Surface Charge Transfer Resistance Research Articles

Double oxygen evolution co-catalysts modified BiVO4 to boost photoelectrochemical water oxidation performance

Improving the kinetics of oxygen evolution is crucial for enhancing the performance of photoelectrochemical (PEC) water splitting. Herein, this study utilizes a chemical self-growth method to grow NiFe tannic acid complex (NFTA) and Co(OH)2 on the surface of BiVO4 photoanode (BiVO4/NFTA/Co). As a result, the synergistic effects of NFTA and Co(OH)2 layers promote the efficiency and stability of BiVO4 photoanode towards PEC water oxidation. The photocurrent density of the obtained BiVO4/NFTA/Co photoanode reaches 4.97 mA cm−2, which is significantly greater than those of BiVO4/NFTA (4.36 mA cm−2), BiVO4/Co (2.51 mA cm−2), BiVO4 (1.34 mA cm−2), respectively. Detailed analysis confirms that NFTA could provide an efficient way to hasten the transfer of photo-generated holes on the photoanode surface and diminish the surface charge transfer resistance. In other hand, Co(OH)2 could be served as a cocatalyst to accelerate charge transfer for efficient oxygen evolution reaction as well as a protective layer to maintain the long-term stability of NFTA on the surface of BiVO4 during water oxidation. Such double oxygen evolution co-catalysts decoration strategy paves an effective pathway to enhance the PEC water oxidation performance of BiVO4 photoanode.

Bilayer coating fabricated on aluminum alloy substrates with superamphiphobicity, corrosion resistance, self-cleaning, and delayed icing properties

To address the functional failure caused by low-surface-tension liquid wetting and adhesion in superhydrophobic materials, in this study, we constructed a bilayer coating with superamphiphobic properties using an epoxy (Ep) bonding layer and an organosilane-grafted attapulgite (OG-ATP) layer through spray-coating technique. The prepared superamphiphobic bilayer coating exhibited high contact angles (CAs) and low sliding angles (SAs) for cold water (25 °C), hot water (85 °C), glycerol, ethylene glycol, peanut oil, and even n-hexadecane (CA = 150.4 ± 0.3° and SA = 7 ± 2°) with surface tension of 27.5 mN m−1. Surface morphology and elemental analysis confirmed the coating's unique micro-nano hierarchical structure and low surface energy characteristics. Electrochemical impedance spectroscopy (EIS) results showed that the surface charge transfer resistance (Rct) and low-frequency modulus (|Z|0.01Hz) were enhanced by 6–7 orders of magnitude compared to the bare aluminum alloy substrate. The anti-corrosion performance remained stable even after 5 days of 3.5 wt% NaCl immersion. Additionally, icing experiments with water droplets at −10 °C and −15 °C revealed a significant delay in ice formation on the substrate protected by the bilayer coating. The development of this protective material, combining superamphiphobicity, self-cleaning, corrosion resistance, and anti-icing functions, will provide advanced technological reserves in areas such as marine, aviation, transportation, and manufacturing.

Enabling Long Cycling with Excellent Structure Stability for High-Nickel Layered Cathodes in Lithium Metal Batteries

Rechargeable Li metal batteries (LMBs) constructed from Li metal and high-Ni NMCs have attracted much attention because of their high energy density in electrical vehicles. The choice of electrolyte plays a crucial role in determining the performance of Li||high-Ni NMC LMBs. In this work, we investigated the electrochemical performance and microstructure change of Li||0.7Nb-NMC 9055 LMBs using ether-based electrolyte (LiFSI–1.2DME–3TTE) and carbonate electrolyte (1 M LiPF6 in EC/DMC). The results show that ether-based electrolyte can significantly enhance the cycling life of Li||0.7Nb-NMC 9055 battery, providing a capacity of 203 mAh/g vs 179 mAh/g (carbonate electrolyte) for 150 cycles without voltage decay at C/5 charge and C/2 discharge rates. The excellent electrochemical performance (capacity and voltage) originates from the perfect structural stability confirmed by the dQ/dV versus V curves, which was further supported by electrochemical impedance spectroscopy test, showing nearly no surface charge transfer resistance increase in ether-based electrolyte, versus continuous growth in carbonate electrolyte.

