For direct utilization of hydrocarbon fuels, a number of alternative anodes have been developed...However, the application of these anodes to actual fuel cell systems is hindered by several critical issues: they are either too expensive to be economically viable (for example, using a noble metal such as Ru or Pd) or outright incompatible with the current YSZ-based SOFCs systems, which have evolved progressively in the past few decades, because of the limited physical, chemical and thermal compatibility of the alternative anodes with YSZ electrolyte during fabrication at high temperatures.

...Although SOFCs powered by H₂-rich syngas derived from steam gasification of carbon may be operated at lower temperatures, the required excess of water not only dilutes the fuel but also increases system complexity and cost because of the need for water management. To date, no stable and desirable power output has been demonstrated by SOFCs with Ni-YSZ-based anodes in CO or gasified carbon through CO₂ gasification below 850 °C. The search for alternative anodes for direct utilization of CO at intermediate temperatures has progressed slowly and the existing SOFCs powered by CO and gasified carbon or coal are still inadequate for practical applications.

—Yang et al.

To counter this problem, the researchers used a vapor deposition process to apply barium oxide nanoparticles (NiO-BaO) to the nickel-YSZ electrode. A continuous BaO film would block the electron path in the anode; the particles, which range in size from 10 to 100 nanometers, form islands on the nickel that do not block the flow of electrons across the electrode surface.

When water vapor introduced into the coal gas stream contacts the barium oxide, it is adsorbed and dissociates into protons and hydroxide (OH) ions. The hydroxide ions move to the nickel surface, where they combine with the carbon atoms being deposited there, forming the intermediate COH. The COH then dissociates into carbon monoxide and hydrogen, which are oxidized to power the fuel cell, ultimately producing carbon dioxide and water. About half of the carbon dioxide is then recirculated back to gasify the coal to coal gas to continue the process.

Unlike alternative anode materials under investigation, the elemental composition of the new anode is very simple and contains no rare earth elements, which helps work towards true cost effectiveness, the researchers said. Moreover, the BaO treatment can be readily incorporated into existing processes for fabrication of advanced SOFCs based on YSZ electrolyte; it does not introduce additional processing steps—vapour deposition is implemented during the firing of the buffer layer—and there are no known compatibility issues that often arise with the use of other alternative anode materials occuring with the BaO, the team said.

This could ultimately be the cleanest, most efficient and cost-effective way of converting coal into electricity. And by providing an exhaust stream of pure carbon dioxide, this technique could also facilitate carbon sequestration without the separation and purification steps now required for conventional coal-burning power plants.

—Meilin Liu, a Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology

The researchers also evaluated the use of propane to power solid oxide fuel cells using the new anode system. Because oxidation of the hydrogen in the propane produces water, no additional water vapor had to be added, and the system operated successfully for a period of time similar to the coal gas system.

Solid oxide fuel cells operate most efficiently at temperatures above 850 °C, and much less carbon is deposited at higher temperatures. However, those operating temperatures require fabrication from special materials that are expensive, and prevent solid oxide fuel cells from being cost-effective for many applications. However, reducing the operating temperature meant worsening the coking problem.

Fuel cells powered by coal gas still produce carbon dioxide, but in a much purer form than the stack gases leaving traditional coal-fired power plants. That would make capturing the carbon dioxide for sequestration less expensive by eliminating large-scale separation and purification steps, Liu noted.

The researchers have so far tested their process for 100 hours, and saw no evidence of carbon build-up. A major challenge ahead is to test the long-term durability of the system for fuel cells that are designed to operate for as long as five years. Researchers must also study the potential impact of possible fuel contaminants on the new electrode.

Forming the barium oxide structures can be done as part of conventional anode fabrication processes, and would not require additional steps. The anodes produced in the technique are compatible with standard solid oxide fuel cell systems that are already being developed for commercial electricity generation, home power generation and automotive applications.

We have started with state-of-the-art technology, and simply modified the surface of the electrode. Because our electrode would be built on existing technology, there is a lower barrier for implementing it in conventional fuel cell systems.

—Mingfei Liu, postdoc in the Center for Innovative Fuel Cell and Battery Technologies

The research was supported by the US Department of Energy’s Office of Basic Energy Sciences, through the HeteroFoaM Center, an Energy Frontier Research Center. The work also involved researchers from Brookhaven National Laboratory, the New Jersey Institute of Technology and Oak Ridge National Laboratory.

Resources

• Lei Yang, YongMan Choi, Wentao Qin, Haiyan Chen, Kevin Blinn, Mingfei Liu, Ping Liu, Jianming Bai, Trevor A. Tyson & Meilin Liu (2011) Promotion of water-mediated carbon removal by nanostructured barium oxide/nickel interfaces in solid oxide fuel cells. Nature Communications 2, 357 doi: 10.1038/ncomms1359