Oxidation Of Zn Research Articles

VOC gas sensor based on hollow cubic assembled nanocrystal Zn2SnO4 for breath analysis

Hollow cubic assembled nanocrystal Zn2SnO4 was synthesized via one-step hydrothermal method for VOC gas sensing applications. The obtained Zn2SnO4 materials had a uniformly hollow cubic structure with an average size of approximately 1 μm and a wall thickness of about 150 nm formed from nanocrystals of around 14 nm. The X-ray absorption near-edge structure results showed no structural disorder and/or lattice damage around the Zn-absorbing atoms and the Zn oxidation state of 2+ in the host lattice of the hollow cubic Zn2SnO4. The gas sensing characteristics of the prepared Zn2SnO4 material were tested to C3H6O, C2H5OH, CH3OH, NH3, H2, and CO at 350 °C–450 °C and results showed that the sensors exhibited a good response to acetone and ethanol gases. The highest response values were 47.80 for 125 ppm of acetone and 7.52 for 10 ppm of ethanol at 450 °C. The Zn2SnO4 hollow cubic sensor demonstrated high sensitivity and selectivity to acetone with good stability and a detection limit of 175 ppb. The VOC sensing mechanism of the hollow cubic Zn2SnO4-based sensor was also discussed. The findings indicated that hollow cubic Zn2SnO4 is a promising material for use in excellent VOC gas sensing application towards breath analysis.

Ni and Zn/ZnO Synergistically Catalyzed Reduction of Bicarbonate into Formate with Water Splitting.

Conversion of CO2 into value-added chemicals with a facile hydrogen source such as water is always of great interest for sustainable development. In this work, a simple and efficient method of reduction of bicarbonate to formate on a simple Ni powder catalyst with water as the facile hydrogen source and Zn as the regenerable reductant is proposed. The Ni catalyst and in situ formed Zn/ZnO exhibited a synergetic catalytic activity in the conversion of bicarbonate into formate, and a good formate yield of 81% was obtained. Detailed studies revealed that the synergetic catalytic activity between Ni and the in situ formed Zn/ZnO was mainly attributed to (i) the inhibited oxidation of Zn by Ni, leading to more interface of Zn/ZnO; (ii) the decreased growth of ZnO crystal along the [0001] direction, and thus increasing the more polar (0001) Zn face and the (0001̅) O face, which have high activity; and (iii) the enhanced generation of more oxygen vacancies at the Zn/ZnO interface to promote the formate yield. This research demonstrates an efficient method of using a simple and nonprecious metal catalyst for the CO2 reduction into value-added chemicals and provides a better understanding of the synergistic catalytic mechanism of Ni and Zn/ZnO.

Mechanism Study of Carbon Coating Effects on Conversion-Type Anode Materials in Lithium-Ion Batteries: Case Study of ZnMn2O4 and ZnO-MnO Composites.

The carbon coating strategy is intensively used in the modification of conversion-type anode materials to improve their cycling stability and rate capability. Thus, it is necessary to elucidate the modification mechanism induced by carbon coating. For this purpose, bare ZnMn2O4, carbon-derivative-coated ZnMn2O4, and carbon-coated ZnO-MnO composite materials have been synthesized and investigated in-depth. Herein, high-temperature synchrotron radiation diffraction is used to monitor the phase transition from ZnMn2O4 to ZnO-MnO composite during the carbonization process. The electrochemical performance has been evaluated by cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The carbon- and carbon-derivative-coated samples display well-improved cycling stability in terms of suppressed electrode polarization, a moderate increase in resistance, and slight capacity variation. The influence of carbon coating on the intrinsic conversion process is investigated by ex situ X-ray absorption spectroscopy, which reveals the evolution of Zn and Mn oxidation states. This result confirms that the strong capacity variation of the bare ZnMn2O4 is induced not only by the reversible charge storage in the solid electrolyte interphase but also by the phase evolution of active materials. Carbon coating is an effective method to prevent the additional oxidation of MnO to Mn3O4, which leads to a stabilization of the main conversion reaction.

Photoinduced Oxygen Reduction Reaction Boosts the Output Voltage of a Zinc-Air Battery.

