Potential Probing Techniques For Future Energy Supply System-Solid Oxide Fuel Cells (SOFCs)

Currently, a brand new worldwide consensus is gradually forming: hydrogen energy acts as an interactive link among a variety of energy systems and is the core of future low carbon-release energy systems. Unswervingly, developing clean renewable energy gives a long-term impact to optimize the world’s energy consumption infrastructure, which is a persistent propelling force toward energy saving and emission reduction.

As a potential clean energy system, a fuel cell can directly convert the released chemical energy into electrical energy for a different purpose than the utilization of electricity. Unlike the traditional internal combustion generation system (e.g. diesel piston generator, etc.), the released chemical energy is obtained from electrocatalytic reduction-oxidation (redox) reactions.

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Moreover, fuel cells exhibit zero emissions in comparison with the harmful emission of NO_x or SO_x from the combustion of traditional petrol or coal systems. Therefore, it shows potential superiority to the conventional energy technology in both conversion efficiency and environmental benignity.

Nowadays, replacing any dominant energy system is no longer as simple as changing the initial fuel selection or energy generation mechanism, the upgrade of overall infrastructure for transmission and applications must also be taken into account. Driven by these fierce power issues, by no means fortuitously, solid oxide fuel cells (SOFCs) has arisen to be a competitive candidate in the fuel cell family because there is no need for a charging or initiation operation to achieve high energy efficiency (easily levels up to 40~60% or even 70%). They do not necessarily follow traditional Carnot-Cycle Thermodynamic Law and only rely on a fast redox reaction predominantly fed by O_2, an oxidizing agent.

If it combines with a heat transfer system for heat-power cogeneration, the energy conversion efficiency can be further increased to as high as 85%. These features cement the important position of SOFCs in the global energy market. Until 2017, the world market for the SOFCs reached around $403.4 million, with a highly rapid expansion to 1.1 billion global market-shares expected in 2025.

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Besides supplying energy for daily-life consumption, the in-system bio-circulating regenerative SOFCs can indeed serve as the embedded inclusion type renewable electrical generations for application in the aerospace industry. Recently, it has been proposed to apply SOFCs in China’s deep-space exploration aircraft called “Lunar-Palace-One-Project”. SOFCs has been chosen due to their ability to optimally supply enough energy for living needs of astronauts at the extreme environment of the moon and additional generation of high-quality water and heat to support a biological internal recycling system.

Fast Ion Conductors

Electrolytes, as the core component of SOFCs, should ensure fast ion transportation with electronic insulation, which can decrease voltage loss and determine electrical performance. Conventionally, this usually could only be ensured for SOFCs at a high working temperature over 800°C. Therefore, to break through the bottleneck problem of high working temperature and corresponding high cost to achieve wider application and further development of SOFCs, we focus on the core part of the technology —electrolytes.

As an essential feature of crystal structure in any materials, the native defects, especially the thermally-driven formation of anion Frenkel (a-Fr) pairs, have been believed to be the initiation of ion conduction. Our previous work on another electrolyte candidate, La₂Hf₂O_7, has also proved such perspective. However, only with certain amounts of a-Fr pairs formed in a lattice, could ion conduction be probed by equipment. Herein, we report a novel in-situ and non-contact luminescence method as a new reliable way of monitoring the complete process of a-Fr pairs formation and ion conduction with the use of density functional theory (DFT) calculations. This is important for thoroughly understanding the ion conduction mechanism and will further benefit controlling properties of materials to realize high conductivity in low temperature.

In the search for proper electrolyte materials for low-temperature SOFCs, we were inspired by La₂Mo₂O₉ (an amorphous-like oxide compound with rather high O-ion conductivity), which has been found to be a fast ion conductor in 2001. In this work, we give a distinct and complementary perspective on the entangled interaction between thermal-driven formed O a-Fr pair (native solubilizer) and Bi³⁺ dopant (competitive inhibitor) in La₂Mo₂O₉ derivatives.

Anion Frenkel Defects with Dopants

Within DFT calculations, the formation energy cost of a-Fr defects in a different position of the La₂Mo₂O₉ has been computed in comparison with the models of Bi-doped La₂Mo₂O₉. These results suggest that with the dopant adoption in the crystal structure, the formation energy cost of a-Fr defects have significantly increased. On the contrary, the difficulties of Bi adoption in the material will be lowered with the existence of a-Fr defects. In other words, the interaction between a-Fr defects and dopants Bi atoms has exhibited a relationship of “native-solubilizers” and “competitive inhibitors,” respectively.

Consequently, the Bi dopants largely suppress the formation of a-Fr defects in the crystal structure until the doping limits. This results in that as the dopant Bi concentration increase to a certain point, the ion conductivity will stop increasing or even start decreasing because the formation of a-Fr pairs has been largely suppressed. The rare-earth based upconversion (UC) luminescence experiments also proved such ion conductivity behaviors change in dependence on the concentration of Bi dopants. An alternation in the ion conductivity has been observed in low Bi dopant concentration sample at 150 ºC, which is consistent with our calculation of a-Fr formation initiation temperature.

As our research team moves forward in this area, we plan to perform a follow-up study on applying such method to screen out more possible candidate materials for low-temperature SOFCs. In long run, building up systematical calculation parameters for material screening will be very urgent and beneficial for energy materials investigation.

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These findings are described in the article entitled Probing oxide-ion conduction in low-temperature SOFCs, recently published in Nano Energy, a high impact academic journal. This work was conducted by Mingzi Sun and Dr. Bolong Huang from the Hong Kong Polytechnic University, in cooperation with Qian He, Qinyuan Zhang, Shi Ye from the South China University of Technology and Xiaojun Kuang from the Guilin University of Technology.