Modified Carbon Nanotubes: Surface Properties and Activity in Oxygen Reduction Reaction

In order to develop highly efficient and stable catalysts for oxygen reduction reaction (ORR) that do not contain precious metals, it is necessary to modify carbon nanotubes (CNT) and define the effect of the modification on their activity in the ORR. In this work, the modification of CNTs included functionalization by treatment in NaOH or HNO3 (soft and hard conditions, respectively) and subsequent doping with nitrogen (melamine was used as a precursor). The main parameters that determine the efficiency of modified CNT in ORR are composition and surface area (XPS, BET), hydrophilic–hydrophobic surface properties (method of standard contact porosimetry (MSP)) and zeta potential (dynamic light scattering method). The activity of CNT in ORR was assessed following half-wave potential, current density within kinetic potential range and the electrochemically active surface area (SEAS). The obtained results show that the modification of CNT with oxygen-containing groups leads to an increase in hydrophilicity and, consequently, SEAS, as well as the total (overall) current. Subsequent doping with nitrogen ensures further increase in SEAS, higher zeta potential and specific activity in ORR, reflected in the shift of the half-wave potential by 150 mV for CNTNaOH-N and 110 mV for CNTHNO3-N relative to CNTNaOH and CNTHNO3, respectively. Moreover, the introduction of N into the structure of CNTHNO3 increases their corrosion stability.

N-Doped Graphene as an Efficient Metal-Free Electrocatalyst for Indirect Nitrate Reduction Reaction

N-doped graphene samples with different N species contents were prepared by a two-step synthesis method and evaluated as electrocatalysts for the nitrate reduction reaction (NORR) for the first time. In an acidic solution with a saturated calomel electrode as reference, the pyridinic-N dominant sample (NGR2) had an onset of 0.932 V and a half-wave potential of 0.833 V, showing the superior activity towards the NORR compared to the pyrrolic-N dominant N-doped graphene (onset potential: 0.850 V, half-wave potential: 0.732 V) and the pure graphene (onset potential: 0.698 V, half-wave potential: 0.506 V). N doping could significantly boost the NORR performance of N-doped graphene, especially the contribution of pyridinic-N. Density functional theory calculation revealed the pyridinic-N facilitated the desorption of NO, which was kinetically involved in the process of the NORR. The findings of this work would be valuable for the development of metal-free NORR electrocatalysts.

Palladium Particles Modified by Mixed-Frequency Square-Wave Potential Treatment to Enhance Electrocatalytic Performance for Formic Acid Oxidation

Palladium catalysts have attracted widespread attention as advanced electrocatalysts for the formic acid oxidation (FAO) due to their excellent electrocatalytic activity and relatively high abundance. At present, electrodeposition methods have been widely developed to prepare small-sized and highly-dispersed Pd electrocatalysts. However, the customary use of surfactants would introduce heterogeneous impurities, which requires complicated removal processes. In this work, we reported a two-step electrochemical method that employed square-wave potential treatment (SWPT) to modify electrodeposited Pd particles without the use of capping agents. Under the SWPT with a mixed frequency, Pd particles show significantly reduced size and more dispersed distribution, exhibiting a high mass activity of 1.43 A mg−1 toward FAO, which is 4.6 times higher than the counterpart of commercial Pd/C. The increase in electrocatalytic activity of FAO is attributed to the highly developed surface of palladium particles uniformly distributed over the support surface.

Seasonal Cycle of Gravity Wave Potential Energy Densities from Lidar and Satellite Observations at 54° and 69°N

We present gravity wave climatologies based on 7 years (2012–18) of lidar and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperatures and reanalysis data at 54° and 69°N in the altitude range 30–70 km. We use 9452 (5044) h of lidar observations at Kühlungsborn [Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR)]. Filtering according to vertical wavelength (λz < 15 km) or period (τ < 8 h) is applied. Gravity wave potential energy densities (GWPED) per unit volume (EpV) and per unit mass (Epm) are derived. GWPED from reanalysis are smaller compared to lidar. The difference increases with altitude in winter and reaches almost two orders of magnitude around 70 km. A seasonal cycle of EpV with maximum values in winter is present at both stations in nearly all lidar and SABER measurements and in reanalysis data. For SABER and for lidar (with λ < 15 km) the winter/summer ratios are a factor of ~2–4, but are significantly smaller for lidar with τ < 8 h. The winter/summer ratios are nearly identical at both stations and are significantly larger for Epm compared to EpV. Lidar and SABER observations show that EpV is larger by a factor of ~2 at Kühlungsborn compared to ALOMAR, independent of season and altitude. Comparison with mean background winds shows that simple scenarios regarding GW filtering, etc., cannot explain the Kühlungsborn–ALOMAR differences. The value of EpV decreases with altitude in nearly all cases. Corresponding EpV-scale heights from lidar are generally larger in winter compared to summer. Above ~55 km, EpV in summer is almost constant with altitude at both stations. The winter–summer difference of EpV scale heights is much smaller or absent in SABER and in reanalysis data.

