Air Cathode Research Articles

Effect of Temperature and Water Content on the Oxidation Behaviour and Cr Evaporation of High-Cr Alloys for SOFC Cathode Air Preheaters

High temperature alloys are being investigated for use in cathode air pre-heater for solid oxide fuel cell systems because of their high thermal conductivity, formability, manufacturability, and superior mechanical properties. However, it is well-known that high temperature alloys often contain a high concentration of Cr which presents a risk of evaporation and contaminating the cathode of the solid oxide fuel cell (SOFC). The oxidation and Cr2O3 evaporation mechanisms of the alloys including alloy 625, SS309, and alloy 318 have been investigated by varying temperatures and water content of the exposure atmosphere. For the influence of water content, the alloys were isothermally exposed at 850 °C in dry air and air containing 1%, 3% and 9% of H2O at a high flow rate for 168 h. For the influence of temperature, the alloys were isothermally exposed at 650 °C, 750 °C and 850 °C in a 6.0 L/min air stream containing 3% H2O for 168 h. The results of this study show that Cr2O3 evaporation and oxidation rates were dramatically reduced with decreasing temperatures for alloy 625 and SS309. Alloy 318 exhibited a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature-dependent Cr2O3 evaporation compared to alloy 625 and SS309. The major effect of water vapour on the three tested materials appeared to be the further enhancement of Cr2O3 evaporation.

Rechargeable zinc-air batteries with an ultra-large discharge capacity per cycle and an ultra-long cycle life.

A conventional two-electrode rechargeable zinc-air battery (RZAB) has two major problems. (1) Opposing requirements for the oxygen reduction (ORR) and oxygen evolution (OER) reactions from the catalyst at the air cathode. (2) Zinc dendrite formation, hydrogen generation and zinc corrosion at the zinc anode. To tackle these problems, we have developed a three-electrode RZAB (T-RZAB) including a hydrophobic discharge cathode, a hydrophilic charge cathode, and a zinc-free anode. The decoupled cathodes enable fast ORR and OER kinetics, and avoid oxidization of the ORR catalyst. The zinc-free anode using tin-coated copper foam that induces the growth of (002)Zn planes, suppresses hydrogen evolution and prevents Zn corrosion. As a result, the T-RZABs have a high discharge capacity per cycle of 800 mAh cm-2 , a low voltage gap between discharge/charge platforms of 0.66V, and an ultra-long cycle life of 5220 hours at a current density of 10mA cm-2 . We have developed a large T-RZAB which has a discharge capacity of 10 ampere-hours per cycle with no obvious degradation after cycling for 1000 hours. Finally, we assembled a T-RZAB pack that had an energy density of 151.8 watt-hours per kilogram and a low cost of 46.7 US dollars per kilowatt-hour. This article is protected by copyright. All rights reserved.

The role of hydrophobicity and active layer thickness on degradation and durability of the air cathode in alkaline fuel cells

The limited cycle life due to the degradation of the cathode is a major problem for commercializing alkaline fuel cells. In this work, a novel combination of in situ and post-mortem analyses have been conducted, along with modifying the thickness and ratio of carbon/binder within the active layer, to investigate the degradation and durability of an alkaline air cathode. In situ analyses such as monitoring the electrode potential and electrochemical impedance spectroscopy are applied. The increased potential loss is likely due to the flooding of electrolyte inside the primary macro structure of the active layer, increasing the mass transfer resistance for oxygen transport. Scanning electron microscopy analysis during the first 24 h of operation reveals that the degree of binder loading has no effect on the electrolyte ingress in the active layer. Higher durability is observed for the cathode with a thicker active layer. X-ray photoelectron spectroscopy data indicates that Pt loss is associated with the elimination of the carbon functional group. Volume rendering of the active layer from Nano-scale X-ray computed tomography has shown a higher porosity and larger pores for the failed cathode. Pt loss, and diminishing of side chain in the binder are observed by thermogravimetric analysis.

