Total Cell Potential Research Articles

Iron Effects in Advanced Alkaline Electrolyzer

Global warming, characterized by an alarming rise in the Earth’s temperature, is one of the serious impacts of industrialization. Transition towards a sustainable future requires the diversification of our energy sources by replacing fossil fuels with renewable energy such as green H2, to lower global CO2 emissions. Alkaline water electrolysis is one of the widely used affordable water splitting techniques that produces hydrogen gas. Since Fe contamination in this system is an unavoidable factor, it is important to understand the optimum Fe concentration for maximum performance. In this work, we are investigating the change in the total cell performance by spiking Fe2+ (0 to 140 ppm) solution at industrial standard conditions (6 M KOH, 80° C).In this work, we are investigating the change in the total cell performance by spiking Fe2+ (0 to 140 ppm) solution at industrial standard conditions (6 M KOH, 80° C). The KOH was purified prior to the experiment making sure the Fe limits are below the ICP-MS detection limit. To fully understand how Fe activates/deactivates the electrode performance, we chose NixCoy(O)OH coated on Ni mesh anode, commercial AWE cathode and Zirfon-220 as separator and performed testing in both 3-electrode set up and zero-gap electrolyzer set up. Our initial results show that, Fe effect on the total cell performance follows a bell curve where the maximum contribution comes from the activation of the anode. It must be noted that by spiking 40 -50 ppm Fe in the electrolyte the total cell potential is reduced by over 1V which is a huge gain. As we spike over 60 ppm the performance starts to go down. This can be explained by the deposition of FeOx on the anode which is a bad OER catalyst in comparison with NixCoy(O)OH. Our anode has a high active surface area from the nano sheet composition along with high conductivity from CoOx. At 20 ppm Fe in the electrolyte, the anode over potential reduced by 49 mV at 10 mA and by ~110mV at 250mA at 80 °C. Unlike bare Ni electrode as cathode, commercial cathodes are very resistant to Fe fouling only leading to a few millivolts of change. It is important to understand that Fe has maximum effect at low temperature, where the total cell potential was reduced by 200mV by adding 40 ppm Fe solution. As we increase the temperature to 80 °C, reaction kinetics gets improved and the optimum Fe concentration for maximum performance decreases.The activation in the anode performance at high Fe concentration has been verified by studying the surface morphology SEM imaging and by taking the EDX analysis. It has been seen that the anode gives maximum performance when we have sites of NixCoyFeO(OH) deposited on the anode catalyst. As our next step, we will be evaluating the changes in the surface morphology of the cathode along with the amount of Fe being deposited at each spiking. This will provide more information on how we should engineer the cell components to achieve the maximum output. Figure 1

Modular Solar-to-Fuel Electrolysis at Low Cell Potentials Enabled by Glycerol Electrooxidation and a Bipolar Membrane Separator.

Solar fuel generation through water electrolysis or electrochemical CO2 reduction is thermodynamically limited when it is paired with oxygen evolution reaction (OER). Glycerol electrooxidation reaction (GEOR) is an alternative anodic reaction with lower anodic electrochemical potential that utilizes a renewable coproduct produced during biodiesel synthesis. We show that GEOR on an Au-Pt-Bi ternary metal electrocatalyst in a model alkaline crude glycerol solution can provide significant cell potential reductions even when paired to reduction reactions in seawater and acidic catholytes via a bipolar membrane (BPM). We showed that the combination of GEOR and a BPM separator lowers the total cell potential by 1 V at an electrolysis current of 10.0 mA cm-2 versus an anode performing anode's OER when paired with hydrogen evolution and CO2 reduction cathodes. The observed voltage reduction was steady for periods of up to 80 h, with minimal glycerol crossover observed through the membrane. These results motivate new, high-performance cell designs for photoelectrochemical solar fuel integrated systems based on glycerol electrooxidation.

