The Science of Hydrogen

Hydrogen is valued because of its versatility. It can be used directly as a truck fuel, or it can be used as a building block to make other fuels and products. Understanding the chemical and physical properties of hydrogen provides insights into how it is utilized as a truck fuel and the systems required to store, distribute, and dispense it.

Hydrogen’s unique chemical and physical properties make it a double-edged sword—offering both vast potential and distinct challenges. Hydrogen (H₂) is diatomic in nature (two hydrogen atoms bonded to one another). It is the smallest and lightest molecule yet is high in energy. It is the most common element, yet it is typically bound to other elements for form compounds including water (H₂O) and hydrocarbon fuels (methane (CH₄) is the simplest hydrocarbon).

Hydrogen Gas Temperature, Pressure and Density factors related to hydrogen truck fueling. Pure hydrogen exists as a gas under standard conditions (68°F (20°C) and 1 atmosphere at sea level). Under these conditions, hydrogen is extremely diffuse (low density) and buoyant (lighter than air). Hydrogen boasts an impressive specific energy of 120 megajoules per kilogram (MJ/kg), nearly three times that of diesel (45.5 MJ/kg). However, hydrogen’s density is only 0.09 g/L, which translates to a low volumetric energy density of 0.01 MJ/L (diesel is about 38 MJ/L). Therefore, in trucks where space is at a premium, hydrogen gas must be highly compressed and stored in tanks at pressures of 5,000 psi or 10,000 psi (35 MPa or 70 MPa). At 70 MPa, up to 120 kg of hydrogen can be loaded into storage tanks, greatly improving truck range. However, even at these high pressures, gaseous hydrogen still occupies more volume than liquid fuels like diesel.

A further complication arises during hydrogen refueling with its rapid expansion from the dispenser into onboard storage tanks. Hydrogen gas is dispensed from a very high-pressure dispenser, which connects to the truck's fueling valve. Upon opening, the hydrogen expands into a lower-pressure fuel tank. During this process, called adiabatic expansion, hydrogen gas heats up as it expands—unlike most other gases. This heating effect must be countered by pre-chilling the hydrogen to around -40°F (-40°C) before dispensing. This is necessary to prevent excessive heating and to maintain proper fueling speed and equipment safety.

Hydrogen Gas Buoyancy. Buoyancy is the force that makes things float. A simple way to think about this is that less dense materials tend to be more buoyant and float on materials that are denser (think of wood floating on water). Buoyancy also applies to gases, where less dense gases float on top of denser gases. Hydrogen, as the smallest and lightest element, is also the least dense and, therefore, the most buoyant. When hydrogen molecules are introduced into a mixture of other gases, like air (which is primarily nitrogen, oxygen, and carbon dioxide), hydrogen molecules rise very quickly. In fact, hydrogen is 14 times lighter than air and rises at an extremely fast rate of 44 mph (20 m/s). This can be beneficial in outdoor environments, where any leakage dissipates very quickly.

In confined spaces, hydrogen's buoyancy and rapid diffusion can become a safety concern. Unlike in open air, where hydrogen quickly rises and disperses, in enclosed areas, hydrogen may accumulate near the ceiling or in pockets, creating a potential explosion hazard if it reaches a flammable concentration and comes into contact with an ignition source. Due to hydrogen's wide flammability range (4% to 75% in air) and low ignition energy, proper ventilation and hydrogen detection systems are essential to prevent dangerous buildups in confined environments.

Hydrogen Gas Diffusivity. Diffusivity is the rate at which molecules intermingle and spread through another medium, whether it be a gas, liquid, or solid, moving from areas of higher concentration to lower concentration. The small size, mass, and density of hydrogen molecules allow them to rapidly diffuse through other materials (visualize water seeping through sand). Hydrogen molecules are so small that they can even permeate through some metals over time, such as steel. This can lead to hydrogen embrittlement, where hydrogen atoms diffuse into the metal, accumulating at imperfections and causing it to become brittle and prone to cracking under stress. To mitigate this, specialty materials and coatings are required to eliminate pores and imperfections in containers, tubing, valves, and seals, preventing leakage and ensuring structural integrity.