Transition metal sulfide/oxide nanoflowers decorated on poly (aniline-2-sulfonic acid) modified polyacrylamide derived carbon cathode catalyst for bioenergy generation in microbial fuel cells

Microbial fuel cells (MFCs) are promising carbon-free energy devices with the potential to harvest power and treat wastewater simultaneously, which is crucial to meet the emerging energy challenges. Despite the high theoretical power output, the performance of MFCs is dependent on the cathodic oxygen reduction reaction (ORR) to generate electricity. Herein, a hybrid nanostructure of nickel-cobalt sulfide (NiCo2S4) nanosheets over NiCo2O4 nanoflowers on in-situ nitrogen and sulfur-doped carbon (NSC) is prepared by direct pyrolysis. Deposition of NiCo2S4 nanosheets improves the effective contact with the electrolyte and increases the catalytic sites for ORR. NiCo2S4/NiCo2O4@NSC shows an oxygen reduction peak at 0.150 V, limiting current of -0.093 mA, close to -0.107 mA for Pt/C and generates optimal power density of 831.74 mW m−2 comparable to Pt/C (857.92 mW m−2) and higher than NiCo2O4@NSC (665.86 mW m−2), NSC (650.28 mW m−2) and NC (485.98 mW m−2) air cathodes. This could be attributed to the high degree of graphitization, appropriate N and S content, a synergistic effect between the metal species and the heteroatoms, and better catalytic surface area utilization. The results suggest that M-N-S-based carbons could be potential cathodes for microbial fuel cells and similar energy devices.

Carbon electrodes supported with nickel oxide derived from olive pits for Li O2 air cathode

Carbon electrodes supported with nickel oxide derived from olive pits for Li O2 air cathode

A Fe Single Atom Seed-Mediated Strategy Toward Fe3 C/FeNC Catalysts with Outstanding Bifunctional ORR/OER Activities.

The discovery of low-cost and high-performance bifunctional oxygen electrocatalysts is vital to the future commercialization of rechargeable zinc-air batteries (ZABs). Herein, a Fe single atom seed-mediated strategy is reported for the fabrication of Fe3 C species closely surrounded by FeN4 C active sites with strong electronic interactions built between them and more importantly, creating optimized coordination environment, via subtly adjusting their ratio, for favorable adsorption energies of oxygen intermediates formed during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Concretely, the voltage difference (ΔE) between the ORR half-wave and OER potential at a current density of 10mA cm-2 for the compositionally-optimized FeNC/Fe3 C-op electrocatalyst is only 0.668V, endowing itself one of the best bifunctional OER/ORR benchmarks. As a demo, ZABs assembled with FeNC/Fe3 C-op as the air cathode deliver a remarkable specific capacity (818.1 mAh gZn -1 ) and a power density (1013.9 mWh gZn -1 ), along with excellent long-term durability (>450h). This work extends the methodology to modulate the activity of FeN4 C atomic site, undoubtedly inspiring wide explorations on the precise design of bifunctional oxygen electrocatalysts.

Coupling MnS and CoS Nanocrystals on Self-Supported Porous N-doped Carbon Nanofibers to Enhance Oxygen Electrocatalytic Performance for Flexible Zn-Air Batteries.

Seeking highly efficient, stable, and cost-effective bifunctional electrocatalysts of rechargeable Zn-air batteries (ZABs) is the top-priority for developing new generation portable electronic devices. For this, the rational and effective structural design, interface engineering, and electron recombination on electrocatalysts should be taken into account to reduce the reaction overpotential and expedite the kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we construct a MnCo-based metal organic framework-derived heterogeneous MnS-CoS nanocrystals, which are anchored on free-standing porous N-doped carbon fibers (PNCFs) by the in situ growth method and vulcanization process. Benefiting from the abundant vacancies and active sites, strong interfacial coupling as well as favorable conductivity, the MnS-CoS/PNCFs composite electrode delivers a mentionable oxygen electrocatalytic activity and stability with a half-wave potential of 0.81 V for ORR and an overpotential of 350 mV for OER in the alkaline medium. Of note, the flexible rechargeable ZAB using MnS-CoS/PNCFs as binder-free air cathode offers high power density of 86.7 mW cm-2, large specific capacity of 563 mA h g-1, and adapts to different bending degree of operation. In addition, the density functional theory calculation clarifies that the heterogeneous MnS-CoS nanocrystals reduces the reaction barrier and enhances the conductivity of the catalyst and the adsorption capacity of the intermediates during the ORR and OER process. This study opens up a new insight to the design of the self-supported air cathode for flexible electronic devices.