Preparation and Properties of Nano Platinum/Carbon Nanotubes/Bacterial Cellulose Conductive Hydrogel Electrode Films

Using the natural nanomesh structure of bacterial cellulose (BC), the good conductivity of multi-walled carbon nanotubes (MWCNTs) and the good electrocatalytic activity of nano platinum (PtNPs), a three-way porous conductive hydrogel of PtNPs and BC was designed. The infiltrating and doping of MWCNTs on BC thin films was realized by ultrasonic assisted electrophoretic deposition process, which ensured the dual characteristics of ionic and electronic conduction of electrode films. The adsorption capacity of BC for chloroplatinic acid and the strong reductivity of sodium borohydride were also used to realize the high-load PtNPs recombination on MWCNTs/BC porous layered conductive hydrogel electrode films. By testing the electrochemical performance of PtNPs/MWCNTs/BC conductive hydrogel electrode film and characterization of microstructure, the electrode film not only has high electroactive area, low surface charge transfer resistance and good diffusion permeability and other electrochemical characteristics, but also shows high catalytic activity for glucose in PBS solution. The most important finding is that the electrode film can selectively catalyze glucose in oxygen-rich PBS solution. The porous layered structure of PtNPs/MWCNTs/BC electrode membrane and the high load dispersion of PtNPs are the reasons for the selective catalytic ability of PtNPs/MWCNTs/BC conductive hydrogel electrode membrane, which can be used for the implanted surface glucose fuel cell.

CuBi2O4/CuO heterojunction coated with electrodeposited ZnO overlayer for stable solar hydrogen evolution

CuBi2O4 has received a lot of attention because of its suitable bandgap and positive onset potential. However, the reported photocurrent density based on the CuBi2O4 photocathode is still far less than its theoretical value. Herein, a layer of amorphous ZnO is coated on the surface of CuBi2O4/CuO heterojunction photocathodes by an electrodeposition method. The photocurrent density achieved by CuBi2O4/CuO/ZnO is 2.1 times that of CuBi2O4/CuO, from 0.56 mA cm−2 to 1.18 mA cm−2 at 0.2 VRHE in 0.5 M Na2SO4. Additionally, after 12 h stability testing, the photocurrent retention rate of CuBi2O4/CuO photocathode with a ZnO protective layer can be maintained at 84%, while only 58% photocurrent retention can be obtained with the bare photocathode. The performance improvement can be attributed to the fact that the electrodeposited amorphous ZnO overlayer provides more active sites, reduces the surface charge transfer resistance, and protects the unstable CuBi2O4/CuO photocathode. This work manifests a ZnO overlayer prepared by the electrodeposition approach is one of efficient strategies to fabricate one durable and high-performance CuBi2O4 based photocathodes.

Magnetron Sputtered Al Co-Doped with Zr-Fe2O3 Photoanode with Fortuitous Al2O3 Passivation Layer to Lower the Onset Potential for Photoelectrochemical Solar Water Splitting

In this paper, we investigate the magnetron sputtering deposition of an Al-layer on Zr-doped FeOOH (Zr-FeOOH) samples to fabricate a Zr/Al co-doped Fe2O3 (Al-Zr/HT) photoanode. An Al-layer is deposited onto Zr-FeOOH through magnetron sputtering and the thickness of the Al deposition is regulated by differing the sputtering time. Electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, Mott-Schottky and time-resolved photoluminescence spectra analyses were used to study, in depth, the correlations between sputtered Al-layer thicknesses and PEC characteristics. High-temperature quenching (800 °C) assists in diffusing the Al3+ in the bulk of the Zr-doped Fe2O3 photoanode, whilst an unintended Al2O3 passivation layer forms on the surface. The optimized Al-Zr/HT photoelectrode achieved 0.945 mA/cm2 at 1.0 VRHE, which is 3-fold higher than that of the bare Zr/HT photoanode. The Al2O3 passivation layer causes a 100 mV cathodic shift in the onset potential. Al co-doping improved the donor density, thus reducing the electron transit time. In addition, the passivation effect of the Al2O3 layer ameliorated the surface charge transfer kinetics. The Al2O3 passivation layer suppressed the surface charge transfer resistance, consequently expediting the hole migration from photoanode to electrolyte. We believe that the thickness-controlled Al-layer sputtering approach could be applicable for various metal oxide photoanodes to lower the onset potential.

