Cathode Environment Research Articles

Potentiostatic and Potential Cycling Dissolution of Polycrystalline Platinum and Platinum Nano-Particle Fuel Cell Catalysts

The dissolution of Pt in aqueous electrolytes has been studied for over forty years, most recently in the context of understanding the observed loss in electrochemically-active surface area (ECA) of cathode electrocatalysts in polymer electrolyte fuel cells. Despite extensive research, there are many unresolved issues regarding the dissolution of nano-particle Pt, such as the source of the observed potential dependence of potentiostatic and potential cycling dissolution rates. To help resolve these issues, in this paper we present results of measurements of the concentration of dissolved Pt and Pt dissolution rates for carbon-supported platinum nano-particles (Pt/C) in dilute perchloric acid, as a mimic of the PEFC cathode environment, as a function of potential and upper potential limit of potential cycling. Also presented, for comparison, are results of similar studies on polycrystalline platinum. In situ Pt LIII X-ray absorption spectroscopy was used to determine the extent of oxidation, the coordination environment, and loss of Pt from the Pt nano-particles to elucidate the mechanism of Pt dissolution. Based on the correlation of these studies with those presented in the literature, mechanisms for Pt dissolution under potentiostatic and potential cycling conditions are proposed.

The corrosion behavior and mechanical properties of CrN/Ni[sbnd]P multilayer coated mild steel as bipolar plates for proton exchange membrane fuel cells

This research mainly studies on the interfacial conductivity and corrosion resistance of mild steel (MS)/electroless plating nickel (EN)/CrN composite coating. The MS/EN/CrN composite coating is deposited on a mild steel substrate using a combination of electroless nickel plating process and closed field unbalanced magnetron sputter ion plating system. The simulative working environment of proton exchange membrane fuel cell (PEMFC) is implemented to test the electrical and corrosion properties of uncoated mild steel and coated samples. Results show that the performance of the MS/EN/CrN coated samples was greatly improved. The SEM results indicate that the surface of MS/EN/CrN is dense and smooth. CrN (111) and Ni (111) are observed in the MS/EN/CrN coating by XRD. The potentiodynamic polarization and EIS tests show that the incorporation of electroless nickel deposits into the CrN coating can provide better corrosion resistance. Potentiostatic polarization tests reveal that MS/EN/CrN coating has the lowest current density both in anodic and cathodic environment. The Interfacial contact resistance (ICR) results show that the ICR of MS/EN/CrN coated sample reduces to 3.3 mΩ cm2 at a compaction force of 140 N/cm2. Moreover, after potentiostatic polarization, the ICR values of the MS/EN/CrN coating are still less than DOE's 2020 target. In addition, compared with other samples, the hardness and adhesion property of the MS/EN/CrN multilayer coating were greatly improved, which is beneficial for resisting the shock and wear during the assembly process of fuel cell stacks.

Electrochemical Properties of Niobium Modified AISI316L Stainless Steel Bipolar Plates for Direct Formic Acid Fuel Cell

AbstractTo ameliorate the corrosion resistance and surface electrical conductivity of AISI316L stainless steel (316L SS) as bipolar plates for direct formic acid fuel cell (DFAFC), a niobium diffusion layer has been successfully prepared on 316L SS substrate via the plasma surface diffusion alloying (PSDA) technique. The niobium modified AISI316L stainless steel (Nb‐316L SS) has a compact and successive niobium diffusion layer with a lower interfacial contact resistance (ICR) compared with bare 316L SS. The electrochemical behaviors of Nb‐316L SS in simulated varied anode (0.05M H2SO4 + 2 ppm HF + xM formic acid (x = 2, 5, 10 and 15) solution at 50 °C) and cathode (0.05M H2SO4 + 2 ppm HF solution at 50 °C, bubbled with air) DFAFC environments are systematically studied. The results show that the corrosion resistance of 316L SS is distinctly bettered in both anode and cathode environments of DFAFC after the PSDA modification. After 4 h potentiostatic polarization tests, the ICR of bare and Nb‐316L SS have increased at the clamping force of 140 N cm−2, but the increasing amplitude of the ICR for Nb‐316L SS is much smaller than that of bare 316L SS.

