Surface Electrochemical Reactions Research Articles

Defect Engineering for Tuning the Photoresponse of Ceria-Based Solid Oxide Photoelectrochemical Cells.

Solid oxide photoelectrochemical cells (SOPECs) with inorganic ion-conducting electrolytes provide an alternative solution for light harvesting and conversion. Exploring potential photoelectrodes for SOPECs and understanding their operation mechanisms are crucial for continuously developing this technology. Here, ceria-based thin films were newly explored as photoelectrodes for SOPEC applications. It was found that the photoresponse of ceria-based thin films can be tuned both by Sm-doping-induced defects and by the heating temperature of SOPECs. The whole process was found to depend on the surface electrochemical redox reactions synergistically with the bulk photoelectric effect. Samarium doping level can selectively switch the open-circuit voltages polarity of SOPECs under illumination, thus shifting the potential of photoelectrodes and changing their photoresponse. The role of defect chemistry engineering in determining such a photoelectrochemical process was discussed. Transient absorption and X-ray photoemission spectroscopies, together with the state-of-the-art in operando X-ray absorption spectroscopy, allowed us to provide a compelling explanation of the experimentally observed switching behavior on the basis of the surface reactions and successive charge balance in the bulk.

Computationally empowered design of emerging earth-abundant electrocatalysts toward electron- /proton-transferring energy conversion

Affordable and sustainable, also often quoted as Pt-free or metal-free, electrocatalysts have a pivotal role in various electrochemical energy conversion systems, which are attracting huge demands in the 21st century. Recent technological development enables us to perform computationally empowered experimental design or discovery of materials constituted of abundant elements (e.g. carbon, nitrogen, boron, etc.) with desirable electrochemical properties. These works indicate that nontraditional candidates, such as insulators being believed to unfavorable, are indeed found to be applicable as active electrocatalysts. This review summarizes state-of-the-art cooperative experimental/theoretical works devoted to understand fundamental aspects of electron- /proton-transferring surface electrochemical reactions from the point of materials. These works are expected to accelerate material research to find out optimal Pt- or metal-free electrocatalysts, even based on material classes which were previously rejected as candidates, toward electrochemical energy conversion devices.

Deep eutectic solvent-assisted in-situ synthesis of nanosheet-packed Ni3S2 porous spheres on Ni foam for high-performance supercapacitors

Herein, self-supported Ni3S2 spherical clusters packed with well-defined nanosheets developed on Ni foam (NF) were rationally fabricated via a novel low-temperature solvothermal sulfurization approach in a choline chloride/ethylene glycol (Ethaline)-based deep eutectic solvent (DES). The DES-based sulfurization process drove an interesting time-dependent surface restructuring and phase transformation that occurred on the Ni substrate, leading to the in-situ formation of a Ni3S2 layer with controllable architecture. Pre-deposition of a Ni interlayer on the NF substrate provides more assessable electrochemical surface area and reaction sites, which favored fast crystal nucleation/growth and structural reconstruction. Benefiting from the integrated design and unique 3D interdigital architecture, the optimized Ni3S2_5/Ni/NF with a sulfurization time of 5 h exhibits a high specific capacitance (specific capacity) of 5,633 mF cm−2 (860.6 μAh cm−2) at a current density of 10 mA cm−2, and maintains 87.7% of initial specific capacitance after 1,000 charge–discharge process at a current density of 20 mA cm−2. This facile DES-driven solvothermal sulfurization strategy for the fabrication of integrated metal sulfides-based electrode materials could be promising for practical applications in high-performance electrochemical devices.

In situ characterizations of photoelectrochemical cells for solar fuels and chemicals

