Section 2: Current Technological Challenges in Hydrogen Energy and Possible Solutions

In the first part of our article series on hydrogen energy, we focused on the question of why hydrogen as an energy carrier and demonstrated that, thanks to its superior properties as an energy carrier, hydrogen is one of the strongest candidates for building a fossil fuel-free and carbon emission-free sustainable future. However, as with all good things, we also mentioned the ongoing technological challenges in a hydrogen-based energy economy. In this second part of the series, we will discuss the difficulties in the widespread use of hydrogen as an energy carrier in our daily life applications and possible solutions.

The vision of an energy system in which hydrogen produced from renewable or low-carbon sources serves as the primary energy carrier replacing fossil fuels in our daily life applications and various sectors is examined under the title “Hydrogen Economy.” This transition aims to reduce greenhouse gas emissions and create a more sustainable energy future. The hydrogen economy addresses the development of hydrogen energy technologies under three main headings: 1) hydrogen production, 2) conversion of hydrogen into energy, and 3) storage and distribution of hydrogen. In this article, we will examine the technological challenges to the widespread adoption of hydrogen energy technologies under these three main headings.

1. Hydrogen production: Hydrogen is the most abundant element in the universe and can be produced from a variety of sources, including water, through different chemical processes. In this context, it can be considered that there is no significant problem in hydrogen production worldwide. However, today, most hydrogen is produced from fossil sources through processes that require high temperatures and pressures and intensive energy input (steam reforming, water-gas shift, coal gasification) [1]. Since hydrogen produced by these methods is largely derived from carbon-based sources and results in high carbon emissions, it is called brown/gray hydrogen and is not suitable for the sustainable clean energy production targeted for the future. Therefore, research on hydrogen production has focused on producing hydrogen entirely from renewable sources or with minimal carbon emissions, i.e., green hydrogen production. Green hydrogen can be produced by splitting water (water splitting) using electrolysis powered by renewable electricity generated from solar and wind energy, or by photocatalytic processes that use solar energy directly. The splitting of one mole of water yields one mole of hydrogen gas (H2) and 0.5 mole of oxygen gas (O2). However, water splitting is a thermodynamically energy-consuming reaction, and direct production of hydrogen gas by electrolysis can only be achieved at high potentials and current densities, which is not economically feasible in practice. For efficient hydrogen production by electrolysis at lower potentials and current densities, active catalysts are needed. A water electrolyzer cell basically consists of an anode, a cathode, and an ion-conducting membrane separating these cells. In a water electrolyzer, H+ ions are reduced at the cathode cell to produce H2 gas (known as the hydrogen evolution reaction, HER), while water is oxidized at the anode cell to produce O2 gas (known as the oxygen evolution reaction, OER). For an efficient electrolyzer system operating at low potential and current density, it is necessary to develop highly active, long-lasting, and economical catalysts that can effectively catalyze both HER and OER. In addition to anode and cathode catalysts, one of the most critical points for the development of efficient water electrolyzers is the design of the ion exchange membrane. Even in the presence of highly effective catalysts, if a low-performance membrane is used, electrolyzers with low efficiency and short lifespans are developed. In this context, many anode and cathode catalysts and membrane designs have been developed for use in water electrolyzers, and research in these areas is ongoing.

Currently, there are commercially available water electrolyzer systems with different hydrogen production capacities on the market [2]. Details of scientific research activities carried out for water electrolyzers, especially catalyst development and membrane design, will be covered in the third part of this article series. In photocatalytic water splitting, where sunlight is used directly as an energy source, photocatalytic materials are needed that can absorb a large portion of the photons from sunlight and produce excited electrons and holes to reduce protons from water to generate hydrogen (HER) and oxidize water to generate oxygen (OER), respectively [3]. Although many photocatalytic materials have been developed for water splitting to date, there is still a need to develop long-lasting photocatalytic materials that can absorb a large portion of the solar spectrum and effectively catalyze both HER and OER. Research in this area is accelerating, and the efficiency of hydrogen production from water by photocatalytic means is increasing every day, and it is predicted that large-scale hydrogen production will be possible in the near future.

