Heterogeneous Electrocatalysts for Efficient Water Oxidation Derived from Metal Phthalocyanine

An efficient and stable iodine-doped nickel hydroxide electrocatalyst for water oxidation: synthesis, electrochemical performance, and stability

The design of oxygen evolution reaction (OER) catalysts with higher stability and activity by economical and convenient methods is considered particularly important for the energy conversion technology. Herein, a simple hydrothermal method was adopted for the synthesis of iodine-doped nickel hydroxide nanoparticles and their OER performance was explored. The electrocatalysts were structurally characterized by powder X-ray diffraction analysis (P-XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), and BET analysis. The electrochemical performance of the electrocatalysts was assessed by cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy. The abundant catalytic active sites, oxygen vacancies, low charge-transfer resistance, and a high pore diameter to pore size ratio of iodine-doped Ni(OH)₂ were responsible for its excellent catalytic activity, whereby OER was initiated even at 1.52 V (vs. RHE) and a 330 mV overpotential was needed to reach a 40 mV cm⁻² current density in 1 M KOH solution. The material also exhibited a low Tafel slope (46 mV dec⁻¹), which suggests faster charge-transfer kinetics as compared to its counterparts tested under the same electrochemical environment. It is worth noting that this facile and effective approach suggests a new way for the fabrication of metal hydroxides rich in oxygen vacancies, thus with the potential to boost the electrochemical performance of energy-related systems.

Oxygen evolution reaction mechanism under alkaline conditions over the iodine-doped Ni(OH)₂ surface.

Spray-Coated Thin-Film Ni-Oxide Nanoflakes as Single Electrocatalysts for Oxygen Evolution and Hydrogen Generation from Water Splitting

Electrochemical water splitting is a key process in many electrochemical energy conversion and storage phenomena. Simple synthesis methods to make highly porous and active nanostructured catalytic materials with large electroactive surface areas are very important to implement water-to-fuel conversion schemes. Herein, ultrafine, transparent thin-film nickel-oxide (NiO x ) nanoflakes are facilely synthesized following a simple spray-coating method from a solution-phase precursor. The NiO x nanoscale structures are grown on the FTO surface in the form of highly uniform smooth thin films. They are employed as promising bifunctional electrocatalysts for the overall water splitting process under alkaline conditions. During water oxidation catalysis, NiO x -SC/FTO initiates the oxygen evolution reaction (OER) at an overpotential η of just 250 mV while generating current decade at just 300 mV and demonstrates well-balanced kinetics toward OER. 10 mA cm-2 current density remains persistent for many hours of continuous electrolysis at just 1.53 VRHE illustrating high robustness of the system. The catalyst also showed substantial activity and durability toward the hydrogen evolution reaction (HER) under the same electrochemical conditions. Tafel slopes of just 57 and 89 mV dec-1 for OER and HER in 0.5 M aqueous KOH solution, respectively, showing high intrinsic kinetics for electrocatalysis. Having high electrochemical surface area and an optimum number of electrochemically active sites, these transparent NiO x thin films can be advantageously combined with photoelectrochemical devices for light-driven water-to-fuel conversion systems.

Cobalt Colloid-derived Efficient and Durable Nanoscale Electrocatalytic Films for High-Activity Water Oxidation

Oxygen evolution reaction is of immense importance and is vitally necessary for devices such as electrolyzers, fuel cells, and other solar and chemical energy conversion devices. The major challenges that remain in this quest are due to the lack of effective catalytic assemblages operating with optimum efficiency and obtainable following much simpler setups and easily accessible methods. Here, we demonstrate that the robust electrocatalytic activity toward water oxidation can be achieved employing straightforwardly obtainable nanoscale electrocatalysts derived from easily made colloidal-cobalt nanoparticles (Co-CNPs) prepared in clean carbonate systems. Thin-film non-noble metal nanoscale electrocatalysts such as simple Co-CNPs/FTO and annealed Co-CNPs/FTO250 and Co-CNPs/FTO500 obtained by depositing Co-CNPs on the FTO substrate are shown to initiate water oxidation at much lower overpotentials such as just 240 mV for Co-CNPs/FTO250 under mildly alkaline conditions while demonstrating an impressive Tafel slope of just 40 mV dec-1. Furthermore, the robust catalyst demonstrated a high electrochemical surface area of 91 cm2 and high turnover frequency and mass activity of 0.26 s-1 and 18.84 mA mg-1, respectively, just at 0.35 V, and superior durability during long-term electrolysis. These outstanding catalytic outcomes using easily prepared Co-CNPs/FTO250-type catalytic systems are comparable and even better than other noble and non-noble metal-based nanoscale catalytic assemblages obtained by much difficult methods. Most advantageously, the colloidal route also offers the easiest approach of incorporating carbon contents in the catalytic layer, which can ultimately increase mechanical stability and mass transfer capability of the system.