Environmental concerns deriving from fossil fuel dependency are driving an energy transition based on sustainable processes to make fuels and chemicals. Solar hydrogen is the pillar of this new green economy, but the technological readiness level of PV electrolysis and direct photoelectrochemical (PEC) electrolysis are still too low to allow broad commercialization. Direct conversion through PEC technology has more potential in the medium–long term but must be first guided by the scientific enhancements to improve device efficiencies. For this purpose, in situ and operando photoelectrochemistry will guide the discovery of new materials and processes to make solar fuels and chemicals in PEC cells. The use of advanced in situ and operando characterizations under working photoelectrochemical (PEC) conditions is reviewed here and anticipated to be a fundamental tool for achieving a basic understanding of new PEC processes and for enabling the large-scale development of PEC technology by 2050, thus delivering fuels and chemicals having zero (or negative) carbon footprint. Hydrogen from solar water splitting is the most popular solar fuel and can be mainly produced by indirect photovoltaic-driven electrolysis (PV electrolysis) and direct photoelectrochemistry. Although PV electrolysis has already been developed on a level of MW-scale pilot plants, PEC technology, which is much less mature, holds several advantages in the long term over PV-electrolysis systems. The key enabling feature to developing PEC technology is the improvement of the photoelectrode materials which are responsible for the absorption of light, and transport of the photo-generated charge carriers to drive the electrochemical surface reaction. These processes are often complex and multistep, spanning multiple timescales and following the simultaneous detection of photoelectrodes modification and formation of reaction intermediates/products can be achieved using eight well-known characterization techniques here presented.

Carbon decorated Li3V2(PO4)3 for high-rate lithium-ion batteries: Electrochemical performance and charge compensation mechanism

Fast charging and high-power delivering batteries are highly demanded in mobile electronics, electric vehicles and grid energy storage, but there are full of challenges. The star-material Li3V2(PO4)3 is demonstrated as a promising high-rate cathode material meeting the above requirements. Herein, we report the carbon decorated Li3V2(PO4)3 (LVP/C) cathode prepared via a facile method, which displays a remarkable high-rate capability and long-term cycling performance. Briefly, the prepared LVP/C delivers a high discharge capacity of 122 mAh g−1 (~93% of the theoretical capacity) at a high rate up to 20 C and a superior capacity retention of 87.1% after 1000 cycles. Importantly, by applying a combination of X-ray absorption spectroscopy and full-range mapping of resonant inelastic X-ray scattering, we clearly elucidate the structural and chemical evolutions of LVP upon various potentials and cycle numbers. We show unambiguous spectroscopic evidences that the evolution of the hybridization strength between V and O in LVP/C as a consequence of lithiation/delithiation is highly reversible both in the bulk and on the surface during the discharge-charge processes even over extended cycles, which should be responsible for the remarkable electrochemical performance of LVP/C. Our present study provides not only an effective synthesis strategy but also deeper insights into the surface and bulk electrochemical reaction mechanism of LVP, which should be beneficial for the further design of high-performance LVP electrode materials.

Simulation of 3-D lithium dendritic evolution under multiple electrochemical states: A parallel phase field approach

The lithium battery is considered as one of the most reliable energy storage methods. The growth of dendrites is a primary challenge for the lithium battery. To simulate 3-D lithium dendrite evolution, a generic parallel computing library was implemented to solve nonlinear phase field governing equations to realize high computation efficiency. Lithium evolution under multiple electrochemical states, including charging, suspending and discharging is studied. Accordingly, growing, healing and necking of dendrites are observed. The driving force, ion concentration and the electric potential are investigated to understand the influence of surface tension and electrochemical reaction on behaviors of lithium. The source code of the program is available to promote the development of the phase field method in this field.

Functionalization of Polymeric Nanofilter Biointerface for Small Biomarker Sensing

