Reverse Current Conditions Research Articles

Boosting the Role of Ir in Alleviating Corrosion of Carbon Support By Alloying with Pt

The degradation of PEMFC is a very complex phenomenon and is affected by various factors. Among the components of PEMFC, an electrode composed of a carbon-supported platinum nanoparticle catalyst (Pt/C) plays an important role in determining the durability of PEMFC. Typically, PEMFCs have an operating voltage range of 0.5–1.0V. During long-term operation under these conditions, Pt nanoparticles agglomerate, reducing the active surface area and gradually reducing electrode performance. On the other hand, abnormal conditions such as low fuel and start/shutdown (SU/SD) cycles produce abnormally high potentials above 1.4V at the electrodes. At this high potential, the carbon support corrodes to form carbon dioxide (C + 2H2O → CO2 + 4H + + 4e–, E0 = 0.207 V), leading to loss of supported Pt nanoparticles and rapid decay or catastrophic failure of electrode performance. Automotive PEMFCs, which are more often exposed to the aforementioned abnormal conditions, have a shorter life (almost thousands of hours) than stationary PEMFCs (nearly tens of thousands of hours). This clearly means that these dynamic conditions will decisively affect the degradation of PEMFC.Extensive research efforts have been made to mitigate the corrosion of carbon supports to improve the durability of PEMFC. First, it improves the corrosion resistance of these materials by modifying the surface properties and crystallinity of carbon. Second, corrosion-resistant and electrically conductive oxides and transition metal carbides have been explored as alternative supports for Pt nanoparticles to remove carbon from the electrode. Electrodes without supports, such as nanostructured thin film electrodes, have also been proposed to remove carbon from the electrode. On the other hand, operating procedures or plant balances (BOPs) have been developed to avoid exposure to cell reversal or reverse current conditions. Despite these studies, the lifetime of automotive PEMFCs is limited due to carbon corrosion, but this is not the only issue. Nevertheless, the mitigation of carbon corrosion remains an important challenge.With a new approach, iridium oxide (IrO2) is incorporated into the Pt/C electrode as an additive that protects carbon from corrosion with the help of sacrificial oxidation of water (or oxygen evolution reaction, OER) demonstrating improved durability in both half-cell and single-cell tests. However, the function of the OER catalyst was not clear in terms of composition and distribution, which is a prerequisite for further improvement of cathode durability. This study aims to gain a mechanical understanding of OER functional Ir-containing Pt/C electrodes. Carbon-supported Pt–Ir alloy catalysts (PtxIry/C) are synthesized in various compositions and are exposed to potential cycles from 1.0 to 1.5 VRHE compared to Pt/C and mixtures of Pt/C and Ir/C (Pt/C + Ir/C, Pt-Ir ratio 85:15). Pt85Ir15/C has been proven to be more resistant to carbon corrosion than Pt/C as well as Pt/C+Ir/C due to Ir atomically distributed in Pt85Ir15/C. The corrosion behavior of the carbon support is thoroughly investigated by the identical location transmission electron microscopy (IL-TEM), differential electrochemical mass spectrometry (DEMS), and X-ray photoelectron spectroscopy (XPS). As a cathode catalyst, Pt85Ir15/C exhibits much higher oxygen reduction reaction (ORR) activity than Pt/C after potential cycles, which is associated with the formation of high-index facets of Pt as revealed by high-resolution TEM (HR-TEM) and inductively coupled plasma mass spectrometry connected to a scanning flow cell (SFC/ICP-MS). This study suggests that the atomic-level distribution of Ir is a key factor in effectively mitigating carbon corrosion and improving PEMFC durability.

Degradation behavior of a polymer electrolyte membrane fuel cell employing metallic bipolar plates under reverse current condition

To examine durability of metallic bipolar plates (BPs) under reverse current conditions, the degradation of PEMFC employing graphite, bare 316L, and CrN-coated 316L BPs is investigated via a 1.4V pulse cycling test. After 20 cycles, the average voltage decay rate at 160mAcm−2 is 6.8, 16.8, and 12.0mVcycle−1 for the single cell using graphite, bare 316L, and CrN-coated 316L BPs, respectively. SEM, EPMA, and TEM analyses of the cathodes that experienced an extraordinary high voltage of 1.4V show that carbon corrosion and Pt migration/agglomeration occur similarly for the single cells, irrespective of the bipolar plate material. In contrast, in the membrane tested with bare 316L and CrN-coated 316L, Fe and Cr are detected; the amounts of Fe and Cr in the membrane are higher for bare 316L than for CrN-coated 316L. The membrane contamination results in a decrease in the ionic conductivity of the membranes, which mainly contributes to the faster performance decay of the single cells employing bare 316L and CrN-coated 316L bipolar plates. Thus, if automotive PEMFCs using metallic BPs are exposed to reverse current conditions upon start/stop cycles, metal contamination of the membrane could accelerate the performance decay in addition to the cathode degradation, such as carbon corrosion and Pt migration/agglomeration.

Effects of residual oxygen partial pressure on the degradation of polymer electrolyte membrane fuel cells under reverse current conditions

A PEMFC stack inevitably experiences reverse current conditions during startup and shutdown processes. Residual oxygen partial pressure in the stack affects the local potential in the cathode facing oxygen in the anode and plays a critical role in cathode degradation. In this study, effects of residual oxygen partial pressure on PEMFC degradation were studied under reverse current conditions. Increasing the residual oxygen partial pressure under repetitive reverse current conditions accelerates performance decay and facilitates cathode degradation. Cathode degradation is revealed by an increase in charge-transfer resistance and a decrease in electrochemical active surface area, which can be attributed to corrosion of the carbon support and dissolution/migration and agglomeration of the platinum catalyst.

Effects of different buffer structures on the avalanche behaviour of high voltage diodes under high reverse current conditions

The effects of different buffer structures on the avalanche and post avalanche behaviour of high voltage silicon diodes under high reverse current conditions has been investigated. After the onset of avalanche, when the diodes are reverse biased, branches both with positive and negative differential resistance may occur in the device characteristics and depend sensitively on the doping profile of the buffer layer. By numerical device simulation the influence of relevant bulk doping and various buffer profiles on these static reverse characteristics are analysed to understand, clarify and illustrate the breakdown behaviour of high voltage pin-diodes. Based upon these considerations an effective restriction of the electric field maximum at the nn+-junction at the cathode side of the diode can be achieved by adapting the doping gradient of the buffer profile.

Effect of load transients on SOFC operation—current reversal on loss of load

The dynamics of solid oxide fuel cell (SOFC) operation have been considered previously, but mainly through the use of one-dimensional codes applied to co-flow fuel cell systems. In this paper several geometries are considered, including cross-flow, co-flow, and counter-flow. The details of the model are provided, and the model is compared with some initial experimental data. For parameters typical of SOFC operation, a variety of transient cases are investigated, including representative load increase and decrease and system shutdown. Of particular note for large load decrease conditions (e.g., shutdown) is the occurrence of reverse current over significant portions of the cell, starting from the moment of load loss up to the point where equilibrated conditions again provide positive current. Consideration is given as to when such reverse current conditions might most significantly impact the reliability of the cell.

Derivation of exact equation for current efficiency under periodic reverse current conditions

Derivation of exact equation for current efficiency under periodic reverse current conditions