Achievable Current Density Research Articles

Anion Exchange Membrane Water Electrolyzer Accelerated Durability Assessment Using Potential Cycling

Anion exchange membrane water electrolyzers (AEMWEs) have shown remarkable progress in terms of voltage efficiency and achievable current density in the past few years. Various methods are employed to assess the durability of AEMWEs at component and cell level, and include constant and cycled current/potential, start/stop cycles, and open-circuit-voltage (OCV) holds. Each technique may impose different levels of stress on individual components, and in cases have aimed to accelerate polymer degradation or to stress the catalyst or catalyst layer integrity. Particularly due to the more fundamental technology readiness level of AEMWE, it is essential to understand the degradation mechanism and failure modes at membrane electrode assembly (MEA) level.In this study, commercial AEM, ionomer, Co3O4 as anode and PtC as cathode catalysts were employed for fabricating MEAs. Square-wave and triangular-wave potential cycling were applied as operational-based stress tests to understand the impact of load cycling on degradation. Square-wave potential cycling appeared to accelerate catalyst layer degradation through ionomer oxidation, which lessened catalyst site-access and enhanced kinetics. On the other hand, triangular-wave potential cycling seemed to stress membrane (thinning) reducing the high frequency resistance (HFR). In both cycling patterns, the dissolution of cobalt (catalyst) and nickel from the porous transport layer (PTL) were observed by inductively coupled plasma-mass spectrometry (ICP-MS) analysis of the feed liquid. Moreover, a slight decrease in double layer capacitance supports degradation of catalyst layer. The motivation of this investigation was to mimic closely representative operation when directly integrated with renewable energy sources. The results discussed here may provide insight as to lifetime differences between different electrolysis technologies and provide pathways to develop stress tests for AEMWE systems and improve cell level durability. Figure 1

Steam Electrolysis Using Solid Acid Electrolytes at Intermediate Temperatures

Solid acid electrolysis cells (SAECs), operating at intermediate temperatures (150-300 °C), has potentials to perform electrochemical conversions using renewable energy resources with a small cell voltage. One of the examples is a steam electrolysis to produce hydrogen and oxygen. The composite of CsH2PO4 solid acid and SiP2O7 matrix is a popular proton-conducting electrolyte in SAECs. The formation of CsH5(PO4)2 at the interface of CsH2PO4 and SiP2O7 provides high conductivity in a wide temperature range from 150 °C to 250 °C.[1] However, CsH5(PO4)2 becomes a liquid phase above 150 °C with a strong acidity, which leads to a low mechanical strength of the electrolyte and a fast cell degradation. In this study, SiO2 precursor was explored to obtain a SiP2O7 matrix, which reduced the generation of CsH5(PO4)2 in the composite and improved the stability of steam electrolysis. Additionally, the cell voltage was greatly reduced by modifying Aquivion proton conducting ionomer on a IrO2/Ti anode, which is likely due to the increasing OER (oxygen evolution reaction) active sites on the IrO2 surface.Commonly used SiP2O7 was prepared from a fumed silica which has a particle size of 3 nm and a surface area of 395 m2/g.[1] Firstly, the chronopotentiometry test was conducted using the traditional CsH2PO4/SiP2O7 electrolyte (molar ratio: 1/2) at 250 °C, the cell voltage rapidly increased after operating for 3 hours (Figure 1. black curve). X-ray diffraction (XRD) patterns showed TiP2O7 generated on the IrO2/Ti anode after the test, suggesting that the corrosion of Ti anode substrate by the strong acid of CsH5(PO4)2 might be the reason for cell degradation. The liquid CsH5(PO4)2 in the electrolyte also caused a severe deformation of the electrolyte pellet, highlighting the necessity to control the amount of CsH5(PO4)2 in the electrolyte. The SiO2 precursor was replaced to the one with particle size of 200 nm and a surface area of 10 m2/g while adjusting the heating time during SiP2O7 synthesis. The resultant SiP2O7 matrix contained a high proportion of hexagonal crystal structure (SiP2O7-hex). Scanning electron microscope (SEM) and differential thermal analysis (DTA) results exhibit that the SiP2O7-hex has a larger particle size and a lower hygroscopicity compared to the traditional SiP2O7. Chronopotentiometry tests was conducted for CsH2PO4/SiP2O7-hex electrolyte with small doping (16%) of CsH5(PO4)2 on the electrolyte surface to ensure a good contact between the electrolyte and the electrode. The steam electrolysis was stably operated for more than 20 hours (red curve in Figure 1), and the electrolyte pellet maintained its original shape after the test. These results suggest that SiP2O7-hex has a low reactivity with CsH2PO4 under humidified conditions, which limits the generation of liquid phase CsH5(PO4)2, and prevents the electrolyte deformation and the cell degradation.However, as seen from Figure 1, the cell voltage increased in the initial stage and finally stabilized at 2 V under only 10 mA cm-2. It is reported that both CsH5(PO4)2 and CsH2PO4 solid acid migrate towards the anode side during OER.[2,3] Energy dispersive X-ray spectroscopy (EDS) confirmed that the Cs salt almost fully covered the IrO2 after the test, which might lead to the high cell voltage. Initially, there were abundant triple phase interfaces (gas-electrolyte-IrO2 interface) where H2O and O2 can be effectively transported. However, as the solid acid migrated on the IrO2 surface with time, the effective gas diffusion pathway was blocked, and the cell voltage gradually increased. By modifying the IrO2/Ti with Aquivion, which is a popular ionomer used in proton exchange membrane fuel cells (PEMFCs) to provide proton pathways and improve water management on the electrode, the cell voltage at 10 mA cm-2 was decreased to 1.6 V. The achievable current density also increased from 30 mA cm-2 to 50 mA cm-2. It is hypothesized that the modification of Aquivion on IrO2 helps to maintain the active sites at the electrolyte-IrO2 interface, thus improving the efficiency of steam electrolysis.Overall, this study highlights the significance to control the amount of conductive liquid phase in the solid acid electrolyte and to maintain the accessibility to the electrochemical active sites. Reference: [1] T. Matsui et al., J. Electrochem. Soc, 2006, 153, A339.[2] M. Wagner et al., Sustainable Energy Fuels, 2020, 4, 5284.[3] N. Fujiwara et al., ChemSusChem, 2021, 14, 417. Figure 1

