Toyota launches proton exchange membrane hydrogen fuel cell

Recently, Toyota Motor Sales of America announced the start-up of a 1.1-megawatt hydrogen fuel cell generator set at Toyota's U.S. headquarters in Torrance. During peak electricity demand periods, these fuel cells will provide half of the electricity supply for the six headquarters buildings.

The proton exchange membrane hydrogen fuel cell used this time was designed and built by Ballard Power Systems. Compared with fuel cells of the same category, this unique proton exchange membrane hydrogen fuel solid-state battery is currently the largest proton exchange membrane fuel cell. As the primary energy supply for hydrogen fuel cells, hydrogen is directly connected to the fuel cell through a pre-installed industrial hydrogen pipeline. This direct energy supply method can greatly reduce the power loss of the power grid itself during peak electricity consumption. In addition, the hydrogen supply station, which is the energy source of Toyota fuel cells and other hybrid vehicle fuel cells, is also connected to these industrial hydrogen pipelines to directly provide them with a source of hydrogen.

As a clean energy source, the 1.1 megawatt power generation capacity of proton exchange membrane fuel cells can provide sufficient electricity for about 765 households on average. This is twice the amount of electricity that Toyota's existing solar panels can generate. It is estimated that the application of proton exchange membrane fuel cells can reduce carbon dioxide emissions by as much as 3.3 million pounds during the peak period of summer electricity consumption, which is equivalent to the total carbon dioxide emissions of 294 cars in a year.

The energy source used by the PEM fuel cell - hydrogen - is provided by Air Products and Chemicals, where the hydrogen is converted from natural gas. The hydrogen PEM fuel cell method has the advantage of zero exhaust emissions compared to the method of purchasing renewable biogas generated by landfills, thereby reducing exhaust emissions during the conversion process.

Toyota expects to save about $130,000 per year in energy purchases from Southern California Edison by using the hydrogen PEM fuel cell. In addition, the hydrogen PEM fuel cell project is also partially funded by the Canadian Sustainable Development Program and the California Self-Generation Incentive Program.

As a new generation of power generation technology, proton exchange membrane fuel cell power generation has a broad application prospect comparable to that of computer technology. After years of basic research and application development, substantial progress has been made in the research of proton exchange membrane fuel cells as automobile power. Micro proton exchange membrane fuel cell portable power supplies and small proton exchange membrane fuel cell mobile power supplies have reached the level of productization, and research on medium and high power proton exchange membrane fuel cell power generation systems has also achieved certain results.

Proton exchange membrane fuel cells (PEMFC) have low operating temperatures and are suitable for frequent starting. They also have the advantages of fast starting, high power density, and long driving range. Therefore, they are considered the best choice for automotive fuel cells and are expected to become one of the power sources to replace current automotive power. However, the high cost, short life, and fuel issues of PEMFC vehicles have seriously restricted their commercialization. This article will explain these three issues.

Cost of Proton Exchange Membrane Fuel Cells

At present, the biggest problem of PEMFC commercialization is that the cost is too high. The current cost of PEMFC is 1000~2000 US dollars/kW. Maybe the cost of fuel cells will drop to 200 US dollars/kW soon, but in order to compete with internal combustion engines, the cost of fuel cells must be reduced to 50 US dollars/kW. To reduce the cost of PEMFC, the cost of three key components (i.e. electrodes, electrolyte membranes and bipolar plates) must be reduced.

1. Electrodes

So far, the cathode and anode effective catalysts of PEMFC are still mainly platinum (Pt), and the excessive platinum loading of the electrode has always been an important factor hindering the development of PEMFC. In order to reduce the use of Pt, major companies around the world have conducted a lot of research. After a lot of research, it has been proved that PEMFC working with hydrogen/air still has good fuel cell performance when the platinum loading is much lower than 40mg/cm2. When the anode uses pure hydrogen as fuel, there is no major technical problem in reducing the platinum loading. When working at high current density, the platinum loading of the anode catalyst as low as 0.025mg/cm2 is sufficient. However, proton exchange membrane fuel cell vehicles almost always use hydrogen produced by reforming carbon-containing fuels such as natural gas, methanol, gasoline or diesel as anode fuel. Since the anode will be poisoned by trace CO in the newly produced hydrogen reforming product, it is difficult to obtain satisfactory fuel cell performance and service life with this type of fuel. The toxicity and dilution effect of CO2 will also reduce the performance of the fuel cell. The catalyst with good toxicity resistance to both CO and CO2 is platinum/ruthenium Pt/Ru (atomic percentage 50:50). It has been proven that a platinum/ruthenium anode containing 0.4 mg/cm2 of platinum can fully withstand the toxicity of 100 mg/L of CO in hydrogen at 90°C and 48.27 kPa. Experiments have shown that using Pt/Ru as a catalyst, the performance of the fuel cell is stable and the working life can exceed 5000 hours. At present, the application of this low-platinum-loaded Pt/Ru anode catalyst in automobiles still needs to further improve the toxicity resistance of the anode catalyst to obtain high power density.

