- Electrochemical Impedance Analysis for Fuel Cell
Electrochemical Impedance Analysis for Fuel Cell | LinkedInMain Topic: PEFC (Polymer Electrolyte Fuel Cell)
(1) PEFC Equivalent Circuit Model
According to Matsumoto, the equivalent circuit model of a PEFC is composed of
- an Ra-Cd high-frequency component and
- an Rr low-frequency component.
Ra-Cd high-frequency component is mainly attributed from the activation polarization on water generation at the cathode. The H2O generation from O2 takes place after
- the H+ diffusion through the electrolyte and ionomers from the anode to the cathode,
- O2 permeation from the air to Pt catalysts at the cathode through the ionomers. This process takes place at the solid-liquid-gas three-phase interface, thus, an electric double layer capacitance must be included in the model.
- the resistance of H+ conduction through the electrolyte and the resistance of electron conduction through the separator, the gas-diffusion layer, and the catalyst layer, and
- the resistance of the gas (mainly O2) diffusion. Note that the flooding mainly occurs at the cathode gas-diffusion layer and the cathode catalyst layer.
- There is a case that FC is not operated at the maximum power. According to Manabe et al., the cold start has been achieved within 1 minute by (1) decreasing the moisture content to ~ 0% upon stop of a fuel cell system, (2) improved water-drainage from MEA upon the stop, and (3) temperature-rise rate maximization upon start of a fuel cell system by (a) reducing the heat capacity (by using the metal-based separator), by (b) pulling out a larger current than usual, and by (c) increasing oxygen concentration overpotential (by lowering air/fuel ratio *1). 『真鍋昇太等、「燃料電池ハイブリッド自動車における燃料電池急速暖気運転の開発」、自動車技術会論文集、2010年、第41巻、p.1379-1385．』
- *1: According to Naganuma, the lowest oxygen excess ratio (= O2-supplied/O2-reacted ratio) for the cold start is 1.03 that can suppress the H2 regeneration at the cathode (so-called pumping H2 generation). 『長沼良明等、「氷点下環境での燃料電池急速暖気制御の開発」、自動車技術会論文集、2013年、第44巻、p.1021-1026．』
(2) PEFC Durability
According to Osaka Gas, the main deterioration factors are
- Electrolyte (ionomers) oxidation (decomposition), which results in the Ra-increase, resulting from the reaction with hydroxy radicals, HO・ and H2O・ which are the products of the reaction between the peroxide, H2O2, and the impurities such as Fe2+ and Cu2+, thus usually the radical quencher, Ce3+ or Mn2+, is added to the layer,
- Anode catalyst deactivation (when the fuel does not contain CO, this does not take place.),
- Cathode catalyst oxidation (then, dissolves)/reduction (then re-precipitates) resulting in the decrease in the specific surface area of catalyst particles and the carbon oxidation resulting in the cathode catalyst loss (Ra increases and Cd decreases), and
- Cathode gas-diffusion deterioration, which results in the Rr-increase, mainly resulting from the water-repellent material loss (see the above-mentioned electrolyte oxidation).
In 2015, Toyota reported that the worst case scenario is 15% power decline per 15 years. The durability has been improved year by year because of new composition/structure catalysts, the new catalyst support, the operation scheme etc.
Supplementary on #3: "Cathode catalyst oxidation/reduction resulting in the decrease in the specific surface area." According to Sugawara (『菅原優、固体高分子形燃料電池模擬環境における白金の溶解機構、ま て り あ、第53巻、第4 号、(2014)、p165-p169．』),
- The PEFC stack voltage can reach 0.6-0.95 V upon FCV-driving, and can reach 1.5 V upon start if air exists at the anode.
- Platinum (Pt) can chemically dissolve at 0.8 V or higher, thus, when the oxidation of Pt at >= 0.8 V and the reduction at < 0.8 V take place over and over again, Pt particles will grow and decrease the specific surface area, resulting in the lower output current limit. Note that oxide surface layer of Pt grains are stable at >= 0.8 V; even so, repeated oxidation/reduction can increase the risk of the Pt particles' dissolution and re-growth.