Utilization of solar energy is of great interest for a sustainable society, and its conversion into electricity in a compact battery is challenging. Herein, a zinc-air battery with the polymer semiconductor polytrithiophene (pTTh) as the cathode is reported for direct conversion of photoenergy into electric energy. Upon irradiation, photoelectrons are generated in the conduction band (CB) of pTTh and then injected into the π2p * orbitals of O2 for its reduction to HO2 - , which is disproportionated to OH- and drives the oxidation of Zn to ZnO at the anode. The discharge voltage was significantly increased to 1.78 V without decay during discharge-charge cycles over 64 h, which corresponds to an energy density increase of 29.0 % as compared to 1.38 V for a zinc-air battery with state-of-the-art Pt/C. The zinc-air battery with an intrinsically different reaction scheme for simultaneous conversion of chemical and photoenergy into electric energy opens a new pathway for utilization of solar energy.

Photoinduced Oxygen Reduction Reaction Boosts the Output Voltage of a Zinc–Air Battery

AbstractUtilization of solar energy is of great interest for a sustainable society, and its conversion into electricity in a compact battery is challenging. Herein, a zinc–air battery with the polymer semiconductor polytrithiophene (pTTh) as the cathode is reported for direct conversion of photoenergy into electric energy. Upon irradiation, photoelectrons are generated in the conduction band (CB) of pTTh and then injected into the π2p* orbitals of O2 for its reduction to HO2−, which is disproportionated to OH− and drives the oxidation of Zn to ZnO at the anode. The discharge voltage was significantly increased to 1.78 V without decay during discharge–charge cycles over 64 h, which corresponds to an energy density increase of 29.0 % as compared to 1.38 V for a zinc–air battery with state‐of‐the‐art Pt/C. The zinc–air battery with an intrinsically different reaction scheme for simultaneous conversion of chemical and photoenergy into electric energy opens a new pathway for utilization of solar energy.

Characterization of Carbon Felt Materials As Negative Electrode Material for Flow-Assisted Zn-Air Batteries

Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost: Zn is abundant in the earth’s crust and alkaline aqueous solutions are cheap and well understood. They also have low toxicity and high chemical stability. To make Zn-air batteries viable for large scale use, issues at the Zn electrode need to be addressed. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging. During the oxidation of Zn occurring in the discharge process in the battery, a sequence of chemical steps occurs producing a passive layer of ZnO on the electrode. Additionally, electrode morphology changes between each cycle produce inconsistent performance. These issues can be improved by using a three-dimensional support structure on which Zn is deposited. Carbon felt electrodes provide an ideal structure for the Zn anode in Zn-air systems as they are porous and highly conductive. These features allow for high surface area for deposition to occur, low mass transport constraints and low internal IR losses of the electrode. Furthermore, the carbon structure provides a consistent structure to return to minimize Zn morphology changes. These structures have been explored for their potential as the anode in Zn electrodes and showed significant promise. No issues with dendrite growth and passivation have been observed in the electrode during cycling. They can achieve 100mA/cm2 of current density at 20mV of overpotential during deposition and dissolution at a flow rate of 15ml/min of electrolyte through the electrode. Additionally, they have been shown to be stable for 100 cycles of cycling in a symmetric cell at 50mA/cm2 while maintain an average overpotential of less than 50mV during each step. Cycling at high flow rates improves the performance of the felt with an average step overpotential of 16mV at 28 ml/min and 28mV overpotential average at 10ml/min, all also at 4M NaOH saturated with Zincate. Increased state of charge of the electrode appears to increase the efficiency of the electrode. During charging steps in 10ml/min cycling experiments overpotential starts out near 40mV and trends towards 20mV during the step. Further, a 50mV CA step at 30ml/min shows current density increase from 100mA/cm2 at the start of the test to 400mA/cm2 at the end of 30 min. This is the result of the increased amount of Zn in the felt system providing more surface area of Zn for deposition to occur directly on. The potential during galvanostatic cycling experiments appears to be somewhat unstable indicating that the basic electrochemical performance of the electrode needs more study. Nucleation of Zn deposits occurs throughout the felt thickness regardless of the applied overpotential or current. As deposition progresses, the location of Zn growth depends on the current applied. At higher currents or overpotentials growth localizes near the membrane separator. It then begins to fill in back towards the current collector. The reverse occurs during discharge of the electrode, material is removed preferentially from the parts of the electrode near the current collector then proceeds towards the membrane separator. Experiments show that this is based on overpotential in the cell and under both potentiostatic and galvanostatic conditions; at low observed or applied potentials growth occurs throughout the cell felt and at higher overpotentials the growth becomes localized. IR drop measurements through the felt indicate that only 20% of the electrode is active during the growth phase of a 1hour, 40mA/cm2 deposition test. This suggests migration of the negatively charged zincate towards the membrane separator is dominant in these conditions. This ultimately reduces the efficiency of the electrode as it increases the overpotential required to achieved the desired current. Methods to counteract this effect with be discussed. Optimized electrode performance including flow rate, electrolyte conditions, and preconditioning of the electrode for cycling of symmetric and full cell setups will also be discussed further. We gratefully acknowledge the support of Dr. Imre Gyuk and the DOE Office of Electricity Delivery and Reliability. Figure 1

