Cell Operating Voltage Research Articles

Controlling Mass Transport in Direct Carbon Dioxide Zero-Gap Electrolyzers via Cell Compression

The integration of renewable energy with industrially relevant CO2 electrolyzers provides a promising pathway to achieving sustainable fuels and chemical generation. In this work, mass transport phenomena in a zero-gap membrane-electrode-assembly (MEA) are investigated by varying the thickness of the gaskets used during cell assembly, which in turn determines the overall applied cell compression (Figure 1a). X-ray computed tomography characterization (XCT) shows changes in the gas diffusion electrode (GDE) thickness and porosity as a result of changing the applied cell compression. Using the experimentally measured GDE properties, a computational model of the electrolyzer is developed to study mass transport effects on the performance of the Ag catalyst for CO2 to CO conversion. Applying high cell compression is found to decrease electrode porosity and increase liquid saturation in the cathode catalyst layer and diffusion media, thereby limiting the mass transport of CO2. The two MEAs assembled and tested (MEA-008 and MEA-010) with 0.008” and 0.010” thick gaskets, respectively, show CO Faradaic efficiencies that are very similar to the model results (Figure 1b). MEA-010, which has higher porosity and thicker GDE, outperforms MEA-008 at high current densities (>100 mA cm-2) due to improved mass transport. However, MEA-010 shows increased cell resistance due to the reduced compression of the conductive fibers, resulting in higher operating cell voltages, highlighting the important trade-off between product selectivity and the voltage required to run the electrolyzer. Figure 1

Facile synthesis of mesoporous Co3O4 anchoring on the g-C3N4 nanosheets for high performance supercapacitor

The rational design of the hybrid supercapacitor is the proficient way to confront the problem addressed in the supercapacitor through the collective advantage of each material and even shows laudable electrochemical properties from intertwined effects. A hydrothermal technique was used to effectively incorporate cobalt oxide into layered g-C3N4 nanosheets for a high-performance-based hybrid supercapacitor. The nanocomposite of Co3O4/g-C3N4 (with 0.01 M Co3O4) achieved a maximum specific capacitance of approximately 455 F/g at a current density of 1 A/g. It exhibited 95 % cyclic stability over 10,000 cycles, without sacrificing its coulombic efficiency in a three-electrode configuration. The enhanced charge storage contribution arises from the effective incorporation of cobalt oxide into the layered carbon nitride, increasing the number of electrochemically active sites for electrolytic ions. A symmetric device was fabricated using a KOH gel-based electrolyte, delivering high energy and power density of 28.13 Wh/kg and 1.68 kW/kg, respectively, with a maximum cell operating voltage of 1.2 V. Furthermore, the device demonstrated excellent cyclic stability of 95 % with constant coulombic efficiency.

Balancing the Co‐Solvent Content in High Entropy Aqueous Electrolytes to Obtain 2.2 V Symmetric Supercapacitors

AbstractThe energy storage capability of supercapacitors (SCs) strongly depend on the operating cell voltage of the electrolytes of choice. In this regard, the inherent distinct electrochemical stability of cations and anions is a factor of relevance for the operating cell voltage. The use of double salts sharing one ion has been described as an approach to circumvent this problem, but whether modifying the solvation structure of cations and anions with different solvent molecules (coordinating and/or non‐coordinating) could help balance their electrochemical stability in SCs has not yet been fully addressed. In this work, electrolytes are prepared by combining solvent mixtures and double salts, specifically 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) and 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EMIMBF4) in mixtures of water (H2O) and dimethysulfoxide (DMSO) as coordinating co‐solvents and acetonitrile (CH3CN) as a weakly coordinating one. It is found that the presence of this latter one helped to enhanced the cation solvation structure (above 9). This increase of the entropic features allows operating at cell voltages of up to 2.2 V and the subsequent enhancement of the energy storage capabilities and capacitance retentions (up to 15 Wh kg−1 and ≈87% after 10 000 cycles, respectively).

