On-Board Hydrogen Storage and Production: An Application of Ammonia Electrolysis

Interest in hydrogen fuel-cell vehicles (HFCVs) has been increasing in popularity over the past decade. This is primarily a result of the shrinking oil reserves which are expected to last only 42 years as of 1998 [1]. HFCVs have also found a niche in the environmental and political fields because they offer a solution for eliminating the harmful air pollutants generated by internal combustion engines such as nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon dioxide (CO2), and sulfur dioxide (SO2) [2-4]. In addition to being environmentally friendly, HFCVs are quiet and convert 50-60% of the energy available in hydrogen to power the automobile rather than the 20-30% efficiency of today's hydrocarbon-dependent vehicles [5, 6].

Producing hydrogen that is cost competitive with gasoline is proving to be difficult. A study in Italy shows that hydrogen-operated vehicles, which utilize untaxed hydrogen, pay a little more at the pump than untaxed gasoline and diesel consumers. However, the health costs in Milan, Italy are expected to decrease by nearly $2 million per year [7]. Most detrimental to the commercialization and market approval of HFCVs is storing hydrogen. Compared to gasoline, hydrogen has 2.7 times more energy content based on weight. Due to its tremendously low density, hydrogen has 25% less energy content than gasoline when based on volume [8]. Because of this, there is no storage technology currently available that allows a vehicle to travel the average 300-mile range that today's internal combustion engines obtain [9].

Liquid ammonia is a non-carbon containing hydrogen-dense (17.6 wt.%) liquid fuel that can be stored at ambient temperature and pressure [10]. Theoretically, ammonia electrolysis requires 95% less energy than water electrolysis (1.55 W-h g-1 H2 versus 33 W-h g-1 H2). In fact, Botte et al. state that hydrogen produced from the electrolysis of ammonia costs roughly $0.89 per kg of H2 opposed to $7.10 per kg of H2 from water electrolysis. These numbers were based on an ammonia cost of $275 per ton and a solar energy cost of $0.214 per kW-h [11]. This low cost is single-handedly a result of the low energy consumption of ammonia electrolysis. Keep in mind, the U.S. Department of Energy's cost of hydrogen goal for 2015, per kg, is $2-3 [9]. Ammonia electrolysis has other uses besides generating hydrogen for mobile applications [11-13].

Tasks

The focus of this paper is on-board hydrogen production with in situ ammonia electrolysis. Within this context, there are three objectives:

1. Develop an anode and cathode for the AEC. The Electrochemical Engineering Research Laboratory (EERL) at Ohio University, has demonstrated that combinations of Pt and Ir minimized the overpotential of the electro-oxidation of ammonia resulting in a decrease in power consumption during electrolysis compared to other metals such as Ru, Rh, Ni, and combinations thereof [11, 14, 15].

2. Design and construct a static alkaline ammonia electrolytic cell. An AEC, that separates hydrogen from the cathode from the nitrogen generated at the anode, was constructed. Since the cathode only requires OH ions according to Eqn. 2, the AEC compartmentally separates the anode and cathode solutions. This prevents any carry over of ammonia into the PEMFC.

3. Determine the feasibility of using ammonia for on-board vehicular hydrogen storage.

A. Perform AEC and PEMFC integration experiments. Synergistic analysis was performed on the AEC and PEMFC. This was carried out using polarization techniques allowing for energy consumption and generation data to be obtained.

B. Analyze the feasibility of ammonia electrolysis as an on-board hydrogen storage system. An analysis was performed to see if an in situ ammonia electrolysis process would meet the 2010 technical targets set forth by the DOE/FreedomCAR and Fuel Partnership.

Results & Conclusions

An Arbin cycler BT2000 was used to measure electrical activities of the setup. Using ammonia as on-board storage of hydrogen has many benefits aside from the atmospheric pressure and temperature required. Using electrolysis to obtain the hydrogen from ammonia takes less energy than that hydrogen produces in a PEMFC. Essentially, an automobile featuring this can be self sustainable until ammonia is depleted.

References

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