What is ion exchange membrane fuel cell? How about its performance?

What are the specifications of the ion exchange membrane fuel cell and its performance?

In 1962, General Electric (GE) designed and developed ion exchange membrane fuel cell (IEM). Scientists at GE made remarkable progress in optimizing the current density parameters of the cell. The use of a highly improved electrolyte showed a significant increase in current density, also at 0.7V, from 17.5w/ft in 1960 ² 35W / cm by 1962 ²。 The new film material is responsible for achieving a ratio of 2kW / ft ³ Better power density per unit volume. Although other performance parameters have been significantly improved, the steady-state output voltage of H2-O2 fuel cell has never exceeded 0.93v. Ge scientists found that the optimization of oxygen electrode can improve cell efficiency and open circuit voltage at the same time.

  1. Performance specifications of IEM fuel cells and battery packs for space applications

The polarization data of propane, propylene and cyclopropane collected by various scientists show that cyclopropane produces the highest current density in these hydrocarbons. Scientists’ research shows that 38ma / cm can be achieved with platinum catalyst and 3mol sulfuric acid electrolyte at about 90 ℃ ² The current density, which will reduce the resistance loss in the battery to about 35 MA / cm ²。 The latest research data show that the current density of propane has reached 90 ~ 100 mA / cm at 0.3-0.4v ² The high-capacity IEM battery was developed to be deployed on the power supply of NASA’s Gemini project. NASA scientists claimed that the 30min, 600mile and high orbit rocket flight in 1960 showed the preliminary feasibility of using IEM fuel cells in space. Two such batteries, each containing 35 battery units with 28V 50W output power, were tested in an orbiting satellite. These fuel cells work intermittently for 30 days under full load conditions, which can be interpreted as 7 days of uninterrupted operation of the satellite.

A portable 200W power supply consisting of IEM fuel cell units operating with air and hydrogen demonstrates its ability to power mobile army radio receivers, transmitter equipment and battlefield surveillance radar devices. A power module composed of fuel cells, which uses diesel or methanol as fuel to produce hydrogen and liquid oxygen as oxidant, has proved its application in submarine propulsion system. This special power system shows more than 2kW / ft ³ It can meet the requirements of hull space.

Thompson, of Cleveland, Ohio, has developed a high-capacity power supply system using fuel cells. The power supply system includes two electrodes, each pressed against the ion exchange membrane and separated by the membrane and the intermediate electrolyte. In the possible applications of manned and unmanned spacecraft, the power system design strictly uses the double membrane battery structure design. Its performance parameters include 2kW / ft ³ Volume power density, low-voltage operation, quick start function of 90% of full power, reliability under space conditions, and power efficiency of 55% ~ 70% in severe space environment.

  1. Fuel cell using low-cost porous silicon substrate material
Fuel cell with porous silicon substrate material
Fuel cell with porous silicon substrate material

The author’s research on medium capacity cells shows that silicon-based fuel cells have the following advantages, such as simple design, lowest cost and small volume. Such a fuel cell unit is most suitable for domestic use, with a power range of about 3 ~ 5 kW. The substrate is porous and highly structured, which can provide faster electrode reaction, and liquid or gaseous electrolyte can be used. This type of fuel cell provides a unique design structure because of the well controlled geometric characteristics of porous silicon structure. Due to the pore size, the value range is 10 ~ 50 mm, and the small deviation in distribution, porous silicon shows significant advantages. Fuel cell designers and scientists believe that this special fuel cell design concept will be a perfect engineering device and the most attractive for home applications. The device manufacturing process is fully mature because it depends on clearly defined silicon processing technology and methods. It is widely used in mass production. In short, the silicon processing methods and quality control technologies used in the manufacture of silicon devices, such as transistors and diodes, can be used to manufacture fuel cells at the lowest cost and complexity. If you are also interested in the relevant knowledge of lithium battery, you can click to enter tycorun lithium battery for reading and learning.

Fuel cell designers believe that there is no other fuel cell technology to prove the feasibility of mass production at the lowest cost. This design concept provides an opportunity to deliver universal and highly scalable fuel cells for applications with key requirements of cost, reliability, weight, size and life.