Corrosion and interfacial contact resistance behavior of electrochemically nitrided 316L SS bipolar plates for proton exchange membrane fuel cells

In the present investigation, electrochemically nitrided surface was developed on 316L stainless steel (SS) through potentiostatic method using an aqueous solution containing 0.1 M HNO3 and 0.5 M KNO3 at room temperature. The formation of nitrides was confirmed by X - ray photoelectron spectroscopy (XPS) and the formed nitrides existed in the form of mixed nitrides viz, CrN, Cr2N and nitrogen incorporated oxides (Cr-O-N). The corrosion behavior of untreated and nitrided SS was studied in 0.5 M H2SO4 and 2 ppm of HF. The untreated and nitrided SS showed decrease in impedance value with increasing potential from OCP (Open Circuit Potential) to proton exchange membrane fuel cell (PEMFC) anode and cathode environments and nitrided SS exhibited comparatively higher impedance values at each potential. The lower passive current density and lower metal ions concentration released for nitrided SS in both PEMFC anode and cathode environments indicated higher corrosion resistance. Before polarization, the interfacial contact resistance (ICR) value of nitrided SS matched well with Department of Energy's (DOE's) required value. After polarization, the lower ICR values for nitrided SS indicate enhanced electrical conductivity.

The study of Ni-based nano-crystalline and amorphous alloy coatings on AISI 304 stainless steel for PEM fuel cell bipolar plate application

Bipolar plates are one of the most important parts of proton exchange membrane fuel cells (PEMFCs) which their performance has appreciable effect on cell operation. In this study, Ni–Mo and Ni–Mo–P alloy coatings were electrodeposited on AISI 304 stainless steel specimens as metallic bipolar plates in order to improve corrosion resistance, hydrophobicity and electrical conductivity of the specimens. Scanning electron microscopy (SEM) micrographs showed that the prepared coatings were uniform and compact. Moreover, X-ray diffraction (XRD) analysis indicated that the coatings were composed of a solid solution where Ni–Mo and Ni–Mo–P coatings had nanocrystalline and amorphous structures, respectively. The potentiodynamic and potentiostatic polarization tests in a simulated PEMFC cathode environment showed that corrosion resistance of the coated specimens was considerably improved compared to that of the bare one. Furthermore, interfacial contact resistance (ICR) before and after potentiostatic test was significantly decreased by applying Ni–Mo and Ni–Mo–P coatings, where ICR values of the as-deposited bipolar plates were reduced by approximately eight times in comparison to that of the bare 304 stainless steel at typical compaction force of 220 N/cm2. Therefore, the investigated Ni–Mo and Ni–Mo–P coatings would be potentially good candidates as metallic bipolar plates in PEMFC. Moreover, Ni–Mo and Ni–Mo–P alloy coatings showed higher surface hydrophobicity than the bare 304 stainless steel, which is advantageous for water management and also for corrosion resistance improvement.

Influence of Ti content on the corrosion properties and contact resistance of CrTiN coating in simulated proton exchange membrane fuel cells

CrTiN coatings with different Ti content are deposited on stainless steel 316L (SS316L) substrate as bipolar plates for proton exchange membrane fuel cells (PEMFCs) by close field unbalanced magnetron sputter ion planting (CFUBMSIP). The effect of Ti incorporation on the surface morphology, phase structure, electrochemical properties, interfacial contact resistance (ICR) and contact angle with water are investigated. Scanning electron microscope (SEM) results indicate that Ti doped coatings are compact and dense. X-ray diffraction (XRD) and nanoindentation hardness results show that Ti incorporated coatings have grain refinement and solid solution phase. From electrochemical test results, CrTiN-4A coating has the highest corrosion potential of 0.1825 V in the anodic PEMFCs environment. It also has the most stable and lowest current density both in simulated anodic and cathodic environments after potentiostatic polarization. Furthermore, electrochemical impedance spectroscopy (EIS) studies reveal that the corrosion resistance of CrTiN-4A coating is higher than other coatings and bare SS316L. ICR results show that CrTiN-4A coating has the lowest ICR of 4.57 mΩ cm2 at a compaction of 1.4 MPa before potentiometric polarization. In addition, the contact angles of CrTiN-4A coating before and after potentiostatic polarization are 109.0° and 99.5°, respectively.

