The electrochemical oxygen reduction reaction (ORR) is the limiting half-reaction of fuel cells, which is mediated by using platinum-based catalysts. Hence, the development of low-cost, active ORR catalysts is highly required to make fuel cell technology commercially available. In this report, transition-metal (TM; Mn, Fe, Co, and Ni) single-doped and multidoped (MD) ZnO nanocrystals (ZNs) were prepared for use as ORR catalysts using a simple precipitation method. The effects of single doping and multidoping on the structure, morphology, and properties of the TM-doped ZNs were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence, X-ray photoelectron microscopy, electron paramagnetic resonance, and Raman and photoluminescence (PL) spectroscopies. The XRD results reveal that synthesized ZnO samples retained a pure hexagonal wurtzite crystal structure, even at high levels of multidoping (nominal 20%). SEM analyses show that the morphology of the prepared ZNs varies with the doping elements, doping mode, and amounts of doping. The observation of peak shifting and peak intensity changes in Raman studies confirms the presence of dopants in ZnO. The PL investigation reveals that the incorporation of dopants into the ZnO structure increases the oxygen vacancies within the materials. The highest oxygen vacancies were present in Mn-doped ZnO and 15% MD ZnO among the single-doped and MD samples, respectively. Linear-sweep voltammetry studies conclude that doped ZnO shows enhanced ORR activity compared to the undoped samples. The Mn-doped ZnO and 15% MD ZnO exhibited the highest ORR activity among the prepared single-doped and MD ZN samples, respectively. In comparison, single doping showed better ORR activity than the multidoping system. The enhanced ORR activity of the synthesized ZN materials correlates with the amount of oxygen vacancies present in the samples. The enhanced activity of TM-doped ZnO suggests that these materials can be used as potential, low-cost electrocatalysts for ORR in fuel cell technology.

No abstract

Water harvesting from the atmosphere using adsorption-based technology holds great promise to solve water scarcity in arid regions. Birnessite (i.e., a layered structure MnO 2 ) can store water molecularly between its layers, providing a path for water adsorption. This work investigates the water sorption characteristic of birnessite from both thermodynamic and kinetic perspectives. The water vapor adsorption on birnessite follows a Type II sorption isotherm. Water molecules quickly adsorb to the interlayers at lower RH region values, while multilayer water−water interactions occur via hydrogen bonding at surfaces and result in condensed water at higher RH. Furthermore, birnessite features excellent solar absorptivity; the temperature can be raised by 87 °C under solar irradiation at a sun flux of ∼900 W/m 2 , providing energy to trigger partial desorption of interlayer water. According to the Do and Do model simulation, birnessite can harvest 0.07 kg of water per kilogram of sample (kg H2O /kg Sample ) per cycle at RH of 23% when the dew point temperature is set to 11 °C. Finally, a device based on the concept of sorption-based atmospheric water harvesting is built to present this application. This inexpensive water adsorption material with solar absorptivity displays an applicable promise for solving water scarcity in arid regions.

Electrocatalytic decomposition of urea for the production of hydrogen, H for clean energy applications, such as in fuel cells, has several potential advantages such as reducing carbon emissions in the energy sector and environmental applications to remove urea from animal and human waste facilities. The study and development of new catalyst materials containing nickel metal, the active site for urea decomposition, is a critical aspect of research in inorganic and materials chemistry. We report the synthesis and application of [NH]NiPO·6HO and β-NiPO using in situ prepared [NH]HPO. The [NH]NiPO·6HO is calcined at varying temperatures and tested for electrocatalytic decomposition of urea. Our results indicate that [NH]NiPO·6HO calcined at 300 °C with an amorphous crystal structure and, for the first time applied for urea electrocatalytic decomposition, had the greatest reported electroactive surface area (ESA) of 142 cm/mg and an onset potential of 0.33 V (SCE) and was stable over a 24-h test period.

Hydrogen evolution reaction (HER) is the cathodic reaction in electrochemical water splitting that produces H 2 in a renewable pathway, primarily to be used in fuel cells. K-OMS-2, a manganese octahedral molecular sieve, has the potential to replace Pt as a catalyst for HER due to its redox and electrochemical properties, which can be further improved by doping metal cations into the K-OMS-2 framework. In this work, we synthesized 1, 5, and 10% vanadium-(V) doped mesoporous K-OMS-2 under mild acidic reaction conditions. The mesoporous nature of the samples was confirmed by BET analysis. X-ray photoelectron spectroscopy confirmed that Mn has 2+, 3 +, and 4+ oxidation states, and V is present as V 4+ in V-K-OMS-2 samples. HER performance of 1% V-K-OMS-2 in an acidic medium shows the best activity with −0.32 V overpotential. The overpotential was lowered by 0.56 V upon doping 1% V into the K-OMS-2 framework. The 1% V-K-OMS-2 material has a Tafel slope of 129 mV/ decay and an electrochemically active surface area (ECSA) of 127.5 cm 2 ECSA.