The move to develop zero-emissions vehicles has mostly focused on working with batteries. Extensive research has been ongoing to decarbonize transportation, which accounts for 22% of global carbon dioxide emissions¹ attributed to people. Most of these emissions have been attributed to light duty; vehicles and batteries have been found to be the most efficient zero-emission energy source for this application (to the extent that electricity is supplied from a renewable source) because approximately 77% of renewable electricity is delivered to the wheels.

But there are other transportation applications that cannot readily use batteries to generate power. Scott Barnett, professor of materials science and engineering at Northwestern University in Evanston, Ill., says, “Marine shipping, which accounts for 4% of global carbon dioxide emissions, is not well suited for using batteries because of the need for large battery packs that add excessive weight to a vessel.”

The marine industry has been going through a transition to decarbonization that started with the implementation of the International Maritime Organization’s (IMO) restriction on the content of sulfur in fuel that was set to 0.5% at the beginning of 2020. A recent TLT article² discussed how the IMO 2020 regulation has led to changes in market demand for marine engine oils. The marine industry is now faced with having to reduce average carbon intensity by a minimum of 40% by 2030 and 70% by 2050, compared to 2008. Greenhouse gas emissions need to be reduced by 50% by 2050 compared to 2008.

Regulatory drivers are prompting the marine industry to look for other decarbonization options besides the use of batteries. Barnett indicated that solid oxide fuel cells are a potential energy producing option for ships. He says, “A solid oxide fuel cell utilizes a solid ceramic as the electrolyte. The two electrodes (anode and cathode) are applied as ceramic coatings on the electrolyte to form the solid oxide fuel cell.”

Barnett continues, “Two advantages realized by solid oxide fuel cells are they can use any type of hydrocarbon fuel that is commercially available in contrast to fuel cells, which require high purity hydrogen. The only concern is that sulfur is present in many fuels, which is incompatible with the solid oxide fuel cell, and has to be removed. Fuel passes over the anode and attracts oxygen from the air that has entered via the cathode. This represents a second advantage as the solid electrolyte separates oxygen from the other components in air and is able to transport it successfully through the membrane to react with the hydrocarbon fuel.”

Thus, the reaction between the fuel and oxygen produces mainly carbon dioxide and steam. Barnett says, “Cooling down the by-product mixture leads to a high concentration of carbon dioxide because the steam separates through condensation.”

The energy generation process used by a solid oxide fuel cell is compatible with an electrically powered ship. The key new invention by Barnett and his colleagues avoids the emission of carbon dioxide into the atmosphere by compressing it and storing it in the same tank used for the fuel. Thus, the ship can be termed a carbon capture fuel cell vehicle (CCFCV). 

Feasibility assessment
A CCFCV contains the solid oxide fuel cell in combination with a dual-chamber storage tank as shown in Figure 1. The storage tank contains a chamber for fuel and a chamber to store the carbon dioxide that is generated by the fuel cell. As the solid oxide fuel cell produces electricity, the volume of fuel declines, and the volume of carbon dioxide in the second chamber increases. The partition between the two chambers is movable and will shrink the fuel chamber and expand the carbon dioxide chamber as electricity is produced.


Figure 1. The carbon capture fuel cell vehicle operates with a dual-chamber storage tank that contains a hydrocarbon-based fuel on the left in one compartment and carbon dioxide in a second compartment on the right. As the vehicle operates, the volume of the chamber on the left declines while the volume on the right increases. Figure courtesy of Northwestern University.

Barnett says, “The CCFCV concept has been in development for a long time. The original application was for passenger car vehicles, but we soon realized that the biggest advantage is achieved for larger, long-range transport vehicles such as ships. The key advantage of the CCFCV is its ability to use conventional liquid fuels that are readily available and, yet, capture carbon dioxide to eliminate emissions. In this manner, the CCFCV will enable ships at the very least to achieve carbon dioxide neutrality. The fuel used by a CCFCV is a lot easier to work with than lithium-ion batteries and hydrogen.”

Barnett and his colleagues conducted a feasibility assessment to evaluate the volume and weight of CCFCV versus other power systems. He says, “We used the compressibility of carbon dioxide gas to estimate the optimum tank size for use on a ship. After figuring in power utilization, we showed that a CCFCV is well suited for use on a ship compared to a battery or hydrogen fuel cell. A CCFCV approach also can be used in light-duty passenger cars, but here batteries and hydrogen fuel cells are strong competition because they can meet the lower energy storage requirements. With their high efficiency, batteries may be preferred for use in light-duty passenger cars because the weight of the battery pack is not detrimental.”

Barnett does not rule out commercialization of light-duty passenger car CCFCV vehicles in the future, in part because battery materials supply limitations may make it difficult to convert all vehicles to batteries by 2050.

Solid oxide fuel cell powered vehicles are being introduced commercially at the current time. Barnett says, “The advantage of not having to introduce a lot of new infrastructure, such as hydrogen, is creating interest in solid oxide fuel cell ships. Future work will involve optimizing the performance of CCFCVs by evaluating new electrode materials and optimizing performance with fossil fuels. In particular, natural gas has become the most common fuel for solid oxide fuel cells.”

Additional information can be found in a recent article¹ or by contacting Barnett at s-barnett@northwestern.edu.

REFERENCES
1. Schmauss, T. and Barnett, S. (2021), “Viability of vehicles utilizing on-board CO₂ capture,” ACS Energy Letters, 6 (9), pp. 3180-3184.
2. Mahajan, K. (2021), “COVID-19, IMO 2020 and decarbonization challenges create entry barriers for new entrants,” TLT, 77 (3), pp. 20-22. Available here.