Anode Feed Research Articles

Experimental Investigation on the Backpressure Effect on the Performance of a High Temperature Proton Exchange Membrane Fuel Cell Stack

This study investigated the impact of backpressure of anode and cathode on the performance of a high-temperature proton exchange membrane fuel cell stack consisting of 30 cells, initially operating at a current density of 0.4 A/cm², temperature of 160 ℃, and 0 bar with pure H2. Subsequently, the backpressure on both sides was varied from 0 bar to 1.5 bar, and the changes in stack performance were recorded throughout the testing period. Additionally, the impact of backpressure was also investigated with reformates (H2/CO2/CO/N2 = 69.4%/22.3%/1.40%/6.9%) as anode feed for the stack. The results showed that increase in operating backpressure improves the fuel cell performance. A stack voltage increase of 20 mV per cell and 40 mV per cell were achieved for the pure hydrogen operation and reformate gas operation respectively when increasing backpressure from 0 bar to 0.5 bar. This signifies potential to improve the system’s overall efficiency with only minor penalty in terms of auxiliary power loss to increase the back pressure.

Experimental Investigation on the Backpressure Effect on the Performance of a High Temperature Proton Exchange Membrane Fuel Cell Stack

This study investigated the impact of backpressure of anode and cathode on the performance of a high-temperature proton exchange membrane fuel cell stack consisting of 30 cells, initially operating at a current density of 0.4 A/cm², temperature of 160 ℃, and 0 bar with pure H2. Subsequently, the backpressure on both sides was varied from 0 bar to 1.5 bar, and the changes in stack performance were recorded throughout the testing period. Additionally, the impact of backpressure was also investigated with reformates (H2/CO2/CO/N2 = 69.4%/22.3%/1.40%/6.9%) as anode feed for the stack. The results showed that increase in operating backpressure improves the fuel cell performance. A stack voltage increase of 20 mV per cell and 40 mV per cell were achieved for the pure hydrogen operation and reformate gas operation respectively when increasing backpressure from 0 bar to 0.5 bar. This signifies potential to improve the system’s overall efficiency with only minor penalty in terms of auxiliary power loss to increase the back pressure.

The Effect of Cation Contamination on the Water Transport for PEM Water Electrolysis

Large-scale hydrogen production by PEM water electrolysis (PEMWE) in the energy sector requires a deep understanding of the system behaviour in failure cases and possible recovery procedures. This includes the purification of the anolyte feed water because the contamination with ions can cause the PEMWE stack to breakdown.[1] For instance, the presence of iron trace cations or other metal cations in the feed water leads to a performance loss due to higher ohmic resistance of the cell and higher iR-free voltages.[2] [3] Moreover, cation contamination might influence the water transport in a PEMWE cell. In principle, the transfer of the water molecules to the cathode is proportional to the proton flux from the cathode to the anode. The proportionality factor is defined as water transport coefficient and is also temperature dependent.[4] In case of the mono- and multivalent cations as trace elements in the feed water, the water loading of the membrane, which is expressed by water molecules per sulfonic acid group of the membrane, λ, decreases over time and leads to successive increase in the ohmic resistance. [5] However, the influence of cation contamination on the water transport in PEMWE is poorly understood to date.In this work, we investigated the effect of cation contamination in the anode feed water on the water crossover for PEMWE. Cation contamination experiments were carried out at a constant current density by adding different concentrations such as K2SO4, Na2SO4, etc. to the anode feed water. All electrochemical measurements were carried out in an in-house test bench equipped with a single cell of 5 cm2 geometric electrode area and potentiostat with booster. A commercially available catalyst coated perfluorosulfonic acid membranes (loading of 0.3 mg cm-2 geo Pt/C and 1 mgcm-2 geo IrOx) was used. The anode and cathode compartments were kept at atmospheric pressure and cell temperature of 80°C. To determine the water crossover from anode to cathode at different current densities, the cathode exhaust was cooled in a heat exchanger and measured by a gravimetric method. [4] Firstly, a conditioning procedure was performed with purified feed water until steady cell performance was achieved. The cell performance was evaluated using polarization curves, electrochemical impedance spectroscopy (EIS) and water crossover measurements. The last two are of particular interest, because they provide information about membrane humidification. The Tafel slope and mass transport overvoltage were established from the polarization curves and high frequency resistance (HFR). We observed a significant rise of the cell voltage by adding cations to the anode water feed within few hours. The analysis of Tafel slope and EIS data also reveal an increase of ohmic and mass transport resistances. In these experiments, we were able to correlate the HFR results with the water crossover as a function of the nature of the cation and its concentrations. Our data shows that the specific water crossover per current density decreases as a consequence of cation contamination. The switch back to purified feed water allows us to monitor the dynamics of the performance recovery process obtained from EIS and water crossover measurements. Additional measurements of the polarization curves and HFR were used to distinguish between reversible and irreversible degradation processes due to the cation contamination.Altogether, we present the effect of cation contamination on the water transport processes and provide deeper insights into the mass transport and performance losses for PEMWE.

