Nickel-Doped Lanthanum Silicate Apatite Boosts Fuel Cell Electrolyte Conductivity

Solid oxide fuel cells promise some of the cleanest electricity generation available today, converting hydrogen and oxygen directly into power with no combustion stage and no greenhouse emissions at the point of use. Yet their commercial breakthrough has been held back by a stubborn problem: the ceramic electrolyte at the heart of the device, typically yttria-stabilized zirconia, only conducts oxide ions efficiently at temperatures approaching 1000 degrees Celsius. That heat demands expensive interconnect materials, accelerates corrosion and degradation of cell components, and shortens operating lifetimes. A new study published in Discover Electrochemistry by researchers at the University of Douala in Cameroon and collaborators reports a compelling way forward, showing that a family of nickel-doped lanthanum silicate oxyapatites can deliver exceptional ionic conductivity at intermediate temperatures of just 500 to 800 degrees Celsius, precisely the range in which next-generation fuel cells are expected to operate.

The material at the center of the work belongs to the apatite family, a class of hexagonal crystals best known from the calcium phosphates that make up tooth enamel and bone. In the lanthanum silicate variant, the structure is built from isolated silicon-oxygen tetrahedra surrounded by large lanthanum cations, and the resulting polyhedral network generates open channels running parallel to the crystallographic c-axis. It is along these channels that oxide ions migrate, a conduction mechanism first recognized in the late 1990s and since then the subject of intense study. Lanthanum silicate apatites offer a high oxygen transference number across a wide range of oxygen partial pressures, meaning nearly all of the current they carry is carried by oxide ions rather than electrons, which is exactly what an electrolyte must do. Their weakness has always been magnitude: compared with doped cerias or gallates, their raw conductivity at intermediate temperatures has lagged behind.

The Cameroonian team, led by Ive R. N. Ntankeu and colleagues, attacked that weakness from two directions simultaneously. First, they chose the oxygen-rich stoichiometry La10Si6O27, which previous work has shown conducts better than the lanthanum-deficient apatites more commonly studied. Second, they substituted nickel onto the silicon site, replacing some of the tetravalent silicon ions with divalent nickel ions. Because each nickel ion carries two fewer positive charges than the silicon it replaces, the crystal must shed oxygen atoms to preserve electrical neutrality, and every oxygen vacancy created is a potential stepping stone for oxide ion migration. The more nickel incorporated, the more vacancies appear in the six-three channels of the apatite structure, and the faster ions can move through them.

The synthesis route itself is a significant part of the story. Conventional solid-state preparation of lanthanum silicate electrolytes requires firing temperatures between 1500 and 1700 degrees Celsius, and even then the products often contain insulating secondary phases that cripple conductivity. The researchers instead used a low-temperature co-precipitation method based on metal succinate precursors. Lanthanum nitrate, nickel nitrate and tetraethyl orthosilicate were dissolved together in deionized water and mixed with sodium succinate, precipitating a single-source metal-organic precursor that contained all three metals in intimate contact. Infrared spectroscopy of the precursors confirmed that the succinate ligands coordinated the metals in a bidentate fashion, with the characteristic carboxylate stretching bands appearing at around 1600 and 1537 per centimeter, and the absence of free carbonyl or hydroxyl bands showing the conversion to succinate was complete.

Thermogravimetric analysis traced the decomposition of these precursors in detail. Water was lost below 200 degrees Celsius, followed by a rapid, combustion-like oxidation of the organic fraction between roughly 260 and 500 degrees Celsius, with total weight losses of about 50 to 52 percent matching the expected formation of the oxyapatite residues. Crucially, no further weight loss occurred above 700 degrees Celsius, which justified the team’s choice of a modest 1000 degree Celsius calcination and sintering temperature, far below the 1500 to 1700 degrees Celsius demanded by solid-state routes. Powder X-ray diffraction confirmed that all four compositions, with nickel contents x equal to 0, 0.2, 0.4 and 0.8, crystallized in the hexagonal apatite structure of space group P63/m, indexed against the standard reference pattern for La10Si6O27.

The diffraction data also revealed the structural fingerprint of successful doping. Because the nickel ion, with a radius of 0.55 angstroms, is considerably larger than the silicon ion it replaces at 0.26 angstroms, the unit cell expands as nickel content rises, and the diffraction peaks shift systematically toward lower angles and larger d-spacings. This lattice expansion is direct evidence that nickel ions genuinely entered the silicon sites of the tetrahedral framework rather than forming separate nickel oxide phases. Energy dispersive X-ray spectroscopy on the sintered pellets independently confirmed the presence of lanthanum, silicon, oxygen and, in the doped samples, nickel, with the nickel signal growing in proportion to the intended doping level and no impurity elements detected.