Band-gap and interface engineering by Ni doping and CoPi deposition of BiVO4 photoanode to boost photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting can efficiently convert solar power into hydrogen energy. Herein, Ni-doped and CoPi-deposited BiVO4 photoanode (Ni:BiVO4/CoPi) is synthesized by electrodeposition-solution drop casting process. The physicochemical characterizations show that Ni partially substitutes the V site, with the remainder being distributed on the sample surface in the form of NiOOH nanoparticles. CoPi is distributed on the BiVO4 surface in the form of amorphous particles. The photocurrent density of Ni:BiVO4/CoPi is 3.2 times more than what is measured of the bare BiVO4 (5.90 mA/cm2 at 1.23 VRHE). Mott-Schottky tests show that Ni doping not only modulates the positions of conduction and valence band for the BiVO4, but that it also increases the carrier density. In comparison with bare BiVO4 (2.54 eV), UV-vis spectra analysis shows that the Ni:BiVO4/CoPi band gap is reduced to 2.43 eV. The narrowed band gap extends light absorption and further facilitates the transport by bulk carriers of the Ni:BiVO4/CoPi. Electrochemical impedance spectroscopy (EIS) results show that the surface charge transfer resistance of the Ni:BiVO4/CoPi (91.49 Ω) is about half the amount of the BiVO4. CoPi, a very effective oxygen evolution co-catalyst, is capable of optimizing the BiVO4 interfacial properties and accelerating injection by surface carriers. This work aims to design a novel Ni:BiVO4/CoPi photoanode through band-gap and interface engineering to boost light absorption, carriers’ transport and water oxidation kinetics for PEC water splitting.

Boosting the photocatalytic hydrogen production performance of graphitic carbon nitride nanosheets by tailoring the cyano groups

Graphitic carbon nitride (g-C3N4) is a promising visible light responsive photocatalyst for solar hydrogen production. However, pristine g-C3N4 suffers from severe charge recombination, resulting in a poor photocatalytic activity. Herein, a facile KOH-assisted sealed heating process is designed to tailor the electronic structure of g-C3N4, leading to a significantly enhanced and stable photocatalytic hydrogen production rate of 225.1 µmol h−1 using only 50 mg of the photocatalyst. An excellent apparent quantum efficiency of 16.82% is achieved at 420 nm. Systematic studies reveal that KOH-assisted sealed heating can generate more cyano groups onto the framework of g-C3N4, which can increase the charge carrier density and reduce the surface charge transfer resistance, promoting charge separation and transfer. The new findings demonstrated in this work provide a facile strategy for the design of low-cost and efficient photocatalyst for solar fuel production.

