Highlights ORR-kinetics study on Pt and Ag by using thin-layer catalysts in a AEMFC at 80 °C. Ag shows a later onset potential for ORR and lower performance at high voltages. The Tafel slope of Pt is 75 mV dec−1 above 0.8 V, for Ag the slope is 160 mV dec−1. For Ag, a voltammetric anodic peak at 0.8 V, indicates formation of Ag2O. Ag performs better than Pt below 0.5 V, but its stability is affected above 0.8 V.

Using stainless steel as material for bipolar plates (BPPs) in proton exchange membrane fuel cells (PEMFCs) carries a perceived risk of corrosion and subsequent metal ion contamination of the membrane electrode as-sembly (MEA). However, assessments in literature on this hazard to PEMFC systems have been based on ex-situ corrosion studies, where general assumptions made on the BPP environment might not be a correct simulation of real on-site conditions. In this contribution, uncoated BPPs from stainless steel grades 304 L, 316 L and 904 L were subjected to in-situ hybrid endurance/stress testing to simulate realistic conditions in operating fuel cell systems and re-evaluate the need of additional corrosion protection. A post analysis of the plates showed no signs of surface dissolution on any of the tested samples and the concentration of iron in the MEA averaged 7 to 10 ppm for uncoated samples and 7 to 11 ppm for coated and graphitic reference tests, displaying a negligible amount of trace metals compared to critical thresholds found in literature. Contact resistance values were stable for all samples and observable changes in cell performance and voltage degradation was confirmed to be un-related to the presence of uncoated bipolar plates. The combined effects of decoupling of bipolar plate surface potentials from electrode potentials and operational control of stable gas flow compositions to sustain stainless steel surface passivation, were identified as explanation for the experimentally observed corrosion resistance of uncoated stainless steel plates in PEMFCs.

Polymer electrolyte fuel cells occupy a key position in implementing the hydrogen economy on a global scale. However, assessments of cost, availability and sustainability of materials currently used to manufacture state-of-the-art fuel cell stacks have given cause for concern. Platinum and platinum-group metals are prohibitively expensive and of low abundance. The benchmark ion-conductive polymer Nafion^TM and related perfluorosulfonic acid-based polymers are difficult to synthesize and environmentally persistent to extreme degrees. Graphitic and carbon composite bipolar plates are unsuitable for mass production and have low recycling potential. In the compiled works, alternative materials were evaluated both for the acidic and alkaline variations of polymer electrolyte fuel cells. Electrochemical characterization was carried out in single cell tests with a focus on finding the limitations in terms of ohmic, charge transfer and transport resistances in the cell, through polarization and impedance measurements.

Carbon coated stainless steel bipolar plates were tested operando in a proton exchange membrane fuel cell (PEMFC) under realistic conditions based on the New European Drive Cycle. Observed trace metal contamination of the MEA was linked to metal dissolution from coating defects caused by manufacturing (Paper I). A theoretical understanding of observed metal dissolution was confirmed experimentally and a concept for preventing metal dissolution was developed for PEMFC bipolar plates (Paper II). The developed concept was extended to uncoated stainless steel bipolar plates and tested successfully for three stainless steel types in operando PEMFC (Paper III)

Anion exchange polymers based on poly(arylene piperidinium) (PAP) were tested as both membranes and ionomers in a comparative study with a commercial reference material, showing higher performance and the significance of ionomer-carbon support interactions (Paper IV). PAP-based ionomers with varying ion exchange capacities were studied to optimize electrodes in anion exchange membrane fuel cells (AEMFC). A combination of high ion exchange capacity ionomer on both cathode and anode was best performing, linked to small water transport resistance in cathode and increased kinetic contribution of the anode HOR (Paper V). The effects of modifying the catalyst layer through the introduction of crosslinked PAP particles were studied in operando AEMFCs. A positive impact on charge transfer and diffusion resistances in electrodes containing particles could be observed. (Paper VI).

Silver nanoparticles were used as catalyst material in the cathode of previously optimized membrane electrode assemblies in AEMFC. The results showed promising performance compared to platinum electrodes based on monetary and sustainability considerations, but also challenges regarding catalyst stability and detrimental silver-ionomer interactions. (Paper VII).

Bränsleceller med polymerelektrolyt har en avgörande roll för implementering av vätgasekonomin globalt. Faktorer som kostnad, tillgänglighet och miljöpåverkan av material som för närvarande används i moderna bränsleceller är dock ett bekymmer. Platina och platinagruppmetaller är dyra och sällsynt förekommande. Den dominerande jonledande polymeren NafionTM och liknande perfluorsulfonsyra-baserade polymerer är krävande att syntetisera och svårnedbrytbara i naturen. Bipolära plattor av grafit och kolkomposit är olämpliga för massproduktion och har låg återvinningspotential. I avhandlingens sammanställda arbeten utvärderades alternativa material både för de sura (PEMFC) och de alkaliska (AEMFC) varianterna av polymerelektrolytbränsleceller. Elektrokemisk karakterisering med polarisations- och impedansmätningar genomfördes i enkelceller för att studera begränsningarna vad gäller laddningsöverföring samt ohmska och transportrelaterade förluster i cellen.

Kolbelagda bipolära plattor av rostfritt stål undersöktes in operando i en PEMFC under realistiska förhållanden baserade på en standardiserad europeisk driftcykel. Spår av metalljoner i cellkomponenterna observerades efter test och kopplades till metallupplösning från defekter orsakade vid tillverkningen (Paper I). Upplösningen visade sig bero på potentialgradienter som uppstod vid förändringar i gassammansättningen och kunde undvikas med bättre reglerade flöden (Paper II). Tre typer av rostfritt stål utan kolbeläggning jämfördes i operando PEMFC med belagda bipolära plattor Inte i något av fallen påvisades någon metalljonupplösning(Paper III).