Insights into estrogenic activity removal using carbon nanotube electrochemical filter

This study reports the performance of a carbon nanotube (CNT) electrochemical filter applied to 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) degradation and their estrogenic activity removal (calculated in terms of E2 equivalent, EQ-E2). The performance of CNT electrochemical filter was assessed at different applied voltages (0–2.5 V) and aqueous matrices (ultrapure water and urban wastewater), using 37 μM of E2 and EE2, a flow rate of 1.5 mL min−1 and 10 mM of Na2SO4, used as supporting electrolyte. Surface characterization of CNT anodic filters was completed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was used to investigate electron transfer mechanisms. The CNT electrochemical filter was successfully applied to E2 and EE2 degradation and removals higher than 95.3% (oxidative fluxes >2.94 ± 0.05 mmol h−1 m−2) were achieved when 2.5 V was applied for both ultrapure water and urban wastewater. CV results indicate that the oxidation in the CNT electrochemical filter is an irreversible process. SEM and XPS results showed evidence of the polymer formation on the CNT surface after 300 min of reaction, which probably reduced the efficiency of the process under low applied voltages. Estrogenic activity was considerably reduced and minimal EQ-E2 levels were observed when 2.5 V was applied. A residual EQ-E2 was observed, likely due to the presence of estrogens, which suggests the non-formation of estrogenic intermediates. At 2.5 V total cell potential, the energy required to remove estrogenic activity was 0.014 ± 0.001 kWh m−3 for ultrapure water and 0.021 ± 0.001 kWh m−3 for post-secondary wastewater. These results suggest a CNT electrochemical filter may have potential to effectively and efficiently remove estrogenic activity and may be a feasible process for wastewater polishing treatment.

Conductive Cotton Filters for Affordable and Efficient Water Purification

It is highly desirable to develop affordable, energy-saving, and highly-effective technologies to alleviate the current water crisis. In this work, we reported a low-cost electrochemical filtration device composing of a conductive cotton filter anode and a Ti foil cathode. The device was operated by gravity feed. The conductive cotton filter anodes were fabricated by a facile dying method to incorporate carbon nanotubes (CNTs) as fillers. The CNTs could serve as adsorbents for pollutants adsorption, as electrocatalysts for pollutants electrooxidation, and as conductive additives to render the cotton filters highly conductive. Cellulose-based cotton could serve as low-cost support to ‘host’ these CNTs. Upon application of external potential, the developed filtration device could not only achieve physically adsorption of organic compounds, but also chemically oxide these compounds on site. Three model organic compounds were employed to evaluate the oxidative capability of the device, i.e., ferrocyanide (a model single-electron-transfer electron donor), methyl orange (MO, a common recalcitrant azo-dye found in aqueous environments), and antibiotic tetracycline (TC, a common antibiotic released from the wastewater treatment plants). The devices exhibited a maximum electrooxidation flux of 0.37 mol/h/m2 for 5.0 mmol/L ferrocyanide, of 0.26 mol/h/m2 for 0.06 mmol/L MO, and of 0.9 mol/h/m2 for 0.2 mmol/L TC under given experimental conditions. The effects of several key operational parameters (e.g., total cell potential, CNT amount, and compound concentration) on the device performance were also studied. This study could shed some light on the good design of effective and affordable water purification devices for point-of-use applications.

Interlaced CNT Electrodes for Bacterial Fouling Reduction of Microfiltration Membranes.

