The effect of doping with N and S heteroatoms on the structural characteristics, stability, and activity of acidic (in HNO3) and alkaline (in NaOH) functionalized carbon nanotubes (CNTs) as the cathodic catalyst for H2/O2 fuel cells with an anion-exchange polymer electrolyte is studied. It is found that the surface groups of functionalized CNTs facilitate the incorporation of heteroatoms into the CNT structure during doping. In its turn, N, S-doping leads to an increase of the hydrophilic surface area and the electrochemically accessible CNT surface area. This correlates with an increase of the nitrogen content and is most pronounced in the case of CNTHNO3-NS specimen, which is synthesized by doping CNTs functionalized in an acidic medium. According to the results of RDE and RRDE studies, CNTHNO3-NS is characterized by the highest activity and selectivity in the oxygen reduction reaction (ORR) to water in an alkaline medium. The calculated number of electrons transferred to an oxygen molecule is 3.2 for this specimen. As a component of membrane-electrode assembly (MEA) cathode of H2/O2 fuel cell, CNTHNO3-NS catalyst is superior to the commercial 60Pt/C catalyst (HiSPEC 9100) in the maximum power density.

A catalyst for the oxygen reduction reaction (ORR) based on the metal-organic framework material MIL-53(Al) modified with palladium was synthesized. Its textural and morphological characteristics were studied using the method of adsorption porosimetry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA-DSC). The electrocatalytic properties of the synthesized material in ORR were studied by voltammetry, using a rotating disk electrode. Corrosion resistance was studied in the CV mode. It was found that the synthesized catalyst is characterized by high corrosion resistance. The tolerance of synthesized catalyst Pyr_MIL-53(Al)_Pd to the methanol was studied. The obtained catalyst was studied in a membrane electrode assembly (MEA) formed by spraying an ionomer suspension onto a gas diffusion layer (GDL). The synthesized Pyr-MIL-53 (Al)_Pd and commercial platinum (60% Pt) (HiSPEC 9100) catalysts were compared in the cathode composition, and 10% PtM (M = Ni, Mo)/CNT catalysts were used on the anode. The power density of the FC (P) was calculated based on the obtained current-voltage curves. Based on the set of characteristics, the synthesized catalyst based on MIL-53 (AL) doped with palladium is superior in efficiency to the commercial platinum catalyst.

To achieve the predicted energy characteristics of the Li–O2 battery (LOB), which are expected to be the highest among known metal-air systems, it is necessary to ensure its long-term cycling at high depth of discharge and high current density. However, in such conditions, the deposition of non-conductive lithium peroxide (Li2O2), a product of LOB discharge, is accelerated on the positive electrode, resulting in the blockage of electronic transport. In this work, using a rotating ring disk electrode (RRDE) in the potentiodynamic regime, the possibility of using CNTs in the active layer (AL) of the positive electrode for long-term LOB discharge in Li+ electrolytes based on DMSO and TEGDME is shown. The direct formation of Li2O2 in the pores of the AL electrode ensures the preservation of a surface fraction that is free of lithium peroxide and accessible for electron transport. The effect of the porous structure is most evident in the DMSO-based electrolyte, which facilitates the formation of Li2O2 on a smooth electrode through the diffusion of the superoxide anion (O2–•) to the solution and its subsequent disproportionation. When oxygen is reduced on the CNTs in the given electrolyte, the formed O2–• is converted to Li2O2 directly in the pores of the AL due to diffusion limitations.

Bimetallic catalysts containing platinum and transition metals (PtM, M = Mo, Ni, CoCr) were synthesized on carbon nanotubes (CNTs) functionalized in an alkaline medium. Their platinum content is 10–15% by mass. PtM/CNTNaOH are active in both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in alkaline electrolytes. Although catalysts based on a single transition metal are inactive in the HOR, their activity in the cathode process of ORR increases relative to CNTNaOH. When using the rotating ring-disk electrode method for ORR, PtM/CNT showed a high selectivity in reducing oxygen directly to water. In HOR, the PtM/CNT catalyst had an activity comparable to that of a commercial monoplatinum catalyst. The results obtained show that it is possible to use the PtM/CNT catalyst in an alkaline fuel cell both as an anode and as a cathode.

