Bipolar Plates for PEM Systems

Abstract

Summary of thesis:

The Bipolar Plate (BPP) is an important component in both Proton Exchange Membrane Fuel Cells (PEMFCs) and Proton Exchange Membrane Water Electrolyzers (PEMWEs). Bipolar plate material and processing constitutes for a large fraction of the cost and weight of a PEM cell stack. The main tasks for the bipolar plates in both systems are to separate single cell in a stack, conduct current between single cells and remove heat from active areas. In addition, the BPPs distribute hydrogen and air in PEMFCs, and in PEMWEs they distribute the water. The material selection is challenging in PEM systems, primarily due to the acidic environment introduced by the proton exchange membrane. In PEMWEs, the high potentials experienced at the oxygen evolving electrode limits the range of suitable materials even further. Consequently, carbon based BPPs are most commonly used in PEMFCs, while titanium BPPs are more common in PEMWEs.

Stainless steels are desired materials for use as BPPs in PEMFCs due to their high electrical conductivity, high mechanical strength and low material and processing costs compared to carbon based BPPs. However, the durability of stainless steels in PEMFCs is problematic, thus preventing their expected influential role as bipolar plate materials in such systems. Investigating the development of electrical resistances and corrosion behavior ex situ in simulated environment or in situ in a real operating PEM system and complementing this with surface sensitive measurements are crucial in developing the next generation of cost efficient and durable bipolar plate materials. Ex situ polarizations of stainless steel (AISI 316L) BPPs were conducted in electrolytes made from various concentrations of H2SO4 with and without addition of halides. It was found that polarization of the stainless steel BPPs in electrolytes with a low pH resulted in higher corrosion currents than polarization in electrolytes with higher pH. In addition, the Interfacial contact resistance (ICR) values obtained after polarization were higher for the BPPs that had been polarized in high pH electrolytes. Auger Electron Spectroscopy analysis revealed that the surface oxides formed on the AISI 316L were thinner on the plates that had been polarized in 0.1 M and 1 M H2SO4 compared to the ones that had been polarized in 0.1 mM and 1.0 mM H2SO4. Polarization over longer periods of time in 1 mM H2SO4 did not cause as low ICR and high corrosion currents as polarization in 0.1 M and 1 M H2SO4. Using very low pH to accelerate the corrosion rate of stainless steel is thus probably not the best solution, as this seems to initiate a thinning of the oxide that would not take place in an operating fuel cell at all. As opposed to what was frequently described in the literature, addition of F- and Cl- to the electrolyte in the amounts expected in an operating PEMFC, did not cause any significant changes in ICR or corrosion current. However, addition of Cl- in amounts 10 times larger than what can be expected in a PEMFC, caused pitting on the AISI 316L surface during polarization. ICR measurements can be used to investigate the electrical conductivity between the Gas Diffusion Layer and the BPP in PEMFCs. Ex situ ICR measurements can be done very easily both before and after polarization, and provides a god idea of how promising a BPP material and/or coating is. However, ex situ measurements do not provide continuous study of how the ICR develops over time. In addition, such tests are usually not conducted under real fuel cell conditions. In situ ICR measurements makes it possible to study the ICR during cell operation, but there are not many examples of such methods in the literature. In this work, the main objective was to develop such a method in cells that had been custom made for a previous research project. Thin gold wires were placed at three different locations in the operating fuel cell, making it possible to measure the ICR between the BPP and GDL directly. Using the average ICR from the three measuring points on the BPP surface was found to be far more accurate than just one measuring point, as the average ICR values were found to be in the same range as the ICR values obtained ex situ after operation. Measuring the local ICR by use of a point measuring setup ex situ, confirmed that the variation in ICR between the three in situ measuring points was caused by uneven current distribution. BPPs for PEMWEs have to withstand the harsh environment caused by high operational potential and acidity from the electrolyte, and they are currently made of titanium. Although it is well-known that titanium easily forms a low conductive oxide, there are surprisingly few studies focusing on testing of substrate materials and coatings for bipolar plate materials for PEMWEs. In this thesis work, a study where several substrate materials were polarized under conditions similar to the ones experienced by BPPs in PEMWE was conducted. Molybdenum, tungsten, Titanium gr. 2, niobium, tantalum, Inconel 625, 254 SMO, AISI 316L and AISI 304L were polarized up to 2.0 VSHE. Weight loss measurements, ICR measurements and scanning electron microscopy imaging were performed before and after polarization. Titanium, niobium and tantalum experienced little or no change in weight during polarization. Tantalum and niobium instead encompassed a substantial increase in ICR. The measured weight loss for AISI 316L, AISI 304L and tungsten showed that only approximately 20 % of the current produced could be attributed to corrosion or dissolution processes. ICR measurements performed after polarization of titanium up to 122 h at 2.0 VSHE Showed that the ICR kept increasing continuously, even though the current stabilized at a more or less constant value. A novel bi-layer coating made from a tantalum base layer with a Tin doped Indium Oxide (ITO) layer on top was investigated for use on titanium BPPs for PEMWE. Several Ta-ITO coated plates were polarized in a parameter study, where pH, potential and temperature of the electrolyte was altered, as well as the duration of the polarization. Before polarization, the Ta-ITO coated samples showed an ICR of 10 mΩ cm2. The ICR measured after polarization at baseline conditions increased for the first 24 hours, and then stabilized at about 30 mΩ cm2. Polarization of the Ta-ITO coated titanium at 1.4 VRHE and 2.0 VRHE resulted in small increases (both at 15 mΩ cm2) in the ICR compared to the unpolarized sample (10 mΩ cm2), but after polarization at 2.5 RHE and 2.6 VRHE, the ICR increased to 102 mΩ cm2 and 503 mΩ cm2, respectively. X-ray photoelectron spectroscopy analysis of the surface showed that the significant increase in ICR is related to a lower concentration of oxygen vacancies, an increase in oxygen to metal ratio and a decrease in metallic character of indium compared to the non-coated and baseline sample. However, neither of the components in a PEMWE will experience potentials significantly above 2.0 VRHE under normal operation. The results obtained in this study showed that Ta-ITO coating is promising for use in PEMWEs.