Investigating Water and Ion Transport in a Novel Cell Design for Seawater Electrolysis

With increasing concerns about the climate change associated with the carbon dioxide emission from utilizing traditional fuels, hydrogen has become a preferred low-carbon energy carrier. For hydrogen to truly be a clean source of energy, it must be produced from a renewable source of energy through water electrolysis. Theoretically, for each kilogram of hydrogen to be produced 9 kg of high-purity water need to be consumed, thus the enormous use of clean water for H2 production may worsen the shortage of fresh water in many parts of the world.Owing to its abundance, seawater has received great attention in the field of hydrogen production; however, there are many challenges associated with the direct use of seawater for electrolysis. In traditional systems, the seawater is first deionized by reverse osmosis (RO) before being fed to the electrolyzer. The RO system adds complexity to the system and requires energy to be expended for the deionization. Therefore, modern systems have aimed to directly electrolyze seawater, but this comes with it own issues including electrode corrosion, scaling from the ions present in saline water, environmental concerns associated with competitive chlorine evolution reaction, and increased membrane resistance limiting the overall performance of saline water electrolysis. Several strategies have been proposed to overcome these challenges including developing selective electrocatalysts, blocking strategy, pH buffer strategy, etc.This project sought to overcome the issues with direct seawater utilization by designing a cell that is driven by osmotic separation, but does not require an external power source. In this device, high concentration acid/base concentrations are maintained in contact with another compartment containing seawater. The acid/base reservoirs act as draw solutions, taking water passively from seawater due to their differences in osmotic pressure. This talk will focus on the cell design and operation, and include both experimental and modeling results. The rate of water transport with various cell operating configurations was quantified alongside the rate of chloride ion crossover through the anion exchange membrane. New methods for chloride mitigation were explored – including cell operating conditions, membrane design and absorption. Multiple cell configurations will be shown and strategies for reducing the cell operating voltage will be discussed.

Smart Electrode Surfaces by Electrolyte Immobilization for Electrocatalytic CO2 Conversion.

The activity and selectivity of molecular catalysts for the electrochemical CO2 reduction reaction (CO2RR) are influenced by the induced electric field at the electrode/electrolyte interface. We present here a novel electrolyte immobilization method to control the electric field at this interface by positively charging the electrode surface with an imidazolium cation organic layer, which significantly favors CO2 conversion to formate, suppresses hydrogen evolution reaction, and diminishes the operating cell voltage. Those results are well supported by our previous DFT calculations studying the mechanistic role of imidazolium cations in solution for CO2 reduction to formate catalyzed by a model molecular catalyst. This smart electrode surface concept based on covalent grafting of imidazolium on a carbon electrode is successfully scaled up for operating at industrially relevant conditions (100 mA cm-2) on an imidazolium-modified carbon-based gas diffusion electrode using a flow cell configuration, where the CO2 conversion to formate process takes place in acidic aqueous solution to avoid carbonate formation and is catalyzed by a model molecular Rh complex in solution. The formate production rate reaches a maximum of 4.6 gHCOO- m-2 min-1 after accumulating a total of 9000 C of charge circulated on the same electrode. Constant formate production and no significant microscopic changes on the imidazolium-modified cathode in consecutive long-term CO2 electrolysis confirmed the high stability of the imidazolium organic layer under operating conditions for CO2RR.

Performance recovery of proton exchange membrane electrolyzer degraded by metal cations contamination

Hydrogen production from renewable electricity offers an eco-friendly alternative to fossil fuels. Proton exchange membrane (PEM) electrolysis is a well-known method for this purpose. Studies have primarily focused on reducing costs of noble catalysts, improving efficiency, managing system degradation, and addressing membrane thinning caused by contaminated cations. However, techniques for PEM recovery post-degradation are still under development. This study investigated the effects of cations on PEM cells using artificial soft water, and analyzed two recovery methods to restore cell performance. Our findings indicate a significant rise in cell operating voltage and a decrease hydrogen production over 8 h of operation with soft water. After introducing both recovery methods, the initial operating value was reinstated in both cases. Only nitric acid treatment, however, achieved hydrogen production levels comparable to those of ultrapure water.