The effect of the support nature and the mass of platinum on the stability, electrochemical characteristics of monoplatinum catalysts, and the reaction path of electrochemical oxygen reduction in alkaline electrolyte is studied. Catalysts with the Pt mass content of 10, 20, 40, 60 wt % are synthesized by the polyol method on carbon nanotubes functionalized in NaOH and doped with nitrogen. The activity, the percentage of hydrogen peroxide formed, and the number of electrons participating in the oxygen reduction reaction are determined from the data obtained by the rotating ring-disk electrode method. For catalysts synthesized on the nitrogen-doped carbon nanotubes, the highest selectivity in the reaction of oxygen reduction to water is observed; the higher Pt surface area at the electrode, the greater is the selectivity, because the contribution of the support surface to the total oxygen reduction reaction decreased. Both the presence of hydrogen peroxide and a decrease in stability result from the decrease in the platinum content in the catalyst.

PtNi catalysts on carbon nanotubes (CNTs) subjected to preliminary treatment are synthesized and studied in the hydrogen oxidation reaction (HOR) in an alkaline electrolyte. A comparison of the structural and morphological and electrochemical characteristics of monoplatinum (Pt/CNTs) and bimetallic (PtNi/CNTs) catalysts in the HOR at equal concentrations of platinum and the same CNTs is conducted. It is found that catalysts synthesized on nanotubes functionalized in an alkali (CNTs NaOH ) are significantly superior to PtNi catalysts synthesized on CNTs doped with nitrogen and monoplatinum catalyst in terms of stability and activity in the HOR. A PtNi/CNTs NaOH catalyst with a weight concentration of platinum of 10% manifests the highest activity in the HOR at a Pt : Ni ratio of 1 : 1. The main parameters providing high characteristics of the bimetallic system are the presence of active sites for the fixation of the metal phase on the CNTs NaOH , concentration of platinum on the surface of the catalyst, and ratio of the metals.

Studies have been carried out to optimize the composition, formation technique and test conditions of membrane electrode assemblies (MEA) of hydrogen-oxygen anion-exchange membranes fuel cells (AEMFC), based on Fumatech anion-exchange membranes. A non-platinum catalytic system based on nitrogen-doped CNT (CNTN) was used in the cathode. PtMo/CNTN catalysts with a reduced content of platinum (10-12 wt.% Pt) were compared with 10 and 60 wt.% Pt/CNTN at the anode. According to the results of studies under model conditions, it was found that the PtMo/CNTN catalyst is significantly superior to the 10 and 60 wt.% Pt/CNTN catalyst in terms of activity in the hydrogen oxidation reaction based on the mass of platinum. The addition of the Fumion ionomer results in minor changes in the electrochemically active surface area and activity in the hydrogen oxidation reaction for each of the catalysts. In this case, the introduction of ionomer-Fumion leads to a partial blocking of the outer surface and the micropore surface, which is most pronounced in the case of the 60Pt/CNTN catalyst. This effect can cause a decrease in the characteristics of MEA AEMFC upon passing from 10PtMo/CNTN to 60Pt/CNTN in the anode active layer. The maximum power density of the optimized MEA based on 10PtMo/CNTN was 62 mW cm-2, which exceeds the literature data obtained under similar test conditions for MEA based on platinum cathode and anode catalysts and Fumatech membranes (41 mW cm-2). A new result of this work is the study of the effect of the ionomer (Fumion) on the characteristics of catalysts. It is shown that the synthesized 10PtMo/CNTN catalyst retains high activity in the presence of an ionomer under model conditions and in the MEA based on it.

PtNi catalysts on carbon nanotubes (CNTs) subjected to preliminary treatment are synthesized and studied in the hydrogen oxidation reaction (HOR) in an alkaline electrolyte. A comparison of the structural and morphological and electrochemical characteristics of monoplatinum (Pt/CNTs) and bimetallic (PtNi/CNTs) catalysts in the HOR at equal concentrations of platinum and the same CNTs is conducted. It is found that catalysts synthesized on nanotubes functionalized in an alkali (CNTsNaOH) are significantly superior to PtNi catalysts synthesized on CNTs doped with nitrogen and monoplatinum catalyst in terms of stability and activity in the HOR. A PtNi/CNTsNaOH catalyst with a weight concentration of platinum of 10% manifests the highest activity in the HOR at a Pt : Ni ratio of 1 : 1. The main parameters providing high characteristics of the bimetallic system are the presence of active sites for the fixation of the metal phase on the CNTsNaOH, concentration of platinum on the surface of the catalyst, and ratio of the metals.