Operatable and Efficient Mitigation Strategies for H2S Poisoning in Proton Exchange Membrane Fuel Cells: Releasing Pt Reactive Sites for Hydrogen Oxidation

The anode feed for hydrogen fuel cell electric vehicles (FCEVs) currently still relies on the reformed fuel that inevitably contains contaminants (e.g., CO, H2S, and NH3). As one of the most sensitive impurities, trace H2S in hydrogen significantly deteriorates the durability of proton exchange membrane fuel cells (PEMFCs), while there is still a lack of effective strategies to mitigate H2S poisoning in PEMFCs. Herein, we present two kinds of facile, operatable, and efficient strategies to mitigate a H2S-poisoned anode, with the aim of improving the durability of PEMFCs by releasing more Pt reactive sites for hydrogen oxidation reaction (HOR) catalysis. The first mitigation strategy is conducted by increasing the cell temperature temporarily when purging with pure hydrogen after H2S poisoning by means of accelerating H2S desorption on Pt sites under high temperatures and thus releasing more available Pt reactive sites for HOR catalysis. A higher temporary temperature leads to a larger recovery percentage of performance, outperforming the effect of the generally adopted pure hydrogen purging operation. Another mitigation strategy is called “internal oxygen permeation”, based on the oxidation of sulfur-adsorbed anode by compelling air diffusion through thin PEM via constructing a pressure differential between the cathode and the anode. The cell was found to be recovered more at a high pressure differential due to the fact that more sulfur species are oxidized by a larger content of permeated oxygen, showing ∼88.7% performance recovery for 0.40 bar pressure differential. The proposed strategies with simplicity, operability, and resilience show promising potential in the application of long-term PEMFCs, which is critical for the durability improvement of FCEVs and cost reduction of hydrogen purification.

Modeling direct ammonia anion-exchange membrane fuel cells

Ammonia has recently been proposed as a promising candidate fuel for anion-exchange membrane fuel cell (AEMFC) technology. Direct ammonia AEMFCs (DA-AEMFCs) are a carbon-free technology that combines the ammonia's high energy density with the fuel cells' high efficiency. However, two major challenges face this technology: ammonia crossover (due to ammonia's high solubility in water) and sluggish ammonia oxidation reaction (AOR). We have developed, applied, and presented a one-dimensional and transient model of a DA-AEMFC system to address these challenges. Excellent agreement is obtained between the experimentally-measured and computationally-simulated performance of DA-AEMFCs operating at 100 and 120 °C with KOH-free anode feed. As the current density increases, the initial cell performance analysis reveals a reduction in the parasitic AOR rate through the cathode. More intriguingly, the results demonstrate a positive impact of ammonia crossover on cell longevity. Crossover drives an AOR within the cathode, which has a detrimental effect on performance but comes with an associated benefit of water generation in this region. The resultant improvement in the cathode hydration reduces the degradation rate of ionomeric material, ultimately increasing cell lifetime. Further studies are required to determine the desired rate of ammonia crossover and its influence on the system's cost-effectiveness.

