Solid oxide fuel cells: A key technology for a clean energy future?

Solid oxide fuel cells (SOFCs) – a highly efficient, stable and flexible technology that generates electricity from eco-friendly fuel sources via an electrochemical reaction – have huge potential for success in the field of clean energy generation. Despite this, the widespread use of SOFCs has not been realized due to the relative expense of materials, cell fabrication and maintenance. Scientists from Russia have now developed an improved procedure for the fabrication of metal-supported SOFCs.

By A. A. Solovyev

Sold oxide fuel cells (SOFCs) generate an electric current by using a solid oxide electrolyte and electrodes made from ceramic or cermet (a mix of ceramic and metallic material) powders to create a flow of electrons. This flow occurs via an oxidation reaction which transfers negatively charged oxygen ions through the electrolyte, pushing electrons out into an object – for example, a lightbulb. The current powers the lightbulb and is carried back into the fuel cell to begin the circuit again.

The performance of SOFCs is closely related to the methods by which the electrolyte and electrodes are produced. One of the main goals in current SOFC research is to optimize the process of synthesizing fuel cell materials. The conventional method involves applying high temperatures to the ceramic or cermet powders to form them into a solid mass. This takes a long time, which worsens the microstructure and electrochemical properties of the fuel cell.

This problem is most clearly demonstrated when trying to manufacture anodes (positively charged electrodes through which electrons leave the fuel cell) of nickel oxide-yttria stabilized zirconia (NiO/YSZ) layers for metal-supported solid oxide fuel cells (i.e. those containing a metallic element, as opposed to being purely ceramic). The most efficient NiO/YSZ cermet anodes are usually manufactured via a process called sintering. This is a method of compacting small particles into a larger solid mass by applying extreme heat (1400°C!). The key characteristic of this process is that the materials being heated join without melting or being liquefied – a more common example of heat-driven sintering is when ice cubes in a glass of water meld together, and an example of pressure-driven sintering is the creation of a hard snowball from lots of individual flakes when they are pressed together.

In metal-supported solid oxide fuel cells, anode sintering in air is undesirable because it leads to oxidation (the formation of a non-conductive oxide layer) of the metal support. But if the sintering process is carried out in a vacuum or inert atmosphere with the goal of preventing this, the opposite happens – nickel oxide reduction (loss of oxygen). The result is a loss of structural uniformity and, consequently, a decrease in the electrochemical characteristics of the anode.

As described in a recent research article published in High Temperature Materials and Processes, this problem can be solved using spark plasma sintering, a technique which rapidly consolidates the ceramic or cermet powder by pulsing it with a direct electric current. This method works quickly (within 5-10 minutes) by directing heat precisely where it is needed – at the microscopic contact points between the powder particles. This results in favorable sintering behavior (less powder decomposition and nickel granule formation) and layers with the desired porosity. Spark plasma sintering is also fast enough to prevent nickel oxide reduction. After spark plasma sintering, a NiO/YSZ anode has a uniform distribution of nickel oxide and yttria stabilized zirconia granules along its cross-section and the formation of large nickel granules does not occur, unlike when using conventional sintering methods.

According to the scientists behind the study, “SOFCs offer high electrical efficiency and reliability, which could be made more accessible via this innovation in manufacturing”.

Read the original article here:

A. S. Ivashutenko, I. V. Ionov, A. S. Maznoy, A. A. Sivkov, A. A. Solovyev: Comparative Evaluation of Spark Plasma and Conventional Sintering of NiO/YSZ Layers for Metal-Supported Solid Oxide Fuel Cells, 05.05.2017.