An ionic liquid based gel polymer electrolyte to be employed in power generating applications

Ionic liquid (IL) based gel polymer electrolytes (GPEs) are being investigated extensively at present as substitutes for conventional GPEs based on a polymer, a salt and solvents. The main reason behind this is the drawbacks in usage of solvents. IL based GPEs have been employed for energy storage devices such as batteries and super capacitors due to their interesting mechanical, physical and electrochemical properties. This study focused on synthesis preparation and characterization of an IL based GPE consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP), zinc trifluoro metha -nesulfonate (Zn(CF3SO3)2 - ZnTF) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (1E3MITF). Thin film samples were prepared using solvent casting method. The optimized composition was found to be 1 PVdF-co-HFP: 1 1E3MITF: 3 ZnTF (by weight basis). This mechanically stable, thin film has the maximum room temperature conductivity of 7.42×10-3 S cm-1. Conductivity variation with temperature follows Vogel - Tamman - Fulcher (VTF) behavior confirming the relation of conductivity mechanism with the free volume theory. The IL based GPE is a purely an ionic conductor having a considerable anionic contribution. It shows stability up to 2.5 V which is very much convenient from a practical point of view. Oxidation and reduction of Zn takes place at the potentials of 0.5 V and –0.5 V, respectively. In addition, Zn platting and stripping occurs only on the Zn electrodes but not on the stainless steel (SS) electrodes. Impedance measurements taken for the GPE continuously for a long period of time exhibited a satisfactory stability with Zn electrodes.

Fabrication and enhanced H2O2-sensing properties of the uniform porous FTO glasses with tunable pore sizes and densities

The pores with tunable sizes and densities have been prepared in the conductive layer of FTO glasses while keeping the conductivity and transparency almost unvaried, by immersing the FTO glasses coated with Zn powder in the mixed aqueous solution of HCl and ethanol. The average pore size and density can be continuously tuned in the ranges of 22–120 nm and 9–120/μm2, respectively, by simply adjusting the Zn concentration and/or ethanol volume ratio. The etching mechanism based on the oxidation of Zn by SnO2 (competing with HCl) has been revealed. Compared with FTO, the porous FTOs made here can greatly increases the detection sensitivity of H2O2 up to 3.7 times, when they work as the host substrate of NiO.

In situ synchrotron x-ray diffraction study of Zn/Bi2O3 electrodes prior to and during discharge of Zn-air batteries: Influence on ZnO deposition

Zn-based electrodes containing different amount of Bi2O3 have been analyzed by synchrotron XRD during the discharging process of a real Zn/air battery. Bi2O3 is clearly detected for pristine Zn-Bi2O3 based electrodes. However, immersion of Zn-Bi2O3 electrodes in 6 M KOH solution before starting the discharge results in the disappearance of Bi2O3 XRD signature in the pattern and the appearance of new peaks indicating the presence of metallic Bi, regardless of the amount of Bi2O3 initially included in the Zn-Bi2O3 electrodes. This behavior together with Open Circuit Voltage measurements points to a spontaneous reduction of Bi3+ to metallic Bi by the oxidation of Zn to Zn2+. Irrespective of the amount of Bi2O3 present in the negative electrodes, Zn XRD peaks diminish as the discharge proceeds. However, the initial amount of Bi2O3 clearly affect to ZnO and Bi metal XRD peaks. ZnO peaks are observed only for electrodes without Bi2O3 or those containing 6% of Bi2O3. Conversely, ZnO peaks do not appear in the XRD patterns obtained for higher quantities of Bi2O3 electrodes. Besides, in this case metallic Bi peaks are observed during all discharging processes, indicating that Bi remained in the electrode. Finally, SEM and EDX studies support these conclusions.

