Air Cathode Research Articles

Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis.

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90V versus RHE and a half-wave potential of 0.72V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

Triethylenediamine cobalt complex encapsulated in a metal–organic framework cage to prepare a cobalt single-atom catalyst with a high Co-N4 density for an efficient oxygen reduction reaction

Transition metal single atom catalysts (TM SACs) are the most promising oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) and metal-air batteries. However, the low density of M-Nx active sites seriously hinders further improvement of the ORR electrocatalytic activity. Here, a strategy for encapsulating nitrogen-rich guest molecules (triethylenediamine cobalt complex, [Co(en)3]3+) was proposed to construct a high-performance cobalt single-atom catalyst (Co-encapsulated SAC/NC). With this strategy, the guest molecules are encapsulated into metal–organic framework (MOF) cages as an additional cobalt source to boost cobalt loading, while abundant nitrogen from guest molecules contributes to the formation of Co-N4 active sites. Remarkably, the resulting Co-encapsulated SAC/NC has a high cobalt loading amount of 4.03 wt%, and spherical aberration-corrected transmission electron microscopy (AC-TEM) has confirmed that most cobalt exists in a single-atom state. As a result, the Co-encapsulated SAC/NC exhibits excellent ORR catalytic performance with a half-wave potential of 0.88 V. Furthermore, Zn-air batteries employing Co-encapsulated SAC/NC as air cathode show high peak power density and excellent cycling stability. Density functional theory (DFT) calculations reveal that adjacent active sites have different rate-determining steps and lower reaction energy barriers than a single active site.

Architecture Optimization of a Single-Chamber Air-Cathode MFC by Increasing the Number of Cathode Electrodes

The aim of this study was the optimization of a single-chamber microbial fuel cell (MFC) architecture, by increasing the number of cathode electrodes. An air cathode single chamber MFC with a modifiable lid and bottom was operated with 4 and 6 Gore-Tex MnO2 cathode electrodes. The anode consisted of graphite granules. It was found that the increase from 4 (total cathodic surface area of 160 cm2) to 6 (total cathodic surface area of 240 cm2) electrodes resulted in an increase of the maximum current and the maximum power output of the cell by approximately 72% and 129%, respectively. Additionally, by increasing the cathodic surface area the internal resistance (Rint) of the unit decreased by approximately 19%. The organic removal from the substrate was not affected by the addition of the new electrodes while it was high across all cases studied (chemical oxygen demand (COD) removal > 89%). The average coulombic efficiency (CE) during the 4-electrode operation was 14.3%, while the corresponding efficiency for 6-electrode operation was 18.5%.

Investigating the effect of anode materials on the performance and microbial community in an integrated chamber-free microbial fuel cell

Conventionally, there are two basic configurations for microbial fuel cell (MFC), one with anode and cathode chambers, as known as dual-chamber MFC, one with only anode chamber and a air–cathode, called single-chamber MFC. However, electrode materials' complex configuration and high resistance limit their practical applications in power generation and wastewater purification. To this end, we constructed three MFCs using carbon felt, Ni/Ti, and Ag-Ni/Ti as anodes based on our previously constructed integrated chamber-free MFC (iMFC). After 21 days of operation, the Ag-Ni/Ti-MFC performed best in bioelectricity output and removal of target pollutants. The bioelectricity output parameters were an open-circuit voltage of 0.773 V, a maximum current density of 552.1429 mA/m2, and a power density of 426.8064 mW/m2. Furthermore, the MFC achieved removals of 67.90% for the chemical oxygen demand (COD), 55.20% for total nitrogen (TN), and 61.83% for ammonia nitrogen (NH3-N). Introducing silver nanoparticles to the transmembrane and outer membrane boosted the charge extraction efficiency in the MFCs. Moreover, 16S rDNA analysis indicated that typical electricity-producing bacteria, including Comamonas, Paraclostridium, and Asaccharospora, had a higher relative abundance in the Ag-Ni/Ti-MFC anode than in other anodes. Overall, the surface structure of different anode materials plays a critical role in bioelectricity generation and wastewater purification using iMFC by affecting the mass transfer of substrates and metabolites.

