The microstructures of the ionomer–catalyst interfaces in the catalyst layers are important for the fuel cell performance because they determine the distribution of the active triple-phase boundaries. Here, we investigate the ionomer–catalyst interactions in hydroxide exchange membrane fuel cells (HEMFCs) using poly(aryl piperidinium) and compare them with proton exchange membrane fuel cells (PEMFCs). It is found that different catalyst layer microstructures are between the two types of fuel cell. The ionomer/carbon (I/C) ratio does not have a remarkable impact on the HEMFC performance, while it has a strong impact on the PEMFC performance, indicating the weaker interaction between the HEMFC ionomer and catalyst. Molecular dynamics simulations demonstrate that the HEMFC ionomer tends to distribute on the carbon support, unlike the PEMFC ionomer, which heavily covers the Pt nanoparticles. These results suggest that the poisoning effect of the ionomer on the catalyst is much weaker in HEMFCs, and the improved ionomer/catalyst interaction is beneficial for the HEMFC performances.

For high-temperature proton exchange membrane fuel cells (HT-PEMFCs), the PA amount in membrane electrode assembly (MEA), which can be tuned by the membrane and binder materials, has a vital influence on cell performance. Herein, to control the PA uptake ability of the MEA, a series of membrane and binder materials functionalized by nitrogen heterocyclic groups with different acidophilic properties were synthesized via superacid-catalyzed copolymerization ofp-terphenyl and functional aldehydes. As expected, the PA-doped TP-2-IM membrane exhibits the highest proton conductivity of 66 mS/cm due to the largest PA uptake, and the corresponding MEA using TP-2-IM membrane displays the highest PPD of 621 mW/cm2 at 160 degrees C. However, the MEA using TP-2-IM as binder demonstrates the poorest cell performance mainly attributed to the limited mass transport resulted from the too strong PA interplay. Notably, owing to the suitable PA uptake ability of TP-4-IM binder, a good balance between the proton conductivity and fuel diffusion in the MEA has been achieved. Thus, the corresponding MEA illustrates the largest electrochemical surface area (ECSA) of 159 cm2/mgPt and exhibits the highest PPD of 821 mW/cm2 at 160 degrees C and 1004 mW/cm2 at 200 degrees C with H2 and O2 as fuel.

The cost-effective sol-gel process for the preparation of phosphoric acid (PA) doped P-PBI membranes using PPA as both condensation reagent and solvent could avoid the use of hazardous organic solvents. However, optimizing the trade-off effect between mechanical properties and proton conductivity of sol-gel membranes by increasing polymer content is ineffective due to the limited solubility of P-PBI in PPA. Herein, we successfully fabricated a series of sol-gel membranes with tunable properties using a highly soluble polybenzimidazole (NPBI). In addition, the sol-gel (5 wt%) N-PBI membrane was further specially dried and post-doped with PA to obtain a re-doped sol-gel N-PBI membrane. Therefore, the properties of PA-doped N-PBI membranes fabricated using three different processes-conv, sol-gel, and re-doped sol-gel-were thoroughly characterized. The sol-gel N-PBI membranes demonstrated a dependence of mechanical properties and proton conductivity on polymer content, with a maximum tensile strength of 7.0 MPa at 7 wt% polymer content and a maximum proton conductivity of 205 mS/cm (200 degrees C) at 2 wt% polymer content. The ADL-controlled re-doped sol-gel N-PBI membrane achieved higher saturated ADL than the sol-gel N-PBI membrane, resulting in higher proton conductivity and fuel cell performance. The sol-gel (2 wt%) N-PBI membrane exhibited an ultra-high H2/O2 peak power density of 1058 mW/cm2 under an optimized operating condition, and the sol-gel (7 wt%) N-PBI membrane achieved stable operation without apparent degradation for over 2000 h at 160 degrees C with 0.3 A/cm2 current density.