2.1 . Hydrogen oxygen power fuel cell fuel cell with porous silicon structure

Structural diagram of oxygen power fuel cell
Structural diagram of oxygen power fuel cell

Scientists are currently studying H2-O2 fuel cells with porous silicon structure, liquid state and acid as electrolyte. The gas diffusion (GD) interface is in the pores of the silicon substrate extending to the whole pore. This special fuel cell design concept provides a spontaneous power supply and an electrolytic cell unit. This device is equipped with a modified silicon electrode for the gaseous reactant, which creates a gas diffusion interface inside the fine pores of the silicon structure extending to the whole pore. This type of fuel cell is most suitable for low to medium power applications, ranging from 1 to 5kW. This fuel cell is ideal for power modules of motorcycles and household appliances. A retired NASA scientist recently demonstrated a prototype design of a fuel cell containing hundreds of such single cells in the form of stacks, using porous silicon electrodes and methanol fuel. The size of porous silicon disc is about 4.5in x 4.5in, and the overall size is close to a shoebox. Scientists predict that the current density is about 180MW / cm ²。 It is assumed that the current density and porous silicon electrode are 4.5 in × 4.5in, the energy generated by a single element will be about 23.5w. Assuming that a stack consists of 100 porous silicon disk units, the electric power generated by the 100 unit fuel cell will be close to (100 × 24 =2400W)2.4kW。 If the stack contains 200 units, the output power is about 4.8KW, which is enough to meet the power demand of a family.

These calculations provide complete information on the physical dimensions of the fuel cell stack capable of generating specific electrical energy. The energy generated by various components does not take into account various losses or battery efficiency.

There is no reliable estimate of the manufacturing cost and total weight of this fuel cell, because the manufacturer has not been contacted so far. However, the author’s best engineering judgment shows that the weight of this special fuel cell pack will not exceed about 20ib. The above fuel cells work like direct methanol fuel cells. Direct methanol fuel cell power supply is most suitable for portable power supply, because direct methanol fuel cell technology has many advantages, such as compact shape, high reliability, reasonable price, and minimum weight and size.

2.2 . Reaction and thermodynamic efficiency of fuel cell

Regardless of the type of single cell, the reaction and thermodynamic efficiency of fuel cell must be carefully considered. These two problems will have an impact on the electric energy obtained from the battery and the total amount of heat released through chemical reaction. In classical thermodynamic theory, reaction enthalpy( Δ HR) is the heat released by chemical reaction and the reaction entropy( Δ SR) indicates the degree of change in system order during the reaction. Exothermic parameters( Δ HR) is negative. Therefore, the entropy of the reaction( Δ SR) becomes less than zero or becomes negative.

To better understand these parameters, consider a simple H2-O2 fuel cell. In addition to the heat generated by the internal resistance of the electrochemical cell through the potential drop, part of the chemical energy is converted into heat in the process of electrochemical energy conversion, and the electrolyte resistance also plays a role in the potential drop.

For H2-O2 fuel cells, thermodynamic efficiency or ideal energy conversion efficiency is always related to the reaction enthalpy of the included chemical process. The ideal thermal efficiency of a single cell can be expressed as

Ideal thermal efficiency of single cell
Ideal thermal efficiency of single cell

Where: E0 is the battery voltage of thermodynamics; E0h is the voltage of the thermal battery.

For this particular fuel cell reaction, the chemical equation of the reaction can be written as

Specific fuel cell reaction representation
Specific fuel cell reaction representation

This clearly shows that the by-product of the chemical reaction is water.

In H2-O2 fuel cells, the typical single cell voltage (E00) under standard conditions or normal temperature and pressure conditions is usually 1.23V, and the thermodynamic cell voltage (e0h) is about 1.48v. For almost all fuel cell reactions, reaction entropy is expected( Δ SR) is less than zero. This means that the heat generated in the surrounding area will be a temperature (T) and reaction entropy( Δ Product of SR). With the conversion from chemical energy to electric energy, additional energy is obtained by absorbing heat from the surrounding environment, so that the theoretically ideal battery efficiency is greater than 100%.

The data show that formic acid produces the highest battery efficiency because the change of free energy of oxidation is greater than that of reactive melting. In other words, as chemical energy is converted to electrical energy, additional energy is obtained by absorbing heat from the surrounding. When the entropy of the reaction( Δ When SR) is less than zero or negative, the efficiency of the cell is less than 100% regardless of the fuel used. This is why hydrogen provides the lowest battery efficiency.

