Is Carbazole the Energy Carrier of the Future?

By Guest Blogger Keith D. Patch

According to Wikipedia, N-ethylcarbazole (C14H13N) was used as a chemical weapon (irritant) in World War I. Now it (and related organic compounds) is being investigated as a hydrogen transport material for fuel cell vehicles.

In 2011 I read some news out of Germany about a “new” method of carrying hydrogen in fuel cell vehicles: Professors Wolfgang Arlt and Peter Wasserscheid (University of Erlangen-Nuremberg) published “A future energy supply based on Liquid Organic Hydrogen Carriers (LOHC)”, which described a scenario where an organic liquid (such as N-ethyl carbazole, or carbazole) is used to carry hydrogen in unpressurized tanks on hydrogen-powered fuel cell vehicles. On-board the vehicle, a reactor uses a heated catalytic reactor to strip hydrogen gas from the carbazole, providing hydrogen fuel to the fuel cell. Later on at a gas station, the spent carbazole is removed from the vehicle’s fuel tank and replaced by fresh, fully-hydrogenated carbazole. The spent carbazole is returned from the gas station to a processing plant to have hydrogen added to it, preparing fresh hydrogen-rich fuel that is ready to be transported back to gas stations. It is a complicated process, but being touted as a potential solution to the problem of efficiently carrying hydrogen on-board vehicles.

The idea has been picked up by numerous German news outlets including Spiegel-online, Financial Times Deutschland and AutoBild. In addition to the flurry of interest in the German press, several German TV channels have broadcast stories. Furthermore, Rainer Bomba, State Secretary at the German Federal Ministry of Transport, Building and Urban Development (BMVBS) has been quoted as saying “This is the future in the automotive industry.” Accordingly, the BMVBS has pledged 400,000 euros to the project.

Professors Arlt and Wasserscheid admit that their “new” technology is based on a 2004 patent from Air Products and Chemicals, Inc. Arlt and Wasserscheid do not supply any new data, and apparently are only beginning to explore the utility of carbazole in the hydrogen economy.

Some history now: starting April 1, 2005, Air Products began a U.S. Department of Energy-funded study on using carbazole as a hydrogen carrier for fuel cell vehicles. Air Products worked on a reactor that used heat and a catalyst to strip hydrogen from the carbazole, as well as process economics and other tasks. To address toxicity questions with carbazole, Air Products conducted various tests. 2008 toxicity test results can be summarized by:

“Toxicology testing revealed that neither N-ethylcarbazole nor perhydrogenated N-ethyl carbazole are mutagenic, as judged by the negative Ames test. However, perhydrogenated N-ethyl carbazole was found to be corrosive and acutely toxic using other standard tests. Thus, while perhydrogenated N-ethyl carbazole remains a suitable model compound for reactor research, it could present concerns for commercial use.”

However, this is not why the DOE stopped work on this project. Carbazole research was halted because it was projected to have no hope of meeting the energy storage goals of the DOE. In the July 02, 2010 DOE Hydrogen Program Record #9017, On-Board Hydrogen Storage Systems – Projected Performance and Cost Parameters, the carbazole system capacities are well below the DOE 2010 targets of 4.5 wt% and 28 g-H2/L:

“Assessments of the liquid organic n-ethyl carbazole for vehicular storage yielded capacities in the range of 2.1-2.8 wt% and 18-21 g-H2/L that are also below the 2010 system targets indicating this will not be a viable vehicular option unless materials with much greater quantities of releasable hydrogen and lower reaction temperature can be found.”

Below is a graph from DOE Hydrogen Program Record #9017, which illustrates how inadequate carbazole is, relative to the universe of alternative hydrogen storage methods. The brown oval (in the lower left corner) labeled LCH2 (Liquid organic Carrier H2) uses data from a 2009 TIAX report to show that carbazole is well below the required 2010 U.S. DOE volumetric and gravimetric hydrogen storage goals. Only Cryo-Compressed Hydrogen [CcH2, or hydrogen pressurized (e.g., to ~350 bar) and stored at cryogenic temperatures] is able to meet the 2015 DOE hydrogen storage goals; all of the other storage methods fall well short of Cryo-Compressed H2.

Carbazole is Well Below U.S. DOE Hydrogen Program Hydrogen Storage Goals (above)

A June, 2011 Argonne National Laboratory (ANL) report (Technical Assessment of Organic Liquid Carrier Hydrogen Storage Systems for Automotive Applications) discusses ANL’s latest technical and economic assessment of carbazole. ANL states:

“… The challenge has been to find suitable organic carriers that have sufficient hydrogen capacity, optimal heat of reaction (∆H), rapid decomposition kinetics, low volatility and long cycle life, and that remain liquid over the working temperature range. Air Products and Chemicals Inc. (APCI) investigated many candidates for potential liquid carriers but no one material could satisfy all the requirements for a viable hydrogen storage system. … a practical storage system cannot be built with this polycyclic aromatic hydrocarbon. The assessment, however, does show the potential of meeting the storage targets with other yet-undiscovered organic liquid carriers that may have the right properties.”

As the DOE and ANL analyses summarized above show, it will require one or more major breakthroughs in order to meet the 2015 DOE hydrogen storage goals, almost tripling the current hydrogen storage performance of carbazole. Good luck to Professors Arlt and Wasserscheid as they try to come up with such a significant breakthrough. The DOE has given up on carbazoles; hopefully the Germans can find an organic liquid that meets all the necessary requirements for a practical energy carrier!