Mechanistic Insight from Acetate Oxidation in Three-Electrode Cells and Flow Cells

Recent years has seen a tremendous growth in finding electrochemical pathways to produce chemicals and fuels because it is believed that pairing low-carbon electricity (solar, wind, nuclear) with renewable carbon sources and electrochemical processes will lead to a reduction in greenhouse gas emissions. One of the possible sources renewable carbon is biomass. During biomass upgrading, one of the byproducts is bio-oil, which contains a large amount of acetic acid. Acetic acid itself is a relatively low value product. Upgrading acetic acid (or its alkaline counterpart acetate) to a wider range of valuable products (e.g., methanol, ethylene) is extremely attractive. This can be done through thermochemical or electrochemical partial oxidation.Though acetic acid reactions have been studied for many years, details about its overall reaction mechanism still remain unknown. The most known and defended pathway in the literature is Kolbe electrolysis, where C-C bonds are formed from dimerization of methyl radicals as shown in Equation 1.2CH3COOH → 2CO2 + C2H6 + 2H+ + 2e- (Equation 1)However, aqueous environments have shown deviations from this dimerization. It is often overlooked that these reactions occur at high potentials and the state and role of the formed surface oxide are not well described in the literature. It is even not known what water-oxide mechanism dominates for the formation of alcohol-based products. Although prior work has shown that a three-electrode batch cell can produce the typical Kolbe products (ethane and carbon dioxide) at high faradaic efficiencies and steady state when the electrolyte bath is held at the pKa (4.7) and high voltage (>3.0V vs. RHE) – our previous studies have shown that the reaction environment (e.g. concentration, addition of supporting electrolyte, pH) can vary the effluent composition and reaction rate considerably. Also, our recent work has shown that transitioning the same catalyst to a lower-water environment in a flow cell can yield a completely different product profile than in aqueous media.Therefore, the purpose of this study is to demonstrate how the catalyst and reacting environment affect the formation of various C1 and C2 products from acetate electrochemical oxidation. To do this, custom-made two-electrode (similar to an alkaline exchange membrane electrolyzer) and three-electrode cells are used. Another variable that is manipulated is the voltage profile, where both constant voltage and pulsed experiments are performed. In the flow cell, several anode electrocatalysts (e.g., Pt, PtRu, NiO, IrOx) were dispersed in inks and sprayed onto porous transport layers. The anode feed was 0.5M potassium acetate. No gases were fed to the cathode, but hydrogen was collected from the exhaust. The anode effluent (gas and liquid mixture) was separated downstream in a simple vessel with UHP helium flowing through the incoming effluent to carry the gas for GC/MS analysis. Liquid samples were analyzed by ex situ and NMR with D2O solvent. Two-electrode and three-electrode results will be combined to provide new insights into acetate oxidation and pathways for future research and development will be discussed.

(Invited) Direct Ammonia Anion-Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) are a promising alternative to the more established proton exchange membrane fuel cell (PEMFC) technology, with advantages mainly related to the reduction in expense of materials used. While Hydrogen is the most popular fuel for AEMFCs, ammonia’s high energy density together with its established transport and distribution technology, make it an attractive candidate as an alternative fuel for AEMFCs.Although early attempts to utilize ammonia as a fuel in AEMFCs resulted in poor performance, recent studies of direct ammonia anion-exchange membrane fuel cells (in short - DAFCs), with advanced anion exchange membranes (AEMs) and electrocatalysts, have yielded new and promising performance measure values (see e.g. [1]) thus motivating further analyses and development of these systems. The two main challenges to be addressed when considering this technology are a slow ammonia oxidation reaction (AOR) and ammonia crossover.In this contribution, we will present a computational analysis of a DAFC, aimed at probing performance and performance stability of this system. A special emphasis will be placed on the effect of ammonia crossover and the resultant parasitic reaction (involving ammonia oxidation in the cathode) on system behavior. Following an appropriate introduction, we will describe our modeling approach, properties, and parameters, where our one-dimensional and transient model of a DAFC, including degradation kinetics capabilities, is conceptually based on our previous studies of hydrogen-fueled AEMFCs (e.g. [2-4]). The computational domain consists of a membrane, anode and cathode gas diffusion layers, and anode and cathode catalyst layers. The model captures species transport and significant electrochemical reactions taking place in the cell. After detailing our modeling approach, we will present calculations aimed at validating our model against experimental data of a DAFC operated with KOH-free anode feed at high temperatures (100 and 120 °C). Following this, we will show and discuss results concerning the effect of ammonia crossover on DAFC performance and performance stability. The initial (un-degraded) cell performance analysis reveals (among other things) a dependence of the parasitic reaction rate through the cathode on the value of current density. More interestingly, the simulated results demonstrate (under certain conditions) a surprising and nontrivial impact of ammonia crossover on cell longevity..

