Hydrogen production is a large and growing industry. The hydrogen economy is believed to play a significant role in the low-carbon economy of the future, where fossil fuels will gradually lose their position as our primary energy source, thus reducing global warming. One of hydrogen's greatest advantages is its suitability for a wide range of fuel cell applications.
Applications using fuel cell technology can be divided into three main categories: portable power generation (mobile devices and portable backup power), stationary power generation (distributed power generation, backup power, and grid-connected power plants), and transportation (cars, public transport, and heavy machinery). The fuel cell market attracts interest from various industries, including the automotive sector, where a lot of research is being done to advance fuel cell technology.
Operating Principle
Fuel cells convert chemical energy directly into electrical energy without a combustion process. The operation of a fuel cell is based on reduction-oxidation (redox) reactions. Energy is released in reactions between hydrogen and oxygen. The advantage of this direct process is its efficiency of over 50%, which can rise to as high as 85% if the heat generated as a byproduct is also recovered. No combustion process in electricity generation can reach such efficiency.
Other benefits include zero emissions at the point of use and quiet operation. Each type of fuel cell technology has its strengths and weaknesses. The three most important technologies currently on the market differ in whether they use a polymer electrolyte membrane (PEM), solid oxide (SO), or molten carbonate (MC) for proton transfer. Among these, PEM technology is the most versatile in its applications, as its low operating temperature (<100°C) makes it suitable for both small and large fuel cells. Solid oxide fuel cells operate at high temperatures, making them best suited for large-scale applications like distributed power generation. PEM fuel cells use pure hydrogen, while SO fuel cells can use natural gas or other hydrocarbons, from which hydrogen is extracted through a reforming process. The high operating temperature of SO fuel cells facilitates the integration of the reforming process into the fuel cell system.
The operating principle of a PEM fuel cell is illustrated in Figure 1. Hydrogen fuel is split into protons (hydrogen ions) and electrons at the anode, with the electrons being transported to the cathode through an external circuit. This generates an electric current. The positively charged carriers (hydrogen atom nuclei) are transported through the cathode's moist membrane, where they react with oxygen to form water. This reaction produces electricity and heat.
Hydrogen fuel reacts at the anode under the influence of a catalyst, producing electrons and hydrogen atom nuclei. These nuclei are transported to the cathode through a proton exchange membrane, where they react with oxygen to form water. The electrons travel through an external circuit, generating an electric current.
Maximizing Fuel Cell Efficiency and lifetime
Various loss sources limit fuel cell efficiency. These include activation losses dictated by the catalytic reaction and catalyst condition, ohmic losses during proton transfer through the electrolyte, concentration losses caused by limited mass transfer rates, and internal currents that occur when fuel diffuses through the membrane without reacting. The mechanisms behind these losses are related to water and temperature management within the cell. If temperature and moisture are not properly controlled, aging of the catalysts and proton transfer medium increases losses, reduces efficiency, and eventually leads to cell failure.
Small PEM cells usually do not require humidification, but larger cells with higher currents may need the polymer membrane to be humidified to prevent drying under load. Proton exchange capacity is directly proportional to the moisture content of the polymer, and a dry polymer restricts the reaction rate and causes losses in the cell.
Another consideration with dry polymers is their lifespan, which is one of the biggest challenges in fuel cell design. On the other hand, if water is not sufficiently removed from the cathode side of the cell, excess water impairs the cell's performance. Measuring the moisture in the fuel stream helps manage the humidification process, estimate water balances, and maintain proper membrane moisture.
Efficiency can be improved by supplying large amounts of reactive gases to the catalytic surfaces. Moist, unreacted fuel from the anode's output can be recirculated back to the anode's input to humidify the fuel stream. This recirculation can lead to the accumulation of contaminants, which is particularly harmful in the case of PEM fuel cells.
Measurement Challenges in Humid Environments
Humidity measurements in fuel cell applications are conducted in very humid environments, where relative humidity typically exceeds 80%. Performing humidity measurements near the condensation point is challenging because condensation can saturate the relative humidity sensor. It can take a long time for the sensor to dry and recover. During this time, measurement data is not available, and the fuel stream's humidity cannot be controlled.
Flowseg; the better way to control process humidity
However, one can accurately control the humidity by measuring the ingredients before mixing them. Flowseg devices can achieve any desired humidity (also ramps and steps) regardless of the application or a dynamic flow, by measuring the ingoing gas mases, and adding the appropriate mass of H2O. Additionally, by knowing the total pressure as well as the partial pressures of hydrogen and water, it is possible to calculate the partial pressures of possible contaminants in the system.