2.3 . DMFC (direct methanol fuel cell) equipment using PEM (proton exchange membrane) structure

Direct methanol fuel cell equipment with PEM structure uses air as a reactant. DMFC equipment has been significantly improved since its development in the 1960s, but they still have certain technical and commercial limitations, including operation problems, water and water management problems, poor reliability, low power density, higher procurement cost and lower conversion efficiency. Most of the disadvantages of a liquid or non porous fuel cell design have been eliminated by minimizing the use of a liquid electrolyte.

2.3.1 design and operation of PEM based DMFC equipment

All fuel cells have two electrodes: a negative electrode and a positive electrode. For direct methanol fuel cells, electric energy is generated by methanol fuel at the negative electrode (also known as anode) and oxygen in the air at the positive electrode (called cathode). When the fuel, catalyst and electrolyte are at a common point called three-phase interface, a chemical reaction occurs to produce electric energy. The current design of direct methanol fuel cell establishes this three-phase interface on the surface of a polymer material called PEM. However, the film limits the reaction zone to the surface or two-dimensional region, which limits the electric power output of the battery, because the generated electric energy is strictly dependent on the active zone. Experienced battery designers have observed a typical power density of 65 ~ 85 MW / cm ², PEM based devices have some inherent technical and commercial limitations [3], which can be briefly summarized as follows.

2.3.1.1 electrochemical efficiency depending on the utilization of methanol fuel

The conversion of methanol (CH3OH) occurs when methanol passes from the anode through the PEM medium and reacts with the catalyst at the cathode. This reduces the battery voltage and wastes a certain amount of fuel. Although the use of diluted methanol minimizes the conversion, it needs to carry additional water, thus reducing the weight and energy density of the device.

2.3.1.2 reasons for reducing conversion efficiency and power density

Fuel cell scientists believe that due to the limitation of methanol chemical reaction, PEM based DMFC equipment using methanol as fuel usually has a working efficiency of 20% ~ 30%. In particular, methanol conversion not only reduces the voltage and power density, but also reduces the conversion efficiency of the device.

2.3.1.3 influence of surrounding environment on performance of PEM based DMFC equipment

If obtaining the maximum electric power from the equipment is the main design requirement, the control of moisture content is very important. According to DMFC design experts, if the DMFC device generates too much water near the device, its performance will be reduced. Moreover, European fuel cell scientists have also expressed serious concern about the vaporization of water in thin-film cells. Fuel cell scientists have observed various problems arising from the surrounding environment. For example, the problem of carbon dioxide pollution in alkaline batteries using thin-film electrodes has emerged. The heat around the battery structure affects the electrical performance and reliability of the battery.

2.3.1.4 factors affecting the structural integrity of PEM based DMFC equipment

The designer of DMFC battery believes that the performance of such equipment may be reduced due to the factors described in this paragraph. Because these devices use air as reactant, any toxicity and pollution in the air will significantly degrade the cathode. The existence of high humidity will reduce the performance of proton exchange membrane. Even low humidity can cause cracks in PEM. In short, operating such a battery under high temperature, high humidity, excessive impact and vibration will significantly reduce the electrical performance, reliability and service life of PEM based equipment.

Despite these disadvantages, this type of fuel cell also has some advantages. It can use the cheapest fuel, gas fuel, liquid fuel, or a combination of both in low and medium temperature environment [6]. The fuel cell provides the following advantages:

 use rich methanol fuel.

 the working temperature of direct methanol fuel cell is lower than 150 ℃.

 it does not produce Noro, and the contact between methanol and acidic membrane is stable and easy to handle.

 the power generated can be changed simply by changing the methanol supply, which is the main advantage of the device.

The current density generated by the continuous battery at the open circuit voltage of 0.7V is greater than 150A / ft ², It has been proved that there is no battery failure or performance degradation after working for more than 3000h. Using carbon anodes, this battery can achieve a maximum battery efficiency of about 35%.

  1. Silicon based DMFC fuel cell
Silicon based DMFC fuel cell
Silicon based DMFC fuel cell

The preliminary research conducted by the author shows that the silicon-based DMFC fuel cell does not seem to have the problems of PEM based DMFC equipment. The research further shows that the silicon-based direct methanol fuel cell device using liquid electrolyte can provide higher power density and portability, especially in the battlefield environment. Using liquid electrolyte can achieve a wider range of reaction zone, which will produce higher power density than PEM based devices. These fuel cell units can also use suitable oxidants, and the equipment can be manufactured at low cost and high reliability.

