Direct Formic Acid Fuel Cells - Thesis in Chemistry

Chemical energy of a fuel such as hydrogen is continuously converted into electrical energy by an electrochemical cell known as a fuel cell (Fuel Cells | Hydrogenic). Usually, this device contains PEM (Proton Exchange Membrane) cell which uses hydrogen gas and oxygen gas as fuels and thus produces water, electricity, and heat as end products.

Cathode: O2 + 4H+ + 4e 2H2OAnode: 2H2 4H+ + 4eOverall: 2H2 + O2 2H2O

Hydrogen is the most abundant element in the universe. It is referred as the simplest and most highly energy efficient element because it's made up of only one proton and electron. Both fuel cells and batteries produce electricity (Hydrogen and Fuel Cells). This involves the conversion of the energy produced by a chemical reaction process. However, unlike batteries, a fuel cell will always produce electricity as long as there is sufficient supply of hydrogen. Thus its charge is never lost.

Classification of fuel cells is determined primarily by the occupation of the type of electrolytes. For instance, liquid phosphoric acid is used as an electrolyte in Phosphoric Acid Fuel Cell (PAFC). The cell operates at temperatures around 150oC to 200oC. The anode contains positively charged hydrogen ions which further migrate through the electrolyte to the cathode. The electrons generated travel through an external circuit from the anode while supplying electric energy and then returning to the cathode. At the cathode, water is formed as a result of the combination of electrons, hydrogen ions, and oxygen which is driven out from the cell. The speed of the reaction is increased by platinum catalyst (Pt) at the electrodes.

Anode reaction: 2H2(g) 4H+ + 4e

Cathode reaction: O2(g) + 4H+ + 4e 2H2O

Overall cell reaction: 2H2 + O2 2H2O

On the other hand, in Solid Oxide Fuel Cells (SOFC), hard ceramic metal compounds such as calcium oxide (CaO) or zirconium oxide (ZrO) are used as an electrolyte. The temperature of the cell operates at about 1000o C, with an efficiency of about 60 percent with an output of up to 100 kW. On the contrary, extraction of hydrogen from the fuel does not need a reformer due to the high temperatures emitted. Recycling of waste heat may be used as an option for increasing the amount of electricity. However, applications of SOFC units usually experience limitations as a result of the high temperatures making the units to appear often large. Leaking apparently cannot be present in solid electrolytes; thus usually they tend to crack. The electrochemical reactions occurring within the cell can be presented as follows:

at the anode: 1/2 O2 + 2e- = Oat the cathode: H2 + 1/2O= = H2O + 2e-overall cell reaction: l/2O2 + H2 = H20

Furthermore, in Alkaline Fuel Cells (AFC), potassium hydroxide (KOH) is the main compound used as an electrolyte. Temperatures usually operate below 100oC. Carbon(IV)oxide should be absent in the reactant gases when a reaction is taking place in the electrodes. This is because it worsens the state of the aqueous electrolyte. During this process, there is presence of hydrogen in the fuel electrode as well as oxygen in the air electrode. An electric current is thus formed. The three key factors to consider in an application are the electrolyte concertation, the temperature of the operation and the AFCs pressure. Moreover, characteristic of electrolytes is improved often when the concertation is high. This reduces the activity of the water as the reaction proceeds more easily.

Anode oxidation of hydrogen: H2 + 2OH- = 2H2O + 2e-

Cathode reduction of oxygen: 12O2 + H2O + 2e- = 2OH-

Total AFC reaction: H2 + 12O2 = H2O

On the contrary, production of electricity in Direct Methanol Fuel Cell (DMFC) is obtained as a result of oxygen reduction and oxidation of methanol. The anode flow field is traversed by an aqueous methanol solution of low molarity which acts as a reducing agent. Once inside the flow channel the solution diffuses. This process takes place in the backing layer which is comprised of both carbon cloth and paper. Oxidation of aqueous methanol generates the current which is collected by the backing layer. Later on, the current is transported laterally to ribs in the current collector plate.

The oxidation reaction occurring at the platinum-ruthenium catalyst of the anode is given by:

CH3OH+H2OCO2 + 6H+ + 6e

At the cathode, oxygen in the air combines with the electrons and protons. This takes place at the platinum catalyst sites thus forming water. This results in a reduction reaction. The reaction is given by:

3/2O2+6H++6e3H2O

Overall reaction is given by:

CH3OH+3/2O2CO2+2H2O

Finally, in Regenerative Fuel Cell, an electrolyzer is combined which then leads to the conversion of solar energy into chemical energy. The energy is stored as pressurized hydrogen and oxygen. In this case, oxygen may be optional. This process can allow the fuel cell to convert the chemical energy into electric power and water. The fuel cell usually has a low-mass with a high energy efficiency. This in turn makes the cell to undergo near 100% charge and discharge cycles in repeated number of times. Significant pressures are necessarily obtained by the electrolyzer for purposes of storing reasonable volumes of gaseous fluids at minimal mass.

