Hapniku elektroredutseerumine süsiniknanomaterjalidel põhinevatel katalüsaatoritel

Väitekirja eletkrooniline versioon ei sisalda publikatsioone.Doktoritöös uuriti hapniku elektrokeemilist redutseerumist erinevatel süsiniknanomaterjalidel. Süsinikmaterjalide pinda modifitseeriti ka raud- ja koobaltftalotsüaniinide ning -porfüriinidega ja uuriti nende elektrokatalüütilisi omadusi. Hapniku elektroredutseerumist uuriti happes töödeldud ning töötlemata süsiniknanotorudega modifitseeritud klaassüsinikelektroodidel 0,5 M H2SO4 lahuses. Tulemused näitasid, et süsiniknanotorude hapetes töötlemine omab materjali elektrokatalüütilistele omadustele märkimisväärset efekti ning nanotorude sünteesi käigus neisse jäävad metallide jäägid määravad paljuski nende materjalide elektrokatalüütilise aktiivsuse hapniku redutseerumisel happelises keskkonnas. Hapniku redutseerumise ekperimendid mitmeseinaliste ja kaheseinaliste süsiniknanotorudega näitasid mõlemat tüüpi nanotorude kõrget elektrokatalüütilist aktiivsust leeliselises keskkonnas. Uuriti hapniku elektrokeemilise redutseerumise pH-sõltuvust ja pindaktiivsete ainete põhjustatud efekti. Mitmeseinaliste süsiniknanotorudega modifitseeritud elektroodidel saadud hapniku redutseerumise pH-sõltuvus sarnaneb kvalitatiivselt poleeritud klaassüsinikelektroodiga saadud andmetele. Mõõtmistulemuste alusel võib väita ka seda, et pindaktiivsete ainete mõju hapniku elektrokeemilisele redutseerumisele on ilmne. Erinevate süsiniknanomaterjalidega modifitseeritud elektroodide hapniku redutseerumise uuringud näitasid, et kõik kasutatud süsinikmaterjalid omasid leeliselistes lahustes suurt katalüütilist aktiivsust. Töö ühe osana valmistati süsiniknanotorude ja metalloftalotsüaniinide ning metalloporfüriinide abil madalatemperatuurilise kütuseelemendi katalüsaatormaterjale. Leiti, et 800 °C juures pürolüüsitud katalüsaator näitab hapniku elektroredutseerumisel kõrgeimat aktiivsust. Lisaks viidi läbi ka katalüsaatormaterjalide uurimine leeliselises OH− ioonvahetusmembraaniga kütuseelemendis. Koobaltftalotsüaniin/süsiniknanotorud katalüsaatori maksimaalne võimsustihedus oli sarnane 20%-lisele Pt/C katalüsaatorile. Porfüriinid ja ftalotsüaniinid seondati redutseeritud grafeenoksiidi pinnale füüsikalise adsorptsiooni meetodil. Elektrokeemilised mõõtmised näitasid grafeenil põhinevate katalüsaatorite suurepäraseid katalüütilisi omadusi hapniku redutseerumisel leeliselises keskkonnas.The electrochemical reduction of oxygen on different nanocarbons and nanocarbon supported metallophthalocyanines and metalloporphyrins has been investigated using the rotating disk electrode (RDE) method. The oxygen reduction reaction (ORR) was studied on carbon nanotubes purified in different acids. The RDE results showed that the acid treatment of multi-walled carbon nanotubes (MWCNTs) has strong effect on the electrocatalytic activity for ORR in acid solution and clearly demonstrated the effect of catalyst impurities remained in CNTs on the kinetics of the ORR. MWCNT and double-walled carbon nanotube (DWCNT) modified electrodes were investigated as catalysts for ORR in alkaline media. The results showed that both MWCNTs and DWCNTs possess excellent electrocatalytic activity towards the ORR in alkaline solution. The pH-dependence of the ORR and the effect of surfactants on this reaction was studied using MWCNT modified glassy carbon (GC) electrodes. The pH dependence of the ORR on MWCNT-modified electrodes follows the same trend as that of bare GC. The effect of surfactants on the reduction of O2 on MWCNT/GC electrodes was evident. The electroreduction of oxygen has been studied on different carbon nanomaterials. The results obtained in this part of research indicate that these nanocarbon materials are highly active for the reduction of oxygen in alkaline solution and this activity might be caused by native quinone-type groups on their surface. Metallophthalocyanines and metalloporphyrins were studied as non-noble metal electrocatalysts for ORR in both acid and alkaline media. It was found that metal porphyrin and phthalocyanine-based electrodes heat-treated at 800 °C yielded the highest electrocatalytic activity. Metallophthalocyanine/MWCNT catalysts were also evaluated in anion-exchange membrane fuel cell. The fuel cell performance of the membrane-electrode assemblies with Co phthalocyanine was found to be similar to the commercial Pt/C catalyst. Finally, the reduced graphene oxide (rGO) nanosheets as advanced electrocatalyst supports were prepared. Metallophthalocyanines and metalloporphyrins modified rGO exhibited excellent electrocatalytic properties for ORR in alkaline media

Are metal-free pristine carbon nanotubes electrocatalytically active?

