NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Position Paper: Hydrogen 

Spillover Limitations for 

Onboard Hydrogen Storage

Thomas Gennett

National Renewable Energy Laboratory

Prepared for the U.S. Department of Energy

January 2015

Milestone Report

NREL/TP-5900-71113

January 2015
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

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303-275-3000 • www.nrel.gov

Position Paper: Hydrogen 

Spillover Limitations for 

Onboard Hydrogen Storage

Thomas Gennett

National Renewable Energy Laboratory

Prepared for the U.S. Department of Energy

January 2015

Suggested Citation

Gennett, Thomas. 2015. Position Paper: Hydrogen Spillover Limitations 

for Onboard Hydrogen Storage. Golden, CO: National Renewable Energy 

Laboratory. NREL/TP-5900-71113.

Milestone Report

NREL/TP-5900-71113

January 2015
NOTICE

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable 

Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding 

provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Fuel Cell 

Technologies Office. The views expressed herein do not necessarily represent the views of the DOE or the U.S. 

Government.

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Energy Laboratory (NREL) at www.nrel.gov/publications.

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Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795.

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1 

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Hydrogen Spillover Limitations for Onboard Hydrogen 

Storage 

The U.S. Department of Energy Office of Energy Efficiency and Renewable Energy (DOE

EERE) Hydrogen Storage sponsored research projects are focused on the development of 

hydrogen storage systems that meet or exceed the DOE’s goals for the onboard hydrogen storage 

in hydrogen-powered vehicles. Recently there has been a tremendous interest and controversy 

concerned with the disparity of results associated with weak chemisorption, i.e., spillover, 

materials for hydrogen storage. The National Renewable Energy Laboratory (NREL) and DOE 

dedicated considerable resources over the last few years to develop/perform the requisite 

measurements in order to establish the validity, capacity, kinetics, and overall performance of 

these materials.

‘Hydrogen spillover’ is a well-known phenomenon in catalysis, involving the dissociative 

chemisorption of hydrogen molecules on metal nanoparticles, followed by the migration of 

atomic hydrogen onto a support. In hydrogen storage applications, this storage enhancement 

mechanism has been investigated in detail most notably via the noble metal decoration of high 

surface area (>2,500 m2

/g) carbon-based supports.1-8 Overall, the observations of hydrogen 

storage that can be attributed to a spillover-type effect vary widely, from a several-fold

enhancement to a negligible effect within the experimental uncertainty.

9

 In part, this wide 

variation may be caused by micro-structural changes resulting from the metal doping, such as 

reduced pore volume and specific surface area. A significant result of the extensive DOE-NREL 

efforts was the first reported spectroscopic (DRIFTS and Neutron Scattering) evidence of 

“spiltover” hydrogen atoms onto a carbon surface. Disappointingly, however, it was only 

observed with modest (<15%) enhancements of sorption resulting in total capacities considerably 

less than the 2020 DOE targets. While larger enhancements up to 50% were observed for low 

hydrogen capacity materials, this was found to be primarily caused by the reduction of the metal 

centers and the formation of water. After multiple hydrogen sorption cycles and once the metal 

was reduced, the observed enhancement was considerably less (<15%). Overall, the spillover 

mechanism onto carbon-based sorbents is not yet fully understood and still under considerable 

debate9-15 due to the absence of consistent empirical results and a fundamental microscopic 

picture. 

It is important to note however, that as interest in the hydrogen storage spillover process peaked 

in the early part of this decade, a considerable amount of new information has come to light 

regarding the intrinsic problems that could complicate the interpretation of empirical results, 

beyond systematic instrumentation errors. The following list provides examples of the items 

most often reported: (1) hydrogenation reactions can occur if the metal nanoparticles are in a 

fully or partially oxidized state;16 (2) residual organics may occlude the noble metal deposition 

precursor from accessing deep into the pores, and these precursors can undergo catalytic 

reduction reactions (which would appear as an erroneous hydrogen sorption) for multiple 

hydrogen sorption/desorption cycles;17 (3) a variation of H2 interaction properties at unsaturated 

atomic sites inside carbon support could lead to irreversible side reactions such as the 

hydrogenation of the carbon (C-H bonds) and oxygen (O-H) sites being the most obvious;10, 12

(4) the variation of substituents that populate the pore structure in very high surface area carbon
2 