Compositional Tuning of Nickel Sulfoselenides for Use As Bifunctional Oxygen Electrocatalysts in Aqueous Electrochemical Energy Storage

Increasing grid-scale deployment of intermittent energy sources necessitates the development of advanced energy storage technologies. Electrochemical energy storage technologies using alkaline electrolytes, especially secondary Zn-air batteries, are uniquely suitable, using inexpensive, non-flammable aqueous electrolytes and making extensive use of earth abundant elements (C, Zn, Mn, etc.). Unfortunately, development of Zn-air batteries is currently limited by electrochemical challenges at both the Zn anode and the air cathode, with the limitations at the air cathode being particularly complex. An ideal cathode would use a bifunctional oxygen electrocatalyst (BOE) capable of efficiently and sequentially catalyzing both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Precious metal-containing materials (Pt, RuO2, etc.) are currently the most efficient BOEs, but their high cost makes them unideal for commercialization. Transition metal chalcogenides (e.g., NiS2, Ni3Se2) have previously been used as BOEs, but are still relatively underdeveloped alternatives to precious metal electrocatalysts.In this presentation, we report the electrocatalytic performance of a series of compositionally-tunable nickel sulfoselenides (Ni3(S,Se)2, NiSSe) for use as BOEs in Zn-air batteries in place of existing precious metal electrocatalysts. These materials have previously shown promise as OER electrocatalysts, but their ORR and BOE performance has not yet been explored. We demonstrate the synthesis of a range of NiSSe materials with variable S/Se composition, and subsequently use multiple characterization techniques, including cyclic voltammetry, X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and others, to explore the performance of these materials as BOE electrocatalysts. The nanoscale NiSSe materials show homogeneous mixing of S/Se over a wide compositional range (10-90% Se), and electrocatalytic testing shows that this S/Se composition has an observable impact on the OER/ORR performance. Roughly 50/50 mixtures showed the best OER performance, while pure Ni3Se2 showed the best ORR performance. As a result, optimum BOE performance of the mixed NiSSe materials was observed near a composition of Ni3S0.4Se1.6; the (electro)chemical stability of this mixed NiSSe was subsequently evaluated using chronoamperometry and XPS. Compared with the binary Ni3S2 and Ni3Se2 compounds, NiSSe was found to be reasonably stable after testing and is therefore a promising candidate for use in air cathodes. As a demonstration, we then tested this NiSSe electrocatalyst in a Zn-air battery and found good agreement with the results from previous the Zn-free experiments. Together, our results show that nickel sulfoselenides are a promising class of tunable electrocatalysts with applications in oxygen electrocatalysis for energy storage.This work was supported by the U.S. Department of Energy, Office of Electricity, and the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Dr. Imre Gyuk, Director of Energy Storage Research, Office of Electricity is thanked for his financial support. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Hybridization of Carbon Nanotube Tissue, MnO2 Catalyst and CFx as an Advanced Air Cathode in Metal–Air Batteries

We first present the utilization of hybrid carbon nano-tube (CNT) – manganese dioxide (MnO2) tissues as air electrodes in nonaqueous aluminum- and lithium-based battery systems. The CNT tissues are impregnated with α-manganese dioxide nanoparticles via a straightforward binder-free ultrasonic treatment, thereby enabling an improved oxygen reduction reaction (ORR) catalytic activity. Half-cell potentiodynamic measurements along with full-cell discharge evaluations of Li- and Al-based battery systems are reported. Higher discharge potentials and capacities were obtained from generic and advanced hybrid CNT–MnO2 air cathodes. The discharge products obtained on the air cathode surface were studied using high-resolution scanning electron microscopy. Their chemical compositions were evaluated using elemental mapping energy-dispersive X-ray spectroscopy. The stability of the α-MnO2 catalyst was studied using X-ray photoelectron spectroscopy and electron paramagnetic resonance spectroscopy, thereby indicating the ongoing formation of an outer MnF2 shell layer, which affected the α-MnO2 functionality, as an efficient ORR catalyst. We conclude this part of the talk by showing that the function of the metal–air battery is enabled by using porous carbon nanotube (CNT)-based tissue substrates loaded with MnO2 catalyst.In contrast, fluorinated graphite (CFx) coating on the CNT substrate provides an additional discharge process, enhancing discharge capacity. Two CFx loadings are investigated, and their performances are compared with that of a pristine CNT tissue. Electrochemical evaluations, including half- and full-cell measurements, are performed to examine the effect of the coating layer thickness on the ability of the cell to function at high discharge rates.

Photoactive Electrocatalyst with Staggered p-n Heterojunction for Rechargeable Zn-Air Batteries

