Hydrogen Energy

The increasing availability of energy from renewable resources such as wind, solar, and hydro provides new opportunities and challenges for energy resilience. Energy supplied from solar and wind parks has daily and seasonal fluctuations, therefore, larger scale energy storage will be a key factor enabling the success of an energy economy based on "green" sources. For large-scale energy storage, one solution is the storage of electrons in chemical bonds, for example, molecular hydrogen that can be utilized in a fuel cell to provide electrons back as needed with water as the only byproduct. As new fundamental and applied research is being developed for large-scale energy storage, we can learn much from the research performed over the past 15 years, focused on materials for onboard hydrogen storage for light-duty vehicles. The energy density of hydrogen on a gravimetric scale, 33 kWh kg−1, is greater than that of petroleum. However, hydrogen is a gas and on a volumetric basis, the energy density is a different story. To this end, the current state of the art for onboard hydrogen storage for fuel-cell electric vehicles (FCEVs) relies on compressed, 700 bar, gaseous hydrogen stored in carbon fiber storage vessels to carry the 5 kg of hydrogen necessary to provide the 500 km driving range. Currently, in the USA vehicle market there are over 5600 FCEVs on the road with more than 35 hydrogen refueling stations in addition to over 20,000 support lifts in operation that use hydrogen fuel cells. Furthermore, fuel cells are being used for backup power in some cell-phone towers and there is a growing interest in hydrogen storage to improve the resiliency of the power grid. In the USA, there is a new H2@Scale Initiative investigating the potential of storing energy from intermittent renewable resources such as wind and solar. The initiative is focused on the Research and Development to make, move, use, and store hydrogen. One of the cost factors for hydrogen at filling stations is the cost to move hydrogen as a compressed gas (250 bar, ca. 20 grams H2/liter). For FCEVs to become more prevalent on the roads there is a critical need to advance beyond compressed gas to move hydrogen from production facilities to city gates where the hydrogen is finally distributed to commercial refueling stations. In many Asian countries and cities, demonstration projects using hydrogen as an energy carrier are coming online, with much more in the works. For example, the 6,000-unit Olympic village planned for the 2020 Tokyo Olympic and Paralympic Games will utilize hydrogen and fuel cells. The Olympic Village will open a hydrogen refueling station for fuel-cell vehicles that will transport athletes and others related to the Games. At the conclusion of the Olympic Games, the Village will be converted into a new city using hydrogen energy. The number of commercially operating hydrogen refueling stations are presently about 100 in Japan, 40 in China, and 10 in Korea—and more are planned to come in the near future. In Europe, hydrogen is gaining more importance as an energy carrier, since surplus energy from renewable sources can be readily converted to hydrogen. The infrastructure of refueling stations is steadily increasing and already 70 stations are in operation in Germany, aiming for 100 stations by the end of 2019. In Paris, Hype taxi was launched in 2015 and now is operating over 100 Toyota Mirai FCEVs as taxis with more than 3.5 million kilometers driven so far. The world′s first hydrogen-powered train got approval for passenger service and is operating in Lower Saxony, Germany, since September 2018. These examples are demonstrating the viability of hydrogen as a future energy carrier for buildings and transportation. However, for hydrogen to move into the main stream, more efficient and safer hydrogen storage technologies are needed. New materials are being investigated and developed for storing hydrogen, either chemically bound in metal or complex hydrides or in liquid carriers, or physically adsorbed on porous materials. This requires an increased effort in fundamental research on hydrogen-related materials and the understanding of their interaction with hydrogen. The invited contributions in this special issue of ChemPhysChem are investigating new materials and approaches to address some of the future directions for moving and storing hydrogen in Asia, Europe, and the USA. The original Articles, Communications, Minireviews, and Reviews herein detail the development of both experimental and computational methods to investigate the fundamental interactions of hydrogen with metals and complex hydrides, that is, chemisorption of hydrogen, and the adsorption of hydrogen to high-surface-area materials, that is, physisorption.