Anode-Fed Anion Exchange Membrane Electrolyzers for Hydrogen Generation Tolerant to Anion Contaminants

Anion exchange membrane water electrolysis has the ability to produce green hydrogen with high voltage efficiencies at low capital cost with zero CO2 emissions. The alkaline environment of these devices allows for the use of economical metal catalysts and anion exchange membranes (AEMs) that conduct hydroxide ions and are less expensive than their proton exchange membrane counterparts. However, research has shown that anion contamination of hydroxide exchange membranes can lead to significant performance losses in anion exchange membrane fuel cells (AEMFCs), particularly when air fed to the oxygen-reducing cathode contains CO2.1 A similar contamination effect occurs in anion exchange membrane electrolyzers (AEMELs), when dissolved CO2 in the electrolyte reacts to form carbonate (CO3 2-) and bicarbonate (HCO3 2-) anions which compete with the hydroxide (OH-) ions that must be conducted through the AEM and ionomer. The presence of these and other anion contaminants can lower the ionic conductivity of the cell. Under high current density operation, an AEMEL undergoes a self-purging process that uses an ionic potential gradient to push hydroxide and anion contaminants through the membrane to the anode. This creates a significant pH gradient between cathode and anode that can lead to concentration polarization which further lowers performance.2 In this work, we use 1-D CO2 transport modeling and experiments to show how altering the electrolyte feed method allows for CO2-tolerant AEMEL operation in several different electrolytes.The unique advantages of AEMELs over AEMFCs and proton exchange membrane electrolyzers (PEMELs) are that (1) this self-purge occurs during normal operation, and (2) they allow for flexibility in the location of the potentially contaminated water feed. In PEMELs, water is intuitively fed to the anode, and cation contaminants are purged through the entire MEA to the cathode. Although water in AEMELs is intuitively fed to the cathode, where it is consumed in the hydrogen evolution reaction, water can diffuse easily through the membrane, allowing for it to be fed as an anolyte to the oxygen-evolving side of the cell. This allows for better contaminant rejection because anions can be concentrated mostly on the anode side of the electrolyzer.The model described in this paper predicts that an anode-fed AEMEL can more easily purge CO2 without contaminating as much of the AEM or inducing as high of a pH gradient due to more rapid self-purging of anions. We find experimentally that AEMELs with DI water anolyte are more tolerant of forced CO2 contamination than those with DI water catholytes, which is likely one of the major reasons for superior anode-feed performance. Furthermore, electrolyzer operation at high current densities can lead to voltage recoveries greater than 200 mV due to self-purging of anions. Although supporting electrolytes such as potassium hydroxide can mitigate catholyte contamination, the anion self-purging shows that electrolyzer operation in DI water and even tap water (containing fluoride, chloride, and nitrates) can improve when employing an anode feed.1. Y. Zheng et al., Energy Environ. Sci., 12, 2806–2819 (2019).2. B. P. Setzler, L. Shi, T. Wang, and Y. Yan, in ECS Meeting s, vol. MA2019-01, p. 1824–1824, IOP Publishing (2019) https://iopscience.iop.org/article/10.1149/MA2019-01/34/1824.