Joint response mechanism of bioanode and biocathode in single chamber biocathode microbial electrochemical systems started-up with a constant and low resistance

Microbial electrochemical systems (MESs), particularly biocathode MESs, are recognized as environmentally friendly technologies in wastewater treatment. Rapid start-up and achievable high current and power density are desirable for scaled-up MES. The high output current can be obtained under low external resistance, but the response behaviors of bioanode and biocathode during start-up stage under low external resistance need to be clarified. In this study, a 10 L single-chamber biocathode microbial electrochemical system with microbial separator was successfully started within 11 days under a low external resistance of 20 Ω. The maximum power density reached 116.6 mW/m2 on the 9th day. However, the competition between oxygen-reducing bacteria and filamentous bacteria in the biocathode caused the decreased output power and power overshoot. Reducing the external resistance not only eliminated the power overshoot but also enhanced the electroactivity and organic removal efficiency of the bioanode. Additionally, it alleviated the constraint of filamentous bacteria on oxygen mass transfer, thereby enhancing the performance of the biocathode. The comprehensive regulation of the bioanode and the biocathode performance was achieved by reducing external resistance. This study analyzed the joint response mechanism of the bioanode and biocathode, aiming to provide guidance for the application of biocathode microbial electrochemical systems.

Anode Catalysts for Electrocatalytic Alcohol Oxidation Coupled to CO2 Reduction in Continuous-Flow Electrolyzers

Thanks to the intense scientific interest, the field of CO2 electrolysis witnessed remarkable progress (industrially relevant partial current densities, selective electrocatalysts towards small molecule products, multi-cell stacks, etc.) in the past decade. However, despite all these efforts, the energy efficiency of CO2 electrolyzers is still relatively low, which is rooted in the large cell voltages. A reason behind this is that the kinetically sluggish water oxidation (OER) is performed at the anode in parallel with CO2 reduction (CO2RR) at the cathode. This issue can be circumvented by coupling CO2RR with alternative anode processes, resulting in smaller cell voltage (better energy efficiency) along with the possible generation of products at the anode with high market value. Thus, maximizing the potential of utilizing waste carbon sources to generate value-added products. Such alternative reactions include, for example, the electrocatalytic oxidation of chloride ions, aliphatic and aromatic alcohols, amines, urea, hydrazine, and numerous biomass-derived substances such as 5-(hydroxymethyl)furfural, sorbitol, etc.In this study, CO2RR to CO was driven at the cathode, and the electrocatalytic oxidation of glycerol (GOR) was performed at the anode of a small (1 cm2 active surface area) microfluidic electrolyzer cell. Ir black was replaced by carbon-supported Pt nanoparticles, while Ag nanoparticles were used as the cathode catalyst. During our experiments, we identified four main factors influencing the GOR activity and selectivity: i) the quality and quantity of the used ionomer, ii) the electrolyte flow rate, iii) glycerol concentration, and iv) the applied anode potential. The latter was particularly important since Pt showed the highest activity toward GOR in a potential window where its surface starts to oxidize. The formation of a continuous oxide layer has to be avoided since it inhibits the alcohol oxidation reaction. By setting optimal conditions, a close to 100% Faradic efficiency (FE) toward CO formation on the cathode and between 70-80% FE toward GOR was achieved above 200 mA cm-2 total current density. The amount of the formed products were tracked by mass spectrometry, gas-, and high-performance liquid chromatography, and nuclear magnetic resonance spectroscopy. Dihydroxyacetone, glycerate, glycolate, oxalate, tartronate, formate, and lactate were identified as GOR products. The missing charge was attributed to the formation and a small amount of CO2. To improve the stability and selectivity of Pt in GOR, semipermeable metal oxide layers were introduced to the top of the catalyst layer. The thin films were synthesized by electrodeposition in different thicknesses. Based on our measurements, a set of criteria were defined regarding the composition of the anode catalyst layer (ionomer, co-catalyst overlayer, etc.), the composition of the electrolyte solution, and the applied electrochemical protocol to allow stable and selective GOR at high achievable current densities.