Small biomolecules contained in body fluids are often recognized as indicators of diseases and health conditions. Therefore, the development of biosensors capable of recognizing significant biomarkers from body fluids such as blood, sweat, tears, and saliva, leads to early diagnosis and contribute to the realization of next-generation medical care. In this regard, we have recently developed the potentiometric biosensor based on the extended-gate field-effect transistor (EG-FET) for small-biomolecule recognition. The catalytically active extended-Au electrodes have the capability to detect various small biomolecules with high sensitivity on the basis of the surface electrochemical reaction. However, since the Au electrode may react with various electrochemically active biomolecules, it is necessary to limit the interfering biomolecules from approaching the Au surface in order to enhance target selectivity. Therefore, a suitable polymeric nanofilter was developed on the Au electrode where interfering biomolecules are prevented from penetrating through the nanofilter, and only the target biomolecules can access the Au sensing surface to generate an electrical signal.1,2 The polymeric nanofilter biointerface is composed of two layers. The first layer is an anchor layer directly grafted on the Au surface, which controls the density of the polymeric nanofilter to make enough space for only small biomolecules to penetrate through and prevent large-biomolecule interferences from approaching the Au sensing surface. Then, a polymeric filter layer as the second layer is precisely grafted from the surface of the anchor layer by atom transfer radical polymerization (ATRP). The filter layer selectively captures the small-biomolecule interferences outside the Debye’s length; therefore, the interfering small-biomolecules will not cause the change in Au surface potential. In our previous study, we designed and developed the polymeric nanofilter by using aryldiazonium reduction chemistry and boronate affinity. Aryldiazonium reduction chemistry enabled the grafting of chemically stable multilayer film on the Au surface, having an adequate density in order to suppress the penetration of large biomolecules. Phenylboronic acid was included in the polymeric filter layer to capture the model interfering small biomolecule, L-DOPA, via a reversible PBA/diol boronate affinity binding. Resultantly, the polymeric nanofilter-coated EG-FET biosensor specifically detected the model target biomolecule, L-cysteine. In this study, we designed and developed the second polymeric layer on the polymeric nanofilter to give an additional function. Although the space-controlled nanofilter suppresses the penetration of the large biomolecules, the non-specific adsorption can cause severe fouling of the nanofilter surface. Therefore, in order to improve the anti-fouling function of the nanofilter interface, poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC) was chosen as the second polymeric layer. PMPC is a bio-inspired polymer that has been widely used as a biocompatible, anti-fouling coating material. In order to investigate the anti-fouling function of pMPC coating, we firstly grafted the pMPC layer directly from the anchor layer surface via surface-initiated ATRP. Then, the response of pMPC-nanofilter coated EG-FET biosensor to human-serum albumin (HSA) was compared with the response of the control (unmodified) sensor. Resultantly, the pMPC-coated sensor suppressed the non-specific response up to 60% compared with the control sensor. Therefore, we verified the successful enhancement of the anti-fouling property on the nanofilter surface.Reference(1) Nishitani, S.; Sakata, T. ACS Appl. Mater. Interfaces 2019, 11 (5), 5561–5569.(2) Himori, S.; Nishitani, S.; Sakata, T. Langmuir 2019, 35 (10), 3701–3709.

The Influence of Surface Stress on the Chemo-Mechanical Behavior of Inverse-Opal-Structured Electrodes for Lithium-Ion Batteries

Inverse opals (IO) are three-dimensional ordered porous microstructures with a large specific surface area and high mechanical stability. They exhibit nanoscale geometric features, where surface stresses gain an appreciable impact on the elastic behavior and electrochemical surface reactions. With this study, we aim to gain an understanding of the influence of an IO cathode’s geometry on its chemo-mechanical behavior. We are particularly interested in the impact of the IO’s pore radius on the mechanical stresses, charge kinetics, and the magnitude of capacity losses. To that end, we performed a Finite Element study considering stress-coupled diffusion, mechanically modulated surface reactions, and surface-stress-induced bulk stresses. An inhomogeneous pressure develops in the polyconcave electrode structure, effecting a local reduction of electrode overpotential. This leads to size-dependent losses in the accessible capacity of the electrode material. Its high surface-to-volume ratio, on the other hand, results in significantly enhanced insertion/extraction rates. With decreasing pore size, we observe both faster insertion and a reduction in the achievable lithiation. An optimal electrode pore radius can thus be determined from balancing the requirements of high charge rate against the surface-stress-induced losses in the accessible capacity.

Integrated 3D foam-like porous Ni3S2 as improved deposition support for high-performance Li–O2 battery

The development of high efficiency air cathode plays important role in regulating the surface electrochemical process to enhance the performance of Li–O2 battery. Herein, we synthesize an integrated porous Ni3S2 on 3D Ni foam substrate through a facile MOF-derived route, the integrated 3D foam-like porous Ni3S2 catalyst can functional as both current collector and catalyst layer. Benefiting from the unique porous structure and the uniform regulate role of the surface electrochemical reaction, the integrated 3D foam-like catalyst cathode delivers nearly 3 times higher discharge capacity and longer cycling performance than that of the pristine Ni foam. More importantly, the morphology of the formed discharge products exhibits lamellar-like structure and has large radial dimension, the Ni3S2 catalyst layer also plays the role of inducing the formation of LiO2 and decreasing the charge overpotential. This study opens a different and facile horizon to fabricate high-efficient air cathode.

