A new study offers a promising answer to one of hydrogen mobility’s most persistent questions: how can we store and deliver hydrogen efficiently in small vehicles without sacrificing performance?
Study: Metal hydride-based hydrogen storage for fuel cell hybrid electric vehicles: Numerical evaluation under real-world operating conditions. Image Credit: Orange Dragon Studio/Shutterstock.com
In research published in the International Journal of Hydrogen Energy, scientists have demonstrated that pairing metal hydride hydrogen storage with smart thermal management - particularly using phase change materials (PCMs) - can dramatically improve the driving range and reliability of fuel cell hybrid microcars.
Rethinking Hydrogen Storage for Small Vehicles
Hydrogen fuel cells are a clean alternative to conventional engines, producing only water as a byproduct. But for compact vehicles, particularly urban microcars, the challenge lies not in generating power, but in storing hydrogen safely and efficiently within tight space constraints.
Metal hydrides have long been considered an attractive solution. These materials store hydrogen in solid form by chemically binding it within a metal matrix, offering higher volumetric density and improved safety compared to compressed gas tanks. However, there is a catch: the process of absorbing and releasing hydrogen is highly temperature sensitive.
When hydrogen is released (a process known as desorption), the reaction absorbs heat. Without sufficient thermal input, hydrogen flow slows or stops, starving the fuel cell and imposing a strict thermal constraint. The hydride bed must then be maintained within a narrow temperature window to sustain adequate hydrogen desorption rates under dynamic load conditions. Despite this well-known limitation, many previous studies have not fully explored how thermal effects interact with real-world vehicle operation.
This latest research takes a more holistic approach.
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Building a Microcar Around the Problem
The University of Rome Tor Vergata research team developed a detailed simulation of a hydrogen-assisted microcar, combining a battery-electric platform with a proton exchange membrane (PEM) fuel cell as a range extender. At the heart of the system is a metal hydride storage tank, modeled to capture heat and mass transfer in realistic driving conditions.
Rather than examining the storage system in isolation, the study integrates it into a full vehicle model using MATLAB and Simulink tools, allowing the researchers to simulate stop-and-go urban driving, parking intervals, and fluctuating energy demand. These conditions closely mirror real-world use.
The vehicle begins each simulation with a fully charged hydrogen tank and a partially charged battery. Its performance is then tracked until the battery reaches a low state of charge. Vehicle range is explicitly constrained by a minimum battery state-of-charge threshold of 10 %, ensuring that hydrogen contribution is evaluated within realistic hybrid operating limits. Key metrics include driving range, hydrogen utilization, and the frequency of forced shutdowns due to insufficient hydrogen supply.
Why Heat Management Changes Everything
The study evaluates three different thermal management strategies, each with markedly different outcomes.
The simplest approach - natural convection - relies on ambient conditions to dissipate or supply heat. In practice, this proved ineffective. The hydride tank could not maintain the temperature needed for steady hydrogen release, leading to frequent interruptions in fuel cell operation. Under these conditions, hydrogen desorption kinetics become the limiting factor, preventing the system from meeting the fuel cell’s demand profile during transient driving cycles. As a result, the vehicle’s range was barely better than a battery-only equivalent.
A more active strategy involved redirecting warm exhaust air from the fuel cell (around 50 °C) to heat the hydride tank. This forced-convection approach improved performance, increasing the driving range by roughly 40 %. However, it still fell short of providing a consistent hydrogen flow under all conditions. The available waste heat (~50 °C) imposes an upper bound on thermal input, which can be insufficient during peak hydrogen demand or prolonged operation.
The most significant performance improvement came from a passive solution: surrounding the hydride tank with a phase change material.
A Passive Solution with Big Impact
Phase change materials store and release heat as they melt and solidify, acting as a thermal buffer. In this system, the PCM absorbs heat when excess is available and releases it during hydrogen desorption, helping to stabilize the tank’s temperature. This relatively simple addition had an astounding effect.
Under simulated driving conditions, with PCM integration, the microcar achieved a driving range of nearly 120 km using a single hydrogen tank. Expanding to a dual-tank configuration pushed the range close to 180 km - roughly three times that of the battery-only baseline.
Equally important, hydrogen utilization efficiency climbed to nearly 99 % during simulation, and the fuel cell supplied around half of the vehicle’s total energy. The number of fuel cell shutdowns dropped sharply, indicating a steady and reliable hydrogen supply. In practical terms, this means fewer interruptions, better energy balance, and a more usable vehicle for everyday urban travel.
Implications for Urban Mobility
The findings arrive at a time when cities are actively exploring low-emission transport solutions, particularly for short-distance and last-mile mobility. Microcars, delivery vehicles, and lightweight urban transport systems are all candidates for electrification - but battery limitations remain a concern.
By combining compact hydrogen storage with efficient thermal management, this approach could offer a viable alternative. The use of PCMs is especially appealing because it reduces the need for complex active heating systems, simplifying design and lowering energy overhead.
Importantly, this also addresses system-level constraints on energy efficiency, component integration, and control stability in hybrid architectures.
The study also highlights the importance of system-level thinking. Rather than treating hydrogen storage, thermal management, and vehicle control as separate challenges, integrating them into a unified model reveals performance gains that might otherwise be missed.
Looking Ahead
While the results are based on simulations, they provide a compelling case for further experimental validation and real-world testing. Scaling the technology, optimizing materials, and refining control strategies will be the key next steps.
Still, the message is clear: thermal management is a supporting factor, but it is also central to unlocking the potential of metal hydride hydrogen storage in small vehicles.
As cities find ways to clean up their transport, innovations like PCM-assisted hydrogen storage could play a crucial role in bridging the gap between battery limitations and the promise of hydrogen power.
Journal Reference
Bartolucci L., Cennamo E., et al. (2026). Metal hydride-based hydrogen storage for fuel cell hybrid electric vehicles: Numerical evaluation under real-world operating conditions. International Journal of Hydrogen Energy, 231, 154877. DOI: 10.1016/j.ijhydene.2026.154877, https://www.sciencedirect.com/science/article/pii/S0360319926015156