Rechargeable zinc-air batteries (RZBs) demonstrate significant potential as next-generation batteries as they are non-flammable, resource-abundant, cost-efficient, feasible for ambient air operation, and have remarkable theoretical energy density (1086 Wh kg-1). However, the development of RZBs needs to address many challenges to speed up their commercialization. For example, low round-trip efficiency during cycling is caused by retarded reactivity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode. An air cathode with deficient electrocatalytic kinetics creates a large voltage gap, especially when the practical charge voltage is > 2.0 V, during the charging and discharging process. Moreover, the cycle life of the RZBs is limited owing to the structural degradation of air cathodes working under high charge voltage. Therefore, it is essential to improve both the round-trip efficiency and structural durability of air cathodes to ensure further development and commercialization of RZBs. For efficient energy generation and storage, photo-enhanced RZBs that combine photochemical and electrochemical energy conversion and storage with the help of solar energy, are a promising research focus. Photo-enhanced RZBs require a photoactive OER/ORR bifunctional cathode that is not merely an electrochemical catalyst but also a photo-active catalyst.In the present study, we developed a photoactive bifunctional air-electrocatalyst with a staggered p-n heterojunction band gap structure, comprising n-type g-C3N4 and p-type copper-doped ZIF-67 (CuZIF-67) composite, to accelerate sluggish OER/ORR processes via a durable structure and stable long-cycle-life performance for photo-enhanced RZBs. g-C3N4/CuZIF-67 has a suitable band gap, which ensures wide-range light absorption in the UV–Vis–NIR region, and stimulates more electrons to participate in ORR/OER electrocatalytic activity. Besides, it has a staggered bandgap structure and a BEF at the p-n junction of the catalyst, which improves electron–hole pairs separation efficiency, lowers overpotential, promotes ORR/OER electrocatalytic activity, and eventually enhances the efficiency of RZBs (the voltage gap was significantly decreased by 14% with simulated solar light irradiation). Moreover, a high concentration of pyrrolic-N groups in the cathode material, resulting in the excellent electrochemical durability of RZBs during charge/discharge cycles. The photo-enhanced RZB based on the g-C3N4/CuZIF-67 bifunctional catalyst exhibits good round-trip efficiency of 60% (2 mA cm-2) with a significantly decreased voltage gap (from 0.92 to 0.81 V) after 1000 cycles, under simulated solar light irradiation. It can be concluded that staggered p-n heterojunction and pyrrolic nitrogen-rich groups play an important role in efficient and durable photo-enhanced Zn-air batteries.

Advancements, Challenges, and Prospects in Rechargeable Solid‐State Lithium‐Air Batteries

AbstractSolid‐state lithium‐air batteries (SSLABs) are attracting widespread research interest as emerging energy storage systems with ultra‐high theoretical energy density. However, due to their relatively short development history, the practical capacity and cyclic performance of SSLABs still fail to meet application requirements. The selection of solid electrolytes and the design and optimization of air cathodes are key factors for developing high‐performance solid‐state lithium‐air batteries. In this review, we focus on recent scientific advances and challenges in SSLABs, providing a comprehensive overview of solid electrolytes, air cathodes, and interface issues. Strategies such as electrolyte modification, composite cathodes, interface engineering, and the addition of catalysts which have been effective in addressing issues related to low ionic conductivity of electrolytes, high interfacial impedance, sluggish kinetics of electrochemical reactions, and poor cycling stability, were reviewed and discussed. Furthermore, this review also discusses the prospects of SSLABs, aiming to inspire and provide references for the development of solid‐state lithium‐air batteries, as well as other metal‐air batteries.

A Redox-Mediated Zinc-Air Fuel Cell

As one of the most promising power sources in converting and storing chemical energy, aqueous zinc-air batteries (ZABs) stand out because of their high theoretical energy density, low cost, high safety of aqueous electrolyte, and environmental friendliness. Three main types of ZABs including primary ZABs, mechanically rechargeable ZABs, and electrically rechargeable ZABs have been developed up to now.[1] Primary ZABs for single use are usually discarded after the Zn anode is consumed or passivated although other parts of battery remain intact, leading to enormous waste. To address it, the depleted anode and failure electrolyte can be physically replaced to recover the capacity in mechanically rechargeable ZABs. But the cumbersome disassembly/reassembly process and the performance deterioration after every recharge process limit its practical application. In addition, it is not feasible to replace zinc in some integrated systems, such as the large-scale ZAB stack. Electrically rechargeable ZABs with bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysts could be returned to the original state by electrochemical charging process, but also face some formidable challenges, such as dendritic zinc deposition, electrode deformation, and hydrogen evolution, limiting the long-term cycling performance. Besides, sluggish kinetics of ORR/OER and Zn passivation result in low energy efficiency, limited depth of discharge and power density. Especially in the resource-constrained and off-grid region, there isn’t external electricity source to charge the electrically rechargeable ZABs which limits their applications. Moreover, the reactions inherently take place on the surface of Zn anode and air cathode, in which the storage and conversion capability of the cell would be essentially constrained by the surface area of electrode and the volume of electrode compartment.The concept of redox targeting (RT) of battery materials offers a feasible solution to address these issues by employing redox mediators (RMs). To tackle the above challenges, here we demonstrate a redox-mediated Zn-air fuel cell (RM-ZAFC) based on the RT reactions of cobalt triisopropanolamine complex (CoTiPA) with O2 and 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) with Zn metal in the catholyte and anolyte, respectively. Both the 4-electron (4 e-) ORR and Zn oxidation reactions are liberated from the electrode surface and occur inside separate reactor tanks resorting to the RT processes. Upon operation, Co(III)TiPA in the catholyte is electrochemically reduced to Co(II)TiPA on the cathode and chemically oxidized back through ORR in the gas diffusion tank (GDT) into which O2 is fed. In conventional ZABs, ORR takes place on the triple-phase reaction interface of the heterogeneous electrocatalyst, and the catalytic performance is closely related to the density of exposed active site, electrical conductivity and reaction energy barrier. As compared, the redox-mediated ORR in GDT occurs homogeneously in the catholyte in which the dissolved RM could provide a large quantity of rection sites with tunable reaction energy barrier. For the negative side, DHPS-2H is firstly oxidized to DHPS on the anode and circulated into anodic tank, where it is regenerated by oxidizing Zn to ZnO. The discharge product ZnO is formed and deposited on the Zn granules loaded in the tank. After fully discharged, the cell can be feasibly refueled by replacing the Zn granules in the tank instead of disassembling and reassembling the whole cell. In this RM-ZAFC, the energy storage unit is decoupled from the power generation unit, thus offering greater operation flexibility and system scalability with modular design than the conventional ZABs. Compared with the commercial hydrogen tank used in hydrogen fuel cells, Zn granules loaded in the anodic tank present much higher volumetric capacity, greater safety and ease for storage and transportation. Therefore, as an energy conversion and storage device, RM-ZAFC would have great promise for automotive, backup, and stationary power applications.[2] In addition, we also propose an efficient and zinc regeneration method by RT reaction between RM and the product ZnO in RM-ZAFC to avoid the waste of ZnO and reduce the cost.