A high voltage zinc–air battery with two isolated electrolytes and moving auxiliary electrodes

A design for a zinc–air battery is presented, which allows use of two different electrolytes for the zinc electrode and the air cathode. The two electrodes are decoupled using two auxiliary electrodes which can exchange their positions to continue the operation even after their capacity is exhausted. A 3-D printed battery module is used to demonstrate the design. The design is demonstrated by employing two different combinations of electrolytes and two auxiliary electrode materials. In the first case, a near neutral aqueous electrolyte is used with the zinc electrode, while an acidic electrolyte is used with the air cathode. This allows the battery to operate with an average discharge voltage of 1.44 V, which is much higher than traditional configuration. In the second case, the battery design was demonstrated by employing a water-in-salt (WIS) electrolyte for the zinc electrode, and a near neutral electrolyte for the air cathode. The battery design therefore opens the possibility of effective use and tuning of hybrid electrolytes in a zinc–air battery, free from any limitation of the capacity arising from the auxiliary electrodes.

Heteroatom-Driven Coordination Fields Altering Single Cerium Atom Sites for Efficient Oxygen Reduction Reaction.

For current single-atom catalysts (SACs), modulating the coordination environments of rare-earth (RE) single atoms with complex electronic orbital and flexible chemical states is still limited. Herein, cerium (Ce) SAs supported on a P, S, and N co-doped hollow carbon substrate (Ce SAs/PSNC) for the oxygen reduction reaction (ORR) are reported. The as-prepared Ce SAs/PSNC possesses a half-wave potential of 0.90V, a turnover frequency value of 52.2s-1 at 0.85V, and excellent stability for the ORR, which exceeds the commercial Pt/C and most recent SACs. Ce SAs/PSNC-based liquid zinc-air batteries (ZABs) exhibit a high and stable open-circuit voltage of 1.49V and a maximum power density of 212mWcm-2 . As the catalyst of the air cathode, it also displays remarkable performance in flexible electronic devices. Theoretical calculations reveal that the introduction of S and P sites induces significant electronic modulations to the Ce SA active sites. The P and S dopings promote the electroactivity of Ce SAs and improve the overall site-to-site electron transfer within the Ce SAs/PSNC. This work offers a unique perspective for modulating RE-based SACs in a complex coordination environment toward superior electrocatalysis and broad applications in energy conversion and storage devices.

Use of reduced-voltage EIS to establish a relation between oxygen concentration and EIS responses of large commercial PEM fuel cell modules

This paper employs equivalent circuit analysis of electrochemical impedance spectra (EIS) signatures of commercial proton exchange membrane fuel cell stacks (PEMFC) to establish a relation between oxygen concentration and EIS response within individual cells of the stack. This paper is part of an ongoing effort to use EIS for detecting and quantifying hydrogen crossover to understand and reduce hydrogen cathode exhaust emissions. Selected results from different sets of experiments performed on Ballard water-cooled and air-cooled PEMFC stacks are presented. A novel reduced-voltage EIS system with the ability to perform impedance measurements over different cell counts is employed to record EIS scans of these stacks. In the first set of experiments, hydrogen was injected into the cathode side of a 30 kW 150+-cell water-cooled module at various rates to emulate the phenomenon of hydrogen crossover at different levels. In the second set of experiments, nitrogen was injected into a target single-cell in an air-cooled stack with over 50 cells (50+-cell) to dilute the cathode available oxygen, and the impedance was measured over various numbers of cells spanning around this target cell. A novel equivalent circuit model with resistors, constant phase elements (CPEs), and a linear finite Warburg parameter was used to study the EIS data via an optimization-based fitting approach in python. Results of these experiments have shown there is a proportional relationship between the Warburg parameter (in the equivalent circuit model used) and hydrogen crossover (at nominal idle and low-current air flows), and that this Warburg parameter increases significantly with reduced cathode air flows. Also, it is concluded that this relationship can be further used, along with decomposition of the EIS scan of a stack into different cell-level scans, to estimate the number of large-leaky cells as well as the total hydrogen crossover rate.