For this study, E690 steel claddings with different contents of TiC were prepared by laser cladding for revealing effects of TiC on microstructure, mechanical properties, and wear behavior of the cladded steel. The microstructure and mechanical properties of the cladded alloys were observed and measured. The wear resistance of the cladded alloys was tested using a ball-on-disc tribometer. The experimental results showed that TiC refined the grain of the cladded alloy obviously. However, excessive TiC could lead to the network-like distribution of submicro TiC particles in the claddings. The microhardness of the cladded alloy with 1% TiC increased to 418.5 +/- 10.7 HV500, and yield strength and ultimate tensile strength (UTS) increased by 11% and 7% compared with those of the cladded alloy without TiC. Excessive TiC decreased the tensile properties to an obvious degree. The addition of TiC improved the wear resistance of the cladded alloy significantly. When normal loads were 5 N and 10 N, the cladded alloy with 5% TiC exhibited high wear resistance, and the wear rates decreased by 59% and 78% compared with those of the cladded alloy without TiC. However, when normal load was 20 N, the cladded alloy with 1% TiC possessed superior wear resistance, and its wear rate decreased by 81% compared with that of the cladded alloy without TiC. The main wear mechanism of the cladded alloys was abrasion wear when normal load was low, but serious plastic deformation played a role in the wear process when normal load was high.

This study further investigates the effect of potential on the corrosion resistance, the self-healing performance and the durability of CrN/Cr-coated SS316L bipolar plates with artificial defects (CR-316) in simulated cathodic HT-PEFC environments by means of electrochemical methods. The self-healing ability initiated by oxygen is relatively weak and needs the assistance of the cathode working potential for sealing. In some cases, the defects have spread over large parts of the bipolar plate. The influence of the potential on the corrosion resistance of the bare 316L and CR-316 specimens in the simulated cathodic HT-PEFC environments were investigated by electrochemical impedance spectroscopy. Moreover, the durability of the CR-316 specimens was examined under the various potential cycles in the simulated cathodic environment of HT-PEFC and O2 atmosphere. After 5000 CV cycles in the potential range of 0.4–1.0 V vs. RHE, the CR-316 specimens could maintain the integrity and good corrosion resistance against the hot phosphoric acid. The results demonstrate the superior performance of CR-316 and make it a prime candidate as a non-precious coating for metallic bipolar plates on the cathode side of HT-PEFCs.

Herein, a ternary Nb-Cr-C film is successfully prepared by arc ion plating for surface modification of titanium using as proton exchange membrane fuel cell (PEMFC) bipolar plates. Film morphology, composition as well as corrosion behavior, conductivity and performance of single cell are investigated. As results revealed, the Nb-Cr-C film is about 500 nm thick without obvious defects. Compared with the bare titanium, the corrosion current density of Nb-Cr-C coated titanium lowered to 0.022 mu A/cm2 and -0.051 mu A/cm2 in simulated PEMFC cathode and anode environments, respectively, accompanied with the interfacial contact resistance lowered to 1.15 m omega cm2, indicating outstanding anti-corrosion and conductivity performance. The single cell with Nb-Cr-C coated titanium bipolar plates reaches its peak power density 1.167 W/cm2 at 2.2 A/cm2, which is comparable with that of graphite bipolar plates and apparently better than that of bare titanium bipolar plates. The remarkable improvements in both ex-situ tests and single cell tests suggest that the Nb-Cr-C coated titanium might be a proper material for PEMFC bipolar plates.

The major challenges for the commercialization of proton exchange membrane fuel cells (PEMFCs) are durability and costs. Prognostics and health management technology is helpful to extend the lifetime and reduce the maintenance costs of PEMFCs. However, the common degradation model, especially in the model-based method and the hybrid method, has the disadvantages of low generality and accuracy. A novel degradation model is proposed by introducing a polarization resistance in this paper to overcome the above disadvantages. Combining the novel degradation model and particle filter, a model-based method is proposed to estimate the state of health (SOH) and predict the future degradation trend (FDT) and the remaining useful life (RUL) of PEMFCs. Then, two actual degradation datasets of PEMFCs are used to validate the model, the RUL errors of the novel degradation model on these two datasets are 12.6% and 12.7%, respectively, while the errors of the common degradation model are 17.8% and 33.4%, respectively. The results prove that the proposed degradation model has higher generality and accuracy than the common model. (c) 2021 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

This paper investigates the general performance and durability performance of a high-temperature polymer electrolyte fuel cell (HT-PEFC) with CrN/Cr-coated SS316L as bipolar plates and compares this with the bare SS316L and graphite bipolar plates. Polarization curves are conducted every 200 h during the 1000 h durability tests for evaluating the overall performances of the HT-PEFCs with different types of bipolar plates. An electrochemical model based on polarization curves, combined with electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM), is used for understanding the degradation mechanisms at play. The CrN/Cr-coated SS316L bipolar plates show excellent corrosion resistance and performance in a real HT-PEFC. Thereby, the degradation rate decrease from ca. 16% of the highest output power with the bare metallic bipolar plates and graphite bipolar plates to almost zero with the CrN/Cr-coated steel bipolar plate.