Experimental Analysis of the Effect of Cathodic CO2 Supply to Industrial Solid Oxide Fuel Cells

The integration of Solid Oxide Fuel Cells (SOFCs) in hybrid power generation systems offers the opportunity to achieve higher electric efficiencies. In the SOS-CO2 cycle [1], a newly developed cycle for blue power production (i.e., power generated starting from natural gas while capturing CO2), the SOFC cathode is supplied with mixtures of CO2 and O2, up to 79% CO2 molar fraction, while the anode is supplied with a reformate feed, containing H2, CH4, CO, CO2, and water vapour. Aim of this work is to experimentally evaluate the performance and the durability of industrial 5x5 cm2 Ni-YSZ anode-supported SOFCs under the SOS-CO2 cycle conditions, focusing on the effects of the CO2-rich cathodic atmosphere. The cells mounted a LSCF-based (La0.6Sr0.4Co0.2Fe0.8O3) single phase cathode, coated by a current collection layer of LSC (La0.6Sr0.4CoO3). Laboratory tests were performed at 700°C, acquiring polarization (I/V) curves and EIS spectra by means of a Horiba – FuelCon C50 Evaluator test station. Initial investigations with 7% humidified H2 and 21/79 O2/CO2 cathodic mixture revealed 25% power loss compared to the base case with cathodic air. The test showed not only that the loss of performance was stable in time, but also that the power reduction was reversible since the initial performance in air was recovered. The EIS spectra showed an increase of the polarization resistance and of the ohmic resistance upon feeding CO2. This latter observation suggested a reduction of the electronic conductivity of the cathode, compatible with the formation of Sr carbonates on the perovskite surface [2–4]. To assess the performance of the SOFC under the operating conditions of the SOS-CO2 cycle, the anode was supplied with reformate. Compared to the base case with humidified hydrogen and air, the current density at 0.7 V decreased by 37% (490 mA/cm2). When supplying the 21/79 O2/CO2 mixture at the cathode, the current density stabilized at 420 mA/cm2 (Figure 1). Durability tests (over 300 h) at 700°C with reformate and 21/79 O2/CO2 mixture highlighted a moderate decrease of the current density at 0.7 V. After these tests, the cathode was characterized post-mortem with XRD, SEM and Raman to analyse the consequences of the prolonged exposure to CO2. To further quantify the effect of CO2 on the cathode, symmetric button cells were also characterized with EIS, measuring ohmic and polarization resistances. The results suggested that the adsorption of CO2 is reversible and indicated a better suitability of LSCF- compared to LSC-based cathodes for operations with CO2-rich mixtures. Experiments performed between 550 and 700°C at varying CO2 and O2 partial pressures allowed to describe the kinetics of the oxygen reduction reaction and extract its activation energy. Overall, the results provided key elements for the integration of SOFCs in the SOS-CO2 blue power technology.