Elucidation of the role of electrolyte nature on electrochemical performance of PANI in energy storage device

To minimize the number of cells during the construction of a device, the working voltage of each cell must be as high as possible. Obtaining an aqueous electrolyte based electrochemical cell with high cell operating voltage is challenging due to the low standard reduction potential of water. The current work shows the effect of the nature and concentration of the electrolyte on the structure of water and its electrochemical stability. A symmetrical PANI cell was constructed in the presence of different electrolytes. It is demonstrated that utilizing ions with high solvation ability (chaotropic ions) and the formation of hydronium/Zundel ions enables the amount of free water to be reduced. As a result, there are no side reactions on the electrodes related to the electrolysis of water, and the main electrochemical reaction contributing to the cell capacity predominates. Moreover, by choosing the appropriate electrolyte it is possible to control the battery or capacitance-like behavior of PANI. The electrochemical performance of a symmetrical PANI cell shows a promising energy density of 7.5 Wh/kg at a power density of 150 kW/kg. The superior electrochemical performance (capacity) is retained after 50 cycles in a potential window of 5 V. It is interesting to highlight that, according to Raman spectroscopy, PANI maintains its chemical stability in a 5 V potential window.

Improved Capacitance of Electropolymerized Aniline Using Magnetic Fields

With the rise in intermittent energy production methods and portable electronics, energy storage devices must continue to improve. Supercapacitors are promising energy storage devices that are known for their rapid charging and discharging, but poor energy density. Experimentally, one can improve the energy density by improving the operating cell voltage and/or improving the overall capacitance, which have traditionally been achieved using difficult, complicated, or expensive syntheses involving additional chemicals or many steps. In this work, we demonstrate a method to improve the capacitance of electropolymerized polyaniline (PANI, a conductive polymer common in supercapacitor applications) with zero additional energy input or chemical additives: the use of a permanent magnet. Using a pulsed-potential polymerization method, we show that the inclusion of a 530 mT magnetic field, placed directly under the surface of the working electrode during electropolymerization, can result in a PANI film with a capacitance of 190.6 mF; compare this to the same polymerization performed in the absence of a magnetic field, which has a significantly lower capacitance of 109.7 mF. Electrochemical impedance spectroscopy indicates that PANIs formed in the presence of magnetic fields demonstrate improved capacitor behavior, as well as lower internal resistance, when compared to PANIs formed in the absence of magnetic fields. To probe the performance and stability of PANI films synthesized in the presence and absence of magnetic fields, galvanostatic charge–discharge was completed for symmetric capacitor configurations. Interestingly, the PANI films formed in the presence of 530 mT magnetic fields maintained their capacitance for over 75,000 cycles, whereas the PANI films formed in the absence of magnet fields suffered serious capacitance losses after only 29,000 cycles. Furthermore, it is shown that performing the polymerization in magnetic fields results in a higher-capacitance polymer film than what is achieved using other methods of forced convection (i.e., mechanical stirring) and outperforms the expected capacitance (based on yield) by 13%, suggesting an influence beyond the magnetohydrodynamic effect.

Two-phase mass transporting under various operating conditions during mode switching of unitized regenerative fuel cell with non-uniform flow channels

ABSTRACT The electrolyzing and fuel cell (FC) modes appear in a unitized regenerative FC. Understanding the operating condition impacts on the dynamic behaviors during mode-switching procedure facilitates better handling of cell working states and increasing the performance behaviors. In the present paper, a two-dimensional transient numerical model is employed to discuss the influence of operating conditions on two-phase species distributions when switching the cell mode into FC. According to the numerical results, it is concluded that when the cell switches from electrolytic cell to the mode of FC, reducing the operating cell voltage can significantly improve the cell performance. At the same time, concentration polarization and temperature heterogeneity are serious. Increasing the gas inlet velocity can improve the cell performance. Providing the reaction gas with the same concentration as the FC mode in advance can improve the temperature uniformity and cell current density values when starting switching the mode but hardly impacts the parameter value and currents after stabilization.