Preventing hydrogen leakage is a critical design challenge due to its small molecular size and high diffusivity. Even the tiniest imperfections in seals or joints can allow hydrogen to escape. Storage systems must be equipped with high-performance seals and non-permeable barriers designed to handle hydrogen at high pressures. Continuous monitoring and regular maintenance are also essential to detect leaks, as hydrogen’s wide flammability range makes even minor leaks potentially hazardous.

Hydrogen gas can be condensed into its liquid state, but under extreme conditions. With a volumetric energy density of approximately 70 MJ/L, liquid hydrogen is significantly more energy-dense than as a gas (about 0.01 MJ/L at 0.1 MPa). This makes liquid hydrogen particularly advantageous in applications like long-distance trucking, where the storage tanks can be sized similarly to those of diesel fuel (diesel fuel has energy density around 38 MJ/L).

However, liquid hydrogen is cryogenic, with a boiling point of -423°F (-253°C) at 0.1 MPa (1 atmosphere). Insulated, dewar-type tanks are required for its storage. At these low temperatures, even a small amount of heat can cause liquid hydrogen to “boil off,” transitioning back into its gas form, which leads to energy losses. Boil-off is unavoidable, making it challenging to maintain hydrogen in its liquid phase for extended periods, especially if the fuel is not being consumed quickly.

Therefore, liquid hydrogen is most effective in high-utilization applications, where fuel is consumed rapidly after being transferred. High utilization is crucial because it minimizes the time that liquid hydrogen remains in storage, reducing the likelihood of boil-off and maximizing the efficiency of the storage system.

Hydrogen and oxygen are both highly reactive elements, and when they react, a tremendous amount of energy is released as they combine to form water (H₂O). There are two types of hydrogen/oxygen chemical reactions: combustion or electrochemical.

Hydrogen-Oxygen Electrochemical Reactions. The hydrogen-oxygen electrochemical reaction is a passive reaction initiated by the contact of hydrogen and oxygen gases with a platinum catalyst, where the resulting energy is directly converted into electrical current. This reaction occurs within a Proton Exchange Membrane (PEM) fuel cell. Structurally similar to a battery, a PEM fuel cell has an anode, a cathode, and an ionic channel membrane (the proton exchange membrane) separating the two. A short description of the chemistry involved follows.

Reaction 1 (Catalytic): Oxidation of Hydrogen. At the anode, hydrogen molecules (H₂) are split into protons (H⁺) and electrons (e⁻) through a catalytic process stimulated by contact with the platinum catalyst. This is known as the oxidation reaction:

                                                                H₂→2H⁺+2e⁻

The protons (H⁺) move through the proton exchange membrane toward the cathode, while the electrons (e⁻) travel through an external circuit, generating an electric current, before eventually reaching the cathode.

Reaction 2 (Catalytic): Simultaneous Reduction of Oxygen and Formation of Water: At the cathode, oxygen molecules (O₂) from the air undergo a reduction reaction by gaining electrons (e⁻) that traveled through the external electrical circuit. This reduction reaction is catalyzed by a platinum-based catalyst. Simultaneously, the reduced oxygen combines with the electrons (e⁻) and the protons (H⁺) that pass through the proton exchange membrane, resulting in the formation of water.

                                                                O₂​+4e⁻+4H⁺→2H₂​O
The oxygen reduction and the proton-electron combination occur in tandem, continuously producing of water and generating electricity.

PEM Fuel Cell Efficiency: Efficiency is the difference between the amount of energy put into the system compared to the useful energy coming out of the system. For a PEM fuel cell, this is the chemical energy of the hydrogen fuel input compared to the electrical energy produced. Typically, a PEM fuel cell operates at an electrical efficiency of 40-60% under optimal conditions with some high efficiency designs reaching close to 65%. Some of the hydrogen’s energy is lost as heat during the catalytic reactions at both the anode and the cathode. Additionally, resistance in the flow of electrons through the external circuit and the flow of protons through the membrane generates further heat (ohmic resistance). Inefficiency can also arise at the anode and cathode during periods of high demand if hydrogen and oxygen are consumed faster than they can be replenished at the electrode surfaces. 