NiCo alloy‐anchored self‐supporting carbon foam as a bifunctional oxygen electrode for rechargeable and flexible Zn–air batteries

AbstractThe design and fabrication of flexible, porous, conductive electrodes with customizable functions become the prime challenge in the development of new‐generation wearable electronics, especially for rechargeable batteries. Here, the NiCo bialloy particulate catalyst‐loaded self‐supporting carbon foam framework (NiCo@SCF) as a flexible electrode has been fabricated through one facile adsorption‐pyrolysis method using a commercial melamine foam. Compared with the electrode with Pt/C and Ir/C benchmark catalysts, the NiCo@SCF electrode exhibited superior bifunctional electrocatalytic performance in alkaline media with a half‐wave potential of 0.906 V for oxygen reduction reaction, an overpotential of 286 mV at j = 10 mA cm−2 for oxygen evolution reaction, and stable bifunctional performance with a small degradation after 20,000 voltammetric cycles. The as‐assembled aqueous zinc–air battery (ZAB) with NiCo@SCF as a self‐supporting air cathode demonstrated a high peak power density of 178.6 mW cm−2 at a current density of 10 mA cm−2 and a stable voltage gap of 0.94 V over a 540 h charge−discharge operation. Remarkably, the as‐assembled flexible solid‐state ZAB with self‐supporting NiCo@SCF as the air cathode presented an engaging peak power density of 80.1 mW cm−2 and excellent durability of 95 h undisrupted operation, showing promise for the design of wearable ZAB. The demonstrated electrode fabrication approach exemplifies a facile, large‐scale avenue toward functional electrodes, potentially extendable to other wearable electronics for broader applications.

Influence of Temperature and Water Vapour on the High-Temperature Oxidation and Cr Evaporation Behaviour of High-Cr Alloys for SOFC Cathode Air Pre-Heaters

The oxidation and Cr2O3 evaporation mechanisms of the stainless steels Inconel 625, SS309 and AluChrom 318 have been investigated by varying temperatures and water content of the exposure atmosphere. The finding of this research demonstrates that Cr2O3 evaporation and oxidation rates were significantly lowered with decreasing temperature for Inconel 625 and SS309. AluChrom 318 showed a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature-dependent Cr2O3 evaporation compared to Inconel 625 and SS309. The major influence of water vapour on the three tested materials appeared to be the further increase of Cr2O3 evaporation.

In-situ gas foaming synthesis of N, S-rich co-doped hierarchically ordered porous carbon as an efficient oxygen reduction reaction catalyst

The design and manufacture of cost-effective and efficient oxygen reduction reaction (ORR) catalysts is critical to the widespread application of multiple energy conversion devices. Herein, a combination of in-situ gas foaming and the hard template method is proposed to construct the N, S-rich co-doped hierarchically ordered porous carbon (NSHOPC) as an effective metal-free electrocatalyst for ORR via carbonizing a mixture of polyallyl thiourea (PATU) and thiourea in silica colloidal crystal template (SiO2-CCT) voids. Benefiting from the hierarchically ordered porous (HOP) architectures and the mass doping of N and S, NSHOPC displays excellent ORR activities (the half-wave potential of 0.889 V in 0.1 M KOH and 0.786 V in 0.5 M H2SO4) and long-term stability, which are all better than those of Pt/C. As the air cathode in a Zn-air battery (ZAB), NSHOPC exhibits a high peak power density of 174.6 mW·cm−2 and long-term discharge stability. The remarkable performance of the as-synthesized NSHOPC signifies broad prospects for actual applications in energy conversion devices.