Functionalization of MWCNTs for Bioelectrocatalysis by Bacterial Two-Domain Laccase from Catenuloplanes japonicus

This study was carried out in order to assess several modifications of carbon nanotube-based nanomaterials for their applications in laccase electrodes and model biofuel cells. The modified MWCNTs served as adapters for the immobilization of laccase from Catenuloplanes japonicus VKM Ac-875 on the surface of electrodes made of graphite rods and graphite paste. The electrochemical properties of the electrodes were tested in linear and cyclic voltammetrical measurements for the determination of the redox potential of the enzyme and achievable current densities. The redox potential of the enzyme was above 500 mV versus NHE, while the highest current densities reached hundreds of µA/cm2. Model biofuel cells on the base of the laccase cathodes had maximal power values from 0.4 to 2 µW. The possibility of practical application of such BFCs was discussed.

Predictability and robustness of anode biofilm to changing potential in microbial electrolysis system

Microbial electrolysis systems (MES) facilitate the process of using waste for efficient production of hydrogen thus resulting in lower energy costs compared to conventional hydrogen production. However, the stability and robustness of anode-respiring biofilms often limit long-term MES application. In this study, a 10 L rotating disc bioelectrochemical reactor was used to analyze the anodic biofilm under rapidly changing processing conditions, including changes in anode potential and shear force. A low complexity biofilm formed by Shewanella oneidensis and Geobacter sulfurreducens was studied to determine the boundary conditions for achievable current density and species interaction in large-scale applications. Demonstrating its robustness to the applied changes, the biofilm produced a stable current density of 1.2 A m−2 over 1.5 months. Furthermore, a mathematical model was developed to predict the behavior of the system in terms of current output, which may allow automatic user-defined control of sub-processes in MES reactors in the future.

Full spectrum photon management of photonic crystal-based aerogels to achieve the multiscale multiphysics regulations and optimizations of PV-TE/T systems

Multicomponent coupling is an effective way to achieve full-spectrum solar energy utilization. However, conventional multicomponent coupled PV systems are deficient in the rational distribution of full-spectrum sunlight. In this study, a photovoltaic-thermoelectric/thermal (PV-TE/T) system with photonic crystal-based aerogels (PCA) is proposed based on a novel strategy for efficient full-spectrum solar energy utilization. Then, a multiscale multiphysics theoretical model is developed to study the effects of full-spectrum selectivity characteristics of PCA on the performance of the PCA-based PV-TE/T system. Comparative analyses between stand-alone PV system, tandem PV-TE system, normal PV-TE/T system and PCA-based PV-TE/T system are performed. The full-spectrum selectivity characteristics of PCA allows the PCA-based PV-TE/T system to not only increase the maximum achievable photon current density of PV component by 2.39%–3.00%, but also to increase the heat source (light absorption) for TE component by at least 2609.84%. When the concentration increases to 20, the electrical efficiency of the stand-alone PV system is already 0%, and the electrical efficiency of the tandem PV-TE starts to decrease to 27.01%. As for the PCA-based PV-TE/T system, the maximum electrical efficiency of the PCA-based PV-TE/T system is 32.64% at the concentration of 93. Meanwhile, the thermal collector of the PCA-based PV-TE/T system absorbs 6277.5 W/m2 of thermal energy. Therefore, the PCA-based PV-TE/T system not only has a high efficiency of full-spectrum solar energy utilization, but also is applicable to higher concentrations, which offers the system the potential for further cost reductions.

