A fuel cell is an electrochemical energy conversion device. A fuel converts the chemicals hydrogen and oxygen into water. In the process, it can generate electricity and energy.
Oxygen (O2) and Hydrogen (H2) enter the fuel cell at the cathode and anode, respectively. A polymer electrolyte membrane acts as a barrier that only positively charged ions (hydrogen ions) can travel through from the anode. Thus, a catalyst causes H2 to split into two positively charged ions and two electrons. H+ can pass through the barrier, whereas electrons travel through external circuit which generates electrical current and energy. Two positively charged ions and two electrons combine into H2 at cathode, which allows to form this hydrogen and oxygen into water. Water flows out of a cell.
Thermodynamics Principles of a Fuel Cell:
The reaction of a fuel cell is simply an electrolysis reaction in reverse. An electrolysis reaction would be similar to using a battery to split water molecules into hydrogen and oxygen. Therefore, a fuel cell reaction brings oxygen and hydrogen together. While some think of batteries when hearing the term “fuel cell”, it is actually the opposite. Batteries store energy, while fuel cells requires a constant source of fuel to produce energy. Fuel cell reaction are just redox reactions and can be divided into electrochemical half-cell reactions, such as:
H2 + ½ O2 —> H2O
The effect of temperature and pressure on cell potential can be analyzed with the following equations:
The maximum electrical work output from this reaction can be related to the Gibbs free-energy change of the reaction:
W=ΔG = -nFE
If reactants and products are both in their standard states, the equation is written as:
ΔGo = -nFEo
In these equations, n refers to the number of moles of electrons in the reaction, E is the reversible potential, F is Faraday’s constant, and delta G is the free energy change.
The enthalpy of a fuel cell is given in the following equation:
ΔG=ΔH-TΔS or ΔH=ΔG+TΔS
Where ΔG is the maximum amount of electrical energy available, ΔH is the total thermal energy available, and TΔS is the amount of heat produced by the fuel cell operating reversibly. If a fuel cell reaction has a negative entropy change, it will generate heat, but if it has a positive entropy change, it can extract heat from the surroundings.
When considering a hydrogen fuel cell, the anode reaction is:
H2 —> 2H+ +2e-
And the cathode reaction is:
½ O2 + 2H+ +2e- —> H2O
The Nernst equation represents the relationship between the ideal standard potential of the fuel cell reaction and the ideal equilibrium potential at other temperatures and pressures of the reactants and products and is given by:
If ideal standard potential is known, the Nernst equation can be used to determine the idea voltage at other temperature and pressures. Additionally, if the cell is operated at higher reactant pressures, the ideal cell potential can be increased at a given temperature. These changes can improve fuel cell performance.
Best Efficiency Based on Thermodynamics:
Thermal efficiency of a fuel cell is defined as the amount of useful energy produced relative to the changes in stored chemical energy, also known as thermal energy. This happens when fuel is reacted with an oxidant. Due to the principle that the operation of a fuel cell is essentially isothermal, where less energy is lost from the system. This along with the idea that fuel cells do not have a temperature ceiling they do not follow the Carnot Cycle, and therefore are not limited to Carnot Efficiency.
The efficiency of a fuel cell based on the First Law of thermodynamics is expressed as Wout/Qin, similar to equation 2.15
Hydrogen and Oxygen can coexist in each other’s presence at moderate temperatures. But if heated to around 500 °C at a high pressure, these two components react violently, typically through an explosion. Fuel cells are inherently two processes. One taking place in the heat engine is a thermal process, and the one in the fuel cell is electrochemical. In an ideal case, electrochemical energy conversion allows the change in Gibbs free energy of the reaction to be made useful as electric energy at the device’s output.
The equation below shows how you can obtain ∆G from the combustion reaction between Hydrogen and Oxygen.
At standard conditions, the chemical energy in a Hydrogen/ Oxygen reaction is 285.8 kJ/mol and the ∆G is 237.1 kJ/mol. Using equation (2.15), we can determine that the thermal efficiency for an ideal fuel cell would be 0.83.
Hydrogen fuel cells are often called the energy source of the future for the promising outlook they share in reducing the world’s dependence on fossil fuels while providing a similar energy output with less pollutants put into the atmosphere. One of the biggest contributing factors is the increased energy production efficiency than other methods that use the Carnot cycle, such as the internal combustion engines typically used for transportation and portable energy generation. One of the reasons hydrogen fuel cells have less of an environmental impact than internal combustion engines for electricity generation is because hydrogen fuel cells directly convert their energy into electricity, whereas others must use a second set of moving parts in order to create electricity. The industry of large scale electricity generation sees the biggest threat to future business because of increasing environmental concerns, thus driving them into searching for more renewable sources of energy. This combatted with an energy grid that could possibly double within the next 25 years could drive the need for more localized and efficient electricity generation and storage, could drive the cost of hydrogen fuel cells down and make their part in reducing greenhouse gas and other environmentally detrimental emissions from the utility sector.
Other industries may see the benefit of hydrogen fuel cells’ environmental impact as well. One of the main problems holding back the market of renewable energy sources today is that their power generation is often intermittent. Wind turbines require windy days, solar generators require sunlight however when these technologies are coupled with hydrogen generators, can create a more steady stream of power generation. Home power generators may also see a benefit from hydrogen fuel cells, as the technology can be scaled from the size of a cell phone battery to large scale power generation. As these low operating temperature fuel cells are quick starting, have low weight, few parts, and produce quiet operation, portable hydrogen fuel cell generators may be a better choice than internal combustion generators, further moving away from the world’s dependence on fossil fuels for generating electricity. The increased demand for portable power in the future could possibly surpass that of current battery technology and resources further driving the need for more environmentally friendly options.
Quite possibly the most important environmental impact of hydrogen fuel cells is the different ways that the hydrogen can be produced as fuel. Hydrogen generated from biomass can have the most positive environmental impact. Biomass can be crops specifically grown for energy production, or it can be municipal or organic waste. Typically these items go directly to landfills and their potential as energy sources is completely wasted, let alone the pollution that these items can cause into the water and atmosphere. Creating energy from our garbage is likely the most positive impact that can be had on the environment. While energy from biomass can only be harnessed 17% by weight, there is an economical advantage because it will cost money to safely dispose of the waste. This factor improves the cost deficit that hydrogen fuel cells experience, by essentially giving them a free source of energy from the daily waste of society.
Hydrocarbons will likely remain the initial source of energy for hydrogen fuel cells in the immediate future as their cost is incredibly low comparatively. As more environmentally friendly ways of producing purer hydrogen fuel emerge, hydrogen fuel cells will have a greater and greater positive impact on the environment. As the hydrogen becomes more pure, water and heat will be the only byproducts of the fuel cell. This heat could be used to further create electricity in large scale high temperature fuel cells, or can be used as a supplemental heat source. With more pure hydrogen from non-hydrocarbon sources used as fuel in real world applications near undetectable levels of NOx emissions because of their low operating temperatures when compared to internal combustion engines. In such cases the water exiting a hydrogen fuel cell can be near drinking level quality. This water itself, could be split using electrolysis in order to create hydrogen fuel for the cell. As environmental demands increase and energy needs begin to exceed the current technology, hydrogen fuel cells will be at the forefront of the switch to cleaner energy production.