Enhancing hydrogen production through anode fed-batch mode and controlled cell voltage in a microbial electrolysis cell fully catalysed by microorganisms

A microbial electrolysis cell (MEC) fully catalysed by microorganisms is an attractive technology because it incorporates the state-of-the-art concept of converting organic waste to hydrogen with less external energy input than conventional electrolysers. In this work, the impact of the anode feed mode on the production of hydrogen by the biocathode was studied. In the first part, three feed modes and MEC performance in terms of hydrogen production were evaluated. The results showed the highest hydrogen production under the continuous mode (14.6 ± 0.4), followed by the fed-batch (12.7 ± 0.4) and batch (0 L m−2 cathode day−1) modes. On one hand, the continuous mode only increased by 15% even though the hydraulic retention time (HRT) (2.78 h) was lower than the fed-batch mode (HRT 5 h). A total replacement (fed-batch) rather than a constant mix of existing anolyte and fresh medium (continuous) was preferable. On the other hand, no hydrogen was produced in batch mode due to the extensive HRT (24 h) and bioanode starvation. In the second part, the fed-batch mode was further evaluated using a chronoamperometry method under a range of applied cell voltages of 0.3–1.6 V. Based on the potential evolution at the electrodes, three main regions were identified depending on the applied cell voltages: the cathode activation (<0.8 V), transition (0.8–1.1 V), and anode limitation (>1.1 V) regions. The maximum hydrogen production recorded was 12.1 ± 2.1 L m−2 cathode day−1 at 1.0 V applied voltage when the oxidation and reduction reactions at the anode and cathode were optimal (2.38 ± 0.61 A m−2). Microbial community analysis of the biocathode revealed that Alpha-, and Deltaproteobacteria were dominant in the samples with >70% abundance. At the genus level, Desulfovibrio sp. was the most abundant in the samples, showing that these microbes may be responsible for hydrogen evolution.

Impact of alternating fuel feeding on a PEMFC stack durability

The integration of PEMFC for transport applications requires cost as well as system complexity reduction. A novel anode circuit architecture named “Alternating Fuel Feeding” (AFF) was developed. It combines on one hand, the benefits of the hydrogen recirculation and on the other hand, the simpleness of Dead-End Anode (DEA). A previous work demonstrated the benefits on the costs and on the anodic line weight. In the present work, investigations on stack durability, from the system scale to the cell scale, are performed during operation in two anode feeding modes: AFF and recirculation. The performances of stacks are evaluated along ageing tests, coupled with local measurements. At the end of life, postmortem analyses are performed on the aged cells. The degradation rates of stack performances are quite similar in both cases while more heterogeneous cell degradation are observed in recirculation mode along with the ageing according to a Fuel Cell Dynamic Load Cycle (FC-DLC).

Effects of Impurities on Pre-Doped and Post-Doped Membranes for High Temperature PEM Fuel Cell Stacks

In this paper, we experimentally investigated two high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stacks for their response to the presence of reformate impurities in an anode gas stream. The investigation was aimed at characterizing the effects of reformate impurities at the stack level, including in humidified conditions and identifying fault features for diagnosis purposes. Two HT-PEMFC stacks of 37 cells each with active areas of 165 cm2 were used with one stack containing a pre-doped membrane with a woven gas diffusion layer (GDL) and the other containing a post-doped membrane with non-woven GDL. Polarization curves and galvanostatic electrochemical impedance spectroscopy (EIS) were used for characterization. We found that both N2 dilution and impurities in the anode feed affected mainly the charge transfer losses, especially on the anode side. We also found that humidification alleviated the poisoning effects of the impurities in the stack with pre-doped membrane electrode assemblies (MEA) and woven GDL but had detrimental effects on the stack with post-doped MEAs and non-woven GDL. We demonstrated that pure and dry hydrogen operation at the end of the tests resulted in significant recovery of the performance losses due to impurities for both stacks even after the humidified reformate operation. This implies that there was only limited acid loss during the test period of around 150 h for each stack.

Bio-assisted treatment of hazardous spent wash via microbial fuel cell. Environmental friendly approach

Microbial fuel cell (MFC) has been utilized for energy generation and wastewater treatment since many decades. Due to the better performance of MFC concerning energy generation and wastewater treatment, the current study was conducted. The aim of current study is to utilize spent wash as anode feed along with impact of various parameters on treatment as well as energy production. Saccharomyces cerevisiae sp. was utilized as biocatalyst while 180 μmol l−1 of methylene blue as a mediator in anode and 350 μmol l−1 potassium ferricyanide was used as an oxidizing agent in cathode chamber for treatment of spent wash. All experiments were carried out through balancing the volume of 1 l for power generation from spent wash in MFC under an optimized parameter of 10% agarose, pH 8, aeration rate 200 ml/min, and 50% substrate concentration. The maximum current and a power density obtained at an optimized concentration are approximately 54.21 mA/m2 and 71.12 mW/m2, respectively. The maximum voltage of 830mV (3.3 mA) was obtained per liter of processed spent wash. The removal of COD and BOD from effluent MFC was 94% remarkably. These results demonstrate that MFC can simultaneously generate electricity and remove COD effectively from spent wash.