Multi-scale multi-physic coupled investigation on the matching and trade-off of conversion and storage of optical, thermal, electrical, and chemical energy in a hybrid system based on a novel full solar spectrum utilization strategy

In order to achieve a reasonable utilization of the full solar spectrum at low-cost, a photovoltaic/thermoelectric/electrolysis (PTE) hybrid system based on a novel full solar spectrum utilization strategy is proposed. The PTE mainly consist of a thermal collector with the spectrally selectively absorption fluid (water), a photovoltaic-thermoelectric (PV–TE) component and a water electrolysis component. Based the structure of the PTE, the full solar spectrum energy can be reasonably and efficiently allocated and utilized. For the complicated coupling physical process in PTE hybrid system, an integrated multi-scale multi-physic theoretical model is developed to optimize and investigate the PTE hybrid system. After the analyses on the multis-scale multi-physic effect of the air insulation layer between the thermal collector and the PV–TE component, it is found that removing air insulation layer leads to the less reflection loss and parasitic absorption, and more the maximum achievable photo current density and the internal heat source for TE component. Moreover, not only does the PV–TE component without the air insulation layer have higher efficiency (27.03%), but the concentration corresponding to the maximum efficiency increase to 31 from 3, which means more output power. By matching the structure and energy between components, the efficiency of the PTE hybrid system can be further improved, while the efficiency and energy storage of the PTE hybrid system can be adjusted according to the demand. Compared to single PV system, the efficiency of the PTE hybrid system has been increased by 15.38% at the concentration of 20, and this difference increases with the concentration. Moreover, the PTE hybrid system can simultaneously generate 200 W/m2 of hydrogen energy at the concentration of 20. In summary, the PTE hybrid system not only offers better performance (generation and storage), but also better integration, providing a guiding idea for the full solar spectrum utilization. Moreover, the multi-scale multi-physic coupled model also provides precise theoretical guidance for the optimization and design.

Pt‐Anchored‐Zirconium Phosphate Nanoplates as High‐Durable Carbon‐Free Oxygen Reduction Reaction Electrocatalyst for PEM Fuel Cell Applications

AbstractCommercially available platinum‐supported carbon (Pt/C) catalysts are the most widely used oxygen reduction reaction (ORR) electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs). However, inadequate active triple‐phase boundary formation and carbon oxidation in Pt/C during PEMFC operation shorten its lifetime and efficiency. In this direction, a new class of carbon‐free electrocatalysts for ORR is prepared by dispersing Pt nanoparticles on ZrP (Zirconium phosphates) nanoplates. In one case (ZrP@Pt), the Pt nanoparticles are found to be closely distributed and completely covering the ZrP nanoplates, whereas in the second case (Pt/ZrP), the Pt nanoparticles selectively restrict dispersion along the edges of the support. ZrP as the support displays an intrinsic proton conductivity of ≈0.5 × 10−4 S cm−1 at 70 °C, with an activation energy (Ea) of 0.19 eV. Pt/ZrP shows better durability after 3000 start‐stop cycles. The mass activity of Pt/ZrP is increased by 4.6 times compared to Pt/C, which exhibits a loss in mass activity by 1.37 times. The single‐cell level validation of ZrP@Pt, Pt/ZrP, and Pt/C as the electrocatalysts in PEMFC at an operating potential of 0.60 V shows the achievable current densities of 0.600, 0.890, and 0.890 A cm−2.

Multi-Atom PGM Based Catalyst for Highly Efficient Oxygen Reduction Reaction(ORR) and Hydrogen Oxidation Reaction (HOR) in Alkaline Environment

Anion exchange membrane fuel cells (AEMFCs) have recently seen significant growth in interest as their achievable current density, peak power density, and longevity have been improved dramatically. Though these advances in performance have been important for demonstrating the feasibility of the technology, nearly all AEMFCs reported in the literature have required a relatively high loading of platinum group metal (PGM)-based catalysts at both the anode and cathode electrodes [1]. However, to take command of the low-temperature fuel cell market, AEMFCs cannot simply reach the same performance as incumbent proton exchange membrane fuel cells (PEMFCs), which have had decades of development and investment. AEMFCs must realize their most widely quoted advantage over PEMFCs and be produced at a much lower cost than PEMFCs. The most likely pathway to acceptably low cost will involve reducing the PGM loading in both electrodes. At the cathode, reasonable PGM-free catalysts exist, as will be shown in this work. At the anode; however, there are no practical contenders that exist to replace PGM-based catalysts. Hence, the most practical approach is to create transitional catalysts with ultra-low PGM content until future PGM-free catalysts can be realized. To reduce the platinum group metal (PGM) loading in anion exchange membrane fuel cell (AEMFC) electrodes, it is important to transition to catalysts with very low PGM content, and eventually to create catalysts that are completely PGM-free. One approach that can be used in both cases is to create atomically dispersed metals on a carbon support. In this work, four catalysts were prepared using a new, simple, scalable Controlled Surface Tension (CST) method: Pt/C, Pt/NC, PtRu/C, and PtRu/NC. CST is unique as it allows for a high density of very small multi-atom clusters, facilitated by altering the surface tension in the synthesis medium. The catalysts were physically characterized using a wide array of techniques, including high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM), extended X-ray absorption fine structure (EXAFS), and X-ray Absorption Near-Edge Structure (XANES). The catalysts were also tested for their oxygen reduction reaction and hydrogen oxidation reaction activity both ex-situ on a rotating ring-disk electrode and in-situ while integrated into the anode (PtRu) and cathode (Pt) of operating AEMFCs. With this new generation of low-PGM materials, it was possible to reduce the PGM loading by a factor of 14 while achieving comparable performance to commercial catalysts with a peak power density approaching 2 W/cm2. AEMFCs were also assembled with ultralow PGM loading (0.05 mgPGM/cm2), where PtRu/NC anodes were paired with Fe–N–C cathodes [2], which allowed for the demonstration of cells with a specific power of 25 W/mgPGM (40 W/mgPt).

