Renewable Energy in Transportation: From the Launch of "Hydrogen Bus"

Preface
The double-decker "Hydrogen Bus" of Hong Kong Citybus, manufactured by Wisdom (Fujian) Motor, is the first hydrogen-powered vehicle licensed to carry passengers in Hong Kong. It is also the first double-decker, three-axle hydrogen-powered bus in the world. As the electric buses and hydrogen buses introduced by KMB and Citybus are gradually put into service, and the related technologies continue to develop and mature, there is a constantly growing interest among the public in the vehicle manufacturing and operating costs, safety, environmental impact and sustainability (Figure 1).¹

The combination of hydrogen and oxygen is an exothermic reaction, that is, energy is released when the chemical reaction is completed. The only reaction product is pure environmentally harmless water. This is well known chemistry knowledge among junior secondary school students. In this way, hydrogen can be regarded as a fuel. Does it qualify as a green energy source? Let’s look at how the Secretary for Environment and Ecology of HKSAR, Mr. Tse Chin-wan, responded to inquiries from members of the Legislative Council in March 2023.²
"Combustion of hydrogen does not emit carbon or other pollutants into the atmosphere... At present, there are still some challenges in the development of hydrogen. Hydrogen is mainly divided into three types: 'grey hydrogen', 'blue hydrogen' and 'green hydrogen'. 'Grey hydrogen' is a by-product of fossil fuels. Although it is cheap, it has high carbon emissions. The manufacturing principle of 'blue hydrogen' is similar to that of 'grey hydrogen', but it uses carbon capture technology to minimize carbon emissions. 'Green hydrogen' is produced by electrolysing water using renewable energy, but supply is very limited. How to reduce its production and transportation costs, as well as losses during the production process, still require in-depth research."
Having a double-decker "Hydrogen Bus" on Hong Kong roads is certainly good news, but the hydrogen injected into the bus at the refuelling station is "grey hydrogen", which is hydrogen produced through fossil fuels such as methane, and is the type with the highest carbon emissions.

Industrial Production of "Grey Hydrogen"
Traditionally, hydrogen is industrially produced using the technology of Steam-Methane Reforming (SMR), a production process widely employed in the petroleum industry. Among many methods of hydrogen production, this method is the cheapest. Today, approximately 50% of global hydrogen production relies on this method. The production process is basically divided into two steps:

1. The raw material, which is natural gas (i.e, methane CH₄), is heated to 700-1100 °C. Under the catalytic conditions of a nickel catalyst, methane and water vapour undergo an exothermic reaction to generate carbon monoxide and hydrogen: CH₄ + H₂O → CO + 3 H₂
2. The second step involves lowering the temperature to 360 °C. Through an exothermic reaction, the carbon monoxide generated in step (1) can reduce water vapour to generate more hydrogen: CO + H₂O → CO₂ + H₂

This is how "grey hydrogen" is produced. For every metric ton of hydrogen produced, 9 metric tons of the greenhouse gas carbon dioxide is generated. Today, countries around the world are committed to reducing carbon emissions and achieving the long-term goal of carbon neutrality. The Hong Kong SAR Government strives to achieve carbon neutrality by 2050, so this SMR hydrogen production method, which is cheap but polluting, must be eliminated soon.

Electrolysis of Water to Produce Hydrogen
In the secondary school electrochemistry module, students learn that electrolysis of water can cause the decomposition of water molecules, producing two parts of hydrogen and one part of oxygen at the cathode and anode respectively. A simple yet interesting experiment can be arranged for students to master the basic knowledge of electrochemistry in practice (Figure 2). The power supply can be a 9 V battery, and graphite pencil leads can be used as the anode and cathode. After the device is turned on, bubbles are released in the test tubes covering the two poles:
2H₂O → 2H₂ + O₂