Safety analysis of proton exchange membrane water electrolysis system

This study presents a novel causality model for the safety analysis of proton exchange membrane water electrolysis (PEMWE). The model is based on the integration of computational fluid dynamics (CFD)-based and Bayesian network (BN)-based modeling approaches. The model is aimed at analyzing evolving hazard scenarios, such as gas permeation/crossover (GP) during PEMWE based on fluid dynamics and electrochemistry of electrolysis. The probabilistic domain is employed to model these scenarios using a BN-based technique to assess the likelihood of hazardous scenario occurrence for the first time. A sensitivity analysis was performed, and it was found that operating pressure is the most significant contributor to GP. Operating cell voltage and pressure are two dominant parameters that can trigger GP with the largest likelihood of 87%. Therefore, gas combinators serve as the most effective safety barrier in avoiding the formation of hazardous H2/O2 gas mixtures. This work provides a robust framework for analyzing accident scenarios and developing safety management strategies. It is expected that this model will help in the development of safer PEMWE systems, which is of great importance for various industrial applications.

Improved desalination via cell voltage extension and simultaneous energy recovery of redox flow desalination using organic supporting electrolyte

Redox flow desalination (RFD) is a promising desalination technology based on a redox flow battery system with intervening channels for water streams. However, the RFD system suffers from a low desalination capacity because its operational cell voltage is limited to approximately 1.2 V to stably operate under a narrow potential window of water. Here, we report a novel strategy for improving the desalination performance of RFD by introducing organic supporting electrolytes for redox reactions, allowing for an extension of the operational cell voltage and effective energy recovery in the RFD system. Using an organic electrolyte (100 mM TEABF4 in acetonitrile) for sustainable redox reactions (V(acac)3-/ V(acac)3 and V(acac)3/ V(acac)3+), the maximum cell voltage was increased to 3.5 V. At an operating cell voltage of 3.0 V, the system exhibited a substantially improved salt removal rate of 2670 µg/cm2/min. This is a 40-fold improvement compared with the salt removal rate (68 µg/cm2/min) in the RFD system utilizing an aqueous supporting electrolyte (100 mM Fe(CN)63-/Fe(CN)64- in 100 mM NaCl). Moreover, with the energy recovery process, the RFD system recovered 30 % of the energy used for desalination, reducing energy consumption even at a high operating cell voltage. The results suggest that the adaptation of organic electrolytes in the RFD system can be a good approach to improve desalination performance via the extension of cell voltage and energy recovery, and it can open new routes for water desalination and environmental applications.

Electrochemical conversion of methane to ethylene, olefins, and paraffins using metal-supported solid oxide cells

Electrochemical oxidative coupling of methane (E-OCM) with Sr2Fe1·5Mo0·5O6−δ (SFM) catalyst is demonstrated with metal-supported solid oxide cells (MS-SOC). SFM anode and Pr6O11 cathode catalysts are loaded into a porous symmetric-architecture cell by infiltration and firing. The most effective chelating agent (citric acid/ethyl glycol) and optimal firing/reducing temperatures (850 °C/750 °C) for the SFM catalyst precursor solution are selected and confirmed by cell testing. Operating temperature, cell voltage, and oxygen concentration at the cathode greatly affect the methane conversion rate and product selectivity by controlling the oxygen ion flow. CH4 conversion of 85.8% is obtained, with C2H4, C2H6, and H2 concentrations of 10.5%, 12.3%, and 25.6% at 800 °C, respectively, in the product exhaust gas. Reasonable stability of the current density and methane conversion is demonstrated during 200 h operation. This research demonstrates technical progress in catalyst and device development for the E-OCM reaction to synthesize valuable chemicals.

Load Regulation Strategies of Solid Oxide Electrolysis System Coupled with External Heat Sources

Solid oxide electrolysis system requires electrical and thermal energy. Five scenarios are defined according to various cases of external heat sources coupling and electrical energy sources. Three methods are proposed to track variations in electrical or thermal load, including constant flow rate (F method), constant steam utilization (Us method) and constant cell operating voltage (V method). Through system process simulation, the applicability of three methods in different scenarios is explored to obtain load regulation strategies. In the scenario coupled with low-temperature heat and intermittent power and the scenario with no heat coupling, the F method is preferred at high loads and the V method is preferred at low loads, while it is the opposite in the scenario coupled with high-temperature heat and intermittent power. The V method is preferred at low loads in the scenario coupled with low-temperature heat and high loads in the scenario coupled with high-temperature heat.

