Hydrogen Fuel Cells: A Short Review

I. Introduction

Our dependence on the use of fossil fuels has not only damaged the environment but also given a greater sense of urgency to transition towards more sustainable forms of energy. Hydrogen fuel cells are at the forefront of energy research and development, with potential for high power output with clean byproducts. Hydrogen fuel research is currently divided into two main types of fuel cells—anionic exchange membrane fuel cells (AEMFCs) and proton exchange membrane fuel cells (PEMFCs). Here we will give an overview of the mechanisms, advantages and disadvantages, and future research regarding these two kinds of fuel cells.

II. Mechanisms

Both PEMFCs and AEMFCs involve reactions between water, hydrogen, and oxygen, and the processes associated with their reduction/oxidation reactions can release energy needed to supply power. PEMFCs are guided by two main reactions: The Hydrogen Oxidation Reaction (HOR), which occurs at the anode and the oxygen reduction reaction (ORR), which occurs at the cathode. From chemical theory, we know that oxidation reactions involve the removal of electrons, while reduction reactions involve the addition of electrons. HOR thus involves the splitting of hydrogen into protons, with a release of electrons in the process, while ORR involves the synthesis of oxygens, protons, and electrons to produce water. HOR, occurring at the anode, allows electrons to flow to the cathode, generating energy in the process. The appeal of this process is that the byproduct of the reaction is water, which is the cleanest byproduct possible. In theory, as long as there is sufficient supply of oxygen and hydrogen at the cathode and anode respectively, PEMFCs will be able to operate continuously.

AEMFCs operate in an alkaline environment. The reaction mechanisms are similar to PEMFCs, but the key difference is that it involves the flow of hydroxide (OH-) ions. The HOR at the anode involves the combination of hydrogen and hydroxide ions to form water with excess electrons, while the ORR at the cathode involves the combination of oxygen, water, and electrons to form hydroxide ions. The hydroxide ions then flow back towards the anode, repeating the cycle again.

III. Advantages & Disadvantages

PEMFCs are the more mature and well-researched in fuel cell technology. They were first developed by General Electric in the 1960s for use in NASA’s first manned space vehicles, such as the Gemini Spacecraft. The early versions of PEMFCs had a short lifetime, around 500 hours, but since then, they have undergone significant research and development. To date, they are regarded for their high power density and their quick start up, which makes them particularly appealing for applications in portable devices and vehicles. PEMFCs are incredibly flexible in their applications, with capabilities ranging from a few watts (enough to power mobile phones and computers) to hundreds of kilowatts for large transport vehicles and ships.

However, PEMFCs face significant challenges, primarily due to the high cost of platinum catalysts used in both the anode and cathode, which increases overall system costs. Additionally, the reliance on pure hydrogen fuel for efficient fuel cell operation presents logistical challenges related to hydrogen production, storage, and distribution. Effective water management is also very important, as excess or insufficient water can negatively impact the fuel cell’s operation.

AEMFCs were developed in the early 20th century, but were not shown to have practicable viability until the 1950s. Though PEMFCs powered the first NASA manned spacecrafts during the Gemini missions, AEMFCs powered the Apollo missions. This led to a wave of AEMFC research during the 1960s and 1970s. AEMFCs offer promising advantages over PEMFCs, most notably their higher operating voltage. Though not clearly understood by chemists, ORR at the cathode occurs more rapidly in the alkaline environment of AEMFCs compared to PEMFCs, which allows the AEMFC to operate at voltages a lot higher than other fuel cells. Additionally, the cost of AEMFCs are significantly lower than PEMFCs, largely due to the potassium hydroxide used as the electrolyte—there are very few standard chemicals that are cheaper than this. AEMFCs operate at even lower temperatures than PEMFCs (around 60-80˚ C), offering benefits in terms of thermal management.

Despite this, AEMFCs still face several challenges. Carbon dioxide poisoning is a major issue for AEMFCs. When CO2 from the air gets into the system, it reacts with hydroxide ions (OH−), creating carbonate compounds. This reaction happens quickly, and just a small amount of CO2—around 400 parts per million, which is typical in the air—can seriously reduce the number of hydroxide ions available in the fuel cell. As these carbonate compounds build up, they can greatly decrease the performance of the AEMFC and cause instability in its output voltage. This can degrade the exchange membranes required for proper diffusion of hydroxide ions, resulting in short-term stability of these membranes. Additionally, developing cost-effective, durable, and efficient catalysts that operate in an alkaline environment remains a current area of research.

In light of a global mission to reduce greenhouse gasses and transition to renewable sources of energy, future prospects of fuel cell research are promising. Researchers are focusing on improving efficiency, durability, and cost-effectiveness across various fuel cell types. Innovations in materials, such as more effective catalysts and membranes, are expected to enhance performance while reducing reliance on expensive metals such as platinum and palladium. Additionally, integrating fuel cells with renewable energy sources and additionally exploring new fuels, like hydrogen and ammonia, will further expand their applications. As research progresses, fuel cells are positioned to play a crucial role in achieving a sustainable, low-carbon future.