General characteristics of Gravity Wave Potential Energy Density at 54 ºN and 69 ºN

&lt;p&gt;&lt;span&gt;We present results of seven years of gravity waves (GW) observations between 2012 and 2018. The measurements were conducted by ground-based lidars in K&amp;#252;hlungsborn (54&amp;#176;N, 12&amp;#176;E) and at ALOMAR (69&amp;#176;N, 16&amp;#176;E). Our analysis technique includes different types of filtering which allow for selection of different ranges from the entire GW-spectrum. We studied &lt;/span&gt;&lt;span&gt;wave&lt;/span&gt;&lt;span&gt; properties as a function of altitude and location and summarized the results in monthly and seasonally mean profiles. &lt;/span&gt;&lt;span&gt;Complementary&lt;/span&gt;&lt;span&gt; data is taken from the satellite-based SABER instrument. Additionally, we consistently applied our analysis technique to the reanalyses data from MERRA-2 and ERA-5. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;A&lt;span&gt; seasonal cycle of &lt;/span&gt;&lt;span&gt;gravity wave potential energy density &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;GWPED&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; with maximum values in winter is present at both stations in nearly all lidar/SABER measurements and in reanalysis data. For SABER and for lidar the winter &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;summer ratios are a factor of &lt;/span&gt;&lt;span&gt;about&amp;#160;3&lt;/span&gt;&lt;span&gt;. The winter &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;summer ratios are nearly identical at both stations. &lt;/span&gt;&lt;span&gt;GWPED&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; from reanalysis are smaller compared to lidar. The difference increases with altitude in winter and reaches almost two orders of magnitude around 70&amp;#160;km.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;GWPEDs per volume&lt;/span&gt;&lt;span&gt; decrease&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; with height &lt;/span&gt;&lt;span&gt;differently for the winter and summer seasons,&lt;/span&gt;&lt;span&gt; irrespective of filtering method and location. &lt;/span&gt;&lt;span&gt;In summer for altitudes above roughly 5&lt;/span&gt;&lt;span&gt;0&lt;/span&gt;&lt;span&gt;&amp;#160;km, GWPED is nearly constant or even increases with height. &lt;/span&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;his feature is very pronounced at ALOMAR and to a lesser extent also &lt;/span&gt;&lt;span&gt;at&lt;/span&gt;&lt;span&gt; K&amp;#252;hlungsborn. This behavior is seen &lt;/span&gt;&lt;span&gt;by both, lidar and SABER&lt;/span&gt;&lt;span&gt;. The observed variation of GWPED with height can not be explained by conservation of wave action alone. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;The &lt;/span&gt;&lt;span&gt;GWPED at K&lt;/span&gt;&lt;span&gt;&amp;#252;&lt;/span&gt;&lt;span&gt;hlungsborn is significantly larger compared to ALOMAR. This observation is opposite to simple scenarios which take into account the potential impact of background winds on GW filtering and Doppler shifts of vertical wavelengths and periods. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;W&lt;/span&gt;&lt;span&gt;e present results of &lt;/span&gt;&lt;span&gt;observations and&lt;/span&gt;&lt;span&gt; analyses &lt;/span&gt;&lt;span&gt;and suggest geophysical explanations of our findings.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Investigating the Spatio-Temporal Distribution of Gravity Wave Potential Energy over the Equatorial Region Using the ERA5 Reanalysis Data

Atmospheric gravity waves play a crucial role in affecting atmospheric circulation, energy transportation, thermal structure, and chemical composition. Using ERA5 temperature data, the present study investigates the tropospheric to the lower mesospheric gravity wave potential energy (EP) over the equatorial region to understand the vertical coupling of the atmosphere. EP is mainly controlled by two factors. The first is zonal wind through wave–mean flow interactions, and thus EP has periodic variations that are correlated to the zonal wind oscillations and enhances around the altitudes of zero-wind shears where the zonal wind reverses. The second is the convections caused by atmospheric circulations and warm oceans, resulting in longitudinal variability in EP. The lower stratospheric and the lower mesospheric EP are negatively correlated. However, warm oceanic conditions can break this wave energy coupling and further enhance the lower mesospheric EP.