The electrochemical characteristics of carbon nanotubes subjected to various types of modification, to increase their activity and stability, in the oxygen reduction reaction in alkaline electrolyte are determined by cyclic voltammetry on a rotating disk and rotating ring-disk electrodes. The measurements were performed on the carbon nanotubes after their functionalization, doping with nitrogen, and subsequent modification with platinum in an amount of up to 20 wt %. The resulting dispersed material in the form of an extremely thin layer was applied to a disk electrode, and the effect of carbon nanotubes’ pretreatment on their efficiency in the oxygen reduction reaction in alkaline electrolyte is studied. The activity is shown to be higher, and the degree of degradation, lower, as the selectivity in the oxygen reduction to water increased. When oxygen was reduced through the intermediate formation of hydrogen peroxide, the degradation of the system under study increased. According to the rotating ring-disk electrode data, the greatest contribution of the reaction with the intermediate Н2О2 formation is observed on the carbon nanotubes after their functionalization, whereas the doping with nitrogen increased the activity and the contribution of the four-electron reaction; the value of n is 3.2. After the modification with platinum, the oxygen reduction reaction proceeds predominantly with the breaking of the O–O-bond and the reduction of oxygen to water. The influence of the support on the platinum-modified nanotubes’ characteristics displays itself in the potential range below 0.70 V, where the electroreduction of oxygen on the platinum-free surface proceeds with the transfer of two electrons and contributes to the overall process. The less hydrogen peroxide formed during the oxygen reduction reaction, the less is the degradation of the catalyst. Further increase in the activity of the carbon nanotubes is required; to this purpose, the number of certain types of nitrogen-containing surface groups facilitating the contribution of the four-electron reaction path of the oxygen reduction reaction must be elevated.

A series of cathodic and anodic catalysts for alkaline H2-O2 fuel cells (FC) deposited on the surface of func-tionalized and nitrogen-doped carbon nanotubes (CNT) was synthesized and studied by various structural and electrochemical methods. It was established that, due to the minor defect structure, CNTNaOH (functionalized in alkali) exhibit high corrosion stability. A further increase in the activity in the oxygen reduction reaction (ORR) and the stability of CNTs is observed following their doping with nitrogen. The high activity of CNTNaOH+N is attributed to pyridine groups and the increased electrical conductivity of this catalyst. It was shown that CNTNaOH comprise effective substrates for hydrogen oxidation reaction catalysts. The PtMo/CNTNaOH system containing 12 wt% Pt at a molar ratio of PtMo= 1:1 is characterized by the highest catalytic activity among investigated anode platinum and non-platinum catalysts. The use of this catalyst made it possible to reduce the content of platinum to 0.1-0.2 mg/cm2, while preserving the characteristics that significantly exceed those of the active layer containing 60Pt/C (HiSPEC 9100) catalyst (0.6 mgPt/cm2).

Platinum deposited on dispersed materials has so far been the most demanded catalyst for creating cathodes for a wide range of electrochemical power sources. This paper sets out to investigate the effect of carbon nanotube (CNT) modification by O, N, and P atoms on the structural, electrocatalytic, and corrosion properties of the as-synthesized monoplatinum catalysts. The investigated Pt/CNTmod catalysts showed an increased electrochemically active platinum surface area and electrical conductivity, as well as an increased catalytic activity in the oxygen reduction reaction (ORR) in alkaline electrolytes. The improved characteristics of Pt/CNT catalysts are explained by alterations in the composition and number of groups, which are formed on the CNT surface, and their electronic structure. By the sum of the main characteristics, Pt/CNTHNO3+N and Pt/CNTHNO3+NP are the most promising catalysts for use as cathode materials in alkaline media.

Platinum catalysts synthesized at carbon nanotubes with the noble metal content of 20 and 40 wt % are studied under model conditions and in cathodes of membrane-electrode assemblies (MEA) of hydrogen–air fuel cells with proton-conducting polymer electrolyte. The effect of the cathode active layer and MEA overall composition on the activity and operation stability of the synthesized catalytic systems is elucidated. Stability against degradation is studied by using the accelerated stress-testing method by the cathode potential repeated cycling over the 0.6–1.3 V range. The synthesized catalysts were shown to possess higher stability against degradation as compared to the commercial 60Pt/C catalysts (HiSPEC). Contribution of the electrochemical, Ohmic, and transport components into the overall voltage losses depends on the total platinum surface area in the active layers, which determines the polarization current density, and on the Pt mass at the support. The higher Ohmic and transport losses in the case of the catalyst with the Pt content of 40 wt %, as compared to the catalyst containing 20 wt % of platinum, are due to the structural characteristics, namely, a decrease in the carbon nanotubes’ pore volume and size when a greater metal load is applied.