Spatial analysis of CO poisoning in high temperature polymer electrolyte membrane fuel cells

The improved tolerance of the High Temperature-Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) to CO allows the use of reformate as an anode feed. However, the presence of several per cent of CO in the reformate, which is inevitable particularly in on-board reformation in automobiles, which otherwise demands complex systems to keep the CO level very low, will significantly lower the cell performance, especially when the HT-PEMFC is operated at 160 °C or below. In this study, a three-dimensional, non-isothermal numerical model is developed and applied to a single straight-channel HT-PEMFC geometry. The model is validated against the experimental data for a broad range of current densities at different CO concentration and operating temperatures. A significant spatial variation in current density distribution is observed in the membrane because the CO sorption is a spatially non-homogeneous process depending on local operating conditions and dilution of the H2 stream. To investigate the local spatial effects on HT-PEMFC operation, the model is applied to a real cell of size 49.4 cm2 with an 8-pass serpentine flow-field at the anode and the cathode. The membrane and anode catalyst layer are segmented into 5×5 array to investigate the spatial resolution of the polarization curves, H2 concentration, current density, and anode polarization loss. The simulation results show that the presence of CO in the anode feed reduces cell performance, however, the results reveal that uniformity in current density distribution in the membrane improves when the cell is operated in potentiostatic mode. The results are discussed in detail with the help of several line plots and multi-dimensional contours. The study also emphasizes on the importance of optimizing the reformate anode feed rate to improve cell performance.

Effect of Reformed Gas Components on the Operation of HT-PEMFC Stacks

Due to still existing limitations in logistic and some risk in handling of compressed gases hydrogen is not a suitable fuel for all applications which can profit from the use of fuel cells. In such cases the use of a downstream reformer to generate hydrogen from another fuel is a viable option. Fuel cell which can tolerate the impurities contained in the reformed gas will avoid the need of sophisticated gas clean-up. Such tolerance is obtained by increasing the operation temperature. BASF has already shown that their Celtec® MEA can tolerated substantial CO and H2S concentration1 which was confirmed by us for MEA of other manufacturers2,3. Here, the effect of impurities on the operation of HT-PEMFC stacks is investigated.In this contribution investigations of the effect of impurities typically found in fuel gas obtained by auto thermal reforming of jet fuels is reported. Tests were conducted on Serenergy S165L-35 stacks with 35 cells of 165 cm2 geometric electrode area. Investigated were in detail the effect of dilution of hydrogen with nitrogen or CO2, the effect of fuel gas humidity and the effect of percent level CO and ppm level H2S impurities.It was found that the performance of the stack at an anode stoichiometry of 1.4 is strongly influenced by hydrogen dilution (cf. fig 1a and b). Thereby no difference is found between dilution with N2 or CO2 (cf. fig. 2) indicating that no relevant reversed water gas shift reaction does occur. High levels of humidification with anode feed dew point exceeding about 60 °C also reduce the stack performance (cf. fig 3). This effect can again be correlated to the dilution with hydrogen (cf. fig. 4). However, after longer test with high due points stack performance is irreversibly reduced (cf. fig 5). Adding CO or H2S to diluted hydrogen further reduces the performance as expected. Unexpectedly the impact of the addition of CO is higher than that of H2S (cf. fig 6). Finally adding CO and H2S has a combined effect (cf. fig 7) though not as strong a described by Schmidt and Baurmeister1.In the contribution further analysis of the different effects e.g, on cell voltage distribution will be given as well as the determination and validation of a suitable operating point for a jet fuel surrogate reformate.The work was financial supported by the European Defence Agency (EDA) through the Ad-Hoc project “IAPUNIT” Contract B 1490 GEM 3 GP, jointly financed by the Federal Republic of Austria, the Federal Republic of Germany, the Kingdom of the Netherlands, the Republic of Slovenia and the Kingdom of Sweden.References T. J. Schmidt and J. Baurmeister, ECS Trans., 3(1), 861 – 869 (2006)C. Cremers, M. S. Rau, A. Niedergesäß, K. Pinkwart, and J. Tübke, ECS Trans., 75(14), 919 – 929 (2016).M. Rau, A. Niedergesäß, C. Cremers, S. Alfaro, T. Steenberg, H.A, Hjuler, Fuel Cells, 16(5), 577 – 583 (2016). Figure 1