Essence - A New Electrochemical Platform

There is a great demand for a fast, on-site screening test, especially during the COVID-19 pandemic. However, most point-of-care (POC) sensors usually suffer from insufficient sensitivity and selectivity. Often these devices have a very high rate of false positives and false negatives. The incorrect data could result in low customer usage (both the patient and the doctor) due to decreased confidence in the results. Portable ELISA-based kits can have low rates of false-positive and false negatives. However, ELISA is inherently complicated and often requires a specialist on-site to ensure the success of each sample's high costs and time.Here we show a new electrochemical platform called ESSENCE. ESSENCE is an Electrochemical Sensor that uses a Shear-Enhanced, flow-through Nanoporous Capacitive Electrode. ESSENCE can overcome current electrochemical sensors' selectivity and sensitivity limitations. The ESSENCE sensor architecture is a microfluidic channel sandwiched between two gold micro-electrode glass slides, formed a non-planar interdigitated porous electrode that improved electric field penetration compared with the traditional planar interdigitated electrode. The porous electrode structure used in this research has a high Zeta potential compared to the bare glass surface. This results in a significantly smaller Debye double-layer (EDL) length than in a clear channel. Due to a smaller EDL length, the relaxation frequency increases, which shifts the EIS signal to a higher frequency range (1kHz to 100MHz) and results in a fast response with a higher signal-to-noise ratio in Electrochemical Impedance Spectroscopy(EIS) measurements.Further, the modular fabrication of the ESSENCE chip allows ESSENCE to target different biomolecules from DNAs to protein cancer biomarkers by simply changing the packed transducer material. However, the packing material’s conductivity represents a significant challenge to the electrochemical response of ESSENCE. The high conductivity of the transducer material shorts the system leading to a pure resistive response. Impedance spectroscopy (EIS) response of ESSENCE cannot be distinguished enough to determine if the sensor is working. ESSENCE has a switchable electrode system for usage in multiple configurations like a material electrode, clear electrode, and working electrode to overcome these limitations. For carbon nanotube (CNT) transducers with functionalized oligos on the CNT, a 20-fold jump in the signal is noticed from the working electrode configuration compared to the other configurations. It is worthwhile to note that depending on the biomolecule (DNA or Protein), the surface charge transfer resistance (Rct) in the EIS signal of ESSENCE can either show a decrease (DNA) or increase (protein) on the binding of the target bio-molecule (target DNA to DNA or target protein to protein). The change in the Rct depends on the charge of the molecule. It has been seen that on the binding of a target protein, the active area of the electrode surface decreased, leading to a reduction in current and a concomitant increase in Rct. On binding of the target DNA, the capture-target DNA pair essentially becomes a part of the electrode due to its charge. This increase in charge is more significant than any reduction in the surface area due to the DNA's binding. This leads to an increase in current to the packed electrode, leading to a concomitant decrease in Rct.The fluid flow through the porous electrode increases mixing, minimizes diffusion, and generates a significantly enhanced fluidic shear force. This force e can unbind most of the false-positive signals generated by non-specific binding, such as physical adhesive or biofouling. The porous electrode also provides a high surface area to volume ratio to capture the target analyte leading to increased sensitivity. We have an automatic fluidic control system in ESSENCE that minimized any errors in the operation of ESSENCE. Our system has high selectivity and sensitivity for DNA (fM sensitivity, selective against non-target DNA), breast cancer biomarker proteins (p53, pg/L sensitivity, selective against non-target HER2). Figure 1

Self-doping of Nb2O5NC by cathodic polarization for enhanced conductivity properties and photoelectrocatalytic performance

A simple novel electrochemical reduction approach was developed for the self-doping of Nb4+ in niobium oxide nanochannels (Nb2O5NC), changing the conductivity, optical properties, and photocatalytic properties of the material. Nb2O5NC was synthesized using different electrolytes: 0.4 wt% HF in 1 M H2SO4 (EI), 0.4 M NH4F in glycerol (EII), and 0.25 g NH4F with 4 vol% water in glycol at 50 °C (EIII). Field emission scanning electron microscopy (FEG-SEM) analysis showed well-organized arrays of Nb2O5 nanochannels produced on Nb foil, with varying tube diameters in the order EII < EI < EIII and film thickness in the order EI < EII < EIII, which drastically affected the photocurrent vs. potential curves. In order to self-dope the Nb2O5, the samples were electrochemically reduced in 0.1 M KH2PO4 buffer solution (pH 10) for 5 min, at −2.5 V vs. Ag/AgCl, resulting in the doped samples denoted P-EI, P-EII, and P-EIII. The results showed that reduction of Nb5+ to Nb4+ occurred for all the Nb2O5NC samples, leading to decreased surface charge transfer resistance between the Nb2O5NC and the electrolyte, as well as increases of the charge carrier density and photocurrent for all the self-doped samples, compared to undoped samples. Sample P-EI was also tested for the degradation of reactive red 120 (RR120) dye, achieving efficient photoelectrocatalytic degradation of a 10 mg L−1 dye solution. These results reveal that the self-doping approach can enhance the photoelectrocatalytic properties of Nb2O5 photoanode, offering an alternative way for the removal of reactive dyes.

