Assessing Pt Demands for Hydrogen Fuel Cell Vehicles

Introduction

Fig. 1: Combined percentage share of hybrid, plug-in hybrid, and all-electric light duty vehicles sold in the U.S. between 1999 - 2001. [4] (Image Source: A. Aleman)

Fig. 2: Combined amount of hybrid, plug-in hybrid, and all- electric light duty vehicles sold in the U.S. between 1999 - 2001. [4] (Image Source: A. Aleman)

Fossil fuels have been employed as a primary source of energy since the first industrial revolution. Their extensive usage and ever-growing demand have resulted in excessive CO₂ emissions into the earth's atmosphere. To decrease dependence on fossil fuels, researchers have further explored reliable and clean energy options.

One promising option is the hydrogen fuel cell. This is a technology that uses oxygen and, in the optimal case, carbon-free hydrogen to directly convert chemical energy into electrical energy and heat. [1,2]

A particularly important type of fuel cell is the proton exchange membrane fuel cell (PEMFC). This consists of a water-based, acidic polymer membrane (typically Nafion) as the electrolyte and precious metal electrocatalysts (notably Pt and Pt-based alloys) on the anode and cathode to perform the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR), respectively. [2,3]

However, while PEMFCs serve as a technology that could decarbonize the transportation sector, the demand for expensive and less abundant Pt electrocatalysts may not be able to compete with the amount of Pt currently required in an internal combustion engine.

Analysis

The combined percentage of hybrid, plug-in hybrid, and all-electric light duty vehicles sold in U.S. has not declined after 2015 and appears to be exponentially increasing after 2019 (Fig. 1). [4] Although it is likely that these vehicles will not all be fuel cell powered, the growth raises the question of whether fuel cell powered vehicles alone could entirely replace the internal combustion engine, even in prinicple. Specifically, is there enough Pt available to supply these fuel cell powered vehicles if they were the sole vehicular alternative to the internal combustion engine?

To address this question, I shall use data from 1999 - 2021 to evaluate how much Pt would have been required to supply all light duty vehicles that do not contain an internal combustion engine. [4] The reasoning is that if Pt demands cannot meet these smaller percentages, then the odds of Pt demands meeting higher percentages are debatable. Additionally, extrapolating future trends may be difficult due to a multiple of outside factors that impact the mining and electric vehicle communities.

On average, in a light duty vehicle that contains an internal combustion engine, the amount of Pt required is 5 g. [5] Regarding fuel cell vehicles, the amount of Pt required varies depending on the brand and model of the vehicle, as each will have different electrocatalyst loadings. The parameters are defined as in Table 1. For evaluation purposes, four commercial fuel cell vehicles were chosen for comparison: Toyota Mirai (Pt_loading = 30 g Pt), Honda Clarity (Pt_loading = 11 g Pt), Hyundai Tucson (Pt_loading = 78 g Pt), and Hyundai Nexo (Pt_loading = 56 g Pt). [6] With the combined number of hybrid, plug-in hybrid, and all-electric light duty vehicles sold in the U.S (N_sold) from Fig. 2, we then have

which is plotted in Fig. 3. The percentage of the Pt demand

is then plotted in Fig. 4. Note that each calculation assumed that all fuel cell vehicles will be one brand. Four brands were chosen merely for a comparison of which brand has potential to solely replace the internal combustion engine.

Fig. 3: Amount of Pt that would have been required each year for the four different fuel cell vehicles described in the text as given by Eq. (1). [6] (Image Source: A. Aleman)

Fig. 4: Percentage of Pt mined worldwide that would have been required to meet the Pt demands of the different fuel cell vehicles, as given by Eq. (2). [7] (Image Source: A. Aleman)

From the comparisons of Figs. 3 and 4, we see that none of the vehicles would have demanded more than the percentage of Pt mined worldwide. However, were demand for such vehicles to increase, vehicles like the Hyundai Tucson and Hyundai Nexo could become problematic, as they already demand up to 60% and 40%, respectively, of the amount of Pt mined worldwide in 2021.

Thus, a vehicle that requires less Pt, such as the Honda Clarity and somewhat the Toyota Mirai, may be the best option Pt demand-wise. However, it is important to note that the amount of Pt in the Honda Clarity is roughly double that required in an internal combustion engine. Additionally, the percentage of Pt mined worldwide that is allocated for the vehicular sector is important to consider when deciding whether such vehicles have room to compete with the internal combustion engine in the future, given the various other applications that require Pt (i.e., electrolyzers, electronics, jewelry, etc.)

Conclusions

Through comparing the amount of Pt that would have been required for four different fuel cell vehicles, it appears that only the Toyota Mirai and Honda Clarity have the potential to compete with the internal combustion engine based on their Pt demands relative to the amount of Pt mined worldwide. However, their Pt demands are still greater than those of the internal combustion engine and it would thus be ideal to reduce the amount of Pt loading (without losing too much vehicular performance and durability) or investigate ways to faster regenerate / recycle Pt. It is important to note that although on average 1.82 × 10⁵ kg of Pt is mined yearly, there are currently more than 100 × 10⁶ kg of world resources of Pt. [7] However, although more Pt could be retrieved to support fuel cell vehicles, increasing Pt mining can negatively impact the environment and communities and should thus be avoided if possible.

© 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.

References

[1] X. Wang et al., "Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design," Chem 5, 486 (2019).

[2] A. Z. Weber, S. Balasubramanian, and P. K. Das, "Proton Exchange Membrane Fuel Cells," in Fuel Cell Engineering, ed. by K. Sundmacher (Academic Press, 2012).

[3] M. C. Williams, "Fuel Cells," in Fuel Cells: Technologies for Fuel Processing, ed. by D. Shekhawa, J. J. Spivey and D. A Berry (Elsevier, 2011).

[4] S. C. Davis and R. G. Boundy, "Transportation Energy Data Book: Edition 40," Oak Ridge National Laboratory, ORNL/TM-2022/2376, June 2022.

[5] T. Nguyen et al., "Platinum Group Metals (PGM) For Light-Duty Vehicles," U.S. Office of Energy Efficiency and Renewable Energy, February 2016.

[6] H.E. Kim, J. Kwon and H. Lee, "Catalytic Approaches Towards Highly Durable Proton Exchange Membrane Fuel Cells With Minimized Pt Use," Chem. Sci. 13, 6782 (2022).

[7] "Mineral Commodity Summaries 1999-2021", U.S. Geological Survey, January 2024.