A Direct Ammonia Fuel Cell with a KOH-Free Anode Feed Generating 180 mW cm−2 at 120 °C

The motivations to operate the direct ammonia fuel cell (DAFC) with no addition of aqueous base, include facilitated fuel management and prevention of corrosion and shunt currents in the DAFC stack. We describe here a polymer electrolyte DAFC, operating at a cell temperature (Tcell) of 120 °C on ammonia and water alone as anode feed, that demonstrated peak power of 180 mW cm−2—a record for this type of fuel cell. We offer a detailed analysis of polarization curves recorded for PE-DAFCs in the Tcell range of 60 °C–120 °C, allowing to estimate the effects of KOH-free operation on the ionic and faradaic resistances, Ri and RF, that determine together the value of Rapp, CL–an, the apparent resistance of the anode catalyst layer. The latter resistance drops ten times between 60 °C and 120 °C as result mainly of the drop in RF. Consequently, DAFC performance at Tcell > 100 °C improves substantially, including the cell performance on a KOH-free anode feed. The demsontrated performance levels reported here for KOH-free DAFCs support further development of this type of ammonia/air fuel cells.

Investigation of cubic Pt alloys for ammonia oxidation reaction

As a promising fuel candidate, ammonia has been successfully used as anode feed in alkaline fuel cells. However, current technology in catalysts for ammonia electro-oxidation reaction (AOR) with respect to both cost and performance is inadequate to ensure large scale commercial application of direct ammonia fuel cells. Recent studies found that alloying Pt with different transition metals and controlling the morphology of catalysts can improve the AOR activity, and thus potentially can solve the cost issue. Herein, (100)-terminated Pt-M nanocubes (M = 3d-transition metals Fe, Co, Ni, Zn) are synthesized via wet-chemistry method and their catalytic activities toward AOR are evaluated. The addition of Fe, Co, Ni and Zn elements can enhance the AOR activity due to decrease in oxophilicity of platinum and bifunctional mechanism. Pt-Zn exhibits the maximum mass activity and specific activity with values of 0.41 A/mgPt and 1.69 mA/cm2 that are 1.6 and 1.8 times higher than Pt nanocubes, respectively. Pt-Fe, Pt-Co and Pt-Ni nanocubes also illustrate higher mass and specific activities compared to Pt nanocubes.

Effects of the Transport/Catalyst Layer Interface and Catalyst Loading on Mass and Charge Transport Phenomena in Polymer Electrolyte Membrane Water Electrolysis Devices

The properties of porous transport layers (PTL) in electrolysis devices and their effects on cell performance have been studied extensively in recent literature. This paper provides a detailed analysis with regards to the transport in the catalyst layer (CL). The work demonstrated that the catalyst loading affects the sensitivity of electrolysis performance to PTL properties, particularly those of the PTL surface in contact with the CL. It was demonstrated that upon reducing catalyst loading, PTL properties had an increased effect on the performance of PEMWE cells. While we observed mild performance variations among PTLs when using a high anode catalyst loading, strong correlations between PTL surface properties and cell performance existed at a low catalyst loading. PTL properties affected performance by influencing the in-plane conductivity and permeability of the CL. The variation of apparent exchange current density and apparent CL bubble coverage with the stoichiometric flow rate was studied at low anode feed rates. This led to the emergence of a PTL grain size effect on apparent bubble coverage at high catalyst loading. We provide a descriptive analysis of the phenomena causing voltage losses in PEMWE devices. These findings are important for electrochemical modeling and designing the PTL/CL interface.