Mass Transport Limitations in Electrochemical Conversion of CO2 to Formic Acid at High Pressure

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental data.

How to maximize geometric current density in testing of fuel cell catalysts by using gas diffusion electrode half-cell setups

Half-cell gas diffusion electrode (GDE) setups have recently been shown to allow characterization of oxygen reduction reaction (ORR) catalysts at fuel cell relevant current densities and potentials while offering the advantage of fast screening and requiring only minimal sample usage. Most recently, publications suggesting best practices for GDE half-cell characterization have highlighted key challenges in running GDE experiments and have presented measurement protocols that allow excellent Inter-lab comparability. This step can be seen as the cornerstone for broad-based utilization of GDE half-cells. However, what is still missing, is an understanding of what limits the maximum achievable current density in GDE measurements. In this study we highlight the influence of various setup parameters (electrode area, electrolyte concentration, as well as size and position of the counter electrode) on the maximum achievable current density in an automated GDE setup. Furthermore, we present the metrics that need to be considered when adapting the electrochemical protocol for recording ORR polarization curves at higher current densities. The findings observed in this study are not limited to testing the ORR, but can be transferred to any other electrocatalytic reaction tested in a half-cell GDE at high current densities.

Electrooxidation of Glycerol on an Electrodeposited Pd-Ni Catalyst on Ni Foam

An increased global demand for clean energy and a sustainable future has led to the intense investigation of hydrogen as a low-carbon energy source in fuel cells and chemical reductant in vital industries such as low-carbon steel production. However, the current status of H2 production through water electrolysis in terms of global H2 availability only makes up around 5%, with the overwhelming bulk of that being formed through the chlor-alkali process. The remaining 95% of H2 production is through steam reforming with coal and natural gas. With the climate crisis expected to be the challenge of the century, it is vital that H2 production be uncoupled from fossil fuels if it is to be a green energy carrier and fuel of a low-carbon global economy.The implementation of direct water electrolysis on a large scale has the dominating issue of the oxygen evolution reaction (OER) on the anode. This reaction is limiting due to it being energy intensive as it proceeds through a four-electron transfer with a high equilibrium potential that exhibits sluggish kinetics, which are only overcome by operating at overpotentials requiring high energy input. The electrochemical oxidation of glycerol, a tri-hydroxyl functionalised alcohol, offers a more thermodynamically favourable anodic reaction to support the concurrent hydrogen evolution reaction at the cathode.Glycerol, a low-cost by-product of biodiesel refineries, has shown to be an excellent platform chemical. Through oxidation it can provide valuable chemical precursors in the food, cosmetic and pharmaceutical industries. These glycerol oxidation products typically include glyceric, tartronic, glycolic, lactic, oxalic and formic acid, to name a few. The specific formation of these products is highly dependent on the experimental conditions. In aqueous electrochemical setups, the chemistry and structure of the catalyst, pH of the electrolyte and the anode potential can all have significant effects on the selectivity of the glycerol oxidation products.The electrodeposition of bimetallic PdNi electrocatalysts on a porous Ni foam substrate (PdNi/NiFoam) provides a facile synthesis route for highly active catalysts for the glycerol electrooxidation reaction (GEOR) at high temperature and alkaline conditions. From previous work on PdNi electrodeposited on an RDE, it was established that a ratio of 2:1, NaOH:glycerol with moderate convection at 80 °C achieved the highest GEOR current density.1 This study, conducted in 2 M NaOH and 1 M glycerol in a divided 3-electrode cell using a Luggin capillary, builds upon the aforementioned previous work, using the optimised experimental conditions, for a porous electrode with a higher Ni composition in the electrocatalyst, approximately 60:40, Pd:Ni. Here the focus is on the achievable anodic current density and glycerol oxidation product selectivity for puriss glycerol in aerated and deaerated atmospheres. It is found through IR-corrected polarisation curves and steady state LSVs, that current densities of 1000 mA cm-2 can be reached at potentials lower than -0.1 V vs. Hg/HgO (~0.9 V vs. RHE), without deactivation. Furthermore, it is seen that in galvanostatic conditions at a current density of 500 mA cm-2 in a deaerated batch solution for 2 h, the PdNi/NiRDE electrocatalyst did not deactivate and provides a glycerol oxidation product selectivity of > 70% for glycerate.The present results emphasise strongly the possibility for the GEOR to be a contributor to not only the production of H2 for the future hydrogen economy but also as a means to convert a waste product into valuable chemical products for further utilisation. Moreover, the results show that even at industrially relevant current densities, good product selectivity is possible. J. White et al., Electrochimica Acta, 139714 (2021) https://doi.org/10.1016/j.electacta.2021.139714. Figure 1. IR-corrected polarisation curve of glycerol oxidation between 1 and 1000 mA cm-2 (from low to high current) on a PdNi catalyst electrodeposited on Ni Foam, in 2 M NaOH and 1M glycerol at 80 oC. No deactivation of the electrocatalyst at 1000 mA cm-2. Figure 1