Lightweight Solid Oxide Cell Design for Ammonia-Fed SOFC Systems

OxEon Energy team has been advancing solid oxide fuel cell (SOFC) technology for over three decades and solid oxide electrolysis cell (SOEC) technology using identical stacks for 15 years.OxEon’s SOEC/SOFC technology builds on the success of the SOEC stack installed on NASA’s Mars Perseverance Rover that produces high-purity O2 by electrolyzing Mars atmosphere CO2. The OxEon team worked with Jet Propulsion Laboratory (JPL) and Massachusetts Institute of Technology (MIT) to validate the MOXIE (Mars Oxygen In-situ resource utilization Experiment) stack.MOXIE stack demonstrated that the operational performance on Mars matched results from pre-launch earth testing. MOXIE stack design forms the basis for developing lightweight fuel cell design suitable for an electric Vertical Take-off and Landing (eVTOL). Demonstration of SOFC performance using carbon-free fuels such as ammonia is another critical objective.A lightweight cell design was developed to increase specific power (kW/kg) of the fuel cell stack. The electrolyte thickness, the most resistive element of the SOFC, was reduced from ~250 microns to 70 microns (72% decrease). The fabrication process results in a dense scandia stabilized zirconia ScSZ electrolyte and a porous yttria stabilized zirconia (YSZ) matrix for cathode (air electrode) infiltration. For the fuel electrode, both an infiltrated and screen printed options are evaluated.Producing an infiltrated electrode with a well-adhered and homogeneous distribution of active material is a challenge that requires multiple infiltration-calcination cycles to deposit sufficient material. Key variables are precursor chemistry to reduce the number of infiltrations and to prevent interaction between electrode layers; intermediate calcination temperatures, and process development using an infiltration equipment that enables volume production. OxEon partnered with National Energy Technology Laboratory (NETL) for electrode infiltration process development and PNNL for validation testing of cells. Full size cells are in production for stack testing.Fuel flexibility was demonstrated in button cells and stacks previously. Button cell and stack performance was nearly identical among hydrogen, ammonia, and reformed natural gas. Durability testing shows similarly stable performance in each fuel.A button cell test in ammonia at 800 °C using the thin electrolyte cell structure measured a button cell area specific resistance (ASR) of 0.25 Ω-cm2 in H2/N2 and NH3 fuel feeds and showed a peak power density of 0.98 W/ cm2 in NH3 fuel feed. Short term hold at 0.7 V showed stable performance.Analysis of additional button cell tests and stack tests using thin electrolyte cell structure will inform test stand design modifications that address balance of plant size requirements.This material is based on research sponsored by Air Force Research Laboratory under agreement number FA8649-22-P-0792. The views expressed are those of the authors and do not necessarily reflect the official policy or position of the Department of the Air Force, the Department of Defense, or the U.S. government. Approved for public release; distribution is unlimited. Public Affairs release approval #_______________. Figure 1

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

Chromium evaporation and oxide scale growth are among the key degradation mechanisms associated with Solid Oxide Fuel Cells (SOFCs). It is often underestimated how the cathode air pre-heater (CAPH) made of high Cr-containing alloys in SOFC systems can contribute to cathode degradation. This study examines the effect of exposure temperature and water content on both degradation mechanisms in Inconel 625, SS309 and AluChrom 318. For the influence of water content, samples of Inconel 625, SS309 and AluChrom 318 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 hours. For the influence of temperature, samples of Inconel 625, SS309 and AluChrom 318 were isothermally exposed at 650°C, 750°C and 850°C in a 6.0 L/min air stream containing 3% H2O for 168 hours. The evaporated Cr (VI) species were collected using the denuder technique. The formed oxide scales were subsequently analysed using XRD and SEM/EDX. The results of the surface characterisation indicated that the oxide scales formed on Inconel 625, SS309 and AluChrom 318 at high temperatures were Cr2O3, (Cr,Mn)3O4 spinel and Al2O3, respectively.The results of this study show that Cr evaporation and oxidation rates were dramatically reduced at the lower temperature used for Inconel 625 and SS309. AluChrom 318 exhibited an increased oxidation rate with increasing temperature, but it demonstrated a reverse trend to the temperature dependent Cr evaporation compared to Inconel 625 and SS309. It was also found that the amount of Cr evaporated from an alloy solely depended on the partial pressure of Cr presented on the oxide surface. At 850°C, the total mass of evaporated Cr(VI) species was found as follows: Cr2O3 (Inconel 625) > (Cr,Mn)3O4 spinel (SS309) >> Al2O3 (AluChrom 318). However, at 650 and 750°C, the total mass of evaporated Cr(VI) species was found to be: (Cr,Mn)3O4 spinel (SS309) > Cr2O3 (Inconel 625) > > Al2O3 (AluChrom 318). The significant effect of water vapour on the three tested materials appeared to be the further enhancement of Cr2O3 evaporation. At 850°C, the amount of evaporated Cr2O3 from the alloys exposed at the same humidity level corresponded to the Cr partial pressure on the surface, as follows: Cr2O3 (Inconel 625) > (Cr,Mn)3O4 spinel (SS309) > Al2O3 (AluChrom 318). The water vapour slightly increased the growth rate of Cr2O3 and (Cr,Mn)3O4 spinel formed on the Inconel 625 and SS309 surface, respectively. However, no evidence of change in the growth rate of the alumina on AluChrom 318 under the influence of water vapour in oxidising atmosphere was observed for the tested period in this study. Figure 1