Heat Losses: The heat generated by the PEM fuel cell is relatively low in temperature, typically around 176°F (80°C), which is favorable for certain applications such as in vehicles.

   Hydrogen-Oxygen Combustion Reactions. Hydrogen internal combustion engines (ICE) rely on the burning of hydrogen gas, an exothermic reaction between hydrogen and oxygen that produces water vapor and releases energy as heat, which powers the engine. The fundamental reaction for hydrogen combustion can be represented as:

Activation Energy (Ea) ⟶​ 2H₂​ + O₂​ → 2H₂​O + Energy

As with other internal combustion fuels, the combustion reaction requires an activation energy, which can be a spark, to initiate the reaction.

There are a few notable characteristics of hydrogen combustion that differentiate it from either diesel or gasoline combustion.

Flame Temperature: The combustion of hydrogen can produce very high flame temperatures, potentially reaching around 2,500°C (4,532°F). This is due to a combination of factors including hydrogen’s high specific energy relative to its mass and its ability to react almost completely with available oxygen.

Combustion Speed: Hydrogen has high diffusivity, leading to fast flame propagation. This can create challenges in engine design, such as pre-ignition and knocking (abnormal combustion). Therefore, engineering is required for the fuel injection system to accommodate these characteristics.

Combustion Products: The primary product of hydrogen-oxygen combustion is water vapor (H₂O). This reaction does not produce carbon dioxide (CO₂) because there is no carbon in the hydrogen fuel. This lack of CO₂ emissions is a major advantage of hydrogen over conventional hydrocarbon fuels (like gasoline and diesel) and a key driver of the shift to hydrogen as a fuel for trucks and other applications. 

Combustion Byproducts: A byproduct of hydrogen combustion nitrogen oxides (NOx), formed in a secondary reaction within the engine. This byproduct is common to all combustion reactions and is independent of the fuel being burned. It occurs because Earth’s atmosphere is 79% nitrogen (N₂) and 21% oxygen (O₂). Although nitrogen and oxygen are stable under normal conditions, high temperatures (above 1,200°C or 2,200°F) can cause them to react and form NOₓ in a process called thermal NOₓ formation. This process is governed by three primary reactions

                                                                N₂ + O → NO + N

                                                                N + O₂ → NO + O

                                                                N + OH → NO + H

Similar to gasoline and diesel engines, hydrogen engines rely on air intake, which introduces a large amount of nitrogen. The fuel-oxygen reaction generates significant heat and pressure in the cylinder, raising the temperature. Any excess oxygen will tend to react with the nitrogen, forming NOₓ. There are several engineering solutions to limit NOₓ formation, including controlling the oxygen level in the combustion chamber and using exhaust gas aftertreatment to remove NOₓ. Conventionally fueled engines already use similar NO_x reduction solutions.

Hydrogen Engine Efficiency: Efficiency is the difference between the amount of energy input to a system and the useful energy output. In the case of hydrogen engines, this refers to the chemical energy of the hydrogen fuel compared to the mechanical energy produced. Overall, hydrogen ICE typically achieve 30% - 40% efficiency which is similar to diesel and gasoline engines. Manufacturers are working to improve this efficiency with some reporting results approaching 50% efficiency. Major areas of energy loss include thermal losses, where heat of combustion is lost through exhaust and engine cooling, friction losses from the mechanical friction between moving parts, incomplete combustion where unburned fuel goes into the exhaust and pumping loses, where work to intake air and expel exhaust further reduces efficiency.

Heat Losses: The heat loses of the hydrogen ICE are significant, typically ranging from 900 – 1,700°F (500 - 900°C).