(3) PEFC Control
Materials mass-transfers are the rate-determining steps: it is obvious from the above-mentioned current-voltage curve and the equivalent circuit model.
- In case the load draws more current from the fuel cell voltage developed by the stack is reduced due to the phenomena are known as the polarization of PEMFC, this can significantly affect the performance. Therefore controllers need to be implemented in such a way that it can control the air and hydrogen flow rate so as to control the output voltage.
- There is another performance related issue encountered in PEMFC stacks known as a slow cold start. At a lower temperature, it becomes difficult for starting the PEMFC stack quickly due to the slowed down fuel cell dynamics because of stacking. Apart from that stacking of a number of fuel cells also significantly increases the temperature. Uncontrolled higher temperature can ultimately result in softening and even destroying the membrane.
- To generate higher current and power from the PEMFC stack it is required to keep the membrane sufficiently wet during the operating cycles. Fuel cells also generate water at the cathodes due to electrochemical reactions. Therefore it is required to precisely control the water flow to and from the PEMFC stack to avoid flooding of electrodes and scarcity of water in cells (Nehrir et al, 2009).
- the reactants subsystem,
- the thermal subsystem,
- the water management subsystem,
- the power conditioning subsystem and the power management subsystem.
- The largest time constant is - 100 s on time-varying temperature, T(t).
- The second largest is - 10 s on the inertia.
- The third is - 1 s on reactants pressure, p(t).
- The fourth, electrochemical reaction, is quite fast.
Thus, the main objective of the fuel cell control system becomes reactants supply.
- Air supply is slower than hydrogen supply, then, hydrogen supply is controlled by receiving the cathode inlet manifold pressure as a pilot signal.
- As such, the compressor drive voltage is controlled by receiving the air mass flow rate or the air pressure, the stack voltage, and the current withdrawn from the stack.
- By withdrawing the current, the stack voltage decreases because of the oxygen depletion; therefore, the cathode must be quickly replenished with oxygen. Otherwise, the stack cannot continue to supply the electric power, and can be deteriorated.
In 2004, Pukrushpan et al. listed up the work required in the future as follows:
- Each component model, such as compressor or blower, manifold, etc.
- Stack humidity model, including flooding.
- Spatially distributed partial pressures and temperature.
- Hydrogen recirculation, which can cause the control delay because of the additional volume.
As of 2020, many technologies have been advanced.
For example,
- #2: As mentioned before, 『酒井政信 他、「高精度内部抵抗センシング技術を用いた自動車用燃料電池湿潤制御システムの開発」、自動車技術会論文集、2015年、第46巻、p.1073-1078．』
- #3: As mentioned before, 『真鍋昇太等、「燃料電池ハイブリッド自動車における燃料電池急速暖気運転の開発」、自動車技術会論文集、2010年、第41巻、p.1379-1385．』
- #4: トヨタFCV「MIRAI」に搭載 -- FCスタック一体化により小型化した水素循環ポンプを新開発。豊田自動織機。
- When a step load change happens, a 2.5 V drop in the output voltage is observed for a few hundreds of ms. This output voltage drop is related to a output power drop, that is produced because the compressor has a slow response.
- Furthermore, this transitory period can damage the fuel cell operation. Therefore, it is important to limit the current variation if the fuel cell is working without another auxiliary source. If there is a auxiliary source, this source must supply the power peak.
- In applications when the fuel cell works with slow variations in its current, its behavior can be described suitably by a stationary model, that is, the polarization curve. Therefore, this model is used, together with the converter model and the load model, to generate the whole plant model necessary to design the control system of the fuel cell operation point. ... A DC/DC converter steps up the low output fuel cell voltage to the standard automotive DC bus 42 V [1]. The control system must maintain the operating point when there are perturbations in state variables of the system.
- As it is mentioned above, the fuel cell system is connected with a DC/DC converter. This converter regulates the output fuel cell voltage. The control signal is the converter duty cycle. To design the control system, it must be taken into account the no-linearity of the all system. The approximate linearization technique allows to obtain a state lineal model which shows the perturbations around the operation point [3]. Figure 5 shows electrical connection scheme between fuel cell system and DC/DC converter, taking into account parasite leaks in inductor and capacitor elements.