Interlaced carbon nanotube electrodes (ICE) were prepared by vacuum filtering a well-dispersed carbon nanotube-Nafion solution through a laser-cut acrylic stencil onto a commercial polyvinylidene fluoride (PVDF) microfiltration (MF) membrane. Dead-end filtration was carried out using 107 and 108 CFU mL-1 Pseudomonas fluorescens to study the effects of the electrochemically active ICE on bacterial density and morphology, as well as to evaluate the bacterial fouling trend and backwash (BW) efficacy, respectively. Finally, a simplified COMSOL model of the ICE electric field was used to help elucidate the antifouling mechanism in solution. At 2 V DC and AC (total cell potential), the average bacterial log removal of the ICE-PVDF increased by ∼1 log compared to the control PVDF (3.5-4 log). Bacterial surface density was affected by the presence and polarity of DC electric potential, being 87-90% lower on the ICE cathode and 59-93% lower on the ICE anode than that on the PVDF after filtration, and BW further reduced the density on the cathode significantly. The optimal operating conditions (2 V AC) reduced the fouling rate by 75% versus the control and achieved up to 96% fouling resistance recovery (FRR) during BW at 8 V AC using 155 mM NaCl. The antifouling performance should mainly be due to electrokinetic effects, and the electric field simulation by COMSOL model suggested electrophoresis and dielectrophoresis as likely mechanisms.

Steps Towards Reactive Hydrogen Pumping

Polymer electrolyte membrane (PEM) fuel cells are an attractive energy conversion device because of the potential for extremely high efficiency and low emissions. Pure hydrogen is an ideal fuel for PEM fuel cells but it is energy intensive to produce. Currently, the main method of producing hydrogen is steam reforming of natural gas. This is a break from the renewable energy future promised by hydrogen fuel cells. A greener approach to producing hydrogen is electrochemical reforming, the most obvious feedstock is water. Water electrolysis occurs at a relatively high potential (1.23 V), which makes it expensive both in terms of materials required and electricity. To fulfill the promise of fuel cells, more efficient methods of producing hydrogen must be found. One way to reduce the energy required is to depolarize the PEM electrolyzer anode by using an alternate feed. This feed must have a lower oxidation potential than water and be generated renewably. Two alternative feeds we have looked at are methanol and phosphomolybdic acid. Oxidation of methanol-water solutions to hydrogen takes place at a theoretical voltage of 0.03 V. Methanol fuel cells are a well-studied problem and the issues are known. The main issue is crossover of methanol to the cathode, lowering the total cell potential. We looked to see if this would be as problematic in a hydrogen pumping environment since there would be no oxygen at the cathode for the methanol to react with. The other feed we examined was phosphomolybdic acid (PMA). PMA is a keggin ion which is readily reduced by biomass substrates in the presence of sunlight or heat.1Our idea was to apply the concept of a depolarized anode electrolyzer to a mediated electrochemical system. This system has the PMA in solution as a combination catalyst and charge carrier. The PMA is reduced by biomass and circulated to an anode where it is oxidized to release the hydrogen removed from the biomass. This gives many of the advantages of a flow battery, where power and capacity scale separately. Our focus has been on studying the anode performance characteristics of PMA. References. 1. Liu W, Mu W, Liu M, Zhang X, Cai H, Deng Y. Solar-induced direct biomass-to-electricity hybrid fuel cell using polyoxometalates as photocatalyst and charge carrier. Nat Commun. 2014;5:3208. doi:10.1038/ncomms4208.

A mechanistic model for electrochemical nutrient recovery systems

Electrochemical membrane technologies such as electrodialysis have been identified as key technologies to enable nutrient recovery from wastewater. However, current electrochemical models are focused on simpler solutions than wastewater and omit key outputs such as pH, or total cell potential. A combined physico-chemical and electrochemical model was developed which includes the mechanisms of competitive transport of ions, implicit inclusion of H+ and OH−, pH (including ionic activity and ion pairing), different factors contributing to total cell potential and a novel method for ion exchange membrane transport. The model outputs compare well with measurements from experiments and simulate secondary effects such as electrode reactions and current leakage. Results found that membrane, rather than boundary layer or bulk resistance was the major contributor to potential drop, and that apparent boundary layers were relatively thick (3 ± 1 mm). Non-ideal solution effects such as ion-pairing and ionic activity had a major impact, particularly on multi-valent Ca2+ ions, which enhances the capability of electrodialysis to recover monovalent nutrient ions such as K+ and NH4+. Decreased resistivity of ion exchange membranes to specific ions (for example, in this case nitrate) could also be detected. The methods here are validated using a comparatively simple synthetic solution of five ionic components, but are able to be easily scaled for a more complex solution, and are also compatible with additional mechanisms such as precipitation, fouling, and scaling.

