ReviewLiquid organic hydrogen carriers☆
Graphical abstract
liquid organic hydrogen carriers (LOHCs) with high hydrogen content, moderate operational temperature, and the compatibility with existing gasoline infrastructure, hold the promises as hydrogen carriers for both onboard application and large scale long-distance H2 transportation.
Introduction
The ever severe global energy and environmental crises impel the change of energy carriers from current fossil fuels to clean and renewable energy sources, among which hydrogen energy has long been viewed as a potential solution. However, the lack of proper onboard hydrogen storage systems meeting the US Department of Energy's (DOE) targets (5.5 wt% and 40 g/L hydrogen capacity in 2020) [1] is one of the bottlenecks for the coming “hydrogen economy” [2], [3] albeit various approaches in storing hydrogen, such as high pressure and cryo-liquid hydrogen, physisorption by porous materials [4], [5], metal hydrides [6], complex hydrides [7] and chemical hydrides [8], have been explored over the years. Compressed hydrogen tank is adopted to store hydrogen onboard by several car companies [9], however, there are concerns on safety and cost [10], [11]. Cryo-liquid hydrogen has high gravimetric hydrogen density and is suitable for large scale hydrogen store. For onboard application, however, the energy cost in liquefaction and boil-off problem are the drawbacks. Research activities over the past two decades mainly devoted to the condensed materials that can store hydrogen chemically or physically. Fig. 1 displays the gravimetric and volumetric hydrogen densities together with the operating temperatures for a number of representative materials systems. Chemical hydrides have relatively high hydrogen contents and moderate dehydrogenation temperatures, but they suffer from irreversibility and energy consuming regeneration. Complex and metal hydride, on the other hand, encounter the drawbacks of unsuitable thermodynamics, sluggish kinetics and/or low hydrogen content. Comparatively, the liquid organic hydrogen carriers (LOHCs) with hydrogen content of 5–8 wt%, reversibility, moderate dehydrogenation temperature, commercial availability and more importantly, the compatibility with existing gasoline infrastructure, hold the promises as hydrogen carriers for both onboard application and large scale long-distance H2 transportation [12], [13], [14]. As a matter of fact, Japanese government has a strategic plan of importing H2 from overseas by means of liquefied hydrogen or organic hydrides [15].
Early researches on liquid organic hydrides for hydrogen storage focused on cycloalkanes [16], i.e., cyclohexane, methylcyclohexane, and decalin etc. The dehydrogenation of cycloalkanes to corresponding aromatics, however, occurs at relatively high temperatures due to the unfavorable enthalpy changes (see Table 1). It was then demonstrated experimentally by Pez et al. [17] and theoretically by Crabtree and coworkers [18] that the incorporation of heteroatoms, such as N or B, into LOHCs reduces the energy input in dehydrogenation. Those findings triggered considerable investigations on heterocycles [13], [14]. More recently, formic acid (FA) with energy density of 4.4 wt% has also been demonstrated as a promising hydrogen carrier. It is obvious that catalysis in hydrogenation and dehydrogenation is an important issue in the application of those LOHCs as hydrogen storage media. Herein below the properties of representative LOHCs, i.e., cycloalkanes, N-heterocycles, and FA, and the catalysts development will be reviewed and discussed, with the hope to provide the readers a brief summary of the on-going activities and perspectives of the near-term direction.
Section snippets
Cycloalkanes
With the advantages of relatively higher hydrogen capacity (6–8 wt%), liquid state at room temperature, high boiling points, production of COx-free hydrogen and low toxicity, cycloalkanes have been proposed and investigated as liquid hydrogen carriers [25]. The candidate cycloalkanes reported by far include cyclohexane, methylcyclohexane, and decalin as shown in Table 1. However, because the dehydrogenation of cycloalkanes is highly endothermic (63–69 kJ/molH2), high temperature for hydrogen
Effect of nitrogen
As shown in Section 2, the reduction of operating temperature for alkane–arene pairs can be kinetically alleviated by employing suitable catalysts. However, the thermodynamic improvement can only be done via compositional alteration, for example, introducing heteroatoms into cycloalkanes to form heterocycles [13], which was first proposed by Pez et al. in a series of key patents [17], [46]. In those patents, it was demonstrated that better reversible de/hydrogenation properties can be obtained
Formic acid (FA)
FA with a hydrogen content of 4.4 wt%, stable and low toxic at ambient condition, easiness in transportation, handling and storage, holds the promises as a safe and convenient liquid hydrogen carrier [77], [78]. As shown in the Scheme 2 the decomposition of FA follows two competitive reactions: the R1 – the dehydrogenation, yielding H2 and CO2 and, the R2 – the dehydration, yielding CO and H2O [78]. In addition, the water-gas shift (WGS) reaction may take apart in affecting the distribution of
Other potential LOHCs
The technical targets of onboard hydrogen storage system for light-duty vehicles are 5.5 wt% in capacity and –40–60 °C for operational temperature, which means that, in this temperature region, at least 1 bar equilibrium hydrogen is needed. The equilibrium pressure strongly depends on the enthalpy and entropy changes in dehydrogenation Reaction [2]. The entropy change of the dehydrogenation mainly comes from the molecular hydrogen, which is about 130 J/K/molH2. Therefore, to reach an
Summary
The design, synthesis and evaluation of new organic hydrides and the development of efficient, non-noble metal catalysts are the areas worthy of intensive research inputs. With the vast versatility of organic hydrides, suitable candidates for both onboard applications and large scale-long distance H2 transport are expectable.
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This work was supported by the Project of the National Natural Science Funds for Distinguished Young Scholar (51225206), and Projects of the National Natural Science Foundation of China (grant nos. U1232120, 51301161, 21473181 and 51472237).