Fuel Cell

A fuel cell is an energy conversion system that converts chemical energy directly into electrical energy, bypassing the traditional combustion - mechanical motion route. A fuel cell is basically an electro-chemical system while the conventional combustion - mechanical motion systems are thermo-chemical systems. The traditional chemical to electrical energy route is characterized by multiple energy conversion steps, each suffering certain losses due to the inherent conversion inefficiencies. Considering an internal combustion engine as a typical conventional energy conversion system, the energy conversion path follows the typical 5 to 6 intermediate steps;

Chemical Thermal Pressure Linear Rotary (Mechanical) Rotary (Electrical) Electrical
η1 η2 η3 η4 η5 η6
ηsystem = η1   x   η2   x   η3   x   η4   x   η5   x   η6

In case of a fuel cell, it would just be Chemical → Electrical without involving the large number of intermediate steps. Now even if each step has a conversion efficiency of say 85% (many steps have much lower efficiencies), then the conventional process would have a chemical to electrical efficiency of 0.85 x 0.85 x 0.85 x 0.85 x 0.85 x 0.85 ~ 38%. The fuel cell itself can typically have efficiency in the range of 50%. It is important to note that in case of conventional systems, the efficiency of many of the steps are much lower than 85% and as such the total system efficiency is significantly dragged down. The typical efficiencies of high power rating diesel electric generators are typically about 40%, brought to the low level by a combination of high and low efficiency intermediate steps. Essentially, the ability to convert chemical energy directly into electrical energy, completely bypassing the traditional combustion - mechanical motion route renders a fuel cell highly efficient in terms of equivalent energy conversion (chemical to electrical efficiency).
Having stated that fuel cells have higher energy conversion efficiency as compared to the conventional combustion - mechanical motion route, it is very important to note that for both fuel cells and systems adopting conventional combustion - mechanical motion route, REDUCTION - OXIDATION reactions (REDOX - reduction of oxidizer and oxidation of fuel) remain the fundamental source of energy. It is the mechanism of harnessing the energy from the REDOX reactions that differs between fuel cells and other systems.
The concept of fuel cell technology, particularly its ability to directly convert chemical to electrical energy, bypassing the intermediate stages, can be understood and appreciated better with a conceptual understanding of the combustion process and the nature of underlying chemistry; the REDOX reactions. A typical combustion process involves a fuel (classic example : Hydrogen - H2) and an oxidizer (classic example : Oxygen - O2) and in the process of combustion, the fuel and oxidizer react through the reduction-oxidation process to generate products of combustion (classic example : Water - H2O) and releasing heat. In the course of formation of the product of combustion (H2O in case of H2 and O2 REDOX reaction), it is very important to note that the fuel looses electrons to the oxidizer. Thus, as a precursor to the reaction, the fuel looses electron(s) (hence the name oxidation) while the oxidizer gains electron(s) (hence the name reduction) and in this respective charged state, the fuel and oxidizer come and stay together to form the product. The attainment of charged state by exchange of electrons and subsequent bond formation takes place only when some initial ignition energy is provided to the gaseous mixture containing the fuel and oxidizer. On the introduction of ignition energy, the fuel gets charged up to eject an electron while the oxidizer gets charged up to receive electrons. Getting a little deeper, in regards to the number of electrons that would have to be transferred between the fuel and the oxidizer to form a stable product, an important concept known as Oxidation Number addresses this aspect. The oxidation number, also called as the oxidation state is the total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom resulting in a new compound. If the oxidation number of an atom is positive then it means the atom loses electrons, and if it is negative, it means the atom gains electrons and if zero, then the atom neither gains nor loses electrons. The sign convention is in line with the concept of negative charge being associated with an electron. Thus, on loss of electron the atom becomes positively charged (hence the positive sign) and an atom becomes negatively charged on gain of electron (hence the negative sign). In regards to Hydrogen and Oxygen, oxidation number of Hydrogen atom is +1 (looses one electron) and that of Oxygen atom is -2 (gains two electrons). Thus, for Hydrogen (H2) to react with Oxygen (O2) to form Water (H2O), two Hydrogen atoms will loose one electron each and the lone Oxygen atom gains the two electrons given out by the two Hydrogen atoms. In case of Carbon reaction to form CO2, the Carbon atom looses 4 electrons while each Oxygen atom gains 2 electrons. The combustion process is as such represented as;

H2 → 2H+ + 2e-       [Fuel oxidation]            ½ O2 + 2e- → O2-       [Oxidizer reduction]

H2 + ½ O2 → H2O       [Overall REDOX reaction]

In a typical combustion process, it is important to note that, subsequent to the introduction of ignition energy into the gaseous mixture, electron exchange and product formation take place in an extremely short time; of the order of Picoseconds and that too when the fuel and the oxidizer are in very close proximity of each other. A fuel cell basically leverages the philosophy of electron exchange between the fuel and the oxidizer as described below.
A fuel cell converts chemical energy to electrical energy by basically following the REDOX reaction approach but following a different path. In a fuel cell, the fuel and the oxidizer are separated by a membrane. The membrane does not permit fuel and oxidizer to pass through it in their electrically neutral state but allows passage when they are charged. The fuel and oxidizer are independently brought in contact with a catalytic agent which enables charging of the fuel and oxidizer. The fuel and the oxidizer side are connected through an external circuit. As the fuel (over certain temperature) comes in contact with the catalyst layer, it looses electrons (Fuel → Fuel+ + e-). The electron moves through the external circuit from the fuel side to the oxidizer side and there the oxidizer grabs the electron in presence of the catalyst (Oxidizer + e- → Oxidizer-). In a final step, the charged fuel moves through the membrane to the oxidizer side and forms the final product by reacting with the charged oxidizer. In a certain variant of fuel cell, the charged oxidizer moves through the membrane to the fuel side and forms the product by reacting with the charged fuel. Thus, the product can be formed either on the fuel side or the oxidizer side depending on which charged specie moves through the membrane. The movement of electrons from the fuel side to the oxidizer side through the external circuit constitutes external electrical work.

01 Fuel cell structure & operation : Elementary discussion Click Here 02 Expression for Fuel cell Electro Motive Force (EMF) Click Here