, and fuel mole fractions ranging from 1% to 10%. Using the recently developed JetSurf 2.0 kinetic model, satisfactory predictions were found for cyclohexane, methyl-, ethyl-, and n-propyl-cyclohexane flames, but the n-butylcyclohexane data were overpredicted by 20 K. The results showed that cyclohexane flames exhibit the highest ignition propensity among all mono-alkylated cyclohexanes and n-hexane due to its higher reactivity and larger diffusivity. The size of mono-alkyl group chain was determined to have no measurable effect on ignition, which is a result of competition between fuel reactivity and diffusivity. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to fuel diffusion and also to H₂/CO and C₁–C₃ hydrocarbon kinetics.     

Experimental and semi-detailed kinetic modeling study of decalin oxidation and pyrolysis over a wide range of conditions

Decalin is the simplest polycyclic alkane (polynaphtenic hydrocarbon) found in liquid fuels (jet fuels, Diesel). In order to better understand the combustion characteristics of decalin, this study provides new experimental data for its oxidation in a jet-stirred reactor. For the first time, stable species concentration profiles were measured in a jet-stirred reactor at a constant mean residence time of 0.1 s and 0.5 s at respectively 1 and 10 atm, over a range of equivalence ratios (? = 0.5–1.5) and temperatures (750–1350 K). The oxidation of decalin under these experimental conditions was modeled using a semi-detailed chemical kinetic reaction mechanism (11,000 reactions involving 360 species) derived from a previously proposed scheme for the ignition of the same fuel in a shock-tube. The proposed mechanism that includes both low- and high-temperature chemistry shows reasonably good agreement with the present experimental data set. It can also represent well decalin pyrolysis and oxidation data available in the literature. Reaction path analyses and sensitivity analyses were conducted to interpret the results.     

Experimental study of minimum ignition energy of lean H2-N2O mixtures

Ignition energies for short duration (<50 ns) spark discharges were measured for undiluted and nitrogen-diluted H₂-N₂O mixtures of equivalence ratios ? = 0.15 and 0.2, dilution of 0% and 20% N₂, and initial pressures of 15–25 kPa. The ignition events were analyzed using statistical tools and the probability of ignition versus spark energy density (spark energy divided by the spark length) was obtained. The simple cylindrical ignition kernel model was compared against the results from the present study. Initial pressure has a significant effect on the width of the probability distribution, ranging from a broad (P = 15 kPa) to a narrow (P = 25 kPa) probability distribution indicating that the statistical variation of median spark energy density increases as initial pressure of the mixture decreases. A change in the equivalence ratio from 0.15 to 0.2 had a small effect on the median spark energy density. The addition of 20% N₂ dilution caused a significant increase in the median spark energy density when compared to no dilution. The extrapolation of the present results to atmospheric pressure, stoichiometric H₂-N₂O indicates that the electrostatic discharge ignition hazards are comparable to or greater than H₂-O₂ mixtures.     

Multi-species time-history measurements during n-hexadecane oxidation behind reflected shock waves

Species concentration time-histories were measured during oxidation for the large, normal-alkane, diesel-surrogate component n-hexadecane. Measurements were performed behind reflected shock waves in an aerosol shock tube, which allowed for high fuel loading without pre-test heating and possible decomposition and oxidation. Experiments were conducted using near-stoichiometric mixtures of n-hexadecane and 4% oxygen in argon at temperatures of 1165–1352 K and pressures near 2 atm. Concentration time-histories were recorded for five species: C₂H₄, CH₄, OH, CO₂, and H₂O. Methane was monitored using DFG laser absorption near 3.4 μm; OH was monitored using UV laser absorption at 306.5 nm; C₂H₄ was monitored using a CO₂ gas laser at 10.5 μm; and CO₂ and H₂O were monitored using tunable DFB diode laser absorption at 2.7 and 2.5 μm, respectively. These time-histories provide critically needed kinetic targets to test and refine large reaction mechanisms. Comparisons were made with the predictions of two diesel-surrogate reaction mechanisms (Westbrook et al. [1]; Ranzi et al. [9]) that include n-hexadecane, and areas of needed improvement in the mechanisms were identified. Comparisons of the intermediate product yields of ethylene for n-hexadecane with those found for other smaller n-alkanes, show that an n-hexadecane mechanism derived from a simple hierarchical extrapolation from a smaller n-alkane mechanism does not properly simulate the experimental measurements.     