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.

The influence of the types and amounts of oxygen (O), nitrogen (N), and/or phosphorus (P) heteroatoms on the surface of carbon nanotubes (CNTs) on stability and catalytic activity in the oxygen reduction reaction (ORR) was investigated in alkaline media. It is shown that functionalization of CNTs leads to growth of the electrochemically active surface and to an increase in activity in the ORR. At the same time, a decrease in stability is observed after functionalization of CNTs under accelerated corrosion testing in alkaline media. These results are most significant on CNTs after functionalization in HNO3, due to the formation of a large number of structural defects. However, subsequent doping with N and/or P atoms provides a further activity increase and enhances the corrosion stability of CNTs. Thus, as shown by the studies of characteristic parameters (electrochemical active surface values (S-EAS);E-1/2; corrosion stability), CNTs doped with N and NP are promising catalytic systems that can be recommended for use as fuel cell cathodes. An important condition for effective doping is the synthesis of carboxyl and carbonyl oxygen-containing groups on the surface of CNTs.

Oxygen electroreduction is the key reaction among processes and devices of practical importance. Accordingly, the efforts of researchers are directed to the development of catalysts for the oxygen reduction reaction, which provided the effective oxygen reduction and stability at their low cost. Carbon nanotubes modified with oxygen- and nitrogen-containing groups, under the oxygen/nitrogen ratio of 1.3, meet these requirements considerably. In this work, we show that carbon nanotubes modified by oxygen and nitrogen accelerate the oxygen reduction reaction both in acid and alkaline electrolytes. Here, the modifying of carbon nanotubes’ surface increased the electrochemical activity in the oxygen reduction reaction significantly which manifests itself in the shift of the half-wave potential in polarization curves in acid electrolyte by ~0.40 V to more positive values in comparison with not modified carbon nanotubes (in acid electrolyte). In alkaline electrolyte, the modified carbon nanotubes approach monoplatinum catalyst in their activity in the oxygen reduction reaction. The stability of modified carbon nanotubes (according to the testing by the potential cycling) is superior to that of the untreated carbon nanotubes. The obtained data show the perspectives of the modified carbon nanotubes’ application as supports for cathodic catalysts of low-temperature hydrogen–air fuel cells. In the direct alkaline alcohol–air fuel cells and metal–air power sources of lithium–air type, the modified carbon nanotubes can be used as catalysts per se. The tailor-made synthesis of desired type and amount of nitrogen-containing groups is suggested to be realizable after the preliminary modification of carbon nanotubes with the oxygen-containing groups.

The effect of discharge depth on the lifetime of lithium–oxygen batteries (LOBs) is studied under cycling in TEGDME and DMSO media. It is shown that in LOBs based on alkali–treated carbon nanotubes (CNTOH and PtCo/CNTOH) combining high outer surface area, volume, and average diameter of pores, cycling reversibility increases while reducing the amount of discharge product formed on the positive electrode. For LOBs with XC-72, reversibility does not depend on the discharge depth owing to the close values of the smallest Li2O2 particle size and average pore size. For each of the materials, replacement of DMSO by TEGDME allows enhancing the LOB lifetime in the studied cycling modes. For CNTOH and PtCo/CNTOH, ratios are determined that characterize variation of LOB parameters when changing the depth of discharge. The calculated ratios allow predicting the LOB characteristics at a change in the discharge mode or solvent type.

— Nanodispersed molybdenum sulfide decorated with platinum nanoparticles is proposed as a new catalytic nanosystem for a Li–O 2 battery. Using platinum as a catalytically active material reduces the overvoltage during charging. Due to the chemical stability of molybdenum sulfide as a support material, the current and power parameters of the system increase to 1000 mA/g, while the number of cycles during uninterrupted operation of the cell reaches 130 (for 500 mA/g). The catalytic system proposed in this study features bifunctional properties and high stability during cycling.