Study of crystal growth and kinetic parameters of Zn/ZnO oxidation in the presence of H2O and CO2

During the H2O/CO2 co-splitting, ZnO crystals were formed by oxidation of Zn giving gaseous product, H2/CO. Micrographs of Zn/ZnO and temperature program reaction (TPRx) of H2O/CO2 splitting showed that the crystal growth was occurred via two phenomena; (1) surface reaction: at temperature from 400 to 550 °C, H2O/CO2 oxidized Zn resulting in the outward growth of the ZnO crystals until the whole surface became inert and totally covered all the unreacted Zn inside the core. At this point, the production rates of H2/CO were temporarily paused; (2) phase changing and core-cracking/shrinking: at temperature 550–600 °C, Zn particles inside the core was phase-changed from solid to liquid/vapor due to the higher temperature. The higher pressure inside the core led to core-cracking, where the Zn travelled to the surface to continue reacting with H2O/CO2. Core-shrinking was then occurred due to the crack of ZnO shell and leakage of Zn. TPRx results showed that CO2 competed H2O in adsorption/dissociation on the Zn during temperature from 400 to 550 °C. Conversions and kinetic parameters were experimental and mathematical determined. The apparent activation energy of H2O and CO2 splitting on Zn/ZnO particles was estimated at 77.63 and 51.11 kJ/mol, while the pre-exponential factors were determined at 4.02 and 0.30 s−1, respectively.

In-Depth Study of Zn Electrode Passivation in Alkaline Solutions for Zn Batteries

Recently, increasing demand for energy storage options has rekindled work on Zn based alkaline batteries, particularly Zn-Air battery systems. Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost, Zn is abundant in the earth’s crust and aqueous alkaline electrolytes are cheap and easy to work with, have a low toxicity and high chemical stability associated with them. To make Zn-air batteries viable for large scale use, issues associated with Zn electrode passivation need to be addressed that lead to a loss of performance during cycling. During the oxidation of Zn to discharge the battery, a passive layer of ZnO is formed on the electrode. This reduces the discharge current possible. Additionally, passivation is linked to electrode morphology changes between cycles, producing inconsistent performance and a reduction in capacity of the battery. Here we describe the current knowledge about Zn passivation and recent work done to better understand Zn passivation in alkaline solutions. Zn passivation is not well understood. There are multiple mechanism of passivation proposed and studies often have presented different results and conclusions despite using the same experimental conditions. Recently there have been calls in the literature for new in-depth studies that clear up the confusion present in the understanding of Zn passivation and dissolution processes in general[1, 2]. The Electrochemical Quartz Crystal Microbalance (EQCM) is a technique which can observe mass changes on an electrode at the order of 15 nanograms. The change in mass observed can be correlated to the charge passed at a given point during an experiment to give a value of mass change per mole of electron passed. This can then be used to determine the equivalent mass of the species being added to or removed to or from the electrode during an electrochemical reaction. EQCM experiments in KOH solutions of various concentrations and saturated to different levels with Zn have been conducted to observe the process by which the passivation layer forms on the surface and the process by which it is removed. Cyclic voltammogram (CV) experiments show that peaks associated with passivation removal give values of 8 to 16 grams per mole electron which corresponds well with the transition of ZnO or Zn(OH)2 to Zn metal on the surface. The transition of the passive layer to active Zn metal on the surface may contribute to electrode shape change. During the anodic dissolution part of the CV sweep, initially values near 32 grams per mole of electron are observed in the EQCM plots, corresponding to direct removal of Zn in a 2-electron reaction. The mass to charge ratios subsequently transition to values near 49 grams per mole electron which would indicate that ZnO or Zn(OH)2 is being formed and removed from the surface. This suggests that the formation of the passivating Zn species is potential dependent and is not solely precipitation of a film on the electrode surface. Linear sweep voltammetry and chronoamperometry experiments will be discussed as well as the impacts of additives to the solution on the electrochemistry of the electrode. Additionally, rotating disc electrode experiments were conducted using a Zn electrode in KOH solutions of various molarities saturated with Zn to understand the mass transport dependence of the passivation behavior. Changes in the location and height of peaks associated with passivation with rotation speed and sweep rate show a mass transport dependence on when passivation occurs but not on the removal of the passivation layer. Dissolution steps were conducted using Zn plate electrodes with different electrolyte concentrations and saturations of Zn at various potentials to observe major influencing factors in the passivation layer formation. These showed that passivation is heavily dependent on the amount of Zn in the solution as well as the potential applied. We conclude that the formation of the passive layer may depend on the ‘escape’ of the ZnO and Zn(OH)­2 species formed during dissolution from the electrode surface. This will be discussed in the context of the various proposed passivation mechanisms and will lead to some thoughts about improved electrode design. Acknowledgement We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). Bockelmann, M., et al., Electrochemical characterization and mathematical modeling of zinc passivation in alkaline solutions: A review. Electrochimica Acta, 2017Mainar, A., et al., Alkaline aqueous electrolytes for secondary zinc-air batteries: an overview. International Journal of Energy Research, 2016 Figure 1

