Fuel cell is a device that can produce electricity without the generation of any pollution when operated on pure H2 fuel. Low-temperature polymer electrolyte membrane fuel cells operate at around 80oC and water management in these cells becomes a challenge. High temperature polymer electrolytic membrane fuel cells operate in the range of 120oC to 200oC with phosphoric acid doped polybenzimidazole as membrane electrolyte [4].
Modeling helps to understand the deep insight of fuel cell processes and thus facilitates the improvement without involving any experimental work. In addition, the burning of fossil fuels also results in the release of carbon monoxide (CO), carbon dioxide (CO2) and nitrogen oxides (NOX). The first fuel cell was invented by Sir William Grove in 1839 in which he first performed electrolysis of dilute sulfuric acid, dissolved hydrogen (H2) and oxygen (O2) in two separate glass tubes.
So several fuel cells are connected in series, called fuel cell stacks, to produce the required power. A fuel cell is an electrochemical conversion device that converts chemical energy of a substance into electrical energy.
Construction
Due to the increase in global warming, which is caused by the emission of greenhouse gas, which in turn resulted from the use of fossil fuel products such as gasoline, diesel, etc. We have a huge responsibility to reduce these emissions and protect our earth. Although we know that fossil fuel emissions are harmful to our nature and fossil fuel reserves are dwindling day by day, we cannot avoid their use because of their ubiquitous demand.
So there is a need to look for an alternative source that produces clean and green energy. He connected an ammeter to the glass tubes in which a small electrical current signal was detected, coming from the oxidation of hydrogen (H2) and the reduction of oxygen (O2) on individual Pt electrodes. The structure of electrodes and electrolyte together is called Membrane Electrode Assembly (MEA), as shown in Fig.
In addition to it, auxiliary parts such as bipolar plate through which the fuel and oxidizer are sent, end plates and gaskets (to shorten the flow of reactant gases) are included in the fuel cell as shown in Fig.
Principle
The difference between the ionic and electronic charges on the two sides causes charges to transfer from the anode side to the cathode side. To facilitate the flow of electrons towards the cathode side, an external charge is connected between the anode and cathode catalyst layers producing electron current.
Types of Fuel Cell
PEMFCs use a thin membrane made of polymer and based on their operating temperature range, they are further divided into. Low temperature PEM fuel cells are generally operated below 80oC and use a thin Nafion based membrane. While HTPEM fuel cells operate in the range of 120-160oC and use phosphoric acid doped polybenzimidazole (PBI) as the membrane electrolyte.
In PEMFC, water is produced on the cathode side while in SOFC it is produced on the anode side as given by Eq.
Recent Developments
Various Losses in a Fuel Cell
To identify the sources of losses, we must analyze the process from reactant delivery to product removal. These losses are attributed to the transport of reactant and oxidant from flow field plate to the catalyst layer through the porous gas diffusion layer. At high current loads, fuel consumption will be more so to use the fuel for the reaction-reactant transport must be fast enough.
To ensure that the surface area to volume ratio of the catalyst layer with a higher current density is high, the layer is made thin. Compared to HOR on the anode side, ORR on the cathode side is slower due to the presence of other gases such as nitrogen. But ions have to pass through the electrolyte, which acts on the jumping mechanism and causes delay in reaching the cathode side.
Various Techniques
- In-situ Techniques
- Charge Transfer
- Hydrogen oxiation kinetics
- Oxygen reduction kinetics
- Equillibrium constant calculation
The voltage of fuel cell is set to a specific value and corresponding current response is monitored. After setting the voltage, it is necessary to give fuel cell some time to relax to reach steady state from dynamic state. This technique is widely accepted in fuel cell research as it is very fast and can be performed in parallel with iV measurement.
The basic idea of this technique is that when the constant current load is interrupted, it results in a time-dependent voltage response, which is indicative of the resistive and capacitive behavior of the fuel cell components. As given by ohm's law, resistance is the ratio of voltage to current applied to direct current which is a. EIS is a dynamic technique which uses alternating current to find out the behavior of the fuel cell.