2. Conversion of hydrogen into energy: On the other hand, since hydrogen is an energy carrier after it is produced, it must be converted into energy by chemical means. Hydrogen combustion engines, also known as hydrogen internal combustion engines, are a type of engine that burns hydrogen fuel to produce power, similar to conventional gasoline or diesel engines, but with almost zero CO2 emission potential [4]. Hydrogen combustion engines are essentially modified versions of existing internal combustion engines (ICE). These engines are being developed as a potential way to decarbonize the transportation sector by offering a cleaner alternative to fossil fuels. However, currently, there is little interest from car manufacturers in hydrogen internal combustion engines due to safety concerns and high costs associated with transporting and burning hydrogen. On the other hand, devices used to convert hydrogen into electrical energy are called hydrogen fuel cells. Like water electrolyzers, hydrogen fuel cells are electrochemical devices consisting essentially of an anode, a cathode, and an ion exchange membrane between these cells [5]. Unlike water electrolyzers, in hydrogen fuel cells, H2 gas is oxidized at the anode cell to form H+ ions, and at the cathode cell, oxygen reduction reaction (ORR) produces oxide (O-2) ions. Thanks to the ion-permeable membrane between the two cells, protons combine with oxide ions formed at the cathode to form water. The electrons produced in these reactions are passed through a circuit to generate electricity. As with water electrolyzers, active anode and cathode catalysts are needed to develop efficient and economical hydrogen fuel cells. However, since the oxidation of H2 gas to H+ ions at the anode is relatively easy, research in hydrogen fuel cells has largely focused on developing economical cathode catalysts that can effectively catalyze ORR. As with water electrolyzers, the type and design of the membrane between the two cells in hydrogen fuel cells is very important.

Today, the most commonly used membranes in hydrogen fuel cells are proton exchange membranes (PEM). PEM fuel cells with different electricity generation capacities are already commercially available [6]. However, due to the catalysts and membranes used, there are still aspects of the technology that need to be improved, as costs are high and lifespans are short. The catalysts developed for PEM fuel cells, the current state of the technology, and future work to be done will be discussed in more detail in the third part of our article series.

3. Hydrogen storage: As summarized above, there are already commercialized technologies for the production of hydrogen and the conversion of produced hydrogen into electrical energy in the hydrogen economy. Of course, there are aspects of these technologies that need to be improved in terms of cost and lifespan. However, compared to the production and conversion of hydrogen into electrical energy, the biggest ongoing problem in the hydrogen economy is the efficient storage and safe transportation of hydrogen, which is the lightest known gas with very low density. If hydrogen produced from various sources by different processes cannot be stored and transported efficiently and safely, it will not be possible to generate the required energy at the desired place and time through hydrogen energy conversion systems, especially hydrogen fuel cells. Therefore, it can be said that the biggest problem in the hydrogen-based energy economy today is the effective and safe storage of hydrogen.

There are many different ways to store hydrogen [7]. Hydrogen storage methods can be grouped under two main headings: physical and material-based. Physical methods are based on storing hydrogen as a gas or liquid in specially designed high-pressure-resistant tanks. Although various special tank designs have been developed for storing hydrogen in gaseous form today, when considering portable systems such as automobiles, larger tanks than the carrier are needed to store large amounts of hydrogen. In addition, these tanks have a high risk of explosion, raising safety concerns. In cryogenic hydrogen storage, hydrogen can be liquefied and stored at very low temperatures in stainless steel tanks with high chromium and nickel content and a special crystal phase called austenite [8]. Although this method allows higher amounts of hydrogen to be stored per unit volume compared to the gas phase, liquefying hydrogen, the lightest known gas, requires a very low temperature of -253°C. Achieving this temperature requires a large amount of energy, which corresponds to about 45% of the energy that would be obtained by using hydrogen as an energy carrier. On the other hand, regardless of how well-designed the tank is, capacity loss due to hydrogen evaporation is inevitable. All these problems limit the energy efficiency of cryogenic hydrogen storage systems and significantly increase the unit cost of the energy obtained. In this context, storing hydrogen as a gas or liquid is not a sustainable solution for the hydrogen storage problem. To achieve high hydrogen storage capacities per unit volume, storing hydrogen in the solid phase is the most advantageous. Solid-phase hydrogen storage can be achieved in two ways based on materials. In the first method, hydrogen can be stored in the form of metal hydrides. In this storage method, although the pressure is relatively low (50 bar) compared to the gas phase, the inner surfaces of specially designed high-pressure-resistant tanks are coated with metals (e.g., palladium, magnesium, aluminum, titanium, nickel, etc.) or their alloys (NiAl, TiFe, etc.) that can interact with hydrogen gas and form hydrides, so that hydrogen is stored in the form of metal hydride (MHx) by binding to metals rather than in the gas phase. The formation of metal hydrides, i.e., hydrogen storage, is exothermic, while the release of stored hydrogen is endothermic and occurs with heat. This method allows more hydrogen to be stored per unit volume compared to gas-phase storage, but heat is required to release the stored hydrogen, and a limited cycle life is obtained in the fill-empty cycle. In addition, over time, the problem of the metal layer wearing out and losing its effectiveness arises.