Gap resonance in the classical dynamics of the current-biased Josephson tunnel junctions

This paper reports a novel time-domain expression for the current-response kernels of a Josephson tunnel junction between BCS superconductors with in general different energy gaps, and its use to simulate the classical dynamics of such junctions. The simulations show a dynamic regime characterized by the resonance between the Josephson oscillations and the gap oscillations in the asymptotics of the current kernels that has not been studied previously. The resonance manifests itself as a hysteresis in the dc current-voltage characteristics of the current-biased junctions in the voltage range above the energy gap, in addition to the usual ``inertial'' hysteresis characteristic for tunnel junctions at voltages below the energy gap. Features of the IV curves related to the gap resonance, including the above-the-gap hysteresis, should manifest themselves in many structures and devices utilizing high-quality Josephson tunnel junctions with relatively large, but achievable current densities.

Electrodeposited PdNi on a Ni rotating disk electrode highly active for glycerol electrooxidation in alkaline conditions

The development of alcohol-based electrolysis to enable the concurrent production of hydrogen with low electricity consumption still faces major challenges in terms of the maximum anodic current density achievable. Whilst noble metals enable a low electrode potential to facilitate alcohol oxidation, the deactivation of the catalyst at higher potentials makes it difficult for the obtained anodic current density to compete with water electrolysis. In this work the effect of significant parameters such as mass transport, glycerol and OH− concentration and electrolyte temperature on the glycerol electrooxidation reaction (GEOR) in alkaline conditions on a bimetallic catalyst PdNi/NiRDE (Pd0.9Ni0.1) has been studied to discern experimental conditions which maximise achievable anodic current density before deactivation occurs. The ratio of NaOH:glycerol in the electrolyte highly affects the rate of the GEOR. A maximum current density of 793 mA cm−2 at -0.125 V vs. Hg/HgO through steady state polarisation curves was achieved at a moderate and intermediate rotation rate of 500 RPM in a 2 M NaOH and 1 M glycerol (ratio of 2) electrolyte at 80 °C. Shown here is a method of catalyst reactivation for enabling the long-term use of the PdNi/NiRDE for electrolysis at optimal conditions for extended periods of time (3 h at 300 mA cm−2 and 10 h at 100 mA cm−2). Through scanning electron microscopy (SEM), X-ray photon electron spectroscopy (XPS) and X-ray diffraction (XRD) it is shown that the electrodeposition of Pd and Ni forms an alloy and that after 10 h of electrolysis the catalyst has chemical and structural stability. This study provides details on parameters significant to the maximising of the GEOR current density and the minimising of the debilitating effect that deactivation has on noble metal based electrocatalysts for the GEOR.