One‐step surface nitridation of CoO for high‐energy‐density lithium‐ion batteries

The surface of micrometer-sized CoO particles is finely tuned by a nitride produced through one-step process with various heat treatments with solid-state urea, which has been frequently used as a surfactant for the nanosynthesis of CoO in aqueous media. After conducting simple surface nitridation processes without a solvent using an autogenic reactor, it is found that the CoO surface is clearly modified by a cobalt-rich nitride (Co5.47N) by X-ray diffraction analyses. As the heat treatment temperature increases, the content of the cobalt nitride also grows. A temperature above 500°C for the surface modification causes an initial specific capacity loss because of the growth of the inert cobalt nitride; however, the surface-modified samples exhibit a more prolonged cycle life than the pristine CoO due to decreased kinetic barrier and enhanced surface stability. Additionally, the surface nitridation leads to a high rate capability, which can solve poor power density for conversion-type anode materials. It is found that the surface nitridation allows a high electrical conductivity and reduces the surface charge transfer resistance.

Valence-engineered MoNi4/MoOx@NF as a Bi-functional electrocatalyst compelling for urea-assisted water splitting reaction

Valence engineering was initially studied to achieve advanced bi-functional electrocatalysts for optimum performance. For this purpose, TMoO4 nanorods (T = Ni, Mn, Cu, and Zn) were grown on nickel foam, which upon reduction treatments, transformed into a composite system MoNi4/MoOx@NF with alloy nanoparticles MoNi4 anchored on MoOx at 3D nickel foam. This composite system was demonstrated to show distinct mixed valence features of Mo0/Mo2+/Mo4+/Mo5+/Mo6+. When taken as a bifunctional electrocatalyst, this composite gives a lower surface charge transfer resistance, a lower Tafel slope, and an excellent electro-catalytic performance with 0.01 V for hydrogen evolution reaction and 1.29 V for urea oxidation reaction vs. reversible hydrogen electrode (RHE) at 10 mA cm−2. Valence engineering of Mo species achieved by tuning the reduction conditions makes the electron density of Mo improved and Tafel-Heyrovsky step significantly increased. Under a reduction atmosphere for 2 h, the composite shows an optimum performance as represented by a much better electron conduction efficiency, a faster rate of Heyrovsky step, and a long-term stability at 10 mA cm−2.

Photoelectrochemical performance of W-doped BiVO4 photoanode

One of the crucial challenges in enhancing the photoelectrochemical (PEC) water splitting performance of BiVO4 photoanode is to improve the charge separation and transfer efficiency. Therefore, in this paper, a novel multilayer gradient W-doped BiVO4 photoanode is fabricated for improved performances in solar water splitting. Firstly, different amounts of W mono-doped BiVO4 photoanodes are prepared, and the W (5%)-doped BiVO4 photoanode reaches highest photocurrent density of 0.61 mA cm−2 at 1.23 V versus reversible hydrogen electrode (RHE). Compared with the photocurrent density of the pure BiVO4 photoanode (0.28 mA cm−2), the enhancement can be attributed to the doped W which acts as an electron donor that could reduce the surface charge transfer resistance and facilitate charge transfer. Furthermore, multilayer gradient W-doped BiVO4 photoanodes are prepared to enhance PEC performances. The BVO-530 achieves a photocurrent density of 1.17 mA cm−2 at 1.23 V versus RHE due to the multilayer gradient structure which forms a diffusion path for electron–holes caused by the gradual increase in the Fermi level. The mechanisms of multilayer gradient W-doped BiVO4 photoanodes are discussed in detail based on PEC measurements. This work provides a new strategy for designing and fabricating photoanode systems to enhance the charge separation and transport for efficient water splitting.

In situ growth of ultrathin Co-MOF nanosheets on α-Fe2O3 hematite nanorods for efficient photoelectrochemical water oxidation

Efficient charge transport is an important factor in photoelectrochemical (PEC) water splitting. The charge transfer at the semiconductor/electrolyte interface is of great importance, especially for the complex water oxidation reaction. In this study, we explored the feasibility of improving charge transfer efficiency at the interface of semiconductor/electrolyte by in situ growth of Co based Metal-Organic Frame work (Co-MOF) through a facile ion-exchanging method. Under optimized conditions, the Co-MOF nanosheet-modified hematite gave a photocurrent density of 2.0 mA cm−2 (200% improvement) at 1.23 VRHE with a cathodic shift of 180 mV in the photocurrent onset potential, in comparison to bare α-Fe2O3 (0.71 mA cm−2@1.23 VRHE). To elucidate the role of Co-MOF, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and Mott-Schottky measurements were carried out. It was found that the atomically distributed Co2+ in Co-MOF possessed excellent hole storage capability and charge transfer efficiency, as evidenced by the high surface capacitance and extremely low surface charge transfer resistance.