Redox flow batteries: Pushing the cell voltage limits for sustainable energy storage

Electrode kinetics of zinc at the anode in an alkaline medium holds a great prospective for energy storage systems due to low redox potential of Zn(OH)42−/Zn redox couple (−1.26 V vs SHE), high capacity, good stability, involves two electron transfer, high reversibility, eco-friendly and low cost. Undoubtedly, enlarging the voltage of the flow cell is the ultimate goal for enhancing the energy density of the system. Here, we demonstrate the increase in the operating cell voltage of Zinc-Polyiodide (ZnI2) flow battery by meticulously switching the anolyte from an acidic/neutral to an alkaline medium. Very interestingly, swapping the electrolyte from neutral to an alkaline medium shows drastic increase in the voltage window from 1.37 V (neutral) to 1.89 V (alkaline) for ZnI2 redox flow battery (RFB). Furthermore, the developed advanced hybrid ZnI2 electrolyte highly improved the rate capability of the RFB, even at 100 mA cm−2, exhibiting a round trip efficiency of 98 % and the corresponding energy efficiency is about 46 %. On the other hand, the ZnI2 flow cell with the given electrolyte condition delivered a maximum coulombic (96 %), voltage (83 %) and energy efficiency (77 %) at 20 mA cm−2. Moreover, the alkaline based ZnI2 RFB performed stably over 90 cycles at 30 mA cm−2. At the same time, 50 cycles at 50 mA cm−2 have also been tested. This reported cycle life is ever maximum cyclability achieved for alkaline based metal halides RFBs. Thus, a creative protocol of switching the anolyte pH from acid/neutral to alkaline medium offers an efficient route to attain high cell voltage and high energy density device when paired with high positive redox potential species.

A Highly Reversible Aqueous Ammonium‐Ion Battery based on α‐MoO3/Ti3C2Tz Anodes and (NH4)xMnO2/CNTs Cathodes

AbstractAqueous ammonium‐ion batteries (AAIBs) are appealing due to their relatively low cost and good rate performance. In general, AAIBs are environmentally friendlier than their non‐aqueous counterparts. However, it is still a challenge to achieve highly reversible AAIBs with decent voltages and energy/power densities. Herein, we report on a full‐cell configuration using α‐MoO3/Ti3C2Tz films as anodes, and (NH4)xMnO2/CNTs films as cathodes in a 1 M ammonium acetate (NH4Ac) electrolyte. At 2 V, the operating cell voltage, OCV, is one of the highest reported for AAIBs. A maximum energy density of ∼32 Wh kg−1 (∼54 Wh L−1) at 0.2 A g−1 and a maximum power density of ∼10 kW kg−1 (∼17 kW L−1) at 10 A g−1 are attained. When the full cells are cycled 2,000 times at 1 A g−1 they retain ∼73 % of their initial capacity. When cycling at 10 A g−1, ∼96 % of capacity is retained after 43,500 cycles. After 10 h, self‐discharge reduces the OCV to ∼72 % of its original value. This work provides a roadmap for developing high performance AAIBs with high voltages and high energy/power densities. Before this is possible it is imperative that the self‐discharge rate be substantially reduced.

Redox-Active Eutectic Electrolyte with Viologen and Ferrocene Derivatives for Flow Batteries.