Membrane-Electrode-Assembly Design Parameters and Their Impact on CO2 Reduction

CO2 electrolyzers that operate at high current densities and selectivities are essential for commercializing CO2 electroreduction and for a sustainable, CO2-neutral green energy paradigm. As part of this, membrane-electrode assemblies (MEAs) are critical components that enable industrial-scale electrochemical CO2 conversion to value-added fuels and chemicals. They consist of humidified gaseous feeds at one or both electrodes and a solid, ion-conducting polymer (ionomer) as the electrolyte. Such assemblies can overcome the experimentally observed ohmic and mass-transfer limitations characteristic of aqueous gas-diffusion electrode (GDE) and planar systems, respectively, which limit their maximum achievable current densities and render these systems impractical for industrial implementation.Foundational design parameters that characterize the ionomer-based catalyst layers unique to MEA systems include the ionomer-to-catalyst (I:Cat) ratio, the catalyst loading, and the catalyst layer thickness. In this talk, the results of a comprehensive experimental study on a Ag-cathode CO2 reduction (CO2R) MEA will be reported, where we systematically analyzed the effects of these design parameters on CO2R MEA performance and selectivity. An optimum, intermediate I:Cat ratio was uncovered and rationalized based on electrochemically-active surface area and ionomer distribution arguments. The effect of decreasing the catalyst loading and/or thickness corroborates previous modeling work, resulting in an optimum range for catalyst layer utilization for CO2R. The effect of MEA anolyte/exchange solution concentration on system performance is also explored, yielding an efficient Ag-cathode CO2R MEA that operates at 200 to 1000 mA/cm2, at CO FEs of 78 to 91%, and with an area-specific resistance of 1 Ω*cm2.The scientific and experimental design learnings from our Ag-cathode MEA studies are subsequently applied to a Cu-cathode MEA, which has farther reaching and impactful applications due to its ability to produce value-added chemical precursors like ethylene and ethanol. The Cu-MEA cathodic I:Cat ratio and catalyst loading are fixed based on settings from the optimized Ag-MEA and additional catalyst layer parameters and operating conditions are examined. This systematic exploration provides deeper insights into the underlying interrelationships between physical phenomena in the Cu-MEA system. Overall, these studies provide a fundamental understanding of the relationships between critical design parameters and inherent operating and materials tradeoffs for practical CO2 reduction devices. Acknowledgements The authors gratefully acknowledge Lawrence Berkeley National Laboratory’s Laboratory Directed Research and Development (LDRD) Grant for funding.

Direct Exfoliation of Conductive MoS2 Using Peroxide for Solid State Sensor and Catalytic Applications

Two-dimensional (2D) materials have attracted much attention over the last decade due to their high performance in nanoelectronic devices. The discovery of graphene opened up many opportunities to investigate and explore other 2D materials. There has been a drive to expand the toolbox of 2D materials to also include insulators and semiconductors with a variety of bandgaps. As a result, a wide range of materials have been discovered or predicted, with molybdenum disulfide (MoS2) being particularly popular. Using the semiconducting phase of MoS2 (2H-MoS2) requires a relatively high voltage to get sufficient conductivity due to the presence of a band gap. However, for applications in batteries, supercapacitors, electrocatalysts and solar cells, a substantially increased conductivity is required in order to achieve reasonable currents.1 The most common source of conductive MoS2 is metallic MoS2 (1T-MoS2) that has been prepared via the lithium intercalation process, which requires inert atmosphere processing and safety procedures.2 Hence, there is a desire to develop a safer and more efficient process to yield conductive MoS2. Defects play a very important role in modulating the electrical properties of MoS2. Sonication of MoS2 in an appropriate solvent results in many disordered structural defects. The most common defects on MoS2 are sulfur defects. These defects increase the energy level of the gap state and eventually deteriorate the device performance. Thiol based molecules are commonly used to reduce the number of sulfur defects on MoS2. Other molecules such as oxygen or organic super acids like bis(trifluoromethane) sulfonamide (TFSI) have also been reported to passivate the surface defect.3 Past research has mainly focused on the theoretical study of defective MoS2 and how to utilize those defects for improving photoluminescent efficiency. However, those defects can also be utilized to improve the conductivity of MoS2 as a safer alternative for applications in batteries, supercapacitors, solar cells, electrocatalyst and sensors.Conductive MoS2 (c-MoS2) can be used as active material for low-cost solid-state chemiresistive pH sensors.4 In chemiresistive sensors, conductivity changes are observed based on direct interactions between the active material and the analyte.5 Even though chemiresistive pH sensors based on exfoliated graphene, carbon nanotubes, or graphitic materials are available, their sensing response is limited to less than 20%.6 On the other hand, MoS2 has attracted great attention as a promising electrocatalyst for the hydrogen evaluation reaction (HER) because of abundant active sites at edge sites and on the basal plane for facilitating hydrogen production. Water splitting is the simplest and most convenient method to generate hydrogen. In industrial applications, an electrocatalyst is commonly used to accelerate the HER and reduce the overpotential. Even though 2H-MoS2 has good catalytic activity at the edge sites, its low electrical conductivity limits the achievable current density, resulting in a high Tafel value and making it unsuitable for practical application in HER.In this work, we show a simple and effective way to prepare few layer c-MoS2 under ambient conditions using 0.06 vol% aqueous hydrogen peroxide. We have demonstrated that the bulk conductivity of the conductive MoS2 that we prepared is up to seven orders of magnitude higher than that of the semiconducting phase of MoS2. The samples were also characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy which showed that hydrogen molybdenum bronze (HxMoO3) and substoichiometric MoO3−y help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. C-MoS2 was further functionalized with thiols to determine the number of residual reactive sites. An important goal of our work is to control the conductivity of the MoS2 thin films in safe and facile ways that enable their application in low-cost chemiresistive sensors in liquid environments. We fabricated chemiresistive pH sensors with centimeter channel lengths while maintaining low measurement voltages. We further measured the catalytic activity of c-MoS2 films in 0.5 M H2SO4 electrolyte solution with three electrode systems using linear sweep voltammetry (LSV) which showed a lower Tafel value at 10 mA/cm2 current density. The lower Tafel value demonstrated that c-MoS2 has potential to use as catalyst for HER. Our study furthers the understanding of conductive forms of MoS2, and also opens up a new pathway for next generation electronic and energy conversion devices.