Quantitative analysis of the cell voltage of SO2-depolarized electrolysis in hybrid sulfur process

SO2-depolarized electrolysis (SDE) is the pivotal reaction in hybrid sulfur process. To date, the total cell potential for an SO2-depolarized electrolyzer has been identified to be controlled dominantly by the sulfuric acid concentration of the anolyte and electrolysis temperature. Potential loss in SDE can be separated into four components, i.e., equilibrium potential, anodic polarization overpotential, cathodic polarization overpotential, and ohmic loss. In this work, the potential individual components of SDE were measured and calculated. Results showed that the anodic polarization overpotential exhibited the highest ratio in the cell voltage of SDE reaction, and the kinetics of the anodic reaction was controlled by a distinct reaction process under different cell potentials. This study increases understanding on SDE and provides assistance to improve its performance.

Degradation of the Common Aqueous Antibiotic Tetracycline using a Carbon Nanotube Electrochemical Filter.

In this work, a carbon nanotube (CNT) electrochemical filter was investigated for treatment of aqueous antibiotics using tetracycline (TC) as a model compound. Electrochemical filtration of 0.2 mM TC at a total cell potential of 2.5 V and a flow rate of 1.5 mL min(-1) (hydraulic residence time <2 s) resulted in an oxidative flux of 0.025 ± 0.001 mol h(-1) m(-2). Replacement of the perforated Ti cathode with a CNT cathode increased the TC oxidative flux by 2.3-fold to 0.020 ± 0.001 mol h(-1) m(-2) at a total cell potential of 1.0 V. Effluent analysis by liquid chromatography-mass spectrometry and disk agar biocidal diffusion tests indicate that the electrochemical filtration process can degrade the TC molecular structure and significantly decrease its antimicrobial activity, respectively. Addition of dissolved natural organic matter (NOM) negatively affected the TC electrooxidation because of competition for CNT sorption and electrooxidation sites. At 2.0 V total cell potential, TC spiked (0.2 mM) into drinking water reservoir and wastewater treatment plant effluent samples had an oxidative flux of 0.015 ± 0.001 and 0.022 ± 0.001 mol h(-1) m(-2), respectively, and an energy requirement of 0.7 kWh kgCOD(-1) or 0.084 kWh m(-3). These results indicate a CNT electrochemical filter may have potential to effectively and efficiently treat antibiotics in water and wastewater effluent.

In-situ electrochemical impedance spectroscopy measurement of anodic reaction in SO2 depolarized electrolysis process

SO2 depolarized electrolysis (SDE) is an important reaction in the hybrid sulfur process, which is one of the most promising approaches for mass hydrogen production without CO2 emission. The potential loss of SDE process could be separated into four components, i.e., reversible cell potential, anode over potential, cathode over potential, and ohmic loss. Thus far, the total cell potential for the SO2 depolarized electrolyzer has been identified to be dominantly controlled by the sulfuric acid concentration of the anolyte and the temperature of the electrolysis process. In this work, an in-situ electrochemical impedance spectroscopy measurement of the anodic SDE reaction was conducted. The results show that anodic over potential is mainly caused by the SO2 oxidation reaction rather than ohmic resistance or mass transfer limitation. This study extends the understanding of the SDE process and gives suggestions for further improvements in SDE performance.