Clarifying the Roles of Oxygen Vacancy in W-Doped BiVO4 for Solar Water Splitting

Most oxide semiconductor photoanode materials for water splitting are synthesized in ambient environment. Oxygen vacancy exists in these samples making them intrinsically n-type at the as-synthesized state. Oxygen vacancy has been widely reported for enhancing the performance of a photoanode by improving the electron conductivity. Besides the effect on the bulk materials properties, oxygen vacancy also plays an important role in the interfacial charge transfer to electrolyte, on which much less attention has been paid in the past. Herein, we found that although air-annealed W-doped BiVO4 has a higher electron density, lower surface charge transfer resistance, and a slightly better light absorption than the O2-annealed sample, the latter displays a higher photocurrent density. Experimentally we found that the enhanced performance comes from a better charge separation efficiency, despite that the presence of oxygen vacancy does lead to a better charge transfer efficiency. Theoretical calculation finds that ...

First report of vertically aligned (Sn,Ir)O2:F solid solution nanotubes: Highly efficient and robust oxygen evolution electrocatalysts for proton exchange membrane based water electrolysis

One dimensional (1D) vertically aligned nanotubes (VANTs) of (Sn0.8Ir0.2)O2:10F are synthesized for the first time by a sacrificial template assisted approach. The aim is to enhance the electrocatalytic activity of F doped (Sn,Ir)O2 solid solution electrocatalyst for oxygen evolution reaction (OER) in proton exchange membrane (PEM) based water electrolysis by generating (Sn0.8Ir0.2)O2:10F nanotubes (NTs). The 1D vertical channels and the high electrochemically active surface area (ECSA ∼38.46 m2g-1) provide for facile electron transport. This results in low surface charge transfer resistance (4.2 Ω cm2), low Tafel slope (58.8 mV dec−1) and excellent electrochemical OER performance with ∼2.3 and ∼2.6 fold higher electrocatalytic activity than 2D thin films of (Sn0.8Ir0.2)O2:10F and benchmark IrO2 electrocatalysts, respectively. Furthermore, (Sn0.8Ir0.2)O2:10F NTs exhibit excellent mass activity (21.67 A g−1), specific activity (0.0056 mAcm−2) and TOF (0.016 s−1), which is ∼2–2.6 fold higher than thin film electrocatalysts at an overpotential of 270 mV, with a total mass loading of 0.3 mg cm−2. In addition, (Sn0.8Ir0.2)O2:10F NTs demonstrate remarkable electrochemical durability - comparable to thin films of (Sn0.8Ir0.2)O2:10F and pure IrO2, operated under identical testing conditions in PEM water electrolysis. These results therefore indicate promise of (Sn0.8Ir0.2)O2:10F NTs as OER electrocatalysts for efficient and sustainable hydrogen production.

An investigation on the role of W doping in BiVO4 photoanodes used for solar water splitting.

n-Type doping has been widely employed to enhance the performance of a photoanode in a photoelectrochemical cell used for water splitting. However, little is known about how doping affects the catalytic activity during surface catalysis. Herein, we took BiVO4 as an example to investigate the effect of doping on surface catalysis from both experimental and theoretical perspectives. To enable an impartial comparison, we have prepared planar BiVO4 thin films with and without W doping. It was found that W doping had no obvious effect on the morphology, crystallinity, and light absorption of the film; however, the photocurrent was significantly enhanced upon W doping. This enhancement is contributed by two important factors: better charge separation efficiency and improved surface charge transfer efficiency. Electrochemical analysis reveals that W doping lowers the surface charge transfer resistance and forms active surface states, facilitating charge transfer. Theoretical analysis shows that W doping activates the V atoms to be reactive sites. Moreover, the adsorption energies and distance between adsorption species (OHads, Oads, and OOHads) involved in the water splitting process become more favorable for surface charge transfer. The current study provides an insight into the roles of W doping in BiVO4, especially during surface catalysis.

