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Carnot Institute of Burgundy - Study of a ceramic fuel cell with proton conduction by X-ray tomography

by Morgane on 19 Jan 2023 at 11h58

Hydrogen energy is one of the industrial fields that is currently the focus of most research. Tomorrow's mobility may well be based on this mode of propulsion, which presents major technical challenges. A look back at a study conducted by the Carnot Burgungy Interdisciplinary Laboratory & FEMTO-ST Institute, France

Back to the study of a ceramic fuel cell with proton conduction by X-ray tomography

A fuel cell is an electrochemical device that converts the chemical energy of hydrogen (H2) directly into electrical energy, heat and water without the electrolyte and electrodes burning up. Several types of fuel cells exist depending on the operating temperature and on the electrolyte and electrode materials. One of the actual challenges in fuel cell research is to reduce the operating temperature which can reach up to 1000°C. PCFC (Protonic Ceramic Fuel Cell) cells operate at around 500-600°C and are based on proton (H+) diffusion. A hydrogen fuel cell is usually made up of a set of cells, stacked together, to allow sufficient energy production for the desired application.

 
 

Carnot interdisciplinary Laboratory of Burgundy

Physics, chemistry and engineering sciences at the service of innovative technologies and materials for industrial applications
 
The Interdisciplinary Carnot Laboratory of Burgundy (ICB), a CNRS-UBFC Joint Research Unit, has 300 physicists, chemists, engineers and technicians based in Burgundy-Franche-Comté, on the sites of Dijon, Le Creusot, Chalon-sur-Saône and Belfort (Sévenans).
 
They develop new functionalities in optics and for the materials of the future, for applications in industry (photonics, metallurgy, industry 4.0, etc.), medicine, high-speed optical communications, information processing on a nanometric scale, energy and quantum technologies.
 
 
 

A study on the behaviour of batteries during reduction phases

There are many types of hydrogen fuel cells on the market today, but research is also very active, in the midst of the replacement of fossil fuels. The main objectives of these projects are to improve the efficiency and durability of these batteries.
 
The study presented below concerns protonic batteries made of ceramic: PCEC Protonic Ceramic Electrochemical Cells, and their behaviour during the reduction phase. It is part of two thesis topics (Mélanie François, defended in 2021, and Victoire Lescure, in progress) within the Carnot Bourgogne interdisciplinary laboratories, as well as the FEMTO-ST Institute, which has been studying the materials of these fuel cells for several years. The objective is to develop, optimize, and physically and electrochemically characterize the various components of a PCEC. This work has been published in the journal Membranes.
 
X-ray tomography is an imaging technique that allows the characterization of the scanned sample in 3 dimensions. The results obtained are presented in the form of virtual sections. The information obtained is generally represented by different levels of grey reflecting the density of the elements of the material scanned. The denser the element, the lighter the grey level, and vice versa for less dense elements.
 
The manufacturing process of proton cells induces structural variations, which can sometimes cause defects to appear. These cells will also be subjected to redox cycles, which will have an impact on their structure. Tomography allows us to characterize these elements with good resolution, in 3 dimensions, on a representative volume of the material studied.
 
The present document illustrates by a series of measurements some examples of characterization of manufacturing defects and observation of the evolution of the ceramic microstructure during the different phases of manufacturing and operation.
 
 
 

Method used

The samples used are half-cells, consisting of a 2.5 µm Yttrium-doped Barium Zirconate (BZY) electrolyte layer, a 65 µm Functional Anode Layer (AFL) and an anode of about 300 µm, both composed of BZY20 and Nickel Oxide (NiO) with different compactnesses.
 
The first layers (Anode and AFL) are produced by strip casting and then sintered at 1350°C. Then the 2.5µm Electrolyte layer is deposited by sputtering. During this deposition, the anode and AFL are heated to 400°C. The assembly then undergoes an annealing step.

Three samples were taken for analysis at different stages of manufacture and use:

• Sample 1: A raw half-cell;
• Sample 2: Half a cell after annealing at 1000°C for 2 hours;
• Sample 3: One half-cell after reduction with H2 dihydrogen, for 2h.