CNT–PVDF composite flow-through electrode for single-pass sequential reduction–oxidation

In this study, a five-layer electrochemical CNT–PVDF filter that is thin, flexible, and stable is demonstrated to be effective and efficient for single-pass nitrobenzene mineralization by sequential reduction–oxidation. Key to the technology is development of a CNT–PVDF membrane that is mechanically stable, electrically conductive, and non-Faradaic, which is used to prevent CNT release from and electrochemical degradation of the Faradaic CNT electrodes. A five-layer electrochemical filter: (1) conductive & protective CNT–PVDF, (2) Faradaic CNT electrode, (3) insulating PVDF separator, (4) Faradaic CNT electrode, and (5) conductive & protective CNT–PVDF is formed by mechanical press. At a total cell potential of 4 V, the sequential electrochemical reduction–oxidation process is able to reduce >99% of the influent nitrobenzene to aniline then oxidize >80% of the aniline to non-aromatic products (mostly carbon dioxide) in a single-pass (∼5 s hydraulic residence time). The mineralization current efficiency is ∼20%, total cell potential is completely (>99%) distributed to the two electrodes, and the energy requirement is ∼36000 kJ per mole of nitrobenzene degraded.

Quantitative Examination of Aqueous Ferrocyanide Oxidation in a Carbon Nanotube Electrochemical Filter: Effects of Flow Rate, Ionic Strength, and Cathode Material

The unique conductivity, small diameter, and high specific surface area of carbon nanotubes (CNTs) make them suitable for a range of nontraditional electrode structures. For example, an electrochemically active CNT microfilter has been shown to be effective for water treatment. The forced convection through the 3D CNT electrode improves mass transfer relative to conventional 2D electrodes, but the effects of several solution and reactor parameters in this system are poorly understood because previous works focused on strongly sorbing organic species capable of undergoing several multielectron transfer reactions. Here, the use of Fe(CN)64–/3– as a model soluble and nonadsorbing redox system enabled a near complete accounting of the predominant electron transfer reactions of anodic Fe(CN)64– oxidation and cathodic water reduction to H2. Subsequently, the effects of liquid flow rate, electrolyte concentration, target molecule concentration, anode potential, and cathode material on anodic Fe(CN)64– oxidation kinetics and thermodynamics in an electrochemical filter were investigated. Electrochemical kinetics were observed to increase linearly with increasing flow rate, and electrolyte concentration had negligible effects in the flow configuration due to convective replenishment of Fe(CN)64– and electrolyte to the CNT surface. In the electron-transfer-limited regime ([Fe(CN)64–]0 = 15 mM), a rate constant for the oxidation of Fe(CN)64– was calculated to be in the range ket = 2332–3705 s–1 or (6–10) × 10–3 cm s–1. The use of a CNT network cathode furthermore enabled the cathodic reaction to proceed at a lower cell potential, indicating that the CNT network catalyzed water reduction. The total cell potential required for facile water reduction decreased from 1.6 V with the Ti cathode to 0.8 V with the CNT cathode, resulting in a total cell energy efficiency (>75%) that approached the current efficiency. Overall, the results quantitatively exemplify some of the advantages of using a 3D CNT electrode in the flow-through configuration.

Application of electrodialysis technique to recover contaminated rinsing water from gold electroplating processes

The aim of the present work was to evaluate the possibility of using an electrodialysis technique to recover rinsing water contaminated with the electrolyte during the process of gold electrodeposition. During the rinsing process the electrolyte, containing gold, silver, cyanide and contaminants, is dragged from the bath with the plated components. The more the rinsing water is used the more it becomes concentrated and therefore water replacement is necessary. In these studies three different types of electrodialysis cell containing two, three and five separate compartments were used. Two kinds of ion selective membranes, a cationic Nafion 450 and an anionic Selemion AMV, were used in the construction of the cells. The total cell potential, membrane potential and polarisation potential as a function of applied current were evaluated. The results show that it is possible to extract an average of 56% of all metal and cyanide from the rinse water working at 30 mA cm−2 for 190 h. However, a more efficient extraction of 75% of all the metals and cyanide was obtained within 270 min of electrodialysis at a current density of 20 mA cm−2. The results of this study showed that electrodialysis is suitable for removing contaminants from rinsing water to be returned to the process.

