The Chemistry of Hydrogen Fuel Cells

In cities like Beijing, citizens have to live every day with a suffocating layer of smog in the atmosphere. Fortunately for most Americans, this is not a very prominent issue. However, with the immense amount of pollution produced from the industries that continuously provide for the needs and luxuries of Americans, it could be.

Pollution has been a problem ever since global warming was coined as a term. There are so many ways to contribute to this problem that is growing at such a fast rate that there seems to be no significant way to stop it.

One small step towards solving this problem is the implementation of fuel cells in vehicles. Fuel cells, specifically hydrogen fuel cells, are one of the most environmentally friendly power sources available. The byproducts of their electricity-making process are solely water and heat! Although frequently compared to the battery, technically speaking, the fuel cell is an electrochemical energy conversion device. It converts hydrogen and oxygen into water, producing energy in the form of electricity in the process. Now, how does energy enter into the equation?

This energy actually comes from the reverse of a relatively familiar process – electrolysis. Electrolysis uses electricity to separate a molecule into its original components. By sending an electric current into a solution through an electrolyte, which ionizes when dissolved in a solvent, the flow of ions is stimulated and allows for the non-spontaneous reaction (the break-up of the molecule) to occur. In 1839, a Welsh scientist named Sir William Robert Grove reversed this process and generated electricity and water from hydrogen. He called his creation a gas voltaic battery, now known today as a hydrogen fuel cell.

There are many types of fuel cells that serve different purposes, but they all have the same general setup. In a hydrogen fuel cell hydrogen atoms enter at the anode, where their electrons are stripped by an oxidation reaction: 2H2 –> 4H++ 4e-. As a result, the hydrogen atoms are ionized and carry a positive charge. The electrons then travel through a wire where they produce a direct electric current (DC) output. In some fuel cells, the positively charged hydrogen ions move through the electrolyte (represented by the proton exchange membrane pictured below) and join with oxygen molecules that enter from the cathode and the electrons returning from the wire. Other fuel cells have the oxygen molecules pick up the electrons first before moving through the electrolyte to the cathode, where the electrons combine with the hydrogen ions to form water, as can be seen in the reduction reaction O2 + 4H+ + 4e- –> 2H2O. In this process, water and heat are formed as a result, meaning that the reaction is exothermic. All these steps can be summarized in the net, or “redox” reaction: 2H2 + O2 => 2H2O + energy. Both products are released from the exhaust and are harmless in terms of pollution.

Graphic credit to Marc Marshall, Schatz Energy Research Center/

Not only are fuel cells eco-friendly, but, in comparison with batteries, they have the ability to last much longer. Unlike batteries, fuel cells can be used as long as there is access to hydrogen and oxygen. Batteries, on the other hand, can’t be refueled. The one exception is rechargeable batteries, but even these will eventually die. Additionally, since fuel cells create electricity chemically, they are not subject to the thermodynamic laws that greatly restrict efficiency. Fuel cells are, thus, more efficient than batteries.

There are six main types of fuel cells: polymer exchange membrane (PEMFC), solid oxide (SOFC), alkaline (AFC), molten-carbonate (MCFC), phosphoric acid (PAFC), and direct methanol (DMFC). They each have advantages and disadvantages and hold different futures in the ever-changing world of technology. For more specific information on each type, check out this link.

Let’s explore one of them. Utilizing the simplest reactions, PEMFC is one of the more promising of the fuel cell family and will most likely be used in homes and transportation. It consists of an anode, a cathode, an electrolyte, and a catalyst, and follows the aforementioned description of how a fuel cell works. Hydrogen gas enters the fuel cell on the anode side and is split into H+ ions and electrons once it comes into contact with the catalyst. The electrons move through the anode to the external electric circuit where they produce an electric current, then return to the cell on the cathode side, where oxygen gas is pumped through. Because of oxygen’s high electronegativity, it attracts and pulls the H+ ions through the exchange membrane and forms, along with the electrons that return to the cell, water molecules. This reaction will produce about 0.7 volts. To increase this voltage to a more useful level, several fuel cells are layered on top of each other and connected by bipolar plates, forming a fuel-cell stack. Try out the simulation here!

Now for how they work in cars! Fuel cell vehicles consist of five distinct components: the fuel cell stack, electric motor, high-output battery, hydrogen storage tank, and power control unit. The fuel cell stack converts the highly pressurized hydrogen gas (to increase driving range) stored in the hydrogen storage tank with the oxygen from the air into electricity, which powers the electric motor. Compared to a conventional internal combustion engine, the electric motor is much quieter, more efficient, and more smooth. The high-output battery stores energy that is generated from regenerative braking and provides supplemental power to the electric motor. Lastly, the power control unit controls and oversees the flow of electricity.

It is clear that hydrogen fuel cells have a bright future ahead, Of course, there are disadvantages to every new breakthrough in science, but with time, those are ensured to be addressed. Even now, these issues are being acknowledged and improved. Recently, an article from USA Today detailed Toyota’s advancement in fuel cell technology with an increase in range and shortening of the time it takes to refuel. With improvements already being seen, this technology is sure to make its way into the transportation and home infrastructure in no time. So keep your eyes peeled for any mention of fuel cells!

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