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materials can lead to multiple side reactions; (5) theory which predicts the existence of 

thermodynamic energy valleys resulting in water formation (which also would appear as an 

erroneous hydrogen sorption);18-20 and most recently (6) results indicating that catalytic activity 

of the metal certainly can enhance adsorption kinetics, but with significant irreversible sorption 

hysteresis attributed to irreversible chemical hydrogenation of the substrate.21-22

Therefore, even though there are validated results in the literature that the hydrogen spillover 

process may result in an observed 10% to 50% increase in hydrogen sorption capacities, it has 

only been verified for well-defined, low surface area materials with inherently low (<0.5 wt.% 

H2) gravimetric capacity—and even in these cases after 2–3 cycles the enhancement is greatly 

reduced. Thus, although there is now definitive spectroscopic evidence in the literature 

(DRIFTS, NMR, RAMAN, Neutron Scattering) of studies that confirm reversible hydrogen 

adsorption/desorption spillover behavior, these confirmations have only been observed for 

materials with low initial hydrogen capacity. 

In a recent paper,23 we address the above issue through systematic density-functional-theory 

calculations. Unlike the conventional experimental study of H chemisorption using an atomic H 

source, hydrogen storage studies use molecular hydrogen sources; therefore, the chemisorption 

state must be in a much lower energy level. In order to lower the total energy, each chemisorbed 

H atom must be paired up with another H atom on the opposite side of the graphene sheet. The 

two H atoms in a pair are strongly correlated and must move cooperatively. In this situation, 

recombination of H atoms on the same side is mostly suppressed because each recombination 

breaks two H pairs and is energetically very unfavorable. Surprisingly, we found that H 

migration can actually happen at room temperature through a cooperative motion of H pairs 

when mediated by water molecules. This study is important for both hydrogen storage 

technology and organic chemistry in terms of C-H bond activation. 

If one assumes that hexagonal patches form and grow simultaneously surrounding a catalytic 

particle, then ideally, six such patches with a size similar to the catalytic particle roughly fills the 

360° angular space, which terminates the spillover around that catalytic particle. That gives a 

maximum area ratio of 6:1 of H covered graphene over that of metal covered graphene. Based on 

this ratio, the lattice parameters, standard atomic weights (12.011 for C, 106.45 for Pd), a 

homogeneous distribution of Pd nanoparticles (3 atomic layer thickness, 2 nm diameter) on 

graphene, and an optimum inter-particle distance of 5–6 nm, the maximum H capacity is derived 

as 2.5–3.0 wt.%. 

Overall, the primary thermodynamic paradox of how a reversible hydrogenation reaction can 

occur at room temperature with the formation of a mobile 30 kJ/mol C-H bond has not been 

adequately addressed. However, as shown in the recent paper by Zhao et al., once the known 

region of influence of the metal center is calculated, along with the metal added to the material in 

the optimal spacing that would allow for 100% hydrogenation of the carbon surface, the 

maximum theoretical capacity would only be 2.75 +/- 0.5 %w/w. Spillover is an interesting 

scientific phenomena; however, the current use of 2 nm metal particles to mediate the sorption of 

hydrogen onto a sorbent is not viable for application to transportation applications. So while the 

investigation of hydrogen sorption via a spillover process is of considerable interest, at this time 

it is still, at a scientific level, at too early of a research and development stage to be considered as 

a viable alternative to other more well-developed hydrogen storage technologies. However, this 
3 

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is not to say that research in this area is not important. Fundamental research, such as work 

sponsored by the Office of Basic Energy Sciences or the National Science Foundation, where the 

mechanism could be established in great detail in order to better define substrate chemistries as 

well as the nano- and meso-scale structures needed to optimize materials suitable for 

incorporation with the spillover mechanism could be beneficial. 

In summary, hydrogen sorption via a spillover process is not considered a viable pathway 

to achieve the 2020 DOE metrics as a hydrogen sorption material for transportation 

applications. 
4 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Acknowledgements 

This work was fully supported by the U.S. Department of Energy under Contract No. DE-AC36-

08GO28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the 

National Renewable Energy Laboratory. Funding provided by the U.S. Department of Energy, 

Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. The views 

and opinions of the authors expressed herein do not necessarily state or reflect those of the 

United States Government or any agency thereof. Neither the United States Government nor any 

agency thereof, nor any of their employees, makes any warranty, expressed or implied, or

assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any 

information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. 
5 

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