Effect of cathode environment on bioelectricity generation using a novel consortium in anode side of a microbial fuel cell

In the present study, two novel dominant genera i.e., Advenella kashmirensis and Desulfovibrio aminophilus in consortium, as confirmed by IlluminaMiSeq has been used in the anode side of a microbial fuel cell reactor to find out the influence of cathode environment on cell performance. A model terminal electron acceptor (TEA), KMnO4 at different pHs has been used in the cathode side. An oxidation peak obtained at −0.478 vs Ag/AgCl (−0.281V vs SHE) corresponding to S0/H2S (−0.280V vs SHE) in cyclic voltammogram confirms the complete mineralization of sulphate to biogenic elemental sulphur in accordance with the microbial community. Higher cathodic pH 10 has a detrimental effect on the cell performance as the power density is reduced to half to that for aqKMnO4 (pH=6.86) catholyte. Solution resistance is found to be low for aqKMnO4 (60ohm) and high i.e. 124.5ohm for aqKMnO4 with pH=10, which corroborates to highest power density obtained for aqKMnO4. Buffering of the catholyte provided stability to the system with lowest internal resistance of 40ohm and comparable performance. The highest current density (25A/m3) and power density (7.8W/m3) is obtained with aqueous KMnO4 at pH 6.86. The developed consortium can be successfully utilized in microbial fuel cells for simultaneous electricity generation and wastewater treatment.

Effect of Ni content on the electrical and corrosion properties of CrNiN coating in simulated proton exchange membrane fuel cell

In this paper, CrNiN coatings with various Ni content are deposited on 304ss bipolar plates by closed field unbalanced magnetron sputter ion plating from CrNi alloy targets. Simulative working environment of proton exchange membrane fuel cell (PEMFC) is applied to test the electrical and corrosion properties of uncoated 304ss and CrNiN-coated samples. The influence of Ni content on microstructure, phase structure, contact angle with water and electrochemical performance is investigated. Results show that all the coated samples significantly enhanced the corrosion resistance of the 304ss, and the CrN-coated 304ss sample without Ni has the best corrosion resistance of 153.8 and −141.9 mV in the simulated anodic and cathodic environments, respectively. Electrochemical impedance spectroscopy (EIS) studies reveal that the resistance of CrN coating is higher than that of other coated samples and 304ss in the cathodic environment. Furthermore, Interfacial contact resistance (ICR) studies revealed that CrN coating has a superior ICR of 11 mΩ cm2 at a compaction force of 160 N cm−2. In addition, the contact angle of the CrNiN coatings with water is approximately 114°, which is beneficial for water management in PEMFC. Analysis result indicates that the enhanced performance of the coated 304ss bipolar plates is related to the high film density determined by closed field unbalanced magnetron sputter ion plating, and the synergistic function of the CrNiN layered structure.

Fast Degradation for High Activity: Oxygen- and Nitrogen-Functionalised Carbon Nanotubes in Solid-Acid Fuel-Cell Electrodes.

Similar to polymer electrolyte membrane fuel cells, the widespread application of solid acid fuel cells (SAFCs) has been hindered partly by the necessity of the use of the precious-metal catalyst Pt in the electrodes. Here we investigate multi-walled carbon nanotubes (MWCNTs) for their potential catalytic activity by using symmetric cell measurements of solid-acid-based electrochemical cells in a cathodic environment. For all measurements, the carbon nanotubes were Pt free and subject to either nitrogen or oxygen plasma treatment. AC impedance spectroscopy of the electrochemical cells, with and without a DC bias, was performed and showed significantly lower initial impedances for oxygen-plasma-treated MWCNTs compared to those treated with a nitrogen plasma. In symmetric cell measurements with a DC bias, the current declines quickly for oxygen-plasma-treated MWCNTs and more slowly, over 12 days, for nitrogen-plasma-treated MWCNTs. To elucidate the degradation mechanisms of the oxygen-plasma-treated MWCNTs under SAFC operating conditions, theoretical calculations were performed using DFT. The results indicate that several degradation mechanisms are likely to occur in parallel through the reduction of the surface oxygen groups that were introduced by the plasma treatment. This finally leads to an inert MWCNT surface and a very low electrode performance. Nitrogen-plasma-treated MWCNTs appear to have a higher stability and may be worthwhile for future investigations.