Alkaline-Acid Zn-H2 O Fuel Cell for the Simultaneous Generation of Hydrogen and Electricity.

An alkaline-acid Zn-H2 O fuel cell is proposed for the simultaneous generation of electricity with an open circuit voltage of about 1.25 V and production of H2 with almost 100 % Faradic efficiency. We demonstrate that, as a result of harvesting energy from both electrochemical neutralization and electrochemical Zn oxidation, the as-developed hybrid cell can deliver a power density of up to 80 mW cm-2 and an energy density of 934 Wh kg-1 and maintain long-term stability for H2 production with an output voltage of 1.16 V at a current density of 10 mA cm-2 .

Alkaline–Acid Zn–H2O Fuel Cell for the Simultaneous Generation of Hydrogen and Electricity

AbstractAn alkaline–acid Zn–H2O fuel cell is proposed for the simultaneous generation of electricity with an open circuit voltage of about 1.25 V and production of H2 with almost 100 % Faradic efficiency. We demonstrate that, as a result of harvesting energy from both electrochemical neutralization and electrochemical Zn oxidation, the as‐developed hybrid cell can deliver a power density of up to 80 mW cm−2 and an energy density of 934 Wh kg−1 and maintain long‐term stability for H2 production with an output voltage of 1.16 V at a current density of 10 mA cm−2.

Formation of ZnO nanowires during anodic oxidation of zinc in bicarbonate electrolytes

Dense arrays of ZnO nanowires were obtained by one-step anodic oxidation of metallic Zn in bicarbonate electrolytes followed by 2h of annealing in air at 200°C. A detailed inspection of the anode morphology at the initial stages of anodizations was performed. It was confirmed that nanowire bundles are initially formed both inside and outside the pits created during the anodic dissolution of the metallic substrate. In consequence, root-like structures are formed inside the cavities. However, no roots are observed for nanowire bundles formed outside the pits, directly on the surface of Zn. The effect of anodizing conditions on the growth of nanowires was studied. It was found that applying higher anodizing potentials and more concentrated electrolytes result in not only the significantly faster nanowires growth, but also the formation of denser NW films. In addition, the electrolyte agitation can also increase the rate of nanowires formation. Finally, we proved for the first time that the previous usage of electrolyte and the orientation of electrodes, can significantly affect the morphology of as-formed anodic films. Thermal treatment of as-anodized samples resulted in the successful conversion of the carbonate-containing precursor to crystalline wurtzite ZnO without the loss of nanowire morphology.