Large-scale industrial electrolysis of water can effectively produce high-purity hydrogen and oxygen. Is the hydrogen obtained by electrolysing water "grey hydrogen", "blue hydrogen", or "green hydrogen"? The answer lies in whether the source of electricity comes from fossil fuels, nuclear energy, or renewable energy. If electricity generated by wind or solar energy is used to electrolyse water, "green hydrogen" is obtained. If electricity is generated with fossil fuels, "grey hydrogen" is the product. At present, industrial hydrogen production via electrolysis of water has higher costs than SMR, and thus only accounts for about 8% of global hydrogen production.3 Nevertheless, with the decline of fossil fuels such as coal, oil and natural gas, and the emerging concept of sustainable development, the hydrogen energy market will likely be dominated by renewable energy in the long term (around 2050). From the perspective of environmental protection, we should expect that the applicability of this method will gradually expand! An article estimates that a hydrogen price of 2.6 yuan/Nm³ will be competitive with gasoline prices.⁴ The cost of water electrolysis is relatively high, mainly depending on electricity prices. Since electricity costs account for about 80% of the entire hydrogen production costs, the key to lowering costs of hydrogen lies in energy consumption. This relies on technology to further improve the efficiency of electrolysing water, or to make full use of renewable energy from solar power.
Many scientists around the world are fully devoted to this research topic that is closely related to people's livelihood, and are committed to developing and producing "green hydrogen" (Figure 3).⁵ For example, researchers specialised in catalysis are working hard to develop various solid catalysts to increase the efficiency of water electrolysis. On the other hand, there are research and development efforts on catalytic photochemical reactions, focusing on how to promote "water splitting" to produce "green hydrogen" using sunlight, which is also attractive direction. A review article titled "Review and Outlook of Hydrogen Production through Catalytic Processes" was published in 2024 in Energy & Fuels.³ It systematically evaluated the development of the scientific community in this field in the past ten years, and some catalytic processes have significant potential for industrialization. Readers can refer to the abstract graphic of the article to grasp a more complete picture of the current research directions of hydrogen energy.

Hydrogen Fuel Cell
The engine that drives electric vehicles requires an Electric Vehicle Battery (EVB) that is highly efficient and capable of supporting continuous long-distance driving. Due to the promising business opportunities for environmentally friendly electric vehicles and fierce competition in the electric vehicle manufacturing industry, breakthrough technologies have been developed over the past decade. The price of EVB has been significantly reduced by 87%. On the other hand, hydrogen-powered buses and cars mainly use hydrogen-driven fuel cells to power their engines (Figure 4). At present, the commercial fuel cell technology is mature. In buses, there are backup bottles of compressed hydrogen that provide fuel for the battery. A fuel cell is different from a primary cell, in that through stable supplies of oxygen and hydrogen fuel, it can continuously provide stable power until the fuel is exhausted. This is unlike ordinary rechargeable batteries, which must be recharged upon exhaustion. From the following reactions, the only product from power generation is water, and thus there is no pollutant emitted to the environment at all.
Anodic reaction: 2H₂ + 2O²⁻ → 2H2O + 4e⁻
Cathodic reaction: O₂ + 4e^– → 2O²⁻
Overall cell reaction: 2H₂ + O₂ → 2H₂O

According to the briefing by Citybus in November 2023,⁶ the Type IV hydrogen storage tanks in its "Hydrogen Bus" have undergone a series of tests and certifications, and can withstand impacts, fires, and even crushes. In case of fire due to hydrogen leakage from the storage tanks and fuel cells at the rear of the vehicle, fire extinguishers will be automatically deployed to put the fire out. Water is the only emission product from power generation, with one litre of water produced for every kilometre travelled. Upon future mass production of these buses, refuelling will only take 10 minutes for the bus to travel 400 kilometres.
The development of hydrogen in Hong Kong is still in its infancy. With the mass production and introduction of hydrogen-powered buses, and the expansion of refuelling facilities, the future transition in hydrogen sources from "grey hydrogen", which emits the most carbon, to "green hydrogen" will be an exciting development.

Postscript
While being a rising star of renewable energy, the low boiling point of -252.9 ^oC and low density of 0.083 g/L under ambient temperature and pressure of hydrogen bring considerable challenges to the storage and transportation of hydrogen, and limit some of its applications.

 

References

1. https://www.hk01.com/article/967004?utm_source=01articlecopy&utm_medium=referral
2. https://www.info.gov.hk/gia/general/202303/29/P2023032900468.htm
3. Energy Fuels 2024, 38, 4, 2601–2629.
4. https://kknews.cc/finance/2538gez.html
5. Science 2011, 333, 6044, 863-866.
6. https://news.now.com/home/local/player?newsId=541193

Article completed on 26 April, 2024
By Prof. Chan W. H.
Translate by Dr. Chan H. T.