Mass spectrometry analyzes to highlight differences between short and long HiPIMS discharges

This study investigates the understanding of the impact on the pulse width duration (30 and 100μs) regarding the plasma dynamic of an HiPIMS discharge by investigating time-resolved and time-integrated ion energy distribution functions. In order to establish an objective comparison, the cathode voltage and the mean power were kept constant at −600V and 7Wcm−2, respectively. Time average measurements revealed that for a pulse as long as 100μs, the ion flux at the substrate is mostly composed of argon ions (18% higher). Instead, for a lower pulse width of 30μs, the ions flux at the substrate seems to be dominated by chromium ions (up to 7%). Furthermore, the huge difference between these discharges resides in the lowest energy part of the IEDFs of singly charged chromium ions where the intensity is actually more important in short HiPIMS pulse. Our results reveal that long HiPiMS pulses are characterized by a longer rarefaction effect close to the chromium target vicinity as compared to shorter pulses. In order to explain these phenomena, time-resolved measurements were carried out and highlighted the fact that low energy ions cannot escape the cathode environment due to the applied high voltage acting as a low-energy ions trap. Current characteristics can be correlated to time-resolved ions fluxes measurements and support this explanation. These findings may help the community to find out that working with shorter pulses (tON≤100μs) can have an important benefit to improve the material properties of most common thin films, but also to enhance the adhesion of coatings on substrates by a metallic pre-treatment.

A comparison of nickel coated and uncoated sintered stainless steel used as bipolar plates in low-temperature fuel cells

Bipolar plates (BP) represent a multifunctional component in PEMFC (Proton Exchange Membrane Fuel Cells). Materials used for bipolar plates should have good corrosion resistance in high acidity and high temperature environments. The most common material for BP is graphite (high thermal and chemical resistivity, high electrical conductivity but low mechanical strength). Recently carbon layers deposited on metallic substrates, such as stainless steel (SS), have been proposed to be used as BP in PEMFC.Sintered stainless steel uncoated and coated with C–Ni coating are being studied to be used as bipolar plates in fuel cell. The study found that the conductive nickel layer has a high corrosion resistance in both anode and cathode environments. The microstructure, surface geometry and wettability of C–Ni layer were analyzed.

Evaluation of Mg–Nd–Zn–Zr magnesium alloy as bipolar plate in simulated polymer electrolyte membrane fuel cell environments

The corrosion behavior and interfacial contact resistance (ICR) of Mg–Nd–Zn–Zr magnesium alloy in simulated PEMFC environments are investigated systematically. The corrosion rate decreases logarithmically with the immersion time due to accumulation of corrosion products. At first, Mg–Nd–Zn–Zr magnesium alloy exhibits lower corrosion rate in anodic environment because the reactions involving formation of hydrogen gas and dissolution of magnesium are inhibited. After long time immersion, it presents higher corrosion rate because of pitting corrosion and cracked corrosion products layer. In the cathodic environment, a compact corrosion products layer composed of amorphous MgCO3 and Mg(OH)2 is formed as a result of precipitation of carbonate-containing corrosion products. The ICR of Mg–Nd–Zn–Zr magnesium alloy immersed in cathodic environment is higher than that in anodic one due to the accumulation of amorphous Mg(OH)2 and MgCO3 corrosion products layer.

Conductive and corrosion behaviors of silver-doped carbon-coated stainless steel as PEMFC bipolar plates

Ni–Cr enrichment on stainless steel SS316L resulting from chemical activation enabled the deposition of carbon by spraying a stable suspension of carbon nanoparticles; trace Ag was deposited in situ to prepare a thin continuous Ag-doped carbon film on a porous carbon-coated SS316L substrate. The corrosion resistance of this film in 0.5 mol·L−1 H2SO4 solution containing 5 ppm F− at 80°C was investigated using polarization tests. The results showed that the surface treatment of the SS316L strongly affected the adhesion of the carbon coating to the stainless steel. Compared to the bare SS316L, the Ag-doped carbon-coated SS316L bipolar plate was remarkably more stable in both the anode and cathode environments of proton exchange membrane fuel cell (PEMFC) and the interface contact resistance between the specimen and Toray 060 carbon paper was reduced from 333.0 mΩ·cm2 to 21.6 mΩ·cm2 at a compaction pressure of 1.2 MPa.

