What are low temperature fuel cells using different electrolytes?

What are low temperature fuel cells using different electrolytes?

In this category, hydrogen-oxygen fuel cells (H2-O2) operating at ambient temperature have received great attention. H2-O2 fuel cells are called HYDROX cells. Scientists believe that, of all possible electrochemical fuels, hydrogen is the most preferable fuel because it reacts very fast, the reaction by-product water does not corrode the electrodes and acts as a useful electrolyte solvent. Furthermore, hydrogen molecules can be easily decomposed and ionized into protons by chemisorption, followed by desorption from a simple catalyst such as commercial nickel at temperatures over 200–250 °C. In this moderate temperature range, this particular cell can produce current densities ranging from 0.7V, 1200mA/cm² to 0.46V, 2000mA/cm². HYDROX cells are considered the most powerful modern fuel cells. The key components of the HYDROX fuel cell are shown in Figure 1, and the device uses a semi-solid electrolyte. The working temperature and pressure are 200℃ and 600Ib/in² respectively. Despite its high current density and high power capacity, the device has relatively long warm-up times, high operating pressures, and requires ultra-high purity hydrogen and oxygen, which significantly increases operating costs. Pratt and Whitney Aircraft Corporation of America has made significant design improvements to the HYDROX fuel cell.

What are low temperature fuel cells using different electrolytes?
Figure 1. High-pressure H2-O2 fuel cell operating at 200°C and 600 lb/in² using double-layer Ni electrodes

1. Performance of low temperature and low pressure fuel cells using aqueous electrolytes
In this particular fuel cell, the electrons released during the oxidation of the fuel pass through an external circuit to generate electricity. The advantage of this type of cell is that the electrochemical process of fuel oxidation minimizes energy consumption through random heat losses. However, the most notable disadvantage is that the power output of the cell is greatly reduced if any fuel other than pure hydrogen is used. In addition, the carbon oxides produced in the oxidation of the fuel can react with the alkaline electrolyte, which not only consumes the electrolyte, but also the carbonate may block the electrode pores, thereby reducing the reliability and efficiency of the fuel cell.

The key feature of this fuel cell is the electrode, known as the DSK, because it contains two metal frameworks—one is the substrate and structure, and the other is an active, highly dispersed framework. The catalytically active framework is embedded into the highly conductive supporting framework. The fuel electrode is made of hot-pressed Ni. In the case of the oxygen electrode, Ag is used to make the active framework form a substrate. The fuel electrode operates efficiently in pure hydrogen, and it has shown current densities as high as 400A/cm² at ambient temperature and low pressure. However, lower current densities around 50A/cm² yield high reliability. Assuming this current density, with a cell voltage of 0.5V, an efficiency of 50%, and an electrode area of ​​1ft² (929cm²), the power produced by this fuel cell can be determined.

Allis-Chalmers Allis-Chalmers Energy has designed, developed and tested several such fuel cells. The fuel used by the company is a gas mixture. The oxidant is oxygen. This fuel is used to operate heavy tractors. The electrical power produced by this particular fuel cell = 50 × 0.5 × 0.5 × 929 = 11.5 (kW). The power output capability of a fuel cell is shown in this calculation example as a function of electrode area, cell efficiency, electrolyte performance, and cell output voltage. A fuel cell like 1ft², with an efficiency of 60%, a polarization voltage of 0.7V, and a current density of 50A/cm², can produce a power output of close to 19.5kW and can be used in heavy-duty tractors for various industrial applications.

The company has developed a four-cell power module using low temperature (65°C) low pressure (5Ib/in²) cells) using hydrogen or ammonia as fuel and oxygen or air as oxidant. The power module has proven 4500h of trouble-free uninterrupted operation. The company has developed and sold 20-cell power modules with complete control, condenser and circulation systems. Commercial power modules have demonstrated a power output of 1kW for sustainable operation. The fuel and oxidant electrodes are made of catalytically porous Ni, each with a thickness of 0.028 in. The thickness of the alkaline electrolyte is 0.030 in. which is preserved on asbestos or similar porous grids. Grids or asbestos must withstand differential pressures in excess of 1,000 Ib/in. The design of fuel cells requires low electrode vessel resistance and precision machining, which will evenly distribute the fuel or oxidant to the electrodes for optimum performance.

2. Output power performance of water fuel cells
Using hydrogen as fuel, with a polarization voltage of 0.78V per cell and a current density of 130A/ft², this particular fuel cell has a watt-per-pound ratio of about 22 and a volumetric power density of 1.5kW/ft³. If the power module has 20 cells, each cell has an output voltage of 0.76V, a power density of 130 A/ft², and a module efficiency of 65%, the total power output will be about 1.32 kW.

With ammonia and a corrosive electrolyte at 77°C, a 0.3V current density of 160A/ft² can be achieved, and an output power ratio per unit volume can be achieved right from the start. However, during the lifetime test, the current density dropped below 50 A/ft² after 700h of continuous operation at 0.3V.

The fuel cell can work with methanol and other alcohols as fuel, acid and alkaline electrolytes, and air or oxygen as oxidant. The design of the prototype cell has demonstrated that the continuous output power of the cell exceeds 1kw, and this device has demonstrated that the current density at 0.3V will be better than 80A/ft³ if the life of the electrodes used exceeds 250 h or more, per unit volume The power output is 0.8kW/ft³, and this battery structure is the same as the H2-O2 battery.