Photoelectrochemical Pyramid Texturization of n-Type Ge (100) Probed By Integrated Electrochemical Chamber/X-Ray Photoelectron Spectroscopy

Recently, Germanium (Ge) has gained a massive attention from different research fields like energy1-2, photonics3-4, biomedicine5, and most importantly, nanoelectronics6. Historically, Ge was the very first semiconductor utilized as a key material for transistors by John Bardeen, William B. Shockley and Walter H. Brattain but was abandoned because of its low purity, narrow band gap and chemically unstable oxide.7 From being left behind, after silicon (Si) found its way to fame, Ge is now enjoying resurgence in the world of CMOS devices.8 Due to the extreme size scaling of Si-based complementary metal-oxide semiconductor (CMOS) devices, performance enhancement can no longer be achieved due to intrinsic mobility issues, which can be compensated by the higher carrier mobility of Ge.9 In this context, Ge is a promising candidate to replace Si. Hence, central understanding in Ge surface and interface chemistry is currently indispensable for the development of novel nanodevice applications. In this work, we report the very first formation of Ge pyramids through electrochemical etching and its surface chemistry in HCl solutions under applied potential conditions. As a starting point, voltammetric (j-V) measurements were performed for a large concentration range of the acid (Figure 1a). In all cases in the plateau region (at 0.8V), a linear increase in photocurrent density with increasing light intensity was measured (Figure 1a inset). These results are attributed to a high oxide solubility and, consequently, low oxide coverage (supported by XPS results). No characteristic oxide passivation is observed during anodic etching of n-type Ge, even in the case of a very dilute 0.05 M HCl solution and current densities as high as ~6.0 mA cm-2. Morphological studies (AFM and SEM) revealed a striking and unexpected anisotropy in the surface chemistry of etching for high HCl concentrations, evidenced by the formation of random pyramids with characteristic (111) facets (Figure 1b). For Si, patterning can be used to further lower reflectance and hence improve light coupling effects. In order to scrutinize if similar effects can be observed for Ge, we fabricated a honeycomb template with circular openings of 1 mm to expose the Ge via photolithography. After photoanodic etching of the patterned Ge sample at 0.8 V in 8 M HCl solution, highly ordered pyramidal structures were obtained which lowered the reflectance further by 6% as presented in Figure 1c. This shows that light coupling effects are very likely vital. Integrated electrochemical etching chamber connected to X-ray photoelectron spectroscopy (XPS) instrument as displayed in Figure 1d, which excludes the effect of O2 and H2O vapor from the atmosphere, was utilized. Core-level Ge3d spectra suggest that the process of pyramid texturization is induced by the presence of Ge-Cl bonds on the Ge (100) surface during photoanodic dissolution (Figure 1e). A strong decrease in Ge-OH upon increasing the HCl concentration from 1 M to 8 M is accompanied by an increase in chemically bonded Cl, supported by the chemical shift of 0.6 eV. As the Cl content is similar based in Cl 2p (data not shown), it is very likely that for lower HCl concentration, with less Ge-Cl, physisorption of the halide ion is more important. While, at high HCl concentration, chemisorption governs. Based on these results, we propose surface electrochemical reaction schemes that relate the observed anisotropy in etching to surface chemistry. We believe that these findings are of fundamental interest and of technological importance for new developments in the fields of energy, photonics, biomedicine, and nanoelectronics.

ZnSnO3 Nanoparticle-Based Piezocatalysts for Ultrasound-Assisted Degradation of Organic Pollutants

Piezocatalysis is a promising alternative of photocatalysis where the strained state of a piezoelectric material is exploited for electrochemical surface reactions under dark conditions. Among vari...

Electrochemistry-enabled atomic layer deposition of copper: Investigation of the deposit growth rate and roughness

Electrochemical atomic layer deposition (e-ALD) presents a low-cost, environmentally friendly and scalable approach for the atomically precise fabrication of semiconductor interconnect materials. In the present work, growth rate and roughness evolution during e-ALD of copper (Cu) were analyzed using anodic stripping coulometry, atomic force microscopy, and diffusion-reaction modeling. Through considerations of the unsteady-state diffusional transport of species and their associated surface electrochemical reactions, a semianalytical e-ALD process model is developed. The model enables quantification of the e-ALD growth rate and the deposit surface roughness as a function of various e-ALD process parameters, i.e., electrolyte composition and deposition time. Model predictions are compared with experimental data and good agreement is noted. The model is used to develop guidelines for the selection of optimal process parameters that enable e-ALD of Cu with minimal roughness evolution and a deposit growth rate close to one monolayer-per-cycle.