Effects of rare-earth element additions to Laves phase-related body-centered-cubic solid solution metal hydride alloys: Thermodynamic and electrochemical properties

The influences of rare-earth element additions on the structural, gaseous phase hydrogen storage, and electrochemical properties of Laves phase-related body centered cubic (BCC) metal hydride alloys were studied using a series of alloys with the design composition of Ti14.6Zr2.1V44.1Cr12.2Mn6.9Fe2.7Co1.4Ni14.7Al0.3RE1.0, where RE (rare earth) = Y, La, Ce, Nd. Samples were prepared by induction melting and then annealed, and only 0.2 to 0.3 at% of the rare-earth element was included in the final samples in the elemental form, which varies from the RENi phase formed in the C14-predominated metal hydride alloy. The Al-content in the final alloy is about 1 at% because of a small reaction to the crucible material. The beneficial effects of rare-earth element addition on the low-temperature performance of the C14-predominated alloy was not observed in this group of BCC/C14 alloys. The gaseous phase and electrochemical hydrogen characteristics were influenced by the rare-earth elements addition mainly through changes in both compositions and abundances of the constituent phase. Y increased reversibility of gaseous phase hydrogen storage. La raised plateau pressure, decreased discharge capacity, and hindered activation and surface electrochemical reaction. Ce increased BCC phase abundance and discharge capacity with reduced room temperature surface charge-transfer resistance. Nd increased C14 phase abundance, decreased the plateau pressure, increased gaseous phase hydrogen storage capacities, and increased discharge capacity and bulk hydrogen diffusion.

A Ni/MH Pouch Cell with High-Capacity Ni(OH)2

Electrochemical performances of a high-capacity and long life β-α core-shell structured Ni0.84Co0.12Al0.04(OH)2 as the positive electrode active material were tested in a pouch design and compared to those of a standard β-Ni0.91Co0.045Zn0.045(OH)2. The core-shell materials were fabricated with a continuous co-precipitation process, which created an Al-poor core and an Al-rich shell during the nucleation and particle growth stages, respectively. The Al-rich shell became α-Ni(OH)2 after electrical activation and remained intact through the cycling. Pouch cells with the high-capacity β-α core-shell positive electrode material show higher charge acceptances and discharge capacities at 0.1C, 0.2C, 0.5C, and 1C, improved self-discharge performances, and reduced internal and surface charge-transfer resistances, at both room temperature and −10 °C when compared to those with the standard positive electrode material. While the high capacity of the core-shell material can be attributed to the α phase with a multi-electron transfer capability, the improvement in high-rate capability (lower resistance) is caused by the unique surface morphology and abundant interface sites at the β-α grain boundaries. Gravimetric energy densities of pouch cells made with the high-capacity and standard positive materials are 127 and 110 Wh·kg−1, respectively. A further improvement in capacity is expected via the continued optimization of pouch design and the use of high-capacity metal hydride alloy.

Performance Comparison between AB5 and Superlattice Metal Hydride Alloys in Sealed Cells

High-power cylindrical nickel metal/hydride batteries using a misch metal-based Al-free superlattice alloy with a composition of La11.3Pr1.7Nd5.1Mg4.5Ni63.6Co13.6Zr0.2 were fabricated and evaluated against those using a standard AB5 metal hydride alloy. At room temperature, cells made with the superlattice alloy showed a 40% lower internal resistance and a 59% lower surface charge-transfer resistance compared to cells made with the AB5 alloy. At a low temperature (−10 °C), cells made with the superlattice alloy demonstrated an 18% lower internal resistance and a 60% lower surface charge-transfer resistance compared to cells made with the AB5 alloy. Cells made with the superlattice alloy exhibited a better charge retention at −10 °C. A cycle life comparison in a regular cell configuration indicated that the Al-free superlattice alloy contributes to a shorter cycle life as a result of the pulverization from the lattice expansion of the main phase.