When the fuel cell rotates in a stable position, the sinusoidal voltage is disturbed with a small amplitude Vo (in mV) and the resulting current response is also sinusoidal with the same period as the voltage, but with a different amplitude Io and with some phase shift φ, as shown in Figure 1. The losses occurring in the fuel cell are identified by comparing them with equivalent circuit models. The general EC model for a fuel cell is given by the Randles circuit [7], which is the combination of two RC constants separated by an R, as shown in Figure 2.
Because ω= 2πf in our case all cos terms will become even and sin terms will disappear, we obtain ist. Because ω= 2πf in our case all cos terms will become even and sin terms will disappear, we obtain ipcosφ. 3.1) Where,i0= exchange current density and expressed as a function of partial pressure or coverage of reactant species with random order dependence.
Model Current density (ia) Exchange current density (ioa) Volmer-Tafel ioa[exp(βaf ηa)−exp(−βcf ηa)] i∗a (KH2pH2). F/RT, βa and βc are symmetry factors for the anode and cathode respectively, pi is the partial pressure of the ith type, ¯pis is the normalized pressure, Kk is the equilibrium constant and i∗a is the flat parameter of all constants. It can be seen from the above equation that the density of the alternating current at the cathode depends on the partial pressure of H2O.
Where, Γ is the total areal density in (mol/m2), θi is the surface coverage fraction of Pt by species. Where, Ed is the desorption energy for the concomitant desorption of H2 on the Pt surface and is reported to be 21 kJ/mol [5].
Numerical model
Species transport
The equilibrium constants Ki are calculated on the basis of adsorption of reactant species and desorption of product species [2]. The sticking coefficient is a new constant introduced to represent the adsorption of gas species on the Pt surface and can be converted into a rate expression as follows. Xi is the mole fraction of species i, µ is the viscosity in kg m−1s−1, p is the total pressure in Pa. DilDGM is given by.
H-matrix elements are given by hkl=. 3.19) Where,Di,Kne is the effective Knudsen diffusion coefficient and is given by.
Charge trasport
Boundary conditions
The dynamics of CO poisoning, i.e. response of the normalized current density, was obtained by the model and is adjusted for the data from ref. The dependence of the order of the exchange current density on the concentration of reactants and products is the result of the derivation of the Butler-Volmer equation. The equilibrium constants that appear in the model are calculated from the adsorption-desorption equilibrium of the reactions [10].
Different HOR models are compared for the data using the same set of tuning parameters, i.e. alternating current density is shown in Fig. We used Arrhenius equation to relate exchange current density and temperature and is given by. As previously mentioned, polarization curves are obtained using Volmer-Tafel model and are shown in Fig.
However, when operating with oxygen at the cathode, it is necessary to compromise at 160oC for air and pure O2 to obtain the best fit as shown in Fig. The calculated current density is the ion flux through the membrane which is equal to the integrated current through the catalyst layers. Since we are sending pure hydrogen to the anode side, the current density and symmetry factor at the anode side are kept constant at 500 (A/cm3) and 0.7 respectively.
The parameters obtained at 160oC and 140oC are substituted into the Arrhenius equation to obtain the same at 150oC. Their values are shown in table 4.2. Arrhenius constants such as i' and E' are used to calculate the exchange current density at 150oC, shown in Table 4.3. To assess the uniqueness of the parameters it is not sufficient to use polarization curves alone.
The aforementioned parameters i∗ and β are adjusted to fit the polarization curves, and the same set of parameters produced predictions of the activation overvoltages at the anode and cathode that agree very well with the experimental data, as shown in Fig. The main goal of the experiment is to study the effect of CO in the reactant on the operation of the fuel cell. The current density obtained with the presence of CO is normalized by dividing it by the current density obtained with pure H2.
The presence of CO caused the current density to drop to nearly 75% of that of pure H2 as shown in Fig. Different HOR and ORR models are presented and it is seen that the same experimental data can be reproduced with all the models by adjusting the fitting parameters ie. exchange current density and symmetrical factor.