Fast-response photoconductive ultraviolet light detectors fabricated using high-quality ZnO films obtained by plasma-assisted molecular beam epitaxy

Abstract ZnO films form the core of numerous ultraviolet (UV) optoelectronic devices. Herein, a ZnO buffer layer produced by thermal plasma oxidation of metallic Zn was used to grow an active ZnO layer, with Zn film and active layer growth achieved by plasma-assisted molecular beam epitaxy. Although low-temperature ZnO buffer layer and Zn film growth was mainly three-dimensional (3D) due to a large lattice mismatch between ZnO/Zn and Si, the use of thermal plasma oxidation changed the growth mode from 3D to 2D, producing a smooth ZnO active layer. The thus fabricated ZnO active layer (sample 2) exhibited a lower tensile stress than that fabricated on a ZnO buffer layer using a conventional method (sample 1). Both samples were used to fabricate UV sensors, with that based on sample 2 exhibiting a faster photoresponse.

Enhancement of electric and magnetic properties by tuning Co cluster in ZnO films via high magnetic field

To reveal the origin of magnetic coupling in dilute magnetic semiconductors (DMSs), this article utilized high magnetic field (HMF) in the process of oxidizing Co/Zn bilayers to study the influence of Co clusters and oxygen vacancy (Vo) content control on magnetic and electrical properties of Co doped ZnO (Co-ZnO) films. Results indicated that HMF can suppress the diffusion of Co and remain the content of Co clusters compared with oxidation growth in the absence of HMF. Moreover, HMF promoted the oxidation of Zn, improved the crystallinity in the films, and lowered the content of Vo. Consequently, the resistivity of the films oxidized in HMF was decreased by 50%, while both the saturation magnetization and coercivity were increased by 100%. This indicates that contribution of more Co clusters to the magnetic properties of the films is greater than that of Vo.

Processes at the Negative Electrode of Alkaline Zinc-Air Batteries

Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large scale systems for energy storage. Such batteries use materials that are low cost (Zinc is abundant in the earth’s crust and alkaline aqueous solutions are cheap and well understood) and have a low toxicity associated with them. To make Zn-air batteries viable for large-scale use, issues at the Zn electrode need to be addressed. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging and electrode passivation during discharge. Additionally electrode morphology changes between each cycle produce inconsistent performance. During the oxidation of Zn occurring in the discharge process in the battery, a sequence of chemical steps occur resulting in eventual production of ZnO on the electrode. Here we report detailed studies of the Zn plating and stripping processes in alkaline environments. These processes are investigated by in situ electrochemical STEM (in situ ec-STEM) measurements that allow us to directly observe nucleation and growth processes as well as the behavior of Zn structures during stripping. These are correlated with electrochemical QCM measurements (EQCM). These methods give us insight into the relative rates of different processes. In situ ec-STEM experiments show that deposition and dissolution processes occurred at different rates and directionality. During deposition, deposits are initially observed grow quickly in all directions consuming available Zinc ions near the electrode surface. In the experiment whose video frames are shown in figure 1, 10,000 µm2 of observed Zinc deposit area was added in the first 30 seconds of deposition and was half of the total observed deposit area for a 3 minute, 250mV overpotential deposition experiment. After this, growth follows the concentration gradient and grows at a linear rate of 3,500 µm2 of observed deposit area per minute. Additionally, the overpotential employed during deposition appeared to impact deposit morphology dramatically, as 250mV deposition produced large boulder type deposits while a 300mV deposition test produced a mossy structure that covered the electrode surface. Dissolution processes observed via in situ ec-STEM with a 300mV underpotential step to anodically strip deposits showed only half of the material area was removed over a 3 min experiment with only a third reduction of the max particle height. Figure 2 shows the video frames from this experiment which began immediately after the experiment from figure 1. Further material removal appears to occur universally along the surface of the deposit, leading to the possibility of the deposit being detached from the electrode surface. EQCM was used to observe the formation and removal of the ZnO passive layer described above. The formation of the passivation layer was observed as a deviation from the rate of Zinc removal in the massograms generated by the QCM as a ZnO layer forms on the electrode surface. EQCM confirmed that the passive layer was the result of a precipitation reaction occurring in the solution near the interface. Further EQCM indicated that during dissolution multiple pathways of removing Zn were possibly occurring at once when looking at the charge transfer with actual mass removed, while deposition steps appeared to have only one reaction path of Zinc deposition occurring. This indicates that the forward and reverse pathways of the Zn reaction are not the same. Motional resistance to the crystal oscillation was collected as well to determine relative roughness of the deposits formed during deposition to correlate to the variations of in deposition seen in the in situ ec-STEM work. To overcome macroscopic issues associated with the Zn electrode, we are exploring using porous carbon negative electrodes similar to those used as air catalyst support materials. These provide a conductive and permanent structure in which Zn can be deposited and removed for battery operation. Operation of this electrode under various circumstances will be discussed. We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User facility. Figure 1