基于前馈模糊逻辑PID 策略的MFC 电压控制和非线性性能分析

Microbial fuel cell (MFC) is a kind of promising clean power supply energy equipment, but serious nonlinearities and disturbances exist when the MFC runs, and it is an important topic to guarantee that the output voltage reaches the setting value quickly and smoothly. Regulating the feeding flow is an effective way to achieve this goal, and especially, the satisfactory results can be achieved by regulating anode feeding flow. In this work, a feedforward fuzzy logic PID algorithm is proposed. The fuzzy logic system is introduced to deal with the non-linear dynamics of MFC, and corresponding PID parameters are calculated according to defuzzification. The magnitude value of the current density is used to simulate the value of the external load. The simulation results indicate that the MFC output voltage can track the setting value quickly and smoothly with the proposed feedforward fuzzy logic PID algorithm. The proposed algorithm is more efficient and robust with respect to anti-disturbance performance and tracking accuracy than other three control methods.

Effects of Ammonia Impurities on the Hydrogen Flow in High and Low Temperature Polymer Electrolyte Fuel Cells

AbstractFuel cells are a promising technology to use as a source of electricity and heat for buildings, and as an electrical power source for electric motors propelling vehicles. They consume hydrogen as fuel and oxygen to produce electricity, heat and water. Conventional fuels, such as natural gas, methanol, or even gasoline are reformed to produce the hydrogen required by the fuel cells. During the reforming process, impurities are inevitably obtained in the hydrogen flow. One of them is ammonia (NH3) that can result in serious damage to the fuel cell operation. In this paper, the effects produced by different concentrations of NH3 present in the hydrogen flow on the membrane electrode assembly (MEA) performance are studied, differentiating between irreversible and recoverable damages. Strictly experimental, the study includes both low and high temperature polymer electrolyte fuel cells (PEFC). The NH3 poisoning effect is analyzed and quantified by comparing the polarization curves. After the poisoning stage, the cells are subjected to a regeneration process (feeding the cell with neat H2) with the aim of knowing the membrane's recovery capacity. The experimental results demonstrate that in low temperature (LT)‐PEFCs, the cell recovers its performance almost completely with a new exposure to neat H2, in spite of the damage previously caused by the presence of traces of NH3 in the anode feed stream. In contrast, in high temperature (HT)‐PEFCs, the cell suffers irreversible damage, even with short time exposure to NH3. The paper concludes with discussing the possible chemical interactions by which NH3 affects the cell performance.

Sulfur diffusion of hydrogen sulfide contaminants to cathode in a micro-tubular solid oxide fuel cell

The contamination effect of hydrogen sulfide on micro-tubular solid oxide fuel cells was investigated using various electrochemical techniques. The cells, mechanically supported by cupper tube, were provided by National Metal and Materials Technology Center, and they contain the electrode materials of nickel and yttria stabilized zirconia cermet for anode and lanthanum strontium manganate for cathode. Pure hydrogen and hydrogen gas containing diluted hydrogen sulfide were repeatedly switched for the anode feed at the constant cell voltage of 0.9 V. Exposure times of 30 and 60 min were applied to study the effects of the poisoning interval. Then, the obtained results were analyzed. Herein, we reported two effects that had not been reported previously: the poisoning interval effect on the same hydrogen sulfide dosage and the poisoning sulfur-transport effect to the cathode surface. In the same dosage experiment, the recovery process was required before complete adsorption of sulfur species, and it supported not to fully block active sites of hydrogen adsorption and oxidation, and thus it would expand a cell lifespan on the contamination. In the analysis, different contamination steps between short and long term exposure time were presented, and thermodynamic parameters were calculated. Also, transport of elemental sulfur from the anode to the cathode surface was presented using the energy dispersive X-ray spectroscopy technique. The results appear to be strong evidence for the transport of sulfur from anode to cathode, and this means that the effect of sulfur diffusion to the cathode should be considered one major reason for performance degradation. This paper provides additional possible reasons regarding the performance degradation on hydrogen sulfide contamination, and serves as a guide to overcome the barrier of the sulfur contamination.