Nonflammable eutectic solvents show great potential to enhance the concentrations of the redox-active materials and the cell voltages for redox flow batteries (RFBs). Herein, we report a promising redox-active eutectic electrolyte (1.5 M total redox species) with viologen and ferrocene derivatives where both of the redox reactions are reversible with a maximum open-circuit voltage of 1.35 V and an energy density of 15.1 Wh L-1, which is relevant to large-scale energy storage. The charge-discharge (from 75 to 25% state of charge) characteristics in a flow cell (0.15 M negolyte and 0.3 M posolyte) showed that it can be cycled with consistent discharge capacity for 12 h (19 cycles), beyond which pressure-driven crossover between the posolyte and negolyte reservoirs leads to capacity decay. This study points to promising new directions toward eutectic electrolyte development for RFBs where we demonstrate increasing the polarity, functionalizing the redox molecules, and separating redox intermediates to prevent undesired side reactions can make improvements in operating cell voltage, energy density, and cyclability.

Fast Ion Transport in Li‐Rich Alloy Anode for High‐Energy‐Density All Solid‐State Lithium Metal Batteries

AbstractAll‐solid‐state Li batteries (ASSLBs) with solid‐polymer electrolytes are considered promising battery systems to achieve improved safety and high energy density. However, Li dendrite formation at the Li anode under high charging current density/capacity has limited their development. To tackle the issue, Li‐metal alloying has been proposed as an alternative strategy to suppress Li dendrite growth in ASSLBs. One drawback of alloying is the relatively lower operating cell voltages, which will inevitably lower energy density compared to cells with pure Li anode. Herein, a Li‐rich Li13In3 alloy electrode (LiRLIA) is proposed, where the Li13In3 alloy scaffold guides Li nucleation and hinders Li dendrite formation. Meanwhile, the free Li can recover Li's potential and facilitate fast charge transfer kinetics to realize high‐energy‐density ASSLBs. Benefitting from the stronger adsorption energy and lower diffusion energy barrier of Li on a Li13In3 substrate, Li prefers to deposit in the 3D Li13In3 scaffold selectively. Therefore, the Li–Li symmetric cell constructed with LiRLIA can operate at a high current density/capacity of 5 mA cm−2/5 mAh cm−2 for almost 1000 h.

Effect of porous transport layer properties on the anode electrode in anion exchange membrane electrolyzers

Anion exchange membrane water electrolyzers (AEMELs) have recently received significant attention due to their potential advantages over existing commercial water electrolysis technologies. However, AEM electrolyzers have not yet met performance and durability targets. One of the critical components in achieving high AEMEL performance is the porous transport layer (PTL) which serves many critical functions in the oxygen evolution reaction (OER) electrode. In this study, several OER PTL attributes were investigated, including: material of construction, fabrication method, feature size, porosity/density, and thickness. It was found that higher porosity helped facilitate multiphase (O2, H2O, etc.) transport; however, as the porosity increased, the catalyst layer adhesion to the PTL decreased, the contact resistance with the bipolar plate increased and the cell operating voltage increased. It was found that nickel-based PTLs allowed for lower operating voltages than similarly-structured stainless steel PTLs. Improved operating voltages were achieved with both fiber felts and sintered structures by porosity optimization. Furthermore, an increase in thickness did not affect transient voltage response, however, it had a negative effect on performance stability. The experimental findings presented here provide important insights for development of PTL materials and structures for efficient and low-cost water electrolysis.

Simulation of Cathode Catalyst Durability Under Fuel Cell Vehicle Operation - the Effect of Temperature

