Review
Brian D. Adamsa and Aicheng Chena, 
Available online 8 June 2011.
We are facing accelerated global warming due to the accumulation of greenhouse gases. A hydrogen-based economy is one potential approach toward maintaining our standard of living while lowering carbon dioxide emissions. Palladium is a unique material with a strong affinity to hydrogen owing to both its catalytic and hydrogen absorbing properties. Palladium has the potential to play a major role in virtually every aspect of the envisioned hydrogen economy, including hydrogen purification, storage, detection, and fuel cells. Major aspects of current research and potential applications of palladium-based nanomaterials in various hydrogen technologies are presented in this review.
Article Outline
Global society has become strongly dependent on the excessive use of fossil fuels1. Oil, coal, and natural gas have been utilized extensively to power automobiles, plants, and factories, causing a dramatic build-up of greenhouse gases in the atmosphere2. Unlike fossil fuels, the combustion of hydrogen does not generate carbon dioxide (CO2), but only water vapor. The ultimate goals of a hydrogen-based economy include the production of hydrogen while generating minimal greenhouse gases, the development of efficient infrastructures for hydrogen storage and transport, and the harnessing of its energy via fuel cells3.
Palladium (Pd) exhibits a number of exceptional properties which enable its application in a myriad of hydrogen technologies. Palladium has the ability to absorb large volumetric quantities of hydrogen at room temperature and atmospheric pressure, and subsequently forms palladium hydride (PdHx). As shown in Fig. 1, the absorbed hydrogen atoms occupy interstitial octahedral sites of the face-centered cubic lattice4. The temperatures and equilibrium pressures required for the formation of various metal hydrides are compared in Table 15. Palladium can form a hydride under ambient conditions and the formed PdHx exhibit the noble character of Pd6. This attractive hydrogen absorbing property has undergone extensive study in both the gas phase[7], [8] and [9] and under electrochemical conditions[10], [11], [12] and [13]. There are several classical, informative reviews, and other literature available in regard to PdHx, which discuss the physical, thermodynamic, and kinetic properties of the system[14], [15], [16], [17], [18], [19], [20] and [21]. Some of the key properties of PdHx are summarized in Table 2.
| Full-size image (46K) High-quality image (199K) |
Fig. 1.
A reconstructed neutron holographic image showing the positions of six Pd atoms surrounding a hydrogen nucleus in an octahedral interstitial site of an fcc lattice.
Reprinted with permission from4. © 2004 American Institute of Physics.
Table 1. Metal hydrides and their hydrogen storage properties.
| Metal | Hydride | wt.% H | Peq, T |
|---|---|---|---|
| Pd | PdH0.6 | 0.56 | 0.02 bar, 298 K |
| Mg | MgH2 | 7.60 | 1 bar, 573 K |
| LaNi5 | LaNi5H6 | 1.37 | 2 bar, 298 K |
| ZrV2 | ZrV2H5.5 | 3.01 | 10−8 bar, 323 K |
| FeTi | FeTiH2 | 1.89 | 5 bar, 303 K |
| Mg2Ni | Mg2NiH4 | 3.59 | 1 bar, 555 K |
| TiV2 | TiV2H4 | 2.60 | 10 bar, 313 K |
Table 2. Key properties of PdHx.
| Property | Value | Reference |
|---|---|---|
| Lattice Constant (nm at 298 K) | ||
| Pd | 0.3890 | 15 |
| PdH0.015 (αmax) | 0.3894 | 15 |
| PdH0.607 (βmin) | 0.4025 | 15 |
| Volumetric Expansion (α/β-phase, %) | 10.4 | 15 |
| Diffusion Coefficient (cm2.s−1 at 298 K) | ||
| α-phase | 3.8 × 10−7 | 15 |
| β-phase | 2.0 × 10−7 | 16 |
| |ΔHplat| (kJ/mol H at 298 K) | 19.1±0.2 | 16 |
| |ΔSplat| (J/K.mol H) | 46.6 | 14 |
Molecular H2 dissociation is the first step in absorption toward the formation of metal hydrides. Other than Pd, most metals require energy input in order to overcome an activation barrier, which necessitates the application of high hydrogen pressures or elevated temperatures. On palladium surfaces, the dissociative adsorption of H2 molecules occurs with little or no activation energy barrier[22] and [23]. Fig. 2 shows the activation-less pathway of H2 dissociation on a Pd5 cluster. The superior dissociative properties of Pd enable it to serve as a catalyst to facilitate hydrogen absorption and desorption in other metal hydrides24. Further, its inherent selectivity for hydrogen, fast sorption kinetics, and reversibility of hydride formation allow Pd-based alloy membranes to attain a high hydrogen gas quality at 99.99999 % purity. Hydrogen sensors are also being investigated that draw on the changes in the properties of Pd upon exposure to hydrogen25.