A rapid compression machine study of the low temperature combustion of cyclohexane at elevated pressures

Ignition delay times have been measured in a rapid compression machine for cyclohexane/O₂/N₂/Ar mixtures with equivalence ratios of 0.5, 1.0 and 2.0 at elevated pressures of up to 40 bar and temperatures between 680 and 910 K. These data clearly show the negative-temperature-coefficient behavior for cyclohexane in the temperature range investigated. The predictions of several detailed kinetic models are compared to these new experimental validation data and these mechanisms have been analyzed to explain the obtained differences with a focus on the crucial peroxy-chemistry of the primary radicals. The presented data are the first set of ignition delay times at elevated pressures for the low-temperature range.     

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Shock tube ignition delay times were measured for DF-2 diesel/21% O₂/argon mixtures at pressures from 2.3 to 8.0 atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300 K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.     

Ignition limit and shock-to-detonation transition mode of n-heptane/air mixture in high-speed wedge flows

In this work, oblique detonation of n-heptane/air mixture in high-speed wedge flows is simulated by solving the reactive Euler equations with a two-dimensional (2D) configuration. This is a first attempt to model complicated hydrocarbon fuel oblique detonation waves (ODWs) with a detailed chemistry (44 species and 112 reactions). Effects of freestream equivalence ratios and velocities are considered, and the abrupt and smooth transition from oblique shock to detonation are predicted. Ignition limit, ODW characteristics, and predictability of the transition mode are discussed. Firstly, homogeneous constant-volume ignition calculations are performed for both fuel-lean and stoichiometric mixtures. The results show that the ignition delay generally increases with the wedge angle. However, a negative wedge angle dependence is observed, due to the negative temperature coefficient effects. The wedge angle range for successful ignition of n-heptane/air mixtures decreases when the wedge length is reduced. From two-dimensional simulations of stationary ODWs, the initiation length generally decreases with the freestream equivalence ratio, but the transition length exhibits weakly non-monotonic dependence. Smooth ODW typically occurs for lean conditions (equivalence ratio < 0.4). The interactions between shock/compression waves and chemical reaction inside the induction zone are also studied with the chemical explosive mode analysis. Moreover, the predictability of the shock-to-detonation transition mode is explored through quantifying the relation between ignition delay and chemical excitation time. It is demonstrated that the ignition delay (the elapsed time of the heat release rate, HRR, reaches the maximum) increases, but the excitation time (the time duration from the instant of 5% maximum HRR to that of the maximum) decreases with the freestream equivalence ratio for the three studied oncoming flow velocities. Smaller excitation time corresponds to stronger pressure waves from the ignition location behind the oblique shock. When the ratio of excitation time to ignition delay is high (e.g., > 0.5 for n-C₇H₁₆, > 0.3 for C₂H₂ and > 0.2 for H₂, based on the existing data compilation in this work), smooth transition is more likely to occur.     

A comparative study of the Ruddlesden-Popper series,Lan+1NinO3n+1 (n = 1, 2 and 3), for solid-oxide fuel-cell cathode applications

A comparative investigation of the much-studied La₂NiO_4+δ (n = 1) phase and the higher-order Ruddlesden-Popper phases, La_n+1Ni_nO_3n+1 (n = 2 and 3), has been undertaken to determine their suitability as cathodes for intermediate-temperature solid-oxide fuel cells. As n is increased, a structural phase transition is observed from tetragonal I4/mmm in the hyperstoichiometric La₂NiO_4.15 (n = 1) to orthorhombic Fmmm in the oxygen-deficient phases, La₃Ni₂O_6.95 (n = 2) and La₄Ni₃O_9.78 (n = 3). High temperature d.c. electrical conductivity measurements reveal a dramatic increase in overall values from n = 1, 2 to 3 with metallic behavior observed for La₄Ni₃O_9.78. Impedance spectroscopy measurements on symmetrical cells with La_0.9Sr_0.10Ga_0.80Mg_0.20O_3−δ (LSGM-9182) as the electrolyte show a systematic improvement in the electrode performance from La₂NiO_4.15 to La₄Ni₃O_9.78 with ∼ 1 Ω cm² observed at 1073 K for the latter. Long-term thermal stability tests show no impurity formation when La₃Ni₂O_6.95 and La₄Ni₃O_9.78 are heated at 1123 K for 2 weeks in air, in contrast to previously reported data for La₂NiO_4.15. The relative thermal expansion coefficients of La₃Ni₂O_6.95 and La₄Ni₃O_9.78 were found to be similar at ∼ 13.2 × 10^− 6 K^− 1 from 348 K to 1173 K in air compared to 13.8 × 10^− 6 K^− 1 for La₂NiO_4.15. Taken together, these observations suggest favourable use for the n = 2 and 3 phases as cathodes in intermediate-temperature solid-oxide fuel cells when compared to the much-studied La₂NiO_4+δ (n = 1) phase.