Rational Design of Co(II) Dominant and Oxygen Vacancy Defective CuCo2O4@CQDs Hollow Spheres for Enhanced Overall Water Splitting and Supercapacitor Performance.

The hierarchical CuCo2O4@carbon quantum dots (CQDs) hollow microspheres constructed by 1D porous nanowires have been successfully prepared through a simple CQDs-induced hydrothermal self-assembly technique. XPS analysis shows the CuCo2O4@CQDs possesses the Co(II)-rich surface associated with the oxygen vacancies, which can effectively boost the Faradaic reactions and oxygen evolution reaction (OER) activity. For example, the as-synthesized 3D porous CuCo2O4@CQDs electrode exhibits high activity toward overall electrochemical water splitting, for example, an overpotential of 290 mV for OER and 331 mV for hydrogen evolution reaction (HER) in alkaline media have been achieved at 10 mA cm-2, respectively. Furthermore, an asymmetric supercapacitor (ASC) (CuCo2O4@CQDs//CNTs) delivers a high energy density of 45.9 Wh kg-1 at 763.4 W kg-1, as well as good cycling ability. The synergy of Co(II)-rich surface, oxygen vacancies, and well-defined 3D hollow structures facilitates the subsequent surface electrochemical reactions. This work presents a facile method to fabricate energetic nanocomposites with highly reactive, durable, and universal functionalities.

Reaction-diffusion-stress coupling model for Li-ion batteries: The role of surface effects on electrochemical performance

Recent years, nanostructured design for battery electrodes has emerged as a promising solution to improve rate capability and reliability of the high-capacity electrodes. However, the surface effects arising from small feature size may affect the multi-physical features of electrodes subject to non-equilibrium electrochemical cycles. In this study, a reaction-diffusion-stress coupling model is developed with the contribution of surface effects, which can capture the interactions between surface electrode reaction, diffusion and solid deformation in a nanoparticle of an electrode under non-equilibrium cyclic charge and discharge. Numerical results show that compressive stress at the surface of an electrode particle during insertion inhibits lithium absorption, while tensile stress at the surface of an electrode particle due to ion extraction suppresses lithium desorption with regard to surface electrochemical reactions. When surface mechanics is taken account into nanoscale electrode study, surface tension is found to be responsible for the increase in mechanical stability, as well as the significant reduction in charge capacity during both the galvanostatic and potentiostatic operation in the meantime. By further introducing tunable surface modulus effects, it is shown that the positive surface modulus aggravates charge capacity loss, whereas the negative surface modulus remedies it for a given potential. The results of the study provide a new idea for nano-surface engineering to optimize the design of electrode to meet required durability and electrochemical performance.

Understanding the Electrochemical Mechanisms Induced by Gradient Mg2+ Distribution of Na-Rich Na3+xV2–xMgx(PO4)3/C for Sodium Ion Batteries

Metal-ion doping can improve the electrochemical performance of Na3V2(PO4)3. However, the reason for the enhanced electrochemical performance and the effects of cation doping on the structure of Na3V2(PO4)3 have yet been probed. Herein, Mg2+ is doped into Na3V2(PO4)3/C according to the first-principles calculation. The results indicate that Mg2+ prefers to reside in the V site and an extra electrochemical active Na+ is introduced to the Na3V2(PO4)3/C crystal to maintain the charge balance. The distribution of Mg2+ in the particle of Na3V2(PO4)3/C is further studied by electrochemical impedance spectroscopy. We find that the highest distribution of Mg2+ on the surface of the particles leads to facile surface electrochemical reactions for Mg2+-doped samples, especially at high rates.