According to the latest progress in international research, the platinum loading of the electrode has been reduced to 0.02 mg/cm2, greatly reducing the cost.

In recent decades, the platinum loading on the membrane electrode has dropped from 10mg/cm2 to 0.02mg/cm2, and the platinum loading has been reduced by nearly 200 times. In the future, the platinum loading may be reduced, but not too much, and a fatal disadvantage of platinum catalysts is that they are easily poisoned by CO and other impurities. Therefore, reducing the amount of platinum in electrocatalysts, seeking cheap catalysts, and improving the performance of electrode catalysts are the main goals of current electrode catalyst research. For cathode catalysts, the research focus is on improving the electrode structure and increasing the utilization rate of catalysts, and on the other hand, it is to seek efficient and cheap catalysts that can replace precious metals. Anode catalysts mainly study catalysts that are resistant to CO poisoning.

2. Electrolyte membrane

The proton exchange membrane is the core component of PEMFC. As an extremely thin film with a thickness of only 50~180um, the proton exchange membrane is the base of the fuel cell electrolyte and electrode active material (catalyst). Its main function is to have selective permeability under certain temperature and humidity conditions, that is, only H ions (protons) are allowed to pass through, while H2 molecules and other ions are not allowed to pass through. At the same time, it has a moderate water content and is stable to the oxidation, reduction and hydrolysis reactions during the operation of the fuel cell. The proton exchange membrane has sufficiently high mechanical strength and structural strength, and the membrane surface is suitable for combining with the catalyst and other properties.

At present, Nafion membranes are widely used in proton exchange membrane fuel cells. Nafion membrane electrolytes have long been used in brine electrolysis in the chlor-alkali industry. Because of its service life of up to 50,000 hours in corrosive environments, it has attracted much attention. Over the years, many important advances in the development of PEMFC are closely related to the use of Nafion membranes. Nafion is a perfluorosulfonic acid polymer with a fluorinated copolymer skeleton. The sulfonic acid groups are chemically linked to it and have a fixed position, while the protons are free to connect and can act as a conductor. The price of proton exchange membranes on the current market is still quite expensive. The price of perfluorosulfonic acid membranes (Nafion) produced by DuPont in the United States is $800/m2.

Ballard has developed a partially fluorinated membrane material that may be suitable for automobiles. The initial performance of this membrane is at least comparable to that of the Dow membrane and Nafion112 membrane in the Mark V single cell, and it has been proven that the performance of this Ballard membrane can meet the requirements of automotive applications, with a working time of more than 4500 hours. Due to the low processing cost of this membrane and the higher production of partially fluorinated polymers, the cost of the membrane can be reduced to $50/m2 if the demand is large.

Nafion membrane electrolyte has good fuel cell performance and service life, but its preparation process is very complicated and expensive. The partially fluorinated sulfonic acid membrane under development has good application prospects due to its reasonable price and good chemical and thermal stability, but its main disadvantage is that it becomes brittle after dehydration.

3. Bipolar Plate

Bipolar plates are the main components of fuel cell stacks. Their functions are to separate the reactant gases, collect current, connect the individual cells in series, and provide channels for the reactant gases to enter the electrodes and for the water to be discharged through the flow field. Usually, the materials of PEMFC bipolar plates are graphite, graphite and polymer composites, or metals.

The most widely used bipolar plate at present is the machined graphite plate. Since pure graphite plates are hard and brittle, they require precise machining, so the cost is relatively high, accounting for about 60% of the cost of the fuel cell stack. The graphite bipolar plate technology is quite mature, but it is difficult to reduce its material cost and processing cost. Therefore, seeking a low-priced, conductive and easy-to-process bipolar plate material is the key to reducing the cost of bipolar plates and making them commercialized.

Metal materials have good electrical conductivity, high mechanical strength, good gas barrier properties, easy processing and easy mass production, and the thickness can be greatly reduced, which can greatly improve the volume power density of the fuel cell stack. They are one of the candidate materials for bipolar plates. However, common metal plates (such as stainless steel, aluminum alloy and titanium plates, etc.) are corroded in the working environment of PEMFC (including thickening of cathode oxide film, corrosion of anode, etc.), resulting in increased contact resistance, contamination of electrode catalysts and membranes, and reduced fuel cell performance. Therefore, when using metal as the bipolar plate material for PEMFC, the surface of the metal must be modified.

The most widely used bipolar plate is graphite bipolar plate, which is processed by precision machining. The processing cost is quite high (the processing cost of each 500cm2 bipolar plate is more than 100 US dollars), accounting for more than 80% of the cost of bipolar plates and 40% to 60% of the cost of fuel cell stacks. In addition, the processing time is relatively long, and it is not easy to mass produce. Therefore, it is necessary to seek a bipolar plate processing method with simple processing method, low operating cost, and easy mass production to reduce the cost of fuel cells.