Fuel cell vehicles (FCVs) emerge to be promising candidates to produce clean power for the transportation sector in order to tackle climate change issues. Although the commercialization of polymer electrolyte membrane fuel cells (PEMFCs) has progressed in the past few decades, there are still obstacles to address, including high cost and limited hydrogen infrastructure. For heavy duty transportation applications, the PEMFC durability is also not yet proven, and extrapolating from lab data to real-world field operating conditions remains a significant challenge [1].In this work, the cathode catalyst degradation in PEMFCs is studied to make an estimation of fuel cell durability in the FCV application. The well-known Butler-Volmer approach is utilized to model platinum dissolution and redeposition, platinum oxidation and platinum ion formation during the fuel cell operation [2]. First, the model is calibrated with the results presented by Ferreira et al. [3] at 80℃; the calibrated model is then validated with Kocha’s results [4] to demonstrate the model’s capability of generating reliable results at different temperatures. In order to realistically predict the cathode degradation pattern and lifetime, the real-life fuel cell operating conditions should be obtained. Therefore, a drive cycle recorded based on a real-life transit bus operation in the city of Victoria is utilized to calculate the input fuel cell voltage profile. The procedure to calculate fuel cell operating voltage profile using the drive cycle has been thoroughly explained by Ahmadi and Kjeang [5]. This procedure employs Newton’s second law to calculate the required cell power density considering the air flow drag force counteracting the vehicle thrust. Finally, polarization curves representing the fuel cell performance are used to calculate the fuel cell voltage profile with the determined required cell power density. Polarization curves measured under a range of temperatures are employed to adequately reflect the effect of temperature on the fuel cell performance. The cell active area is considered to be 500 cm2 and the stack is assumed to contain 225 cells, representing a nominal stack power of 74 kW at 80℃. The fuel cell operation and degradation are simulated at three different temperatures: 60, 70, and 80℃. The change of remaining electrochemically active surface area (ECSA) with time is calculated as the output of the model.Temperature highly affects the electrochemical reactions of platinum dissolution in several ways. First, it affects the Tafel slope in the platinum dissolution reaction. Second, it substantially impacts on platinum degradation reaction rate constant [6]. The Arrhenius approach is taken in this study to apply the effect of temperature on the reaction rate constants. Fuel cell voltage loss over time is determined by assuming simple Tafel kinetics. The cathode lifetime is calculated at 0.6 A/cm2 and is estimated by considering 10% voltage drop as the failure criterion. Fig. 1 shows the simulated change of remaining ECSA over time and the resulting fuel cell lifetime for the three different cell temperatures. The results indicate that the cathode lifetime increases 168% when the cell temperature drops from 80℃ to 60℃ due to a significantly lower platinum degradation rate at 60℃. The results show relatively low cathode catalyst lifetimes for a transit bus. The reason is attributed to the bus drive cycle used as the model input. The drive cycle contains a great portion of deceleration and idling time which leads to a fuel cell operation close to open circuit voltage (OCV), thus experiencing a high degradation rate.In this regard, the present modeling framework could become a useful tool for fuel cell and FCV developers to predict lifetime for new products and systems prior to commercial release. Scientifically, the present model can also be used to further explore key influencing factors on fuel cell durability and develop more durable cell, stack, and system designs for a targeted FCV application. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs, and Simon Fraser University Community Trust Endowment Fund.

Vanadium dioxide sulphur-doped reduced graphene oxide composite as novel electrode material for electrochemical capacitor

In this study, vanadium dioxide (VO2) combined with sulphur-doped reduced graphene oxide (S-rGO) as a composite material is reported. Vanadium dioxide sulphur-doped reduced graphene oxide (VO2@S-rGO) composite was successfully synthesized via a rapid solvothermal technique by varying the S-rGO concentration on VO2. The structural, morphological, and textural properties revealed the successful integration of S into the rGO matrix, leading to a good interaction between S-rGO and VO2 in the composite materials. The optimized composite (VO2@S-rGO-1) presented better electrochemical performance and exhibits a specific capacitance of 204.2 F g−1 in 0.5 M K2SO4 electrolyte at a specific current of 0.5 A g−1 within a large stable potential window of 0.8 V vs Ag/AgCl. VO2@S-rGO-1 and cocoa-derived activated carbon (Cocoa-AC) were used as positive and negative electrodes, respectively, to fabricate an asymmetric supercapacitor device (VO2@S-rGO-1//Cocoa-AC). The device displayed excellent specific energy of 32.6 W h kg−1 with a corresponding specific power of 426.0 W kg−1 at 0.5 A g−1 in an operating cell voltage of 1.7 V. The stability test exhibited an excellent columbic efficiency of 99.9 % and capacitance retention of 75 % up to 10,000 galvanostatic charge/discharge at 10 A g−1. Overall, VO2@S-rGO-1//Cocoa-AC asymmetric device demonstrated excellent electrochemical properties and is thus a promising candidate for wide use in energy storage systems.