Higher Performance and Durability of Fuel Cells with High-quality Cathode Air

Higher Performance and Durability of Fuel Cells with High-quality Cathode Air

Synergistic poisoning effect of NH3 and NOx gases in the cathode air on proton exchange membrane fuel cell

Nitrogen oxides (NOx) and ammonia (NH3) are coexisting in fuel cell vehicles’ working condition such as urban industrial areas, which are harmful impurities to proton exchange membrane fuel cell (PEMFC). In this work, an experimental investigation between NH3 alone, NOx alone, and 1:1 mixed NH3/NOx are conducted at different concentrations. Voltage time curve and electrochemical impedance spectroscopy (EIS) analyses show that mixed gases cause slighter poison effect than single gas. Mixed 20 ppm NOx and 20 ppm NH3 caused a −14 mV/h voltage decay rate, while that of 20 ppm NOx alone and 20 ppm NH3 alone were −5 mV/h and −18 mV/h. Meanwhile, by comparing the poison and recovery processes between different impurity gases at different concentrations, mechanism of synergistic poisoning is proposed. A fast voltage decay rate caused by NOx and an irreversible performance loss caused by NH3 are observed in sequence at higher concentration, which due to the different adsorption energy of NOx and NH3 and the different reactions occurring in the membrane electrode assemblies (MEAs).

Solvent-free synthesis of Co/CoNx encapsulated by N-doped, sucrose-derived porous carbon as bifunctional air cathode catalyst for high-energy rechargeable zinc-air batteries

Rechargeable zinc-air batteries have a substantial theoretical energy density while potentially being manufactured at a minimized cost. Hence, developing low-cost bifunctional electrocatalysts is of crucial importance. Herein, the synthesis of an electrocatalyst containing zinc/cobalt species encapsulated in a nitrogen-doped carbon matrix modulated with a large number of carbon nanotubes (ZnCo@CN) was developed through a facile thermal method under solvent-free conditions. Using bio-based sucrose in the presence of inorganic catalysts (Zn/Co) resulted in a Zn/Co carbon foam-like structure. Subsequently, the high-temperature treatment with the addition of 2-methylimidazole resulted in a unique material (ZnCo@CN). The incorporation of highly active species dispersed in hierarchical porous carbon presented an excellent bifunctional electrocatalyst for ORR (half-wave potential of 0.85 V) and OER (overpotential of 327 mV). The assembled zinc-air battery with ZnCo@CN as a bifunctional air cathode exhibited an excellent power density of 206 mW cm-2 and an astonishing energy density of 983 Wh Kg-1.

Hierarchically porous hydrogel electrolyte prepared from interpenetrating polymer networks for flexible Zn-Air batteries

Gel polymer electrolyte (GPE) based flexible zinc-air batteries (ZABs) are considered promising power sources for next-generation wearable devices because of their high specific energy density, low cost, high safety, and environmental friendliness. However, the liquid component in GPE is susceptible to evaporation through the air cathode, drastically reducing the cell performance and lifetime. Poly(vinyl alcohol) (PVA) is the most widely adopted ion conductive polymer, but its poor water retention and low ionic conductivity hinder the widespread commercialization of flexible ZABs. A hierarchically porous (HP) PVA and poly(acrylic acid) (PAA) hybrid GPE is developed to address the aforementioned issues. The polymerization of acrylic acid in a PVA matrix forms a semi-interpenetrating polymer network involving a unique HP structure inside the membrane. The PAA-rich lotus-seed-like pores fundamentally improve electrolyte uptake and retention properties, and enhance ionic conductivity. The HP-PVA/PAA hybrid GPE delivers excellent cycling stability, rate characteristics, and power output in ZABs. Furthermore, HP-PVA/PAA based flexible ZABs incorporated into the customized strap demonstrates their feasibility for powering the wearable smart watch.

