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| Fig. 1: Toyota Mirai hydrogen fuel cell vehicle. (source: Wikimedia Commons) |
Electrolyzers are a promising sustainable energy technology to generate renewable hydrogen at large scale. [1] The renewable hydrogen produced can then be used to power fuel cell vehicles (Fig. 1), helping accelerate the transition towards increased usage of greener vehicles. However, storing the hydrogen generated from electrolyzers is challenging due to hydrogen's low volumetric energy density. [2] A potential route to storing such hydrogen includes developing and utilizing metals that can intercalate hydrogen, where hydrogen is inserted into the lattice of the metal. [3]
Currently, a few metals (either alone or in combination with other species) have been observed to intercalate hydrogen under specific operating conditions relating to pressure, temperature, and applied potential. [4] For the purposes of this analysis, I will focus here primarily on the amount of hydrogen that can be adsorbed per kilogram of metal. Thus, pressure, temperature, and applied potential will not be included in the cost analysis. In particular, I will calculate the total amount and cost of metal that would be required to fuel a 2021 Toyota Mirai. The total cost for each metal will then be compared to recent costs of refueling the hydrogen tanks in the fuel cell vehicle. In a more extensive cost analysis, pressure and temperature costs should be considered.
Pd, Mg, FeTi, and LaNi5 are chosen for comparison because of their varying costs and hydrogen capacities. [4] The drivetrain of the 2021 Toyota Mirai contains 3 hydrogen tanks that each contain 5.6 kg of hydrogen, resulting in 16.8 kg of hydrogen stored in total within the drivetrain. [5] In Table 1, the total amount of metal required (represented by Mmetal required) to store 16. 8 kg of hydrogen (represented by Mhydrogen) in the vehicle was calculated using Eq. (1) as follows:
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(1) |
where the weight percent (wt.%) of hydrogen intercalated by each metal was considered. [4] All calculated values and symbol definitions are represented in Tables 1 and 3, respectively.
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| Table 1: Metal, its respective hydride, and reported weight percent of hydrogen. [4] Mmetal required values were calculated with Eq. (1). |
We see that Mg is able to store the most hydrogen and consequently requires the least amount of metal to store the 16.8 kg of hydrogen required for the 2021 Toyota Mirai. However, it is important to also consider the individual costs of the metals required to fuel the vehicle.
The price in dollars of each metal per kilogram (represented by Pmetal) obtained from the 2023 USGS Minerals Commodity Summaries is shown in Table 2. [6] Given that FeTi is a mixture of two metals, the prices of Fe and Ti were both considered. I take the total cost of each metal required to be
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(2) |
where the price of each metal per kilogram was multiplied by total amount of metal required to store 16.8 kg of hydrogen to fuel the vehicle (represented by Mmetal required. It is assumed that the FeTi material is a 50-50 split between the metals. This results in Mmetal required for FeTi from Table 1 being divided by 2 and then multiplied by Pmetal for Fe and Ti separately. Likewise, for LaNi5, it is assumed that the material is a 1:5 split between the metals. This results in Mmetal required of LaNi5 from Table 1 being multiplied by (1/6) and (5/6) to yield the total mass of La and Ni in the material, respectively. Then, the individual masses are multiplied by Pmetal for La and Ni separately. All symbols in Table 2 are defined in Table 3.
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| Table 2: Metal, its respective price per kg in 2023, and the total metal price to store 16.8 kg of hydrogen, calculated with Eq. (2). [6] |
This analysis finds that Mg is the optimal choice for hydrogen storage when compared to the other metals, both in hydrogen capacity and price.
However, there are other important factors to consider, such as the long-term stability of Mg and its hydride and the operating conditions, including notably high temperature (573°K). [4] Interestingly, Pd hydride is the only one that is sustained at low pressures (0.02 bar) and room temperature (298°K). However, to employ Pd hydride at large scale, Pd prices must drop significantly. [3,4] Addiontally, LaNi5 hydride is sustained at room temperature (298°K) and under relatively low pressures (2 bar) when compared to a gas cylinder. [4] However, as in the case of Pd, it would be ideal to find ways to decrease the overall material price. This may be possible through decreased Ni loading and increased La loading, although this depends on the modified material's stability and hydrogen capacity.
When comparing metal hydrides with hydrogen refueling prices, the 2023 Annual Assessment of the Hydrogen Refueling Network in California reported that in August 2023, the price of hydrogen was $36/kg, a record-high. [7] Given this price, to refuel a 2021 Toyota Mirai that stores 16.8 kg of hydrogen would cost $604.8. However, if metal hydrides were used, it is possible that the fuel tank would be smaller, while also potentially increasing the range of the car on one tank of fuel. Additionally, the hydrogen tanks in the 2021 Toyota Mirai have a nominal working pressure of 70 mPa (or 700 bar), internal volumes ranging from 25.3 - 64.9 L, and weights ranging from 22.5 - 43.0 kg (ranges are provided as the three tanks are different sizes, all under the same pressure). [5] Given such large vessels that contain hydrogen under high pressure, metal hydrides could be a better alternative for hydrogen storage safety-wise, as many hydrides (for instance, Pd, Mg, and LaNi5) can store hydrogen at significantly lower pressures. Altogether, if metal hydrides are stable enough to endure prolonged vehicular cycling, then the fuel tank size in the vehicle may be decreased, while also mitigating the potential safety hazards that may arise from the highly pressurized tanks in the vehicle.
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| Table 3: Parameters for calculations performed in this work. |
© Ashton Aleman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. Smolinka, E. T. Ojong, and J. Garche, "Hydrogen Production From Renewable Energies - Electrolyzer Technologies," in Electrochemical Energy Storage for Renewable Sources and Grid Balancing, ed. by P. T. Moseley and J. Garche (Elsevier, 2014).
[2] A. Züttel, "Hydrogen Storage Methods," Naturwiss. 91, 157 (2004).
[3] A. T. Landers et al., "Dynamics and Hysteresis of Hydrogen Intercalation and Deintercalation in Palladium Electrodes: A Multimodal in situ X-Ray Diffraction, Coulometry, and Computational Study," Chem. Mater. 33, 5872 (2021).
[4] B. D. Adams and A. Chen, "The Role of Palladium in a Hydrogen Economy," Mater. Today 14, No. 6, 282 (June 2011).
[5] "Toyota Technical Review," Vol. 66, Toyota Motor Corporation, March 2021.
[6] "Mineral Commodity Summaries 2023," U.S. Geological Survey, January 2024.
[7] "Joint Agency Staff Report on Assembly Bill 8: 2023 Annual Assessment of the Hydrogen Refueling Network in California," California Energy Commission, CEC-600-2023-069, December 2023.
