| 1. |
- Ritopecki, Milica S. S., et al.
(författare)
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The Local Coordination Effects on the Reactivity and Speciation of Active Sites in Graphene-Embedded Single-Atom Catalysts over Wide pH and Potential Range
- 2022
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Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 12:23
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the catalytic performance of different materials is of crucial importance for achieving further technological advancements. This especially relates to the behaviors of different classes of catalysts under operating conditions. Here, we analyzed the effects of local coordination of metal centers (Mn, Fe, Co) in graphene-embedded single-atom catalysts (SACs). We started with well-known M@N-4-graphene catalysts and systematically replaced nitrogen atoms with oxygen or sulfur atoms to obtain M@OxNy-graphene and M@SxNy-graphene SACs (x + y = 4). We show that local coordination strongly affects the electronic structure and reactivity towards hydrogen and oxygen species. However, stability is even more affected. Using the concept of Pourbaix plots, we show that the replacement of nitrogen atoms in metal coordinating centers with O or S destabilized the SACs towards dissolution, while the metal centers were easily covered by O and OH, acting as additional ligands at high anodic potentials and high pH values. Thus, not only should local coordination be considered in terms of the activity of SACs, but it is also necessary to consider its effects on the speciation of SAC active centers under different potentials and pH conditions.
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| 2. |
- Dobrota, Ana S., et al.
(författare)
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First-principles analysis of aluminium interaction with nitrogen-doped graphene nanoribbons : From adatom bonding to various
- 2022
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Ingår i: Materials Today Communications. - : Elsevier BV. - 2352-4928. ; 31
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Tidskriftsartikel (refereegranskat)abstract
- Enhancing aluminium interaction with graphene-based materials is of crucial importance for the development of Al-storage materials and novel functional materials via atomically precise doping. Here, DFT calculations are employed to investigate Al interactions with non-doped and N-doped graphene nanoribbons (GNRs) and address the impact of the edge sites and N-containing defects on the material's reactivity towards Al. The presence of edges does not influence the energetics of Al adsorption significantly (compared to pristine graphene sheet). On the other hand, N-doping of graphene nanoribbons is found to affect the adsorption energy of Al to an extent that strongly depends on the type of N-containing defect. The introduction of edge-NO group and doping with in -plane pyridinic N result in Al adsorption nearly twice as strong as on pristine graphene. Moreover, double n-type doping via N and Al significantly alters the electronic structure of Al,N-containing GNRs. Our results suggest that selectively doped GNRs with pyridinic N can have enhanced Al-storage capacity and could be potentially used for selective Al electrosorption and removal. On the other hand, Al,N-containing GNRs with pyridinic N could also be used in resistive sensors for mechanical deformation. Namely, strain along the longitudinal axis of these dual doped GNRs does not affect the binding of Al but tunes the bandgap and causes more than 700-fold change in the conductivity. Thus, careful defect engineering and selective doping of GNRs with N (and Al) could lead to novel multifunctional materials with exceptional properties. [GRAPHICS]
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| 3. |
- Dobrota, Ana S., et al.
(författare)
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Surface pourbaix plots of M@N-4-graphene single-atom electrocatalysts from density functional theory thermodynamic modeling
- 2022
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 412
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Tidskriftsartikel (refereegranskat)abstract
- Single-atom catalysts (SACs) are rapidly developing in various application areas, including electrocatalysis of different reactions, usually taking place under harsh pH/electrode potential conditions. Thus, a full atomic-level understanding of the nature of the active sites under realistic electrochemical conditions is needed, having in mind that the state of SACs active centers could be altered by the adsorption of spectating species. In this contribution, Density Functional Theory is employed to conduct thermodynamic analysis of SACs with metal atoms (Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, or Au) embedded into N4 moiety in graphene. Various surface electrochemical processes on such SACs are considered, their Pourbaix plots are constructed, and their activity, selectivity, and stability under operating conditions are discussed. It is demonstrated how adsorption of H, O and OH can cause blockage and restructuring of the active sites and alter the electronic structure. Furthermore, when one deals with metals with lower D-band filling, it is shown that metal center oxidation is preferred over the oxidation of carbon lattice. The effect of the state of the metal center on the reactivity of the carbon lattice is discussed in the case of Fe@N-4-graphene. Finally, a possible strategy for confirming the changes in the architecture of the SACs' active site by analyzing their vibration spectra is suggested.
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| 4. |
- Gebremariam, Goitom K., et al.
(författare)
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Hydrogen Evolution Volcano(es)-From Acidic to Neutral and Alkaline Solutions
- 2022
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 12:12
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Tidskriftsartikel (refereegranskat)abstract
- As the global energy crisis continues, efficient hydrogen production is one of the hottest topics these days. In this sense, establishing catalytic trends for hydrogen production is essential for choosing proper H-2 generation technology and catalytic material. Volcano plots for hydrogen evolution in acidic media are well-known, while a volcano plot in alkaline media was constructed ten years ago using theoretically calculated hydrogen binding energies. Here, for the first time, we show that the volcano-type relationships are largely maintained in a wide range of pH values, from acidic to neutral and alkaline solutions. We do this using theoretically calculated hydrogen binding energies on clean metallic surfaces and experimentally measured hydrogen evolution overpotentials. When metallic surfaces are exposed to high anodic potentials, hydrogen evolution can be boosted or significantly impeded, depending on the type of metal and the electrolyte in which the reaction occurs. Such effects are discussed here and can be used to properly tailor catalytic materials for hydrogen production via different water electrolysis technologies.
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| 5. |
- Jovanovic, Aleksandar Z., et al.
(författare)
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Enhancement of hydrogen evolution reaction kinetics in alkaline media by fast galvanic displacement of nickel with rhodium-From smooth surfaces to electrodeposited nickel foams
- 2022
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 414
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Tidskriftsartikel (refereegranskat)abstract
- Energy-efficient hydrogen production is one of the key factors for advancing hydrogen-based economy. Alkaline water electrolysis is the main route for the production of high-purity hydrogen, but further improvements of hydrogen evolution reaction (HER) catalysts are still needed. Industrial alkaline electrolysis relies on Ni-based catalysts, and here we describe a drastic improvement of HER activity of Ni in alkaline media using several model catalysts for HER, obtained upon nickel surface modification in the aqueous solution of rhodium salts, where a spontaneous deposition of rhodium takes place, based on the chemical displacement reaction 3Ni + 2Rh3+ = 3Ni2+ + 2Rh. In the case of smooth Ni-poly electrodes, HER activity surpasses the activity of Pt-poly after just 30 s of exchange with Rh. SEM analysis showed that Rh is uniformly distributed, and that surface roughness changes are lower than 10%, which is in agreement with the electrochemical measurements. Furthermore, XPS analysis has shown effective incorporation of Rh in the surface, while DFT calculations suggest that hydrogen binding is significantly weakened on the Rh-modified Ni surfaces. Such tuning of the hydrogen binding energy is seen as the main factor governing HER activity improvements. The same galvanic displacement protocols were employed for nickel foam electrodes and electrodeposited Ni on Ti mesh. In both cases, somewhat longer Rh exchange times are needed to obtain superior activities than for the smooth Ni surface, but within 10 min. HER overpotentials corresponding to -10 mA cm-2 for nickel foam and electrodeposited Ni electrodes, after modification with Rh, amounted to only -0.07 and -0.09 V, respectively. Thus, it is suggested that a fast spontaneous displacement of Ni with Rh could effectively boost HER in alkaline media with minor cost penalties with regards to energy saving in the electrolysis process.
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