Fabrication of in operando, self-growing, core-shell solid electrolyte interphase on LiFePO4 electrodes for preventing undesirable high-temperature effects in Li-ion batteries

Electric vehicles (EVs) have attracted attention all over the world because of their advantages, such as energy conservation and carbon emission reduction. Li-ion batteries (LIBs) for energy storage in EVs are used because of their high energy density. LiFePO4 is one of the most preferred cathode materials for EVs because of several advantages, such as high specific energy, long life cycle, environmental safety, and low cost. However, at high temperatures, LiFePO4 devices exhibit iron dissolution, which limits the use of LiFePO4 in EVs. This study fabricates an in operando, self-growing, solid electrolyte interphase (SEI) on the LiFePO4 electrode by using an atmospheric pressure plasma jet (APPJ) to prevent iron dissolution and extend its life cycle. The APPJ treatment results in the plasma-induced grafting of hydrophilic functional groups on the LiFePO4 surface, which are used for the in operando synthesis of an amorphous a-FePO4·H2O thin layer on the surface and subsequent electrochemical reaction with ethylene carbonate to form a unique core-shell SEI layer. Contact angle measurements and optical emission and Raman spectroscopy analyses reveal the surface characteristics of LiFePO4 after the APPJ treatment. TEM images confirm that the SEI was formed by an amorphous layer with a thickness of 2.73 nm. High-temperature (60 °C) testing reveals that the SEI considerably improved the cycle performance by suppressing iron dissolution. The present study fabricates an in operando, self-growing, core-shell SEI on a LiFePO4 electrode to improve the high-temperature performance of LIBs.

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.

Identifying “Optimal” Electrocatalysts: Impact of Operating Potential and Charge Transfer Model

Understanding the influence of potential on electrochemical surface reaction kinetics remains a challenge in identifying catalytic materials for numerous important reactions including water splitting (OER), hydrogen evolution (HER), and CO2 reduction, among others. Limitations in computational methods, complicated by the unique environment of the electrode–electrolyte interface, have compelled many studies to focus on the thermodynamics of reaction schemes and to generalize inferences about the kinetics of charge transfer. In instances where activation barrier estimates are available, they are typically assumed to follow the empirical Butler–Volmer (BV) model. In this Perspective, we illustrate that the relative magnitudes and potential-dependences of elementary barriers can have a marked effect on the properties of a catalyst deemed “optimal” for a given reaction. We use a simple pseudosteady-state analysis of two sequential surface-mediated charge transfers to assess the degree of rate control of each s...

Increase in the Surface Catalytic Ability by Addition of Palladium in C14 Metal Hydride Alloy

A combination of analytic tools and electrochemical testing was employed to study the contributions of Palladium (Pd) in a Zr-based AB2 metal hydride alloy (Ti12Zr22.8V10 Cr7.5Mn8.1Co7Ni32.2Al0.4). Pd enters the A-site of both the C14 and C15 Laves phases and shrinks the unit cell volumes, which results in a decrease of both gaseous phase and electrochemical hydrogen storage capacities. On the other hand, the addition of Pd benefits both the bulk transport of hydrogen and the surface electrochemical reaction. Improvements in high-rate dischargeability and low-temperature performances are solely due to an increase in surface catalytic ability. Addition of Pd also decreases the surface reactive area, but such properties can be mediated through incorporation of additional modifications with rare earth elements. A review of Pd-addition to other hydrogen storage materials is also included.

Synthesis of α-Bi2Mo3O12/TiO2 Nanotube Arrays for Photoelectrochemical COD Detection Application.

One-dimensional anodic TiO2 nanotube arrays hold great potential as a photoelectrochemical sensor for the determination of chemical oxygen demand (COD). In this work, we report a warm synthesis of modified TiO2 nanotube arrays with enhanced photoelectrochemical determination performance. Herein, a bismuth-based semiconductor (α-Bi2Mo3O12) was introduced into TiO2 nanotube arrays by sequential chemical bath deposition (CBD) at room temperature. Field-emission scanning electron microscopy, X-ray diffraction, Raman, and X-ray photoelectron spectroscopy were used to investigate the morphologies, structures, and elemental analysis of the products. The photoelectrochemical properties of TiO2 and α-Bi2Mo3O12/TiO2 NTAs were measured by amperometry and cyclic votammetry methods. The α-Bi2Mo3O12/TiO2 nanotube arrays decrease the background photocurrent and increase the current response to organics at the same time, both of which are beneficial to enhancing the photoelectrochemical detection performance. The optimized α-Bi2Mo3O12/TiO2 NTAs with enhanced photoelectrochemical detection performance can achieve a detection sensitivity of 2.05 μA·cm-2/(mg·L-1) and a COD detection range of 0.366-208.9 mg/L respectively. With the α-Bi2Mo3O12 modification, the surface electrochemical reactions of TiO2 NTAs were regulated, the mechanisms of which were also further studied.