Model Based Comparative Analysis of GDC and YSZ Based Solid Oxide Fuel Cells

Designing solid oxide fuel cell (SOFCs) stacks for transportation applications places new challenges on SOFC architecture and operation with respect to power density, dynamic response, and thermal management. Recent developments in SOFCs with either thin-film yttria stabilized zirconia (YSZ) electrolytes (thickness ≤ 5 μm) or highly conductive, gadolinia-doped ceria (GDC) electrolytes (thicknesses ≥10 μm) enable thigh-power-density stack operation at inlet temperatures well below 700ºC with the potential for using compliant seals and lower cost ferrous interconnect materials. Applications, such as hybridized aircraft engines, can use such SOFC stacks if they meet strict requirements with respect to specific power and thermal management. Highly conductive GDC-based electrolytes offer pathways to high power-density SOFCs at operating temperatures below 650ºC, but the rise in mixed ionic electronic conductivity (MIEC) above 650ºC reduces open circuit voltages and thus fuel cell efficiency [1,2]. This necessitates thermal management to maintain operating temperatures and mitigate thermo-mechanical stresses within an SOFC. Thin-film YSZ electrolytes offer lower power densities below 650ºC, but avoid the penalties associated with MIEC behavior at higher temperatures and therefore reduce the challenges associated with thermal management at high power densities. In this work, a model-based approach is used to compare SOFC stack operation with thin-film YSZ electrolytes and with GDC-based electrolytes for operation on hydrogen and reformate fuels at high-power densities relevant for transportation applications.To calibrate models to experimental cell performance, polarization curves of a commercially available YSZ-based cell operating between 600-750ºC and of a high-power-density GDC cell operating between 500-650ºC are used to fit electrochemical model parameters for both electrolytes. Modeling of the YSZ based cell can neglect electronic charge transfer through the membrane electrolyte, but GDC-cell modeling must account for the multiple charge carriers (ceria polarons and oxygen vacancies) in the electrolyte [3]. Electrochemical parameters for ionic conductivities and electrochemical reaction rate parameters for oxide cathodes and nickel-electrolyte-composite anodes are fit to the experimental polarization data over the broad range of operating temperatures. Global rate expressions to account for internal reforming on the active nickel surfaces of anode are adopted from literature [4,5]. The calibrated cell model for each cell type is adopted to a down-the-channel discretized cell simulation to predict heat loss and temperature gradients at high current densities.This presentation compares GDC- and YSZ-based SOFCs in high power-density applications from fundamental and practical perspectives. The open-circuit voltages for GDC-based cells drops with increasing temperature to about 0.85 V/cell at 650°C which is lower than the Nernst open-circuit voltage of YSZ-based cells (≥ 1.05 V/cell) at similar conditions. Both types of cells can sustain power densities around 1.0 W/cm2 with high excess cathode air ratios ≥ 5 to keep temperature gradients near 10°C/cm. These results show that GDC-based cells support higher power densities at lower cell flow inlet temperatures < 600°C, whereas thin-film YSZ cells provide superior performance at flow inlets that drive cell temperatures > 600°C. Higher stack power densities > 1.0 W/cm2 must be supported by stack designs that alleviate high thermal stresses and allow steeper gradients of temperature. A summary of stack design and operational strategies that meet the stringent requirements of high power-density operation will be presented in the light of the modeling resuls..

The Development of a Backup SOFC Generator at Upstart Power

Upstart Power is developing a state-of-the-art SOFC generator that provides backup power solutions for residential and industrial customers. The Upgen NXG™ fuel cell enables grid independence and pairs seamlessly with solar and/or standalone battery storage to provide extended backup power and 24/7/365 energy solutions. The system has dual fuel capability using either propane or natural gas and exports AC power via Enphase micro-inverters. Field trials are being conducted including a batch of 10 units in Puerto Rico running on propane in collaboration with Sunnova.The key technologies enabling the Upgen NXG™ system’s fast cycling and overall simplicity include:1) Integrated Hot Zone with advanced Thermal Management2) Custom dry CPOx reactor (waterless)3) Control strategy based on two thermocouple setpointsThe core of the NXG system is the Hot Zone which integrates the CPOx reactor, fuel cell stack, heat exchanger, flame sensor, spark ignitor and exhaust system. Figure 1 shows the complete subsystem layout of the NXG system, including the fuel filter, blowers, power electronics and controls. The core component of the Hot Zone is a single piece structure that comprises a thermally conductive element that conducts heat from the combustion chamber to the stack inlet. This enables both rapid start-up and enhanced performance from the stack, without the risk of cell failure due to thermal stress. Figure 2 shows a Hot Zone demonstrating 1000 rapid thermal/power cycles from operating temperature down to 200°C. The chart shows power per cycle with a heat-up time of 40 minutes. Power output was increased after 800 cycles with higher power test cycles run throughout the test.The dry CPOx reactor converts fuel gas into hydrogen and carbon monoxide for use in the fuel cell without the use of any additional water. The CPOx can operate at very low O:C ratios (< 1.2) providing stack fuel efficiencies up to 40% and is controlled by a single thermocouple in the reactor core. The other system control setpoint is the temperature in the core of the hot zone which provides a simple system control scheme with minimal yet reliable sensors.Figure 3 shows real time cycle data from the Upgen NXG™ system, with a start-up time of less than 60 minutes with 9 hours of steady state AC power exporting to the grid via Enphase micro-inverters. The cathode air flow is controlled by the thermocouple in the core of the Hot Zone (Radiated).In summary, the Upgen backup power systems outperform ICE generators and battery-only backup by providing unlimited backup solar + storage systems that are more robust, cost-effective and have lower emissions. The single wall mounted unit minimizes connections and simplifies installation over an ICE generator that typically sits on an outdoor concrete pad. With near silent operation and the absence of deadly CO emissions, the Upgen NXG™ is a safe and environmentally friendly backup power solution for residential solar and industrial customers. Figure 1