Each sample was shaped by manual cutting to the dimensions 1x1x0.35 µm3 :
 Battery CT scan
 
 
 
  
EsayTom XL | RX Solutions

X-ray CT at Carnot Institute

An RX Solutions EasyTom XL Ultra micro-tomograph has been used to perform the X-ray CT studies. The acquisitions were implemented with the following components:
 
• A 160kV tube, equipped with a LaB6 cathode and Tungsten target
• A planar sensor, pixel size 127µm, CsI scintillator
 
The acquisition conditions are as follows:
 
• X-ray tube voltage: 100kV
• Target current: 15µA
• Voxel size: 0.35µm
• Acquisition time: 11h
 

Scan 1: Raw sample

The reconstructed sections on this first sample show a defect in the electrolyte (lack of material). By studying the space surrounding this defect, an abnormally large NiO grain is observed. It is then possible to trace the origin of the defect by looking at the physical properties of the materials studied.
 
Thanks to this correlation between the manufacturing process of the cell and the defect encountered, it is possible to improve this process and to reduce the probability of generating this type of defect.
 
 
The lack of material in the electrolyte layer at this level can be explained by looking at the difference in thermal expansion coefficients that exist between BZY, which makes up the electrolyte, and NiO, which makes up the AFL, during the deposition of the layer at around 450°C. This temperature causes a strong expansion of the NiO grain, followed by a strong shrinkage upon cooling, compared to BZY.
 
The mechanical stresses exerted on an already very thin layer of BZY then cause it to break: part of it, at the level of the break, remains attached to the NiO grain, while the rest of the electrolyte is attached to the AFL layer.
 

Scan 2: Annealed sample

The tomographic analysis of the annealed sample showed defects similar to those already observed in the raw sample (related to the growth of NiO grains). On the other hand, new defects were observed with different causes, without being correlated to the annealing stage of the sample. 
 

Scan 3: Sample after reduction

The analysis of the last sample from the first reduction phase of the cell revealed the impact of this reaction on the structure of the material. This change in structure is probably linked to a variation in the crystallographic structure of the NiO grains (NaCl-type structure), which, after reduction, become Ni grains (face-centred cubic, and therefore more compact), and can sometimes induce or accentuate defects such as collapses in the very thin electrolyte layer. 
 
This type of defect comes from:
 
• The lack of material created by the chemical reduction reaction (air layer around the Ni grains);
• The difference in thermal expansion between Nickel and Nickel Oxide: the thermal expansion coefficient of Ni is higher than that of NiO. Combined with the strong adhesion between the electrolyte layer and the AFL layer, this can lead to cracking.
 
 
 
The results obtained make it possible to highlight various defects in the structure of the samples. It also allows the characterisation of the microstructure and its evolution at different stages of the manufacturing process. The major advantage of tomography lies in the possibility of characterising and controlling the samples in three dimensions, and this at fine resolutions. It allows volumetric control of the sample at resolutions that are difficult to achieve with other control methods.

In the case of this study, tomography allowed the identification of several defects in the electrolyte layer, which make it permeable to elements other than hydrogen. This type of defect can affect the efficiency of the cell and even its life span. It has also enabled us to understand their origins.

Tomography has also made it possible to confirm the completion of certain processes specific to the different stages of production or the life of the sample.

This method of analysis, applied here to ceramic samples, can be used directly for other types of proton cells.
 
 
 
Click here to download the full application note here
 

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Since 2007, L'Oréal has been committed to an ambitious policy of sustainable development, with the goal of reducing plastic packaging. In this context, X-ray tomography is a state-of-the-art 3D imaging technology, which allows us to explore new packaging in its entirety and thus ensure the best quality and performance for the consumer. Read more here

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The technical progress expected in the automotive industry in the coming years is important and very challenging, particularly with the development of electric vehicles. Renault Group has made innovation one of the keys to its success with its Guyancourt Technocentre, one of the largest automotive research and development center in Europe, using X-ray CT. Read more here

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