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 (S_EAS). The obtained results show that the modification of CNT with oxygen-containing groups leads to an increase in hydrophilicity and, consequently, S_EAS, as well as the total (overall) current. Subsequent doping with nitrogen ensures further increase in S_EAS, higher zeta potential and specific activity in ORR, reflected in the shift of the half-wave potential by 150 mV for CNT_NaOH-N and 110 mV for CNT_HNO3-N relative to CNT_NaOH and CNT_HNO3, respectively. Moreover, the introduction of N into the structure of CNT_HNO3 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.

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/CNT_mod 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/CNT_HNO3+N and Pt/CNT_HNO3+NP are the most promising catalysts for use as cathode materials in alkaline media.

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/CNT_NaOH 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 CNT_NaOH. 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.

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 (CNT_N) was used in the cathode. PtMo/CNT_N catalysts with a reduced content of platinum (10–12 wt.% Pt) were compared with 10 and 60 wt.% Pt/CNT_N at the anode. According to the results of studies under model conditions, it was found that the PtMo/CNT_N catalyst is significantly superior to the 10 and 60 wt.% Pt/CNT_N 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/CNT_N catalyst. This effect can cause a decrease in the characteristics of MEA AEMFC upon passing from 10PtMo/CNT_N to 60Pt/CNT_N in the anode active layer. The maximum power density of the optimized MEA based on 10PtMo/CNT_N 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/CNT_N catalyst retains high activity in the presence of an ionomer under model conditions and in the MEA based on it.

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.