In situ observation of the electrochemical behavior of Li–CO2/O2 batteries in an environmental transmission electron microscope

AbstractLi–CO2/O2 batteries, a promising energy storage technology, not only provide ultrahigh discharge capacity but also capture CO2 and turn it into renewable energy. Their electrochemical reaction pathways' ambiguity, however, creates a hurdle for their practical application. This study used copper selenide (CuSe) nanosheets as the air cathode medium in an environmental transmission electron microscope to in situ study Li–CO2/O2 (mix CO2 as well as O2 at a volume ratio of 1:1) and Li–O2 batteries as well as Li–CO2 batteries. Primary discharge reactions take place successively in the Li–CO2/O2–CuSe nanobattery: (I) 4Li+ + O2 + 4e− → 2Li2O; (II) Li2O + CO2 → Li2CO3. The charge reaction proceeded via (III) 2Li2CO3 → 4Li+ + 2CO2 + O2 + 4e−. However, Li–O2 and Li–CO2 nanobatteries showed poor cycling stability, suggesting the difficulty in the direct decomposition of the discharge product. The fluctuations of the Li–CO2/O2 battery's electrochemistry were also shown to depend heavily on O2. The CuSe‐based Li–CO2/O2 battery showed exceptional electrochemical performance. The Li–CO2/O2 battery offered a discharge capacity apex of 15,492 mAh g−1 and stable cycling 60 times at 100 mA g−1. Our research offers crucial insight into the electrochemical behavior of Li–CO2/O2, Li–O2, and Li–CO2 nanobatteries, which may help the creation of high‐performance Li–CO2/O2 batteries for energy storage applications.

Well-dispersed NiFe nanoalloy embedded on N-doped carbon nanofibers as free-standing air cathode for all-solid-state flexible zinc-air battery

Flexible zinc-air battery (ZAB) is a viable candidate for energy storage in upcoming flexible-wearable technologies. Non-noble transition metal alloys are used to synthesize oxygen evolution reaction (OER) catalysts to facilitate this development. However, inhibiting self-aggregation and promoting high OER activity in alloy nanoparticles is still challenging. Anchoring metal alloys to carbon support materials may produce a strong metal alloy‑carbon support couple, thereby preventing nanoparticle aggregation. Here, the combination of polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), pyrrole, and Ni/Fe salts resulted in homogeneously NiFe nanoparticles embedded in the free-standing nitrogen-doped‑carbon nanofiber (NiFe-N@CNF), produced via electrospinning and followed by carbonization. The NiFe-N@CNF shows a high OER activity with an overpotential of 351 mV. Importantly, good structural stability of NiFe-N@CNF results in superior OER stability with no voltage change after 14 h of chronopotentiometry stability test. The constructed rechargeable aqueous ZAB with NiFe-N@CNF as the air–cathode displays high peak power density (179 mW cm−2) and high stability up to 500 h. With its flexible properties, free-standing NiFe-N@CNF is also used as all-solid-state ZAB cathode, exhibiting stable performance under both flat and bending conditions, proving the promising application of NiFe-N@CNF for flexible-wearable technologies.