하이드라진으로 환원시킨 그래핀을 코팅한 오스테나이트와 마르텐사이트 스테인리스 강 고체고분자형 연료전지 금속 분리판의 전기화학적 특성 평가

Graphene was coated on austenitic and martensitic stainless steels to simulate the metallic bipolar plate of proton exchange membrane fuel cell (PEMFC). Graphene oxide (GO) was synthesized and was reduced to reduced graphene oxide (rGO) via a hydrazine process. rGO was confirmed by FE-SEM, Raman spectroscopy and XPS. Interfacial contact resistance (ICR) between the bipolar plate and the gas diffusion layer (GDL) was measured to confirm the electrical conductivity. Both ICR and corrosion current density decreased on graphene coated stainless steels. Corrosion resistance was also improved with immersion time in cathodic environments and satisfied the criteria of the Department of Energy (DOE), USA. The total concentrations of metal ions dissolved from graphene coated stainless steels were reduced. Furthermore hydrophobicity was improved by increasing the contact angle.

Investigation of multi-coating process treated magnesium alloy as bipolar plate in polymer electrolyte membrane fuel cell

Magnesium alloy bipolar plates offer great advantage in weight reduction of polymer electrolyte membrane fuel cell. An attempt to improve the corrosion resistance and surface conductivity of magnesium alloy has been made by a medium electroless plating Ni coating prior to PVD deposition of carbon film, envisaging the application of these coating/substrate systems as bipolar plate in PEMFC. The results show that Ni + C coated magnesium alloy GW83 has the best corrosion resistance of 8.65 μA cm−2 and 0.80 μA cm−2 in the simulated cathodic and anodic environments. The release of metallic ions is also greatly inhibited by Ni + C coating. The interfacial contact resistance (ICR) of the magnesium alloy GW83 is decreased from 196.5 mΩ-cm2 to 2.97 mΩ-cm2 by Ni + C coating at the compaction force of 150 N-cm−2. After potentiostatic tests, Ni + C coated sample has the lowest ICR of 15.6 mΩ-cm2 and 52.6 mΩ-cm2 in the anode and cathode environment, respectively.

Bioelectrochemical system as an oxidising filter for soluble and particulate organic matter removal from municipal wastewater

A bioelectrochemical system (BES) configuration was designed and tested as a polishing process to remove soluble chemical oxygen demand (SCOD) and suspended solids (SS) from municipal wastewater. The configuration was designed such that the anodic carbon felt could behave as a filter enabling removal and bioelectrochemical oxidation of particulate and soluble organic matter from wastewater. Three reactors (each with approximately 10L liquid volume) were set up to evaluate three different operational settings: (i) close-circuit operation enabling current production; (ii) open-circuit operation without current production; and (iii) control reactor without electrode to account for the effect of electrode as a filter. The reactors were operated for over 200days during which key process parameters (SCOD removal, SS removal, turbidity, electricity production, electrolyte pH) were quantified. The results indicated that current production (maximal current of approximately 120mA) could increase the SCOD removal efficacy from 11% to 77% at a SCOD removal rate of approximately 0.1kgSCOD/m3d. No measurable difference between SS removal efficiencies in open and close circuit (83%), and clogging of the electrode filter were observed. A high coulombic efficiency (more than 80%, based on the SCOD removed) and low internal resistance (2.7Ω) were achieved, indicating that the configuration is suitable for handling real wastewater containing particulate matter. However, active maintenance of a neutral pH at the anode and an acidic cathodic environment (pH<7) were essential to sustain the optimum performance of the system. Overall, this study affirms the potential of using a BES to serve as both a physical filter and an oxidising medium for effluent polishing.