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Fig. 2.
Potential energy surface for a H2 molecule bound to the planar Pd5 cluster. The H2 dissociation path on the potential surface is indicated by the thick line with crosses. The calculations were performed by the DFT/B3P86 method.
Reprinted with permission from23. © 2004 Plenum Publishing Corporation.
Carbon-supported Pt is commonly used as an electrocatalyst in low temperature fuel cells that utilize hydrogen or small organic molecule feedstocks26. The high cost of Pt and the limited world supply are barriers to widespread use[27] and [28]. Pd is of great interest as a substitute material for Pt, not only due to its chemical similarity to Pt, but also because of its significantly reduced cost (approximately 1/5 that of Pt) and 50 fold greater abundance26. The extensive use of Pd for hydrogen storage remains impractical, however, due to its relatively high price. Nevertheless, with the application of appropriate cost-saving methods such as alloying with less expensive metals and utilizing nanoscale materials, palladium has the potential to play a significant role in many aspects of the hydrogen economy, from purifying hydrogen to harnessing its energy via fuel cells. In this review, the utilization of Pd as a material for applications in several hydrogen technologies is examined. The current state of research and implications regarding the next steps in the development of such materials are also discussed.
Hydrogen purification
Hydrogen may be produced from either hydrocarbons or water by employing a wide array of techniques. The most environmentally friendly production technique involves the electrolysis of water, which uses off-peak power or power from renewable sources such as wind turbines or solar cells. Unfortunately, 
70 % of the electricity in the US is generated through combustion of coal and natural gas. Currently, the majority of H2 is produced in the form of syngas that is generated by the gasification of hydrocarbons, which generates large amounts of CO and CO2.
A high purity hydrogen gas stream is crucial to the performance of fuel cells since certain species poison the catalysts even at very low concentrations29. In the case of CO, a strong adsorbing species, less than 10 ppm is typically required30. The application of a highly selective palladium-based membrane may be utilized to purify the gas, and is currently the most advanced method of doing so30. A major drawback for membranes that are comprised of pure Pd is hydrogen embrittlement31. The absorption of hydrogen into Pd occurs in two distinct phases. The α-phase appears at low concentrations of hydrogen (solid solution), and the β-phase forms at high concentrations of hydrogen (metal hydride). A schematic phase diagram for PdHx is displayed in Fig. 3.
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Fig. 3.
Phase diagram for PdHx.
Reprinted with permission from6. © 2009 Elsevier.
Associated with the α/β-phase transition is an expansion of the face-centered cubic crystal lattice. This large expansion of the lattice, due to hydrogen embrittlement, can cause the cracking of the membrane. In general, βmin decreases and αmax increases in conjunction with elevated temperature. As a result, the width of the co-existence region or miscibility gap (βmin − αmax) decreases with rising temperatures until a critical threshold Tc = 570 K (bulk Pd) is reached. Above Tc, the miscibility gap is no longer observed. For this reason, membranes that are comprised of pure Pd are held above 570 K in order to avoid hydrogen embrittlement6.
It has been shown that the size of the Pd particles has an effect on this phase transition32. The miscibility gap and Tc are decreased with the use of nanoparticles[33] and [34]. A comparison of the hydrogen absorption isotherms for Pd bulk and Pd nanoparticles is illustrated in Fig. 4. The hysteresis in the PC isotherm of Pd nanoparticles (2.6 ± 0.4 nm in diameter) disappeared at 393 K. The absence of hysteresis, in addition to the absence of a plateau region in the equilibrium pressure, implies that the critical state of the α- and β-phases has been attained. Thus, a remarkable decrease in Tc was recognized in Pd nanoparticles19.
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Fig. 4.
Hydrogen absorption isotherms for Pd bulk (black) and Pd nanoparticles of size 7.0 ± 0.4 nm in diameter (blue) and 2.6 ± 0.4 nm in diameter (red).