Metal‐free nanostructured‐carbon inks for a sustainable fabrication of zinc/air batteries: From ORR activity to a simple prototype

AbstractZinc/air batteries are convenient energy storage devices for both small and massive applications. While future perspectives indicate the need for low‐cost components and sustainable fabrication processes, the battery performance is in part controlled by the kinetics of the oxygen reduction reaction (ORR), which typically involves transition metals as catalysts. In this context, we prepare a series of metal‐free water‐based carbon inks, which are tested for their catalytic performance, once deposited on a gas‐diffusion substrate, in the air cathode of a simple battery prototype. The inks contain a variety of well‐defined carbon nanomaterials and additives, exhibiting different physicochemical properties that critically influence the interaction with the gas diffusion hydrophobic substrate. The intrinsic ORR catalytic activity of the ink material is also analyzed on a glassy carbon electrode by the rotating ring‐disc electrode (RRDE) method and specific capacitance measurements. The discharge capacity on our zinc/air battery prototype correlates well with the intrinsic catalytic activity in the RRDE. However, only the activity in the RRDE does not actually assure the performance on the commercial cathode of the prototype, since other chemical compatibility issues play a role. Thus, we highlight the importance of catalyst testing, not only on the RRDE but also under realistic device conditions.

Hydrophobic and Homogeneous Conductive Carbon Matrix for High-Rate Non-Alkaline Zinc-Air Batteries.

Non-alkaline zinc-air batteries (ZABs) that use reversible O2 /ZnO2 chemistry exhibit excellent stability and superior reversibility compared to conventional alkaline ZABs. Unlike alkaline ZABs, ZnO2 discharge products are generated on the surface of the air cathodes in non-alkaline ZABs, requiring more gas-liquid-solid three-phase reaction interfaces. However, the kinetics of reported ZABs based on carbon black (CB) is far from satisfactory due to the insufficient reaction areas. The rational structural design of the air cathode is an effective way to increase active surfaces to further enhance the performance of non-alkaline ZABs. In this study, multi-walled carbon nanotubes (MW-CNTs) with unique mesoporous structures and high pore volumes are selected to replace CB in the air cathode preparation. Due to the larger electrochemically active surface area, superior hydrophobicity, and uniform electroconductibility of MW-CNTs-based cathodes, primary ZABs exhibit high specific capacity (704 mAh gZn-1 ) with a Zn utilization ratio of 85.85% at 1.0mA cm-2 , excellent discharge rate performance, and negligible self-discharge. Furthermore, rechargeable ZABs also demonstrate outstanding rate capability and excellent cycling stability at various current densities. This work provides a fundamental understanding of the criteria for the cathode design of non-alkaline ZABs, thus opening a new pathway for more sustainable ZABs.

Regulation of oxygen vacancies and electronic structures by substituting Ba2+ at A-sites of LaNi0.5Mn0.5O3 double perovskites enabling high-performance catalysts for Mg-air batteries

The air cathodes impact the electrochemical performance of primary Mg-air batteries greatly, in which the activity of catalysts plays a decisive role. Perovskites attract significant attention for their high oxygen reduction reaction (ORR) catalytic activity, unique structure, simple synthesis, and low cost. Herein, a series of La1-xBaxNi0.5Mn0.5O3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) catalysts fabricated by substituting Ba2+ for La3+ at the A-sites of LaNi0.5Mn0.5O3 are reported. As a result, the catalytic performance toward ORR exhibits a volcano curve with the increase of doping amount and the highest catalytic activity was achieved at 30 % of Ba2+ content. Importantly, the experimental and computational results explain the enhancement by the large O2 adsorption capacity, the formation of oxygen vacancies, the increase in the content of Mn3+ as an adsorption active site, and the upshift of the Mn-3d band center toward the Fermi level, thereby facilitating the O2 adsorption and reducing the energy barrier of the rate determining step of ORR. The Mg-air battery with the La0.7Ba0.3Ni0.5Mn0.5O3 catalyst displays a high discharge voltage of 1.37 V and a maximum power density of 65.224 mW·cm-2. Therefore, this new ORR catalyst presents great potential in practical Mg-air batteries.

Effects of chemical oxygen demand/nitrogen on electrochemical performances and denitrification efficiency in single-chamber microbial fuel cells: Insights from electron transfer and bacterial communities

The electrochemical performances and denitrification efficiency of microbial fuel cells (MFCs) are often limited by chemical oxygen demand/nitrogen (COD/N) of wastewater. To overcome this limitation, single-chamber air cathode MFCs with varying COD/N (16/1, 8/1, and 4/1) were established to investigate their electrochemical performances, denitrification efficiency, and bacterial communities. The optimal COD/N for maximizing electricity generation and denitrification efficiency was 8/1, as supported by the greatest corrected coulomb efficiency (13.6%) and electron transfer rate (2.36 C/h for electricity generation, 39.77 C/h for denitrification). As COD/N decreased, the electrochemically active genus Geobacter was replaced by the denitrifying genera Un._f_Burkholderiaceae, Dechlorosoma, and Petrimonas. These results indicated that the efficiency of electricity generation and denitrification was not solely determined by the abundance of electrochemically active and denitrifying bacteria. The presence of a faster electron transfer pathway, possibly direct interspecies electron transfer, enhanced simultaneous electricity generation and denitrification in MFCs with COD/N of 8/1.