These calculations assume that the power density of proton exchange membrane based equipment is 65-85 MW / cm ², The power density of silicon-based DMFC equipment is about 180 ~ 205 MW / cm ²。 These calculations show that the silicon-based DMFC device can meet the power requirements of some household and industrial applications.

It is obvious from the above calculation that silicon-based direct methanol fuel cell equipment can be manufactured with larger electrodes to achieve higher bulk energy density. Fuel cells can be stacked to meet specific power requirements. In addition, the size of the surface area can be changed to accommodate the fuel cell in a given space. In other words, one-dimensional dimensions can be slightly smaller or larger than other dimensions to accommodate devices in a given available space.

The following are the unique design characteristics and potential benefits of silicon-based DMFC.

(1) DMFC provides a closed system operation: fuel cell designers believe that porous silicon electrodes can be easily assembled into cells and stacks with minimal separation. Methanol fuel and oxidant react at the position of catalyst in porous silicon structure to produce electric energy. After the electrochemical reaction is completed, the residual or residual fuel can be removed from the cell through the electrode with a continuously flowing liquid.

(2) DMFC allows efficient non air oxygen supply operation: since the fuel does not use air as oxidant, it can be used in special operating environments, such as underwater or smoke filled buildings, without reliability or life loss.

(3) DMFC uses all methanol fuel single cells: the design of DMFC adopts the cycle operation process and continues to operate until all methanol fuel available in replaceable fuel container or box is consumed. If further CH3OH supply is required, another box filled with CH3OH can be inserted to maintain a constant power supply.

(4) DMFC can be used as a reservoir to collect excess water: the fuel box is the source of oxidant required by the cell. At the same time, it can also be used as a reservoir to collect excess water generated in the electrochemical process. Because of this, these fuel cells do not expose the stack to the ambient air or discharge hot water vapor around the fuel cell.

(5) DMFC can be customized for any application with minimum cost and complexity: key components related to DMFC assembly, such as electrodes, micropumps (MPS), heat exchangers and fuel cartridges, can be fabricated and modularized according to the specified size for any application. This flexibility provides the best equipment performance, maximum design flexibility and lowest design cost. In order to show the flexibility of the design, the natural material box can produce 4, 8, 12 or 16h continuous operation by simply replacing the box. This allows suppliers of electronic, electrical and mechanical components to provide the most suitable multiple choices or combinations for specific applications. This personalized approach enables fuel cell customers to choose the size of the fuel cell most suitable for their application.

(6) DMFC utilizes a mature and risk-free Technology: according to the discussion of the advantages of silicon-based DMFC, silicon-based fuel cells can be manufactured using the existing mature and cost-effective infrastructure in the semiconductor processing industry. In addition, the quality control technology and reliability standard test widely used in the semiconductor industry can be adopted without any additional cost. In other words, the manufacturing cost of these devices will be very low. In addition, the manufacturing of some fuel cell components, such as heat exchanger, MP, electrode, fuel box and printed circuit board, can be outsourced if necessary to further reduce manufacturing costs and procurement delays. Due to its advantages in cost, performance and reliability, silicon-based DMFC is a key technology of portable electronic, electrical and mechanical equipment for various applications in the future, from automobile to portable power supply to unmanned aerial vehicle (UAV) to communication satellite.

(7) Component requirements and design of silicon-based DMFC: the key components of silicon-based DMFC include a micropump with fluid header, anode and cathode electrodes, power management battery, scrubber equipment, fluid sensor, monitoring device capable of continuously monitoring temperature and pressure, active elements of stack and methanol fuel box.

Among all these components, micropump MP is the most important component of silicon-based direct methanol fuel cell, and its manufacturing uses the principle of nanotechnology. MP adopts electrostatic or piezoelectric actuation mechanism. The pump requires several milliwatts of power, with or without valves and no moving parts, so it provides ultra-high reliability. This pump is most suitable for fuel delivery applications, in which case constant and uniform fuel flow is the main requirement. Most other components are off the shelf and delivered immediately on the market. Different fuel capacity cartridges can be purchased on the free market.

Read more: The Basics of Batteries for Space and Communication Satellites