|Fig. 1: Count of Google Scholar search results for the keyword "hydrogen economy" (with quotation marks).|
The concept of a "hydrogen economy", namely an energy system operating on hydrogen, was proposed decades ago. In the context of transitioning into a less carbon-intensive energy future, there has been a sustained research interest in this concept as well as its technological, economic, and policy aspects, as can be reflected by the search results for the keyword "hydrogen economy" in Google Scholar (Fig. 1). The reality for hydrogen, however, is not always a rosy picture. In 2009, the US Department of Energy cut its Fiscal Year 2010 budget for fuel cell technologies by $100 million, and the emphasis has been shifted from transportation to buildings. [1,2] In the meantime, international automotive companies have continued their effort in pursuing large-scale hydrogen programs.  Topics surrounding hydrogen production, purification, storage, and use are also still actively investigated in research communities worldwide. The contrast in the attitudes towards hydrogen prompts the thinking of its role in the future energy landscape.
In an interview with the MIT Technology Review, the then Secretary of Energy Steven Chu summarized the challenges hydrogen encountered as a potential transportation fuel, citing production from natural gas, inefficient storage, and lack of a distribution infrastructure and readiness of the fuel cell technology. 
A common misconception of hydrogen is to view it as an energy carrier and needs to be produced from an energy source. The most common hydrogen production pathway is from steam reforming of methane (sourced from natural gas), known as steam-methane reforming: 
A subsequent water-gas shift reaction is often required to increase the yield of hydrogen:
For use in proton exchange polymer fuel cells, the primary fuel cell type for transportation applications, 99.99+% hydrogen and a CO concentration below 10 ppm is recommended to avoid catalyst poisoning. [5-7] This calls for a purification step, which commonly involves adsorption or cryogenic processes and requires additional energy expenditure. 
Since the current production and purification scheme largely relies on fossil fuels, the status of hydrogen as a "clean" fuel is compromised. If fossil fuels remain the dominant energy source for producing and purifying hydrogen, carbon reduction measures, such as carbon capture and sequestration (CCS), will need to be integrated into the scheme for hydrogen to become sufficiently clean. This is true for both stationary and mobile applications of hydrogen. Implementation of CCS in the latter faces an even higher barrier because either infrastructure will be needed for hydrogen distribution (assuming CCS facilities are installed at centralized production sites) or some form of onboard CCS will need to be devised. Currently, the most technologically-mature CCS technology for stationary emissions is encumbered by its size along with other issues.  For onboard CCS to become a viable option, one would have to think outside of the box to go around the size issue. Alternatively, hydrogen can be generated from renewable sources. One promising technology is biomass gasification, but subsequent separation and upgrading will be more demanding due to the presence of a significant amount of tar in the product gas.  The argument of centralized versus distributed production would also apply to hydrogen generated from renewables.
Two intrinsic properties - the large scale and irreversibility - are immediately linked to any discussion of a "hydrogen economy". They dictate which solutions to hydrogen distribution and storage are feasible and explain the resistance hydrogen research and development are experiencing. It has been estimated that a transition to a hydrogen economy in the US alone would cost between $200 billion and $500 billion.  An investment as such would have to be justified by a portfolio of much more developed and robust technologies, rendering a quick and radical change towards a hydrogen-based energy system unlikely.
On small scales, an emerging "membrane reactor" concept has shown potential for more efficient hydrogen production and purification. The key to this concept is the integration of reaction catalysts with hydrogen-separation membranes, enabling hydrogen generation and purification in a single, compact unit. Since hydrogen is separated as it is produced, the steam-methane reforming and water-gas shift reactions will be shifted towards the product side, leading to higher hydrogen yields.  Metallic membrane materials envisioned for the membrane reactor concept work by a mechanism that intrinsically allows high-purity hydrogen to be produced. The compact size of a membrane reactor would offer flexibility enabling onsite production in a timely manner, thus reducing the need for extensive distribution networks and storage of large quantities. Membrane reactors are still in the precommercial phase, but its ability to supply high-purity hydrogen as well as compactness may offer a partial solution to decarbonizing the transportation sector, provided that infrastructure exists to collect and sequester the CO2 generated onboard. 
The role hydrogen will play also depends on the geographical environment and resources available. Abundant geothermal and hydroelectric resources in places like Iceland allow hydrogen to be produced economically from electrolysis of water while maintaining its low-carbon status.  The relatively close-knit bus system in the capital region of Iceland - where two-thirds of the country's population resides - makes adoption of hydrogen as a fuel for buses a less formidable task than it would in countries where the energy infrastructure depends on fossil fuels and the population is more scattered. [12,13] The availability of low-cost, low-carbon hydrogen as well as concentrated CO2 streams from geothermal sources allows synthetic fuel conversion to become profitable and renewable, as demonstrated by the Icelandic-American company Carbon Recycling International.  Admittedly, Iceland may not be a duplicable example, but the message here is that there are no cookie-cutter solutions to hydrogen or fossil fuel replacement and climate change mitigation in general. New technologies should be developed from creativity coupled with understanding of local and global energy landscapes.
Independent of its potential as a transportation fuel, hydrogen is an important chemical feedstock and energy carrier and will not be easily wiped out in an energy system where low-carbon technologies are evolving to play bigger roles. Just as other energy technologies, hydrogen is not a standalone solution and is dependent upon the development of other technologies such as distribution networks and CCS. While an immediate transition to a hydrogen economy on the global scale may not be foreseeable, each small step towards a more renewable energy future counts, and somewhere in this future, hydrogen will find its own place.
© Mengyao Yuan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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