Electrocatalytic oxidation of 2-propanol on PtxIr100-x bifunctional electrocatalysts – A thin-film materials library study

Due to the high demand for renewable and infrastructure compatible energy conversion and storage technologies, research on organic fuel cells receives increasing interest again recently. Organic fuels such as alcohols provide an attractive avenue to overcome the drawbacks of hydrogen as an energy carrier. Particularly interesting are secondary alcohols that almost exclusively form ketones as the final oxidation product, as they can be utilized in “zero emission” concepts without CO2 as a by-product. The state-of-the-art electrocatalyst in secondary alcohol oxidation is Pt-Ru, which demonstrates low onset potentials for the oxidation of the most facile secondary alcohol isopropanol. Yet, the achievable current densities are still relatively low and decrease rapidly due to the formed product acetone, which can poison the catalyst surface over time. Therefore, there is an inevitable need for the development of novel electrocatalyst materials circumventing these challenges. In this study, we employ a high-throughput electrochemical approach coupled to on-line inductively-coupled plasma mass spectrometry to map the composition-dependent activity and stability of PtxIr100-x alloy electrocatalysts toward the electro-oxidation of isopropanol. The activity and stability of magnetron sputtered PtxIr100-x material libraries are studied in 0.1 M HClO4 both in the absence and presence of isopropanol. The highest current densities are achieved for the sample containing the least amount of Ir (3.4 at.%), with a continuous decrease with the increasing amount of Ir. The alloys are inactive towards the oxidation of isopropanol when the amount of Ir exceeded 80 at%. The presence of isopropanol also has a notable effect on stability: while dissolution rates do not change in the case of pure Pt and Ir, a significant increase in stability is observed for the PtxIr100-x thin-film samples at all applied upper potential limits. This is explained by the strong adsorption of acetone on the surface of the catalyst that inhibits the formation of surface oxides.

Facile synthesis of Ni/NiO nanocomposites: the effect of Ni content in NiO upon the oxygen evolution reaction within alkaline media.

We present the facile synthesis of Ni/NiO nanocomposites, via a solution combustion methodology, where the composition of metallic Ni within NiO is controlled by varying the annealing time, from 4 minutes up to 8 hours. The various Ni/NiO nanocomposites are studied via electrically wiring them upon screen-printed graphite macroelectrodes by physical deposition. Subsequently their electrochemical activity, towards the oxygen evolution reaction (OER), is assessed within (ultra-pure) alkaline media (1.0 M KOH). An optimal annealing time of 2 hours is found, which gives rise to an electrochemical oxidation potential (recorded at 10 mA cm−2) of 231 mV (vs. Ag/AgCl 1.46 vs. RHE). These values show the Ni/NiO nanocomposites to be significantly more electrocatalytic than a bare/unmodified SPE (460 mV vs. Ag/AgCl). A remarkable percentage increase (134%) in achievable current density is realised by the former over that of the latter. Tafel analysis and turn over frequency is reported with a likely underlying mechanism for the Ni/NiO nanocomposites towards the OER proposed. In the former case, Tafel analysis is overviewed for general multi-step overall electrochemical reaction processes, which can be used to assist other researchers in determining mechanistic information, such as electron transfer and rate determining steps, when exploring the OER. The optimal Ni/NiO nanocomposite exhibits promising stability at the potential of +231 mV, retaining near 100% of its achievable current density after 28 hours. Due to the facile and rapid fabrication methodology of the Ni/NiO nanocomposites, such an approach is ideally suited towards the mass production of highly active and stable electrocatalysts for application within the anodic catalyst layers of commercial alkaline electrolysers.

Enhancing the efficiency of the hydrogen evolution reaction utilising Fe3P bulk modified screen-printed electrodes via the application of a magnetic field.

We report the fabrication and optimisation of Fe3P bulk modified screen-printed electrochemical platforms (SPEs) for the hydrogen evolution reaction (HER) within acidic media. We optimise the achievable current density towards the HER of the Fe3P SPEs by utilising ball-milled Fe3P variants and increasing the mass percentage of Fe3P incorporated into the SPEs. Additionally, the synergy of the application of a variable weak (constant) external magnetic field (330 mT to 40 mT) beneficially augments the current density output by 56%. This paper not only highlights the benefits of physical catalyst optimisation but also demonstrates a methodology to further enhance the cathodic efficiency of the HER with the facile application of a weak (constant) magnetic field.