Reprinted from33. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
Hydrogen embrittlement may also be controlled at ambient temperatures by alloying Pd with metals which have larger atoms than Pd itself. In PdAg, for example, which is the most common hydrogen extraction metal alloy used by industry35, the lattice is pre-expanded by the silver atoms and thus is less influenced by hydrogen than a pure Pd lattice36. An optimal hydrogen permeation value is achieved at a silver content of 23 wt.%35. The isotherms for PdAg alloys do not show a well-defined plateau as in the case of Pd, and the lattice expands continuously in this region. The α/β-phase transition results in a volumetric expansion of only 0.38 % for this alloy37. The same effect has been seen with Pd-Cd nanostructures. A composition of Pd-Cd (15 at.%) was found to eliminate the α/β-phase transition with optimal hydrogen solubility and, consequently, increased the kinetics of the absorption process38. The lattice constant relationship for the Pd-Cd alloys along with the potential dependence of hydrogen electrosorption is shown in Fig. 539.
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Fig. 5.
(a) Vegard's plot showing the dependence of the fcc lattice constant on the normalized atomic composition of Cd in Pd-Cd alloys (b) and an overlay showing the H/(Pd+Cd) ratio (triangles) and time to reach steady state hydrogen sorption (circles) versus potential for the Pd and Pd-Cd(15 %) electrodes.
Reprinted with permission from[38] and [39]. ©2009, 2010 American Chemical Society.
In heterogeneous catalysis, it is observed that poisons (e.g., sulphur, arsenic, and carbon monoxide) deactivate catalyzed reactions through irreversible adsorption at active sites40. Recently, palladium-based sorbents were shown to effectively remove the contaminants mercury, arsenic, selenium, and phosphorus from high temperature syngas[41], [42] and [43]. There is a competition between the poison and the reactant for available adsorption sites at the catalyst surface. In the case of Pd-based hydrogen selective membranes, poisoning species tend to inhibit the dissociative adsorption of hydrogen at the surface, which is the initial step in the absorption/desorption process. Pd is especially vulnerable to hydrogen sulphide (H2S) and carbon monoxide (CO), which are among the contaminant species that are present in the hydrocarbon fuels used for hydrogen production[30] and [44]. The alloying of Pd with Cu has been reported to offer a significant tolerance to H2S in the hydrogen gas stream when a PdCu membrane is operated at high temperatures (> 908 K)[44] and [45]. On the other hand, both Pd and PdCu membranes suffer greatly from the effects of H2S at lower temperatures, forming Pd4S and Pd-Cu-S layers, respectively, which nullify hydrogen dissociation or permeation46.
Electrochemical hydrogen absorption experiments with Pd electrodes in the presence of CO have revealed that the adsorption of CO strongly blocks the capacity for hydrogen insertion as well as its removal from the metal[47] and [48]. The hydrogen permeance of Pd0.75Ag0.25 membranes has also been measured in the presence of pure H2 or mixtures of H2 with CO and H2O at various temperatures49. A strong access inhibition by CO, owing to the robust interaction between CO and Pd was observed; however, the effect was eliminated at temperatures higher than 623 K. Much of the work done with respect to the poisoning species of Pd-based hydrogen permeable membranes has suggested that conducting hydrogen purification at elevated temperatures is likely to decrease the problematic effects described above.
Hydrogen storage
Hydrogen storage and transportation are among the most challenging issues to overcome, as they are critical prerequisites to the realization of a hydrogen-based economy. There are three primary classes of material that are currently under study for hydrogen storage in automotive applications: organic chemical hydrides, metal hydrides, and carbon materials for physisorption50. In 2002, the US Department of Energy set quantitative goals for the hydrogen content of storage devices (6 wt.% by 2010, then 9 wt.% by 2015). These targets have not been met, and consequently, the DOE has revised the target to 5.5 wt.% by 201551. The critical properties that a hydrogen storage material should possess include that it be (i) lightweight, (ii) inexpensive and readily available, (iii) accommodating for high volumetric and gravimetric densities of hydrogen, and exhibit (iv) rapid sorption kinetics, (v) easy activation, (vi) low temperature dissociation or decomposition, (vii) appropriate thermodynamic properties, (viii) long-term cycling stability, and (ix) high reversibility52. Palladium should not be considered as a sole hydrogen storage material since it is somewhat expensive and has a low gravimetric hydrogen density53. However, it does satisfy all of the other requirements that define a hydrogen storage material, which enable it to “assist” in the hydrogenation and dehydrogenation of other materials.
Palladium acts not only as a catalyst to facilitate the uptake and dissociation of hydrogen in other metal hydrides54, it can also protect the surface from corrosion, allowing for the study of air-sensitive samples in ambient conditions[55], [56] and