Scanning electron microscopy showed short rod-shaped particles with hexagonal bases, consistent with the crystallography, that coalesced into dense, well-connected microstructures with grain sizes of approximately two micrometers and few pores. That density matters enormously for an electrolyte, since pores would otherwise provide shortcuts for gas crossover rather than controlled ion migration. One caveat emerged for the intermediate composition: a minor secondary phase of lanthanum pyrosilicate, La2Si2O7, was detected by X-ray diffraction, most prominently in the x equal to 0.4 sample. The researchers attribute this to the different solubilities and precipitation mechanisms of the lanthanum and silicon precursors, and note that this non-conducting phase measurably depressed the conductivity of that particular composition.

The electrochemical measurements, performed by complex impedance spectroscopy on sintered pellets between 450 and 750 degrees Celsius, delivered the headline result. Fitting the Nyquist plots with equivalent circuits of resistors and constant phase elements allowed the team to separate grain interior, grain boundary and electrode contributions to the total resistance. Conductivity rose steadily with both temperature and nickel content, exactly as the vacancy-mediated conduction model predicts. The undoped La10Si6O27 showed an activation energy of 0.23 electron volts, while the most heavily doped composition, La10Si5.2Ni0.8O27-delta, reached a conductivity of 2.07 times ten to the minus two siemens per centimeter at 750 degrees Celsius with an activation energy of just 0.15 electron volts. That value exceeds most figures reported in the literature, including 2.04 times ten to the minus two siemens per centimeter for an aluminum-iron co-doped apatite at 700 degrees Celsius and, by more than an order of magnitude, the 1.21 times ten to the minus three siemens per centimeter achieved previously for nickel-doped lanthanum-deficient apatites.

The Arrhenius behavior of all samples, with straight lines when the logarithm of conductivity is plotted against inverse temperature, confirms that oxide ion transport in these materials is a thermally activated hopping process, and the systematic fall in activation energy with doping shows that nickel substitution does not merely add vacancies but genuinely lowers the energy barrier for ion migration through the apatite channels. For the field of intermediate-temperature solid oxide fuel cells, the implications are considerable. An electrolyte that conducts this well at 750 degrees Celsius relaxes the thermal and mechanical demands on every other component of the cell, from the nickel-based anode to the ferritic steel interconnects, opening the door to cheaper, longer-lived devices. The work also demonstrates that solution-based precursor chemistry can produce phase-pure, dense apatite ceramics at temperatures low enough to be industrially attractive, sidestepping the secondary-phase problems that have long plagued high-temperature synthesis routes.

Challenges remain before these oxyapatites reach a working stack. The secondary pyrosilicate phase must be suppressed, likely through tighter control of the precipitation chemistry, and the long-term chemical stability of nickel in a reducing fuel atmosphere will need verification, since nickel is also a well-known electrocatalyst that could behave differently when exposed to hydrogen at the anode side. Nevertheless, the combination of a low-temperature synthesis route, a deliberately chosen oxygen-rich parent composition and a dopant strategy that exploits both ionic radius and charge mismatch has produced one of the most conductive lanthanum silicate apatites yet reported. As the global push for clean, efficient and affordable energy conversion intensifies, this modest ceramic powder from Douala may prove to be a meaningful step toward fuel cells that work hard without running hot.

Subject of Research: Nickel-doped lanthanum silicate oxyapatite electrolytes for intermediate-temperature solid oxide fuel cells

Article Title: Synthesis, structural and electrochemical properties of oxyapatites La10Si6−xNixO27−δ as electrolyte material for solid oxide fuel cells

Article References: Ntankeu, I. R. N., Mbah, H. J. N., Mabou, J. B. L., Nforna, E. A., & Tchieta, P. G. (2026). Synthesis, structural and electrochemical properties of oxyapatites La10Si6−xNixO27−δ as electrolyte material for solid oxide fuel cells. Discover Electrochemistry, 3(1), Article 39. https://doi.org/10.1007/s44373-026-00130-5

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00130-5

Keywords: solid oxide fuel cells, oxyapatite, lanthanum silicate, nickel doping, oxide ion conductivity, co-precipitation synthesis, electrolyte materials, impedance spectroscopy, oxygen vacancies, intermediate temperature, hexagonal crystal structure, clean energy