This paper concerns the corrosion behaviors of CrN/Cr-SS316L (with artificial defects) in a simulated high-temperature polymer electrolyte membrane fuel cell (HT-PEFC) cathode environment and the effect of oxygen on its corrosion processes. The overall anti-corrosion performance of CrN/Cr-SS316L (with artificial defects) was evaluated using the potentiodynamic polarization method under the environment with either N-2 or O-2 purging atmosphere. Electrochemical impedance spectroscopy was used to clarify the mechanisms of the corrosion processes. Scanning electron microscopy was then carried out to investigate the morphological changes of coated specimens before and after corrosion tests under different gas purging atmospheres. The existence of O-2 is essential for the CrN/Cr coating with surface defects maintaining good corrosion resistance in an HT-PEFC and oxygen could initialize the selfhealing ability of CrN/Cr coating with defects in the hot phosphoric acid. On the basis of the experimental findings, the mechanisms of the influence of oxygen on the corrosion processes of the CrN/Cr-SS316L in the simulated cathodic environment of an HT-PEFC are ultimately proposed. (C) 2019 The Electrochemical Society.

The time dependence of the open circuit potential under oxygen and air is characterized by half-cell experiments in the temperature range of 30 degrees C to 80 degrees C. The data is analyzed with the aid of a macroscopic model that captures the effect of a coupled reaction of platinum surface oxidation and the oxygen reduction reaction. The aim of the model is to facilitate an understanding of the principle reactions from an engineering perspective. Two modeling approaches, namely 'gas electrode' and 'flooded electrode', are compared. It can be shown that the difference between the theoretical Nernst potential and open circuit potential can be described by two major effects: gas solubility in the electrolyte and platinum surface oxidation. The fact that platinum surface oxidation does not lead to a 'fully oxidized surface' has strong implications for the design of accelerated stress tests, which is briefly discussed. (C) The Author(s) 2019. Published by ECS.

High-temperature polymer electrolyte fuel cells (HT-PEFCs) operate in the range of 120 – 180 °C and currently employ phosphoric acid doped polybenzimidazole membranes. This, in turn, represents a quite aggressive environment for the components of a fuel cell [1]. Due to this reason, bipolar plates are presently made of graphitic composite materials. Indeed, they show long-term stability, but otherwise, they exhibit high mass and volume as well as a high effort of manufacturing. Therefore metallic materials come into play as promising candidates for the bipolar plate [2]. They provide some crucial advantages, like enhanced volumetric and gravimetric power density for stacks, increased ductility, high mechanical stability and the capability of low-cost mass production. Due to the corrosion of the metals, it is not feasible to employ bare commercially available stainless steels or Ni-based alloys as metallic bipolar plates for HT-PEFC application. One possibility to reduce the corrosion rate and to keep the benefits of metals could be applying conductive and inert coatings [2]. For coatings without self-healing ability, localized corrosion should occur at the damaged site and continuous corrode the substrate under the coating. Self-healing ability can be given to the films by introducing a metallic component into the films: The oxidation of the metallic component will heal flaws in the films [3]. According to this idea, CrN/Cr bilayer coating has been studied. Using an electrochemical cell, which was specially designed for HT-PEFC experimental requirements, we analyzed CrN/Cr bilayer coated 316L and bare 316L by means of potentiodynamic, potentiostatic and free corrosion potential monitor tests combined with electrochemical impedance spectroscopy. By purging different gases (nitrogen, oxygen) into the electrolyte, we have also investigated the influence of oxygen on the corrosion resistance ability of CrN/Cr coating under simulated cathodic HT-PEFC environments. Additionally, ex-situ characterizations using SEM+EDX, XPS and ICP-OES were carried out in order to investigate the metallic surface, corrosion production and the corresponding dissolved ion species in the electrolyte. The CrN/Cr bilayer coating shows good anti-corrosion capability in the simulated cathodic environment of HT-PEFC with the existence of oxygen, i.e. no cracks or holes on the surface of the samples after potentiostatic tests at either 0.65 V or free corrosion potential for more than 4 hours at 130 º C. Cathodic operation conditions including a relatively high potential and the presence of oxygen lead to the oxidation of chromium on the damaged site, therefore healed the damaged site and beneficial for providing sufficient corrosion resistance to the cathodic HT-PEFC environment. References: [1] C. Hartnig and T. J. Schmidt, Electrochimica Acta, 56, 4237 (2011). [2] V. Weissbecker, U. Reimer, K. Wippermann, & W. Lehnert, ECS Transactions, 58(1), 693-704(2013). [3] M.Yasuda , N. Akao, N. Hara, K. Sugimoto, Self-healing Corrosion Protection Ability of Composition-Gradient Al 2 O 3 ⋅ Nb Nanocomposite Thin Films, J. Electrochemical Society, 2003, 150(10): B481-B487