Sammendrag av avhandlingen:

Bipolar plater for PEM systemer

Samtidig som det globale energibehovet øker, blir det også mer og mer tydelig hvordan utslipp av klimagasser påvirker drivhuseffekten. "Proton Exchange Membrane" (PEM) brenselceller benytter hydrogen og oksygen til å produsere vann. Utslipp av klimagasser og andre skadelige stoffer fra et slikt system er derfor så godt som null. Man kan også bruke PEM systemer til å produsere hydrogen, ved å tilføre systemet elektrisk energi i en elektrolyseprosess.

PEM systemer består typisk av flere elektrokjemiske celler, som settes sammen til en såkalt "stack". I en slik stack er bipolare plater viktige komponenter, da de har som hovedoppgaver å separere og lede strøm mellom enkeltceller. Bipolare plater må være motstandsdyktige mot korrosjon under det utfordrerne miljøet i PEM systemer, samtidig som de må være elektrisk ledende. Dette gjør materialvalget for slike plater meget vanskelig.

Arbeidet i dette ph.d. prosjektet har hatt som overordnet mål å øke levetiden til PEM systemer, samtidig som kostnadene holdes lave. Utvikling av testprosedyrer for bipolare plater har vært en viktig del av dette studiet, og en metode for in situ testing av kontaktmotstand var en av disse. Disse metodene har videre blitt brukt til å undersøke både substratmaterialer og belegg. Bipolare plater laget av rustfritt stål ble polarisert i elektrolytter med ulik pH. Resultatene viste at man ikke bør endre pH for å akselerere testene, da det fører til at prosessene som finner sted på overflaten av materialet ikke lenger blir realistiske.

Ni ulike materialer ble testet under forhold lik de man forventer i en elektrolysecelle. Testene viste at veldig få materialer tilfredsstiller kravene om høy ledningsevne, lav korrosjon og lave kostnader. Et nytt belegg bestående av Tantal og IndiumTinnOksid (ITO) ble undersøkt for bruk på bipolare plater i PEM elektrolysører. Belegget viste lovende resultater, og vil bli jobbet videre med etter avsluttet ph.d.