High Temperature Fuel Cell and Steam Electrolyzer Research and Development

Solid Oxide Fuel Cells

In an SOFC, oxygen from air is reduced to ions at the cathode, which diffuse through the electrolyte and oxidize the fuel (hydrogen) at the anode to produce water while liberating electrons into the external electric circuit to generate electricity.

Fuel cells based on ceramic electrolytes are called Solid Oxide Fuel Cells (SOFC). These high temperature (600-1000°C) cells convert a broad spectrum of fuels such as: hydrogen, carbon monoxide, methane, and other gaseous hydrocarbons into electricity through an electrochemical process. To increase voltage output, cells are “stacked” together in electrical series using high-temperature chromium alloy bipolar plates to connect the anodes and cathodes of adjoining cells together.

Solid oxide fuel cell systems based on clean fuels derived from coal have the potential to provide a stable, environmentally-friendly source of electricity. Currently, cathode electrochemical activity and stability remain two of the largest opportunities for improving fuel cell power density (thereby reducing cost) and lifetime.

Argonne National Laboratory has a long history in SOFC development. As part of the Solid State Energy Conversion Alliance (SECA) Core Technology Program, Argonne has developed novel SOFC cathodes, gas seals, and cell interconnection materials (bipolar plate), in addition to solving  issues of chromium poisoning of the cathode arising from the chromium in the bipolar plate.

Currently, Argonne is teamed with other national laboratories and universities to investigate the interactions between molecular oxygen and the cathode (see figure). A complex series of reactions occurs at this interface to allow the transport of the oxide ions through the electrolyte to the anode, where they react with the fuel to form water (and carbon dioxide in the case of a hydrocarbon or other carbonaceous fuel).  Efficiency losses of SOFCs are typically concentrated at the cathode-oxygen interface.  Argonne is working towards a fundamental understanding of these reactions in order to guide the development of efficient, clean, and cost-effective SOFC systems for energy production.

High Temperature Steam Electrolysis

Hydrogen for use in automotive fuel cells or other portable applications can be made from steam using high temperature steam electrolysis (HTSE), which process takes advantage of SOFC technology to split steam into hydrogen and oxygen at elevated temperatures. This process can use the heat from high temperature industrial process, solar furnace, or nuclear reactor to lower the amount of electrical energy needed to split water, which gives it an advantage over conventional water electrolysis. Also, unlike the current state-of-the-art commercial method for making hydrogen, steam methane reforming, hydrogen production by HTSE does not release any greenhouse gases.

As part of the Nuclear Hydrogen Initiative, Argonne has conducted studies of the causes of HTSE component degradation over extended operating time periods. We have developed methods for determining areas where degradation has occurred using X-ray fluorescence mapping, and X-ray Absorption Near Edge Structure (XANES) spectroscopic techniques at the Advanced Photon Source. The results obtained at the APS were complemented by analysis by electron microscopy, energy dispersive spectroscopy (see figure below).

Images of a used solid oxide electrolysis cell (a) set up at the APS; (b) X-ray fluorescence map of Ni Ka with labels showing where XANES measurements were made.  These maps identify regions of degradation that are further analyzed by electron microscopy. The XANES analysis identified the oxidation state of the nickel and the amount of nickel oxide in the electrode.

Scientist Jennifer Mawdsley sets up a “half-cell” measurement to study the effects of the steam-to-hydrogen ratio on the performance of a cathode material in the steam electrolysis cell.

To improve performance and durability of electrolysis cells and stacks, group scientists also conduct anode and cathode development work. Initially, simple “half-cell” measurements are set up to test individual anode or cathode materials in a single atmosphere environment. Cells are made with the candidate electrode applied to each side of a thick electrolyte substrate. The material is then tested in a single atmosphere (air or steam-hydrogen) simultaneously as a cathode and anode in electrolysis and fuel cell mode.

Successful candidate anode and cathode material combinations are then applied to an electrolyte substrate and tested as “full cells” where air and steam-hydrogen mixtures flow to either side of the cell. Cell and electrode polarization measurements and long-term tests are performed to determine cell performance and lifetime assessments.

Funding

This work is funded by the Fuel Cell Technologies Program of the DOE Office of Energy Efficiency and Renewable Energy.

December 2009