The reaction is given by:

At cathode: H2O + 2e H2 + O2

At anode: O2 1/2O2 + 2e

Overall: H2O 1/2O2 + H2

Operation of a Proton Exchange Membrane Fuel Cell (PEM) is determined by a proton conducting membrane such as perfluorosulphonic acid. The membrane carries the hydrogen ions and protons from the anode to the cathode (NFCRC Tutorial: Proton Exchange Membrane Fuel Cell). During this process, an oxidation reaction occurs at the anode in which H2 generates protons while the electrons are released. Once the electrons are released, they pass through an external circuit thus arriving at the cathode. Diffusion occurs through the membrane as the protons solvates with the water molecules. A reduction reaction occurs at the cathode where protons and electrons react with the O2 to form water. The result obtained in the electrochemical reaction can be presented as follows:

At the anode: H2 = 2H+ + 2e- At the cathode: 1/2O2 + 2H+ + 2e- = H2OWith the overall cell reaction: l/2O2 + H2 = H20

Direct Formic Acid Fuel Cells are a type of Proton Exchange Membrane Fuel Cell that uses formic acid as the source of protons. The cells can only be used in small application due to the limited power they produce of about 50watts. This type of cell is usually preferred over other types of liquid fuel cells like methanol because it has a higher potential for miniaturization which gives it a high-power output. Formic acid, being a fast-electro-oxidation, non-flammable and non-toxic, formic acid enhances the availability of fuel in the cell which makes the cell efficient in its use. Other fuel cells such as methanol experience a decrease regarding their performance due to their toxic nature and permeability through the electrolyte.

Dehydrogenation and Dehydration are the two kinds of reactions experienced when direct formic acid fuel cells react with platinum (Pt). The dehydrogenation reaction process occurs when the formic acid is oxidized to produce CO2. The process can be presented as follows:

HCOOH> CO2 + 2H+ + 2e-

Likewise, the reaction process in dehydration produces CO. The reaction mechanism can be presented as follows:

HCOOH + Pt0 >Pt-CO + H2O

Pt0+ H2O >Pt-OH + H+ + e-

Pt-CO + Pt-OH >CO2 + H+ e

Overall reaction: HCOOH>CO2 + 2H+ + 2e-

Indirect pathway comes as a result of the formic acid being oxidized on Pt. This, in turn, produces CO which blocks the site of reaction on the surface of the catalyst. Contrarily, Palladium (Pd) is preferred over Pt because it gives a direct pathway. In other words, the efficiency of the cell is determined by dehydrogenation reaction. This is presented as follows:

Direct pathway: HCOOHCO2+2H++2e-

Indirect pathway: HCOOHCO+H2OCO2+2H++2e-

According to the research conducted by Hong et al., it has been proven that when formic acid is at a higher concentration, the cell tends to lose its performance due to the higher fuel supply rate. (N, Uwitonze, and Chen YX). It may be due to some various factors such as catalyst poisoning, crossover of the formic acid through the membrane from the anode to the cathode, the carbon cloth experiencing diffusion barriers and also the dehydration of the membrane.

In a fuel cell system, the catalyst plays an important role in determining the cells performance. Fuel cell catalysts are categorized into two, Platinum (Pt) and Palladium (Pd). The Pt-based catalyst is involved in oxidizing carbon monoxide (CO) and hydrocarbons. This is done so under excessive oxygen conditions for effective catalyzing oxidation reactions. Although it has a high melting point, platinum does not show any indication of its overall thermal durability. This is because it's not subjected to high temperatures during use. This makes it suitable for the low-temperature fuel cell. Its ability to be efficiently recycled also contributes importantly to its function. Unlike other catalytic materials, it can also minimize its interaction with poisons such as Sulphur compounds to the metal surfaces. On the anodic oxidation reaction of formic acid, Pt is usually in the form of nanoparticles which makes the reaction to be increased regarding the catalytic activity.

Regarding promoting oxidation, Pd based catalyst is more efficient in alkaline media compared to the Pt-based catalyst. However, unlike Pt catalyst, they can be easily poisoned by chemical compounds (N, Uwitonze, and Chen YX). This has been observed in many experiments when Pd is used as the anode. Slow and continuous decay is a common sign. Formic acid oxidation efficiency of Pd has been improvised in various ways such as alloying in which other metals are combined with the catalyst. These include Pd-Pt, Pd-Au, Pd-Cu, and Pd-Ni. This approach prevents aggregation, and the electrocatalytic activity of Pd is thus maximized.

Oxidation potential in Pd is much higher compared to Pt. Likewise its oxides are considered to be more stable. The catalyst is widely used in automotive industries as it minimizes the toxic effect emitted from a combustion engine similar to Pt. Distribution of Pd in fuel cell technology has recently been reviewed and analyzed on its recent development for formic acid and alcohol oxidation together with oxygen reduction reaction (Meng, Hui et al.). The survey showed that 35.12% is for the formic acid oxidation, 30.99% for alcohol oxidation and 19.42% for ORR (oxygen reduction reaction).

Fig: The distribution of Pd in Fuel cell technologies.

Blends are an example of transitional fuels. They comprise of blended conventional and alternative fuels which vary in percentages (Alternative Fuels Data Center: Fuel Blends). This is regarded as a way of petroleum conservation. E10 (10% ethanol/90% gasoline), B5 (5% biodiesel/95% diesel), and B2 (2% biodiesel/98% diesel) are some of the standard low-level blended fuels. Blended fuels can also be in the form of compressed natural gas (HCNG) or hydrogen. However, they cannot be categorized as low-level blends. Ethanol fuel is commonly used in vehicles when blended with gasoline because it rarely contaminates the environment. Biodiesel is considered as a renewable source for fuel consumption. It can also be used in diesel engines as it produces less toxic pollutants and greenhouse gases in comparison to petroleum diesel.

With properties such as being a light element and having an extremely low volumetric energy density, hydrogen has limiting factors when used as a blended fuel (Soloveichik, Grigorii L). Usually, the oxidation process in a fuel occurs by pumping air through the cathode where hydrogen is stored on site. When usin...