In this context, the most promising method for solving the hydrogen storage problem is the chemical bonding of hydrogen in compounds [9]. In this method, called chemical hydrogen storage, hydrogen forms covalently bonded compounds with various atoms, and when needed, these bonds are broken by thermal or chemical means to release hydrogen gas. The most commonly used element for this is boron (B), of which our country holds 72% of the world’s reserves. The boron atom can form various hydride compounds with hydrogen that have high hydrogen content by mass and volume (such as sodium borohydride (NaBH4), ammonia borane (H3NBH3), etc.). The hydrogen stored in these compounds can be efficiently released at room temperature through catalytic processes when desired. However, the critical issue in these processes is the development of effective, economical, and reusable long-life catalysts that can release hydrogen from chemical hydrogen storage materials. In our country, as well as our own research group, many research groups are actively working on the development of catalysts for hydrogen storage and production in different chemical hydrogen storage materials, mainly sodium borohydride and ammonia borane [10]. In addition to boron compounds, studies on the use of formic acid (HCOOH), a simple liquid organic molecule, as a hydrogen storage material and hydrogen carrier are progressing rapidly. Although formic acid has a lower hydrogen content compared to the boron compounds mentioned above, it is attracting interest as a chemical hydrogen material due to its advantages such as being inexpensive, liquid, non-toxic, and producing only gaseous products upon dehydrogenation [11]. However, hydrogen production from formic acid is thermodynamically more difficult than hydrogen production from boron compounds. Therefore, studies on catalyst development for hydrogen production from formic acid are more limited compared to those for boron compounds. Recent studies have shown that much more efficient hydrogen release can be achieved from formic acid and other chemical hydrogen storage materials through photocatalytic applications stimulated by sunlight. Research in this direction is continuing at an increasing pace.

In conclusion, hydrogen is the best energy carrier that can replace fossil fuels for a sustainable world. However, significant technological developments and time are still needed for the widespread use of hydrogen in our daily lives. Especially if hydrogen can be stored efficiently and safely in the solid phase and the costs of hydrogen fuel cells due to catalysts and membranes are reduced, it is highly likely that we will experience a hydrogen-based energy era in the near future.

References:

[1] Dash, S.K.; Chakraborty, S.; Elangovan, D. A Brief Review of Hydrogen Production Methods and Their Challenges. Energies 2023, 16, 1141.

[2] https://www.xh2.tech/

[3] S. Nishioka, F. E. Osterloh, X. Wang, T. E. Mallouk, K. Maeda Nature Reviews Methods Primers 2023, 3, 42. 

[4] www1.eere.energy.gov/hydrogenandfuelcells/tech_validation/pdfs/fcm03r0.pdf

[5] Yılmaz, A., Ünvar, S., Ekmen, M., Aydın, S. 2017, Applied Sciences, 12, 185-192.

[6] G. AlZohbi, A. Almoaikel, L. AlShuhail, Energy Reports, 2023, 9, 28-34.

[7] https://zepp.solutions/en/pem-fuel-cell-systems/

[8] C. A. Mukwanje, A. Faik, M. Nachtane, Polymer Composites. 2025;1–23.

[9] https://www.energy.gov/eere/fuelcells/chemical-hydrogen-storage-materials

[10] J-J Long, H-C Wu, Y-T Liu, Y-Y Ding, Q-Lu Yao, O Metin, Z-H Lu, cMAT, 2024, 1, e10

[11] A.K. Singh, S. Singh, A. Kumar, Catal. Sci. Tech. 2016, 6, 12-40