Temperature-dependent growth of ZnO structures by thermal oxidation of Zn coatings electrodeposited on steel substrates and their photocatalytic activities

Abstract In this work, zinc oxide (ZnO) nanostructures were successfully synthesized by thermal oxidation of zinc (Zn) coated steel substrates. Zn coatings were electrodeposited on the mild carbon steel sheet in the sulfate bath by DC current. The zinc coated samples were oxidized in air at distinct annealing temperatures between 400 °C and 800 °C. The phase structure and surface morphology of the ZnO films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The photocatalytic activity of these ZnO layers was examined by means of decomposition of methylene blue (MB) aqueous solutions under UV lamp irradiation for various duration. The findings illustrated that annealing temperatures had a big effect on the morphology and structure of the ZnO layers. The annealed layers showed significantly enhanced photoactivity activity than the pure Zn layer under UV-irradiation. The sample with ZnO nanostructures oxidized at 800 °C exhibited a better photocatalytic degradation of MB than the other samples. This paper can provide an important contribution to the development of efficient photocatalysts for the solution of the environmental pollution problems.

Experimental investigation of heterogeneous hydrolysis with Zn vapor under a temperature gradient

The hydrolysis step of the Zn/ZnO thermochemical cycle for hydrogen production is experimentally investigated in a laboratory-scale tube-reactor. The current work uses a new approach in which the heterogeneous oxidation of gaseous Zn with steam is carried out under a negative axial temperature gradient in order to improve cycle efficiency by reducing the proportion of steam and inert carrier gas used. It is shown that complete conversion of Zn to ZnO is possible at steam-to-Zn stoichiometries greater than 5.0. As the steam-to-Zn stoichiometry approaches unity at reduced inert gas fractions, condensation of Zn on the reactor walls becomes more likely. In addition, the observed gas-phase equilibrium shift toward increased production of ZnO at temperatures under 800 K is consistent with earlier theoretical predictions. While complete conversion with low inert gas and steam usage was not achieved, our approach shows great improvement over previous aerosol-based approaches when considering the total amounts of steam and inert gas used per unit of hydrogen produced. Therefore, the current temperature gradient approach is promising for the design of an efficient reactor for water splitting via Zn vapor.

Coupled Concentrating Optics, Heat Transfer, and Thermochemical Modeling of a 100-kWth High-Temperature Solar Reactor for the Thermal Dissociation of ZnO

This work reports a numerical investigation of the transient operation of a 100-kWth solar reactor for performing the high-temperature step of the Zn/ZnO thermochemical cycle. This two-step redox cycle comprises (1) the endothermal dissociation of ZnO to Zn and O2 above 2000 K using concentrated solar energy, and (2) the subsequent oxidation of Zn with H2O/CO2 to produce H2/CO. The performance of the 100-kWth solar reactor is investigated using a dynamic numerical model consisting of two coupled submodels. The first is a Monte Carlo (MC) ray-tracing model applied to compute the spatial distribution maps of incident solar flux absorbed on the reactor surfaces when subjected to concentrated solar irradiation delivered by the PROMES-CNRS MegaWatt Solar Furnace (MWSF). The second is a heat transfer and thermochemical model that uses the computed maps of absorbed solar flux as radiation boundary condition to simulate the coupled processes of chemical reaction and heat transfer by radiation, convection, and conduction. Experimental validation of the solar reactor model is accomplished by comparing solar radiative power input, temperatures, and ZnO dissociation rates with measured data acquired with the 100-kWth solar reactor at the MWSF. Experimentally obtained solar-to-chemical energy conversion efficiencies are reported and the various energy flows are quantified. The model shows the prominent influence of reaction kinetics on the attainable energy conversion efficiencies, revealing the potential of achieving ηsolar-to-chemical = 16% provided the mass transport limitations on the ZnO reaction interface were overcome.