Constructing Co/Fe-Nx dual-site catalyst based on Co0.72Fe0.28 alloy nanoparticles anchored on hollow hierarchical porous carbon framework for enhanced oxygen reduction reaction and ZABs

Hollow hierarchical porous nitrogen-doped carbon (HNC) derived from ZIF-8@ZIF-67 (ZIF: zeolitic-imidazolate framework) is a promising nanocomposite electrocatalyst rich in active sites. However, the single active species (Co-Nx) constrains its application. By introducing Fe in the precursor preparation, this work increased the active species of the catalyst to Co/Fe-Nx species and Co0.72Fe0.28 nanoparticles. Due to the advantages of abundant active sites and the synergy of multiple active species, Co,Fe-HNC-1 exhibited excellent oxygen reduction reaction (ORR) performance in 0.1 M KOH electrolyte with an onset potential (Eonset = 0.92 V vs. RHE) and a half-wave potential (E1/2 = 0.85 V vs. RHE) comparable to those of commercial Pt/C, as well as better long-term stability and methanol resistance. Meanwhile, Co,Fe-HNC-1 additionally demonstrated oxygen evolution reaction (OER) activity. Co,Fe-HNC-1 also performed well as an air cathode in a zinc-air battery (ZAB) with a peak power density of 85.00 mW cm-2 and outstanding stability with only 1.8 % loss after more than 200 galvanostatic cycles at 10 mA cm-2. This work provides a new perspective for the rational design and preparation of electrocatalysts based on HNC nanocomposites.

Multifunctional bypass valve for water management and surge protection in a proton-exchange membrane fuel cell supply-air system

A multifunctional valve for air supply was placed downstream of the compressor in a fuel cell system to manage cathode air characteristics under different load changes and configurations. The advantages of using the four-way valve, with different ports for cathode flow, throttling, and bypassing, were analyzed through modeling and simulation. The model focused on the air supply system, which included a centrifugal compressor, intercooler, membrane humidifier, valve, and mixer. The simulation demonstrated that placing the valve right after the compressor was more effective in controlling the compressor surge than a traditional back pressure valve. During compressor surge, opening the throttling port at 18° could suppress the surge and stabilize the cathode flow. The balance between surge protection and stack performance under transient conditions was also discussed, and an optimal valve opening for throttle flow was presented. Furthermore, bypassing the humidifier by 20% of the supply air reduced the cathode inlet relative humidity by the same percentage, contributing to water management in the stack, particularly under flooding conditions.

Embedded Platinum–Cobalt Nanoalloys in Biomass-Derived Laser-Induced Graphene as Stable, Air-Breathing Cathodes for Zinc–Air Batteries

Fuel cells and metal–air batteries hold significant promise to help decarbonize transportation and the electricity grid. To encourage widespread adoption of these technologies, improvements to their air-breathing cathodes are required, which address problems such as the high cost of platinum (Pt) catalysts used to boost the kinetics of the oxygen reduction reaction (ORR) as well as the stability of Pt/C interfaces over long-term cycling. In this paper, we demonstrate a facile approach to reduce Pt content to less than 2 wt % by interfacing Pt with CoOx as well-dispersed nanoparticles entrapped within a highly conductive laser-induced graphene (LIG) matrix. Laser-induced carbonization of polymerized furfural alcohol preloaded with Co and Pt precursors resulted in the formation of a mixture of spherical nanoalloys PtCoOx and core (CoOx)–shell (Pt) structures. This LIG-PtCoOx electrode exhibited a low onset and half-wave potential in alkaline media, which closely approached a benchmark Pt/C. The effectiveness of LIG-PtCoOx was demonstrated by its performance in rotating disk and rotating ring disk electrode studies versus commercial Pt/C with the same concentration of the catalyst, which resulted in 4-fold greater mass activity and more than 6-fold higher specific activity, which are reflected in a high turnover frequency (TOF). The resulting material was tested as an air–cathode for zinc (Zn)–air batteries leading to improved stability (118 h of operation) and rechargeability (0.75 V voltage gap), exhibiting a higher peak power density compared to batteries assembled with the commercial benchmark Pt/C cathodes with similar composition.