Corrosion kinetics of 316L stainless steel bipolar plate with chromiumcarbide coating in simulated PEMFC cathodic environment

Stainless steel with chromium carbide coating is an ideal candidate for bipolar plates. However, the coating still cannot resist the corrosion of a proton exchange membrane fuel cell (PEMFC) environment. In this work, the corrosion kinetics of 316L stainless steel with chromium carbide is investigated in simulated PEMFC cathodic environment by combining electrochemical tests with morphology and microstructure analysis. SEM results reveal that the steel’s surface is completely coated by Cr and chromium carbide but there are pinholes in the coating. After the coated 316L stainless steel is polarized, the diffraction peak of Fe oxide is found. EIS results indicate that the capacitive resistance and the reaction resistance first slowly decrease (2–32h) and then increase. The potentiostatic transient curve declines sharply within 2000s and then decreases slightly. The pinholes, which exist in the coating, result in pitting corrosion. The corrosion kinetics of the coated 316L stainless steel are modeled and accords the following equation: i0=7.6341t−0.5, with the corrosion rate controlled by ion migration in the pinholes.

Increase in the corrosion resistance of stainless steel bipolar plates by the formation of thermally-induced Cr-nitride

Cr-nitride layer is formed on the surface of Cr-electroplated stainless steel to use metallic bipolar plates of polymer electrolyte membrane fuel cell (PEMFC). The dense and continuous Cr-nitride layer is confirmed by scanning electron microscope (SEM) results. The corrosion resistance results in aggressive environments of PEMFC reveal that the corrosion current density of 0.5 μA cm−2 and 0.5 μA cm−2 are presented in anodic and cathodic environments of PEMFC, respectively. The highest protective efficiency of 99.9% is recorded in the cathodic condition. The dense Cr-nitride layer provides sufficient corrosion resistance. Moreover, the interfacial contact resistance (ICR) of Cr-nitride layer is significantly decreased up to 15 mΩ cm−2 compared with pure stainless steel and the ICR is maintained after corrosion tests. This analysis indicates that the improvement is originating solely from dense Cr-nitride layer on the surface.

Influence of Cr-C film composition on electrical and corrosion properties of 316L stainless steel as bipolar plates for PEMFCs

Stainless steel (SS) has received much attention in recent years as an alternative material for bipolar plates (BPPs) in proton exchange membrane fuel cells (PEMFCs). However, stainless steel BPPs are still required to combat corrosion in the acid environment without forming oxidants, passive layers and metal ions. In this study, multilayered chromium-carbon (Cr-C) film is deposited on SS316L sheet as BPPs using closed field unbalanced magnetron sputter ion plating (CFUBMSIP). Five films with different composition are deposited and the influence of chromium content adjusted by sputtering current is characterized by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Interfacial contact resistance (ICR) between coated SS316L sheets and gas diffusion layer (GDL) is measured. Potentiodynamic and potentiostatic tests are conducted to evaluate the corrosion resistance of the coated samples. Experimental results show that Cr-C film with content of Cr0.75C5 presents the optimal performance, i.e. the interfacial contact resistance(ICR) reduces to 1.4 mΩ cm2 under a compaction pressure of 1.4 MPa, and the corrosion current density reaches 1.046 μA cm−2 in the PEMFCs cathodic environment (0.5 M H2SO4 + 5 ppm HF solution at 70 °C, 0.6 V vs. SCE). The performance variation of five films can be preliminarily explained by the crystallographic structure and chemical composition analysis.

Copper Electrodeposition Kinetics Measured by Alternating Current Voltammetry and the Role of Ferrous Species

Impurities and additives play a key role in copper electrodeposition, in particular in upstream processes such as electrowinning or electrorefining. One common impurity is iron, mostly present as iron species Fe(II) in highly concentrated sulfuric acid solutions and in a cathodic environment. Herein, the kinetics of copper electrodeposition from such solutions have been investigated using a copper rotating disk electrode and alternating current voltammetry (ACV). For a concentration of proton of 1.84 M and a concentration of Fe(II) ions of 0.054 M, the deposition kinetics are slow enough to separately observe the two electron transfer steps involved in copper reduction: an observation unique to ACV. The results suggest that Fe(II) ions affect the electrodeposition kinetic by slowing down reaction kinetics, in particular slowing the second electron transfer reaction.