Using an electrochemical cell that was specially-designed for HT-PEFC experimental requirements, we analyzed a CrN/Cr-coated SS316L using potentiodynamic, potentiostatic and free corrosion potential monitor tests in the simulated cathodic environment of an HT-PEFC. By purging different gases (nitrogen, oxygen) into the electrolyte, we also investigated the influence of oxygen on the corrosion resistance of the CrN/Cr coating in simulated cathodic HT-PEFC environments. Additionally, ex-situ characterizations using SEM+EDX and ICR were carried out to investigate the metallic surface, anti-corrosion properties and interfacial contact resistance of samples before and after corrosion tests were carried out. Cathodic operation conditions included relatively high potential and the presence of oxygen beneficial for CrN/Cr coating so as to provide sufficient corrosion resistance to the cathodic HT-PEFC environment. Therefore, the CrN/Cr coatings are promising for the application in the cathode side of HT-PEFCs.

Increasing attention is being paid to the use of metallic materials as a replacement for non-porous graphite in the bipolar plates of polymer membrane fuel cells, including high-temperature polymer membrane fuel cells (HT-PEFCs). This work investigates the corrosion and electrical properties of SS316L stainless steel in the simulated anode and cathode environments of HT-PEFCs. The influence of gases on the free corrosion potential (E-corr), free corrosion current (i(corr)) and dynamic formation of passive layers were analyzed in 85 wt% phosphoric acid at RT and 130 degrees C by means of potentiodynamic and potentiostatic tests, together with open circuit potential (OCP) measurements. The working potential of the anode (0.05V) is located in the active corrosion region, while the working potential of the cathode (0.65V) is located in the passive corrosion region. The potentiostatic tests show that the corrosion rate of SS316L in the simulated anode environment of an HT-PEFC is 10 times higher than that in the simulated cathode environment of an HT-PEFC. The free corrosion potentials, immediately noted after potentiostatic tests, show that the existence of oxygen could improve the stability of the passive layer formed during the potentiostatic stage. Scanning electron microscopy (SEM) results showed different morphologies of the corroded surface. Inductively-coupled plasma optical emission spectrometry (ICP-OES) and interfacial contact resistance (ICR) was then used to determine the levels of metal ions in the solution after corrosion and the influence of the passive layer on the ICR of metallic bipolar-plates separately made from SS316L. X-ray photoelectron spectroscopy (XPS) was used to investigate the distribution of elements on the surface of samples before and after the corrosion tests. These ex-situ measurements showed that in the anode environment, SS316L undergoes active corrosion, which results in a higher level of leaching metal ions and lower value of ICR compared to that in a cathode environment, which has a passive corrosion environment. The specimen in the cathode environment showed lower values of ICR and the number of leaching metal ions when exposed to oxygen compared with the nitrogen atmosphere. The passive layer formed in a simulated cathode environment with an oxygen purge shows the best corrosion resistance within hot phosphoric acid. The XPS results indicate that this is a Cr-rich layer. The thickness of the surface films was estimated to range from 1.1-2.2 nm. (C) 2018 The Electrochemical Society.