The Environmental Engineering Hub

Petroleum is made of gaseous, liquid, and solid hydrocarbon-based chemical compounds from sedimentary rock deposits around the planet. Crude petroleum, when refined, provides high-value liquid feeds, solvents, lubricants, and other products. Petroleum-based fuels make up almost one half of the energy supply in the world. Simple distillation is enough to make gasoline, diesel, aviation fuel and kerosene out of petroleum. How much is obtained, in terms of fractions, from the crude oil depends on the origin of the supply. When fuel cells are considered, it is important to understand the physical and combustion characteristics of the fuel, as well as its chemical composition (it is this factor that determines the fuel processing type).  Different technologies have to be employed to convert the many fraction types of the petroleum into hydrogen for FCs.  A special case is when the fuel is catalytically converted and generates various trace compounds that may be poisonous for the

There are four major causes of voltage drops in fuel cells: Activation losses: they are caused because of the low speed of the reactions that take place on the surface of the electrodes: Because the electrons have to move to and from the electrode, a portion of the generated voltage is lost to drive the chemical reaction.  Fuel crossover and internal currents: this energy loss results from the portion of the fuel that passes through the electrolyte and the electron movement through it. In an ideal case, the fuel should not have that behavior, but real cases always have that phenomenon. Ohmic losses: this voltage drop comes from the resistance of the materials of the fuel cell to the transport of electrons. Mass transport or concentration losses: these happen due to the change in concentration of the reactants at the surface of the electrodes. Reference: LARMINIE, James; DICKS, Andrew. Fuel Cell Systems Explained. 2. ed. West Sussex, England: Wiley & Sons Ltd., 2003.

The Gibbs free energy changes vary with temperature, pressure and gas concentration in fuel cells. Take into account the following generation reaction: j J + k K → m M Where k moles of K react with j moles of J to generate m moles of M. Both the reactants and products have an associated 'activity'. We can call this 'activity' a, aj and ak for the reactants and am for the product activity. When gases behave close to ideal conditions (as is the case with fuel cells), we know that: a=P/P0 Where P is the pressure/partial pressure of the gas and the standard pressure is P0 (around 0.1 MPa). This simple equation is useful because fuel cells are, in a general way, gas reactors. When dissolved chemicals are involved, the activity can be linked to the molarity or strength of the solution. The case of water in fuel cells is complex to deal with, but in steam form, it can be stated that the activity of water is equal to the partial pressure of water divided by

Most fuel cells use hydrogen as fuel because of its high reactivity for the electrochemical reaction in the anode and the water release from oxidation, which is harmless to the environment. Thus, vehicles that employ PEMFCs (Proton-exchange Membrane Fuel Cells) may be classified as zero-emission. However, hydrogen is not readily available in most places and it has be generated before fuel cells can be used. One common way of producing hydrogen is through the electrolysis of water, which is the reverse process of a fuel cell. Even though the method may seem perverse, it is in fact a very convenient way of providing hydrogen for mobile fuel cells. Another method of providing fuel is through biological processes that can break down fossil or bio-fuel. Some of these methods are based on enzymes, bacteria or light. In other cases, hydrogen is produced in large central plants, or by electrolysers and is stored for use in fuel cells. There is already something of an infrastruct

Most of the time, the designer doesn't have the choice between air and oxygen when assembling a PEM fuel cell. Oxygen is employed in systems that don't depend on air to operate, such as submarine and spacecraft (when that is not the case, air is used). However, the performance of a PEM fuel cell is greatly improved when oxygen is used. That happens because of three factors: The increase in partial pressure of oxygen makes the 'no loss' open circuit voltage rise, as appointed by the Equation of Nernst (https://en.wikipedia.org/wiki/Nernst_equation). Use of better catalyst sites reduces the activation over-voltage. The mass transport or concentration over-voltage losses are reduced by the increase in the limiting current, which is an event caused by the absence of nitrogen(a gas that contributes for this kind of loss at high current densities. Some results have showed that the change from air to oxygen in a PEMFC can increase performance by as much as 30%.  R

In large PEMFC systems, the hydrogen fuel usually comes from a fuel reforming system. These systems always involve a reaction that produces carbon monoxide, like the reaction between steam and methane: CH 4 + H 2 O → 3H 2 + CO Fuel cells that work in high temperatures can use this carbon monoxide as fuel, which is different from FCs that employ platinum as part of the catalyst, since small amounts of the compound can negatively affect the anode. One of the strategies to overcome this limitation is the transformation of carbon monoxide to carbon dioxide through the increased insertion of steam:   CO + H 2 O → H 2 + CO 2 Which is a reaction commonly known as the water gas shift reaction . However, this process never fully transforms all the CO to CO2. The best systems usually leave 0.25 to 0.5% of the original concentrations of CO in the PEMFC. What the carbon monoxide does it to occupy platinum catalyst sites because of its relative affinity, which prevents the

The thing that puts most sponsors off financing FC applications if the cost. However, there are several advantages that balances that and makes researching FCs a way to pave for a promising future. These are: Efficiency: FCs are, in general, more efficient than combustion engines. Adding to that, small FC systems can be just as efficient as large ones.  Simplicity: since FCs have no or few moving parts, they are considered to have a simple and reliable design.  Low emissions: the only by-product of a FC is pure water, thus making them almost emission-less. That characteristic is specially well received in car engine applications, since there is an international trend to reduce emissions from cars. Silence: since they have no or few moving parts, FCs tend to be very quiet during operation.   These advantages are particularly interesting in combined heat and power systems and on mobile power systems, like vehicles, portable computers, mobile telephones and military