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| Fig. 1: Schematic of a proton exchange membrane fuel cell (PEMFC), showing hydrogen oxidation at the anode, proton transport through the polymer electrolyte, electron flow through an external circuit, and oxygen reduction at the cathode to form water and heat. (Source: Wikimedia Commons) |
Hydrogen fuel cell cars generate electricity through electrochemical reactions between hydrogen and oxygen - which powers an electric motor that drives the wheels without any combustion process. They offer a driving experience that is similar to battery electrical vehicles, including components like smooth acceleration and regenerative braking, but use compressed hydrogen tanks that refuel in just minutes (refueling time for a 650 km trip takes ~280s) rather than requiring hours to charge a large battery. [1] This makes these vehicles particularly functionally appealing for drivers.
Most fuel cell cars utilize a proton exchange membrane fuel cell (PEMFC) stack which consists of hundreds of individual fuel cells that are layered together for sufficient power output (typically around 100 kW for passenger vehicles like sedans or SUVs). [2] Hydrogen gas from high- pressure onboard tanks is supplied to the anode side of each cell, where a thin platinum-based catalyst layer triggers an oxidation reaction where each H2 molecule dissociates into two protons (H+ ions) and two electrons (2e-). [3]
At the core of the PEMFC lies a solid polymer electrolyte membrane which is often made from selectively permeable materials like Nafion. [4] This selectively permeable membrane layer allows for the conduction of only protons across to the cathode side under an electrochemical gradient while blocking electrons and reactant gases to prevent short-circuiting. This gradient separation then forces electrons to flow externally through a circuit, generating direct-current (DC) electricity to power the vehicles traction motor, air-conditioning, lights, and other systems. Meanwhile, at the cathode component, the protons combine with electrons and oxygen (O2) from ambient air in the following reduction reaction, which produces liquid water/vapor as the sole byproduct: O2 + 2H2+ + 2e- → 2 H2O. The schematic of a functioning PEMFC can be seen in Fig. 1.
The entire stack operates at moderate temperatures of 60-85°C with humidified gases to keep the membrane hydrated and conductive, allowing for relatively quick engine startup even in cooler weather. [4]
As there is no combustion or high exhaust temperatures, the tailpipe emissions are simply water vapor and warm air - which eliminates local pollutants like nitrogen oxides (NOx), carbon monoxide (CO), or particulates that can burden internal combustion engines. [5]
Vehicular PEMFC systems typically convert 50-60% of hydrogen's chemical energy into usable electricity during real-world driving, which is significantly more efficient than gasoline engines that usually convert only 20-25% of fuel energy into motion. [6,7]
Well-to-Wheel (WTW) analyses evaluate the full energy chain and environmental impact of vehicle fuels, from raw resource extraction (well") through production, distribution, and delivery to the vehicle itself (tank"), and finally vehicle operation (wheels). In a WTW analysis conducted between a hydrogen fuel cell electric vehicle (HFCEV, Toyota Mirai) and a conventional internal combustion engine vehicle (ICEV, Mazda 3), it was confirmed that the HFCEV used 5% - 33% less WTW fossil energy and has 15% - 45% lower WTW greenhouse gas emissions compared to the gasoline ICEV. [8]
While HFCEVs and BEVs both deliver zero tailpipe emissions, they differ in their energy efficiency, refueling, range, and suitability. A recent 2025 study conducted by Togun et al., found that BEVs are significantly more energy-efficient, converting 70-90% of stored electricity into motion. [9] However, despite this efficiency gap, the researchers determined that HFCEVs still remain a critical solution for long-distance and heavy-duty applications (such as trucks and buses) as they provide driving ranges that are comparable to gasoline vehicles (300-400 miles) and refuel in just 3-5 minutes. [9]
Although HFCEVs offer appealing advantages, their practicality for widespread use is limited by several key factors. The PEMFC stacks currently cost around $75 per kilowatt in the US, which exceeds the necessary cost ($35 per kW) for FCEVs to be competitive. [10] Durability is also a major factor, as fuel cells last about 4,000 operational hours which is below the 8,000 hour goal that is set for light-duty vehicles. [11] Moreover, hydrogen refueling infrastructure is sparse, with roughly 70 stations across America (most of them concentrated in California) - which restricts convenience for consumers and vehicle deployment outside those areas. [12] Until there are reductions in system costs, improvements in longevity, and expansion of refueling stations, HFCEVs will remain products that are niche and primarily suited for highly specific applications rather than broad mass-market adoption. These constraints continue to be a primary focus on ongoing research and policy efforts that are aimed at enabling HFCEVs to reach their full potential in everyday transportation.
© Nipun Gorantia. 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.
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