Low platinum fuel cells for hydrogen vehicles

Scientists from Sweden's Chalmers University of Technology have modeled low platinum (Pt) fuel cells for use in hydrogen fuel cell vehicles (FCVs). They have based their work on previous experimental findings about low-loading Pt catalysts to approximate the development's automotive application.

“This [is] a modeling work, and from the modeling to the real application, there are many challenges to consider, for instance, whether these or similar catalysts can be produced at a large scale,” corresponding author, Tatiana Santos Andrade, told pv magazine.

In hydrogen fuel cells Pt is used as a catalyst material in the cathode that is supplied with oxygen in the process of making electric current. Commercialized FCVs, such as Toyota Mirai, use 0.31 mg/cm2 of Pt on the cathode, while experimental researchers have been able to lower it down to 0.01 mg/cm2 at a cell level.

“The amount of platinum is considered to be the bottleneck for the spread commercialization of fuel cells,” the research group explained. “Platinum is pointed out as responsible for about 40–55% of the total cost of the stack fuel. Due to the low kinetic of the oxygen reduction reaction that takes place in the cathode, the current Pt-loading on that electrode is the critical design part accounting for about 85% of the total platinum of the fuel cell stack.”

In their analysis, the team scaled up four types of low-loading fuel cells from the literature – two with 0.033-0.035 mg-Pt/cm2 and two with 0.01 mg-Pt/ cm2. The systems were referred to as E1 – 3.3 Pt, E2 -3.5 Pt, E3 – 1.0 Pt, and E4 – 1.0 Pt, respectively. E1 and E2 were grouped as low-loading Pt catalysts, while E3 and E4 were ultra-low-loading Pt catalysts.

“The first two are catalysts consisting of platinum-cobalt core-shell nanoparticles in a platinum-free catalytic substrate with slightly different Pt loading,” the academics stressed. “The last two are channeled-mesoporous carbon (CMC) particles with PtFe with slightly different channel porous.”

Based on those cells' power-efficiency curves, the researchers were able to upscale them to a stack and system level and compare them to the Toyota Mirai. The Mirai has 370 cells with an area of 237 cm2, a maximum power of 114 kW, and a cathode Pt loading of 0.31 mg/cm2. The researchers compared the commercial system to two models – one in which the low-Pt fuel cells system has the same stack size, of 370 cells, and the other with the same maximum power of 114 kW.

In the same stack size scenario, the researcher found E1 to have a maximum power of 62-68 kW, E2 to have 70-77 kW, E3 to have 48-53 kW, and E4 with 45-49 kW. However, although their power reached 52% of the commercial system at best, at lower power the fuel cell systems comprising low-loading Pt catalysts presented a higher efficiency.

“When a relation of power (product of voltage and current) is established, it emphasizes the high variation in efficiency at high power values for samples E1-E4,” the scientists said. “The more dramatic drop compared to the commercial FCV stack indicates that these materials demonstrated less stable performance at different power ranges.”

In the second model, when the same power was the target, E1 needed 623-677 cells, E2 552-600, E3 795-864, and E4 860-935. “If just the Pt amount is considered, even with the increased number of cells, the samples would still represent a Pt reduction of 81%, 82%, 93%, and 92% compared to the commercial FCV for samples E1-E4, respectively. It can lower the stack cost by about 27–45%,” they emphasized.

Following those results, they modeled the system in an FCV and simulated it with the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). The car had a battery with a state of charge (SOC) of 20%-95%, a 4 kg hydrogen tank, and a control strategy that switched between the fuel cells and the battery when needed.

According to their results, in the commercial car and the E1-E4 cases, the power requirement from the fuel cell is always lower than 40 kW under the WLTP test. Therefore, all models could supply competitive results compared to the 628 km range of the reference car.

Under the same rack size, E1 had a range of 646km-651km, E2 had 641km-646km, E3 had 632km-638km, and E4 had 617km-623km. In optimizing the same maximum power, E1 had a range of 662km-665km, E2 had 654km-656km, E3 had 632km-638km, and E4 had 646km-647km.

“In catalyst development, researchers are usually focused on improving the maximum power, which is a relevant metric to consider, while the whole fuel cell efficiency profile, e.g. at lower to medium power, is usually overlooked,” Andrade stated. “That can be a relevant factor in making the fuel cell suitable for vehicle applications, since the fuel cell is usually oversized for a vehicle. I hope this paper can function as a bridge between material science research and vehicle implementation.”

Their findings were presented in the paper “Low platinum fuel cell as an enabler for the hydrogen fuel cell vehicle,” published in the Journal of Power Sources.