Metal-free (i.e., residual metallic impurities-blocked) carbon nanotubes (CNTs) do show electrocatalytic activity for H2 evolution, O2 evolution and O2 reduction reactions (HER, OER & ORR) in alkaline solutions, but their activities strongly depend on the number of walls or inner tubes with a maximum for CNTs with 2–3 walls

Wood and Black Liquor-Based N-Doped Activated Carbon for Energy Application

The research was funded by the Latvian Council of Science project “Nanostructured Nitrogenated Carbon Materials as Promoters in Energy Harvesting and Storage Technologies”, project No LZP-2018/1-0194, “New biomass origin materials hybrid carbon composites for energy storage” project No LZP-2020/2-0019 and postdoc project “Nitrogen and phosphorus-containing biomass based activated carbons for fuel cells and supercapacitors” project No 1.1.1.2/VIAA/4/20/596.Fuel cells, batteries and supercapacitors are critical to meet the rising global demand for clean, sustainable energy. Biomass-derived activated carbon can be obtained with tailored properties to fulfil the extensive need for low-cost, high-performance, catalyst and electrode materials. To investigate the possibility of nanoporous nitrogen-doped carbon materials as catalysts in fuel cells and electrodes in lithium-ion batteries, biomass precursors were thermochemically activated with NaOH at 800 °C, nitrogen was introduced using dicyandiamide and doping was performed at 800 °C. The chemical composition, porous structure, texture and electrochemical properties of the obtained materials change depending on the biomass precursor used. It has been found that the most promising precursor of the obtained materials is wood char, both as an oxygen reduction catalyst in fuel cells, which shows better properties than the commercial 20% Pt/C catalyst, and as an anode material in Li-ion batteries. However, catalysts based on black liquor and hybrid material have comparable properties with commercial 20% Pt/C catalyst and can be considered as a cheaper alternative. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Latvian Council of Science LZP-2018/1-0194, LZP-2020/2-0019; postdoc project 1.1.1.2/VIAA/4/20/596; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Identification of Active Sites for Oxygen Reduction Reaction on Nitrogen- and Sulfur-Codoped Carbon Catalysts

This research was financially supported by ERA.Net RUS Plus funding mechanism (Project HeDoCat) and by the European Regional Development Fund project TK134.Nitrogen- and sulfur-codoped carbon catalysts were prepared as electrocatalytic materials for the oxygen reduction reaction (ORR). Herein, we propose a novel and effective one-pot synthetic approach to prepare a NS-doped carbon catalyst by using the mixture of graphene oxide and multi-walled carbon nanotubes as a carbon support. Successful NS-doping of carbon and formation of the catalytically active sites were confirmed by X-ray photoelectron spectroscopy and with energy dispersion spectroscopy. The ORR activity of NS-codoped carbon was investigated by using a rotating disc electrode method. The NS-doped carbon shows superior ORR performance in alkaline media, and the electrocatalytic mechanism for the reduction of oxygen was well explained by density functional theory calculations of graphene sheets.ERA.Net RUS Plus Project HeDoCat; ERDF TK134; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Electrode modification using nanocomposites of electropolymerised cobalt phthalocyanines supported on multiwalled carbon nanotubes

A polymer of tetra(4)-(4,6-diaminopyrimidin-2-ylthio) phthalocyaninatocobalt(II) (CoPyPc) has been deposited over a multiwalled carbon nanotube (MWCNT) platform and its electrocatalytic properties investigated side by side with polymerized cobalt tetraamino phthalocyanine (CoTAPc). X-ray photoelectron spectroscopy, scanning electron microscopy and cyclic voltammetry studies were used for characterization of the prepared polymers of cobalt phthalocyanine derivatives and their nanocomposites. L-Cysteine was used as a test analyte for the electrocatalytic activity of the nanocomposites of polymerized cobalt phthalocyanines and multiwalled carbon nanotubes. The electrocatalytic activity of both polymerized cobalt phthalocyanines was found to be superior when polymerization was done on top of MWCNTs compared to bare glassy carbon electrode. A higher sensitivity for L-cysteine detection was obtained on CoTAPc compared to CoPyPc

A MnOx-graphitic carbon composite from CO2 for sustainable Li-ion battery anodes

The increasing concentration of CO2 in the atmosphere is the leading cause of the greenhouse gas effect. Carbon capture and storage is an important topic to develop sustainable technologies. Molten salt CO2 capture and electrochemical transformation represent a suitable process to produce various carbon products, such as carbon nanofibers, carbon nanotubes, graphite, and graphene. The employment of graphitic anode materials for Li-ion batteries coming from CO2 capture is ideal for increasing battery sustainability. Moreover, the addition of transition metal oxides represents a suitable strategy for new negative electrodes because conversion reactions lead to high specific capacities. Among them, manganese has gained attention due to its multiple valence states and numerous possible crystalline structures. In the present work, a MnOx-graphitic carbon composite obtained by electrolysis of CO(2)via molten Li2CO3 is characterized and used to prepare a negative electrode for LIBs with an environmentally sustainable aqueous process