Quantification of Voltage Loss in the PEM Reactor for Electrolyzing Hydrogen Bromide to Produce Hydrogen

Simultaneous conversion of gaseous hydrogen bromide to bromine and hydrogen can be accomplished in a Proton Exchange Membrane (PEM) electrolyzer. The cell voltage required to carry out this conversion is a function of design and operating conditions. The cell voltage is composed of the equilibrium cell potential as well as individual potential losses in the cell (e.g. ohmic, activation, mass transfer etc). All these potential contributions are greatly influenced by the water content in the membrane and at the catalytic surface. Therefore, it is necessary to accurately predict the water transport across the cell to quantify the individual voltage losses. The equilibrium cell potential is predicted by Nernst equation coupled with vapor-liquid equilibrium (VLE) calculations for the Br2-HBr-H2O and H2-H2O system. The model for predicting the total cell potential shows good agreement to the data over a range of current densities.

Quantifying Individual Potential Contributions of the Hybrid Sulfur Electrolyzer

The hybrid sulfur cycle has been investigated as a means to produce clean hydrogen efficiently on a large scale by first decomposing to , , and and then electrochemically oxidizing back to with the cogeneration of . Thus far, it has been determined that the total cell potential for the hybrid sulfur electrolyzer is controlled mainly by water transport in the cell. Water is required at the anode to participate in the oxidation of to and to hydrate the membrane. In addition, water transport to the anode influences the concentration of the sulfuric acid produced. The resulting sulfuric acid concentration at the anode influences the equilibrium potential of and the reaction kinetics for oxidation and the average conductivity of the membrane. A final contribution to the potential loss is the diffusion of through the sulfuric acid to the catalyst site. Here, we extend our understanding of water transport to predict the individual contributions to the total cell potential.

Utility of an Empirical Method of Modeling Combined Zero Gap/Attached Electrode Membrane Chlor‐Alkali Cells

An extensive survey of the Docktor‐Ingenieur Dissertationen of Jakob Jörissen and Klaus‐R. Menschig, both originally from the Universität Dortmund, is presented in regard to the empirical modeling of membrane chlor‐alkali cells and how it can be applied to a combined zero gap/attached porous electrode layer membrane cell. Particular emphasis is placed on Menschig's work on zero gap (ZG) and attached porous electrode layer (APEL) membrane chlor‐alkali cells, the first such research to appear in the open literature. Menschig developed various computer programs to characterize these ZG and APEL membrane chlor‐alkali cells. He characterized these cells by using the following parameters: the current density distribution over the membrane, the species concentrations on the membrane surfaces, equivalent diffusion layer thicknesses for the mesh electrodes/current collectors and attached porous electrode layers, and the electrode overpotentials and equilibrium potentials using the surface concentrations for the ZG and APEL cell configurations. He used empirical equations first presented by Jörissen for gap membrane cells combined with his own experimental observations for a cell which used Nafion™ 390, a bilayer perfluorosulfonic acid membrane, to determine values for these parameters. His empirical relations describe the dependence of the flux of OH− from catholyte to anolyte as a function of catholyte caustic concentration and the membrane potential drop as a function of catholyte caustic and anolyte salt concentrations. By using the experimental values for total cell potential, current density, and cell outlet concentrations with the empirical equations, Menschig calculated values for the characterizing parameters mentioned above. He used these values and other information (e.g., membrane and porous electrode layer conductivity) to predict the total cell potential for the ZG configuration. With prior knowledge of total cell potential and current efficiency for corresponding APEL and ZG cell configurations, membrane surface concentrations were derived and used in the prediction of total cell potential for a combined zero gap/attached electrode cell.

Membrane Potentials of Pyrex Glass Electrodes

Potentials across Pyrex glass electrodes immersed in fused sodium‐silver nitrates were measured. The membrane potential across the glass was small, so that the total cell potential was close to the electrode potential. The membrane potential showed a minimum at low silver concentration, which was attributed to the two‐phase structure of the glass. These potentials were shown to be roughly consistent with the ion exchange theory for membrane potentials and measurements of ionic distribution and mobility in the Pyrex, although exact quantitative calculations of the potentials were not possible.