Anjonbytarpolymerer baserade på poly(arylenpiperidinium) (PAP) undersöktes både som membran och jonomer. I en jämförande studie med ett kommersiellt Aemion-material visade PAP högre prestanda. Vidare visade studien att interaktionen mellan jonomer och bärarkol hade betydelse för prestandan (Paper IV). PAP-baserade jonomerer med varierande jonbyteskapacitet studerades för att optimera elektroder i AEMFC. En kombination med jonomer med hög jonbyteskapacitet på både katod och anod var bäst, och kopplad till litet vattentransportmotstånd i katoden och ökat kinetiskt bidrag från anodreaktionen (Paper V). Effekterna av att modifiera katalysatorskiktet genom införandet av tvärbundna PAP-partiklar studerades i operando AEMFCs. En positiv inverkan på laddningsöverföring och diffusionsmotstånd i elektroder som innehåller partiklar kunde observeras. (Paper VI). 

Slutligen användes silvernanopartiklar som katodkatalysator i AEMFC med elektroder optimerade med PAP. Resultaten visade lovande prestanda jämfört med platinaelektroder, utifrån kostnads- och hållbarhetsaspekter, men också utmaningar gällande katalysatorns stabilitet och interaktionen mellan silver och jonomer. (Paper VII).

The bipolar plate (BPP) is a component with vast cost-reduction potential in proton exchange membrane fuel cells (PEMFCs). Apart from mechanical and heat transfer requirements, the most desired BPP properties are high corrosion and low electrical contact resistance. In this study we confirm that due to ionic decoupling between BPPs and electrodes, the surface potentials of the BPPs remain stable even at varying operation loads. These mild potentials, in combination with low metal-ion leeching due to passive-transpassive-passive dissolution in stainless steels, suggest that low-cost carbon-coated stainless steel can readily be used as a BPP in PEMFCs. To prove this, single-fuel cell tests were carried out under realistic driving conditions, including electrochemical analysis, in-situ contact-resistance measurements, and post-mortem investigation of the membrane electrode assembly (MEA) by inductively coupled plasma trace-metal analysis, combined with electron microscopy and Auger spectroscopy of the BPPs. The results show that due to the ionic decoupling, conditions at the BPP surfaces are much less corrosive than previously thought. Furthermore, carbon-coated stainless-steel BPPs prove to be unaffected by global hydrogen starvation, which causes severe MEA degradation independent of the presence or absence of BPPs.

Awakening interest in anion exchange membrane fuel cells (AEMFC) for low temperature applications has led to an increased demand for high-performing polymers stable under alkaline conditions. In this study a poly(p-terphenylene piperidinium)-based membrane and ionomer was synthesized and applied in membrane electrode assemblies (MEAs), with porous gas-diffusion electrodes based on Pt catalysts supported by VULCAN (R) and high surface area carbon, respectively. The MEAs were evaluated in AEMFC single-cell tests. In order to identify specific beneficial characteristics of the polymer, the results were compared to reference tests using a commercial Aemion T-polymer. Steady-state polarisation performance measurements were carried out as well as electrode characterisations via cyclic voltammetry and electrochemical impedance spectroscopy, in addition to ex-situ characterisation of the polymer and the membrane electrode assemblies. Poly(p-terphenylene piperidinium)based (PAP) membranes showed great potential with an in-situ measured average ohmic resistance of 0.09 Omega cm(2). Mass transport limitations at higher current densities were observed for high surface area carbon electrodes, leading to an overall higher performance with the use of VULCAN (R). Properties of the ionomer related to water uptake capabilities were observed to inhibit performance as well. The higher water uptake of PAP-based ionomers appears to be a key property for increasing electrode performance.

Trace-metal contamination poses a threat to performance and stability of proton exchange membrane fuel cells (PEMFCs). In this study the source of origin and degree of metal dissolution from carbon-coated 316L bipolar plates (BPPs) are evaluated after a long-term PEMFC test run under conditions resembling a real-life automotive application. Despite intact carbon-coating, metal dissolution occurs from uncoated oxycarbide stains on the plates? surface. Which correlates with post-mortem detection of chromium, iron and nickel in the membrane electrode assembly. The rate of cell voltage decrease throughout the high current operations is found to be twice as high in the presence of metal ions. Metal dissolution can be correlated with transients in cell voltage during dynamic current load cycling as a result of temporary global fuel starvation. The observed difference in metal dissolution on the anode and cathode BPP indicates weak galvanic coupling between the bipolar plate(s) and the electrode layer(s). ? 2020 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

The kinetics of oxygen reduction reaction (ORR) on Ag and Pt thin-layer electrodes was studied in an anion exchange membrane fuel cell (AEMFC). The two-dimensional nature of these layers minimizes the effects of current distribution and mass transport. The ORR activities were evaluated at 80 °C and 100 % RH via polarization curves. Compared to Pt, Ag displays a later onset potential and a lower performance at high voltages. For Ag an anodic peak at 0.82 V was obtained by cyclic voltammetry. This peak indicating formation of Ag2O was also identified in SEM micrographs. At 100 % O2, the Tafel slope for Pt above 0.8 V was 75 mV dec−1. For Ag this slope, verified via electrochemical impedance spectroscopy, was 160 mV dec−1. By decoupling the first proton- and electron-transfer step of an associative ORR mechanism, a theoretical model captures the Tafel-slope response of Pt when the first proton transfer is the rate-determining step (rds). If the electron transfer is the rds, the theoretical slope fits well with the Tafel behavior of Ag. In fuel cell conditions, Ag performs better than Pt below 0.5 V, but the stability of Ag is compromised above 0.8 V.