The Energy Conversion of Hydrogen Fuel Cells

Articles in discussion:

• Fuel Cell Conversion — Student Energy

• Fuel Cells — U.S. Department of Energy

• Understanding How a Hydrogen Fuel Cell Works: A Comprehensive Guide — Lhyfe Heroes

• Fuel Cell Article (ScienceDirect)

• Proton-Exchange Membrane Fuel Cells — Wikipedia

• Benefits of Hydrogen — AFDC (Alternative Fuels Data Center)

• Hydrogen Emissions — AFDC Vehicles

Introduction

Hydrogen fuel cells are an emerging alternative technology to counteract internal combustion engines and other fuel-based systems. These fuel cells are comparable to rechargeable batteries because they can be continuously recharged and generate electricity at sustainable operating conditions, including temperature, reaction requirements, etc. This alternative method has also gained prominence with hydrocarbons, given the more efficient, cleaner, and quieter energy conversion the cells can provide. When looking at hydrogen fuel cells, we focus on the conversion process to energy via chemical reactions between hydrogen and oxygen, leading to the eventual production of electromechanical energy.

Overview

There are six types of fuel cells based on the electrolyte that they employ, and they are separated into their required category based on electro-chemical reaction, operating temperature range, and kind of catalysts required:

·   Polymer electrolyte membrane (PEM) fuel cells

·   Direct methanol fuel cells

·   Alkaline fuel cells

·   Phosphoric acid fuel cells

·   Molten carbonate fuel cells

·   Solid oxide fuel cells

·   Reversible fuel cells

Hydrogen is generally stored in the PEM fuel cell systems given their ability to deliver high power density, is advantageous due to low weight and volume requirements, and can be easily transported and stored.

Components

Hydrogen fuel cells are held together by two bipolar plates with a membrane in between. The plates operate as the cathode and anode on each side and produce less than 1V under normal operating conditions. Gaskets are added around the edges of the plates to create a seal that avoids leakage of hydrogen and oxygen from the cell. 

Hydrogen fuel cells comprise a three-layer membrane, also known as the membrane electrode assembly (MEA). 

The first layer is the polymer electrolyte membrane (PEM), which conducts positively charged ions and blocks the electrons. The second layer is known as the catalyst, which is on both sides of the PEM and enables hydrogen molecules to be split into protons and electrons on the anode side and oxygen reduction on the cathode side, producing excess water. 

Collection and Storage

The hydrogen is produced from steam by reforming hydrocarbons, yielding less harmful volumes of CO2 when compared to methane.

The hydrogen is stored the the cells along with oxygen molecules. These can be stored at room temperature and have an operating temperature range of 60°-110° C. 

Fuel Conversion

The fuel conversion of hydrogen fuel cells occurs through a chemical reaction process where the hydrogen molecules are oxidized and split into protons and electrons, combining hydrogen and oxygen to produce electricity, water, and heat. 

Two separate reactions occur in the anode and cathode. At the cathode, oxygen molecules are reduced, forming water. The reaction is defined as: . At the anode, hydrogen molecules are split into protons and electrons and energy is produced. The reaction is defined as: .

This yields an overall combined reaction defined as: 

Throughout this whole process, the PEM fuel cell can produce in a range of 0.01-250 kW depending on the size of the cell and what system is utilizing it. Hydrogen fuel cells are also estimated to have an electrical efficiency of 40-55%, much higher than their hydrocarbon counterparts which have around 25-30% efficiency.

Inside the conversion process, water is converted in the cathode but it is also evaporated at the same rate it is created due to the excess heat produced in the anode. This is attributed to the membrane required to remain hydrated to continue operating and providing 0 resistance to the current of the electrons flowing through it. If the membrane dries, this can create a short circuit that can permanently damage the fuel cell. This is the main reason why the size of the fuel cell is directly proportional to the energy it can output, given the amounts of water it can produce must be evaporated at an equal rate to ensure membrane integrity and operational efficiency. 

The result of the entire process is direct current electricity that can be converted for their respective applications, whether it be small systems such as cars, or large-scale power and cogeneration applications.   

Current State and Challenges

Hydrogen fuel cells have consistently proven to be a more efficient alternative to their hydrocarbon counterparts. Excess heat is harnessed more efficiently and more energy can be extracted from the fuel, while compounded with the environmental advantage and its thermodynamic advantage. This is due to it being a chemical reaction rather than a gas combustion system. Even so, hydrogen fuel cells face major financial and transition challenges in order for it to be successful. 

For instance, it is expected that by 2050 the global energy demand will double or triple. Hydrogen fuel cells are considered to be the alternative energy source that could fulfill the holes in future demands. However, the transition to both hydrogen-based energy and other forms of alternative energy (solar, wind, geothermal, etc.) requires several decades and substantial economic investment. 

Although some regions are embracing the transition and beginning investments, such as the European Union investing 2.8 billion euros over a 10-year period, it is not sufficient to catalyze the transition. 

Furthermore, the cost of creating affordable hydrogen fuel cells is not sufficient to boost production of hydrogen-powered systems. For example, an 80 kW fuel cell for a vehicle can cost between $55-$67 per kW in the United States, much more expensive than its hydrocarbon and electricity-based counterparts. 

Environmental Impacts

Advantages

One of the main advantages hydrogen fuel cells have is that they do not emit carbon dioxide or other harmful gases that are notorious in their hydrocarbon counterpart. Overall, the application of hydrogen fuel cells is considered to be derived from zero- to low-emission sources. 

The following chart depicts the percent-change of  certain mixed-use or single-alternative methods of vehicle-based energy production when compared to gas-powered vehicles:

It can be seen how Liquid Hydrogen (Electro LH2, Renew) and Green Hydrogen (Electrro GH2, Renew) have the largest percent changes when compared to gas and other renewable energy sources. 

Disadvantages
There are not many known or deeply studied disadvantages to hydrogen fuel cells, but there exist certain risk. It is known that the fuel conversion process does not emit greenhouse gases, but the development of the fuel cell itself may have indirect emissions via material procurement and hydrogen collection. Even so, it is safe to assume that the indirect pollution caused is much lower than fossil fuels. 

Disclaimer: The reflections shared here are my own technical and analytical perspectives. They are not definitive statements of fact or policy positions. I welcome thoughtful discussion; feel free to contact me if you’d like to explore these ideas further.