Surface-assembled non-noble metal nanoscale Ni-colloidal thin-films as efficient electrocatalysts for water oxidation

A highly operative and inexpensive water oxidation scheme using an efficient nanoscale electrocatalyst is vastly demanded for optimum H2 production, CO2 reduction, and has attracted increased attention for chemical energy conversion. We present here a simple route to make efficient electrocatalytic colloidal nanoparticles of nickel out of mere metal ions in a simple borate buffer system. The simple and annealed Ni-colloidal nanoparticles (Ni-CNPs) resulted in a facile transformation into ultrafine films, which further activated the catalysts, while initiating OER just at the overpotential η = 250 mV (1.48 V vs. RHE) under benign conditions. They also showed high porosity and favorable kinetics while displaying impressive Tafel slopes of just 51 mV dec-1, and a high TOF value of 0.79 s-1 at 0.35 V was observed for Ni-CNPs/FTO500. These electrocatalysts also showed long-term stability during the bulk water electrolysis experiment conducted for a continuous 20 hours without notable catalytic degradation, which ensures their economic benefits. The electrochemical data, CVs, kinetic study, short-term durability, extended catalytic stability, SEM analysis, and other supporting data provide compelling evidence that these non-precious, metal-based, electroactive, catalytic, colloidal thin-films (simple and annealed) with nanoscale morphological attributes presented promising catalytic performance under the conditions used herein.

Highly Sensitive and Selective Detection of Arsenic Using Electrogenerated Nanotextured Gold Assemblage

Arsenic is considered as a toxic heavy metal which is highly detrimental to ecological systems, and long-term exposure to it is highly dangerous to life as it can cause serious health effects. Timely detection of traces of active arsenic (As³⁺) is very crucial, and the development of simple, cost-effective methods is imperative to address the presence of arsenic in water and food chain. Herein, we present an extensive study on chemical-free electrogenerated nanotextured gold assemblage for the detection of ultralow levels of As³⁺ in water up to 0.08 ppb concentration. The gold nanotextured electrode (Au/GNE) is developed on simple Au foil via electrochemical oxidation-reduction sweeps in a metal-ion-free electrolyte solution. The ultrafine nanoscale morphological attributes of Au/GNE substrate are studied by scanning electron microscopy. Square wave anodic stripping voltammetry (ASV) response for different concentrations of arsenites is determined and directly correlated with As³⁺ detection regarding the type of electrolyte solution, deposition potential, and deposition time. The average of three standard curves are linear from 0.1 ppb up to 9 ppb (n = 15) with a linear regression coefficient R ² = 0.9932. Under optimized conditions, a superior sensitivity of 39.54 μA ppb^-1 cm^-2 is observed with a lower detection limit of 0.1 ppb (1.3 nM) (based on the visual analysis of calibration curve) and 0.08 ppb (1.06 nM) (based on the standard deviation of linear regression). Furthermore, the electrochemical Au/GNE is also applicable for arsenic detection in a complex system containing Cu²⁺, Ni²⁺, Fe²⁺, Pb²⁺, Hg²⁺, and other ions for the selective and sensitive analysis. Au/GNE substrate also possesses remarkable reproducibility and high stability for arsenic detection during repeated analysis and thus can be employed for prolonged applications and reiterating analyses. This electrochemically generated nanotextured electrode is also applicable for As³⁺ detection and analysis in a real water sample under optimized conditions. Therefore, fabrication conditions and analytical and electroanalytical performances justify that because of its low cost, easy preparation method and assembly, high reproducibility, and robustness, nanosensor Au/GNE can be scaled up for further applications.