P2-type layered oxides are attractive cathode active materials for sodium-ion batteries, however, these materials typically suffer from detrimental Na+/vacancy orderings. In this work, we investigate the origin as well as the influence of the transition metal ratio on Na+/vacancy orderings in P2-type cathode materials. A combination of X-ray diffraction (XRD), neutron diffraction, advanced electrochemical methods, operando XRD and DFT calculations is applied to study Na+/vacancy orderings in P2-NaxNi1/3Mn2/3O2 and P2-NaxMn3/4Ni1/4O2. In P2-NaxNi1/3Mn2/3O2, a honeycomb Ni/Mn superstructure leads to charge ordering within the transition metal slab and pronounced Na+/vacancy orderings, causing distinct voltage jumps at specific sodium contents (x = 2/3, 1/2 and 1/3). For P2-Na0.60Mn3/4Ni1/4O2, the Ni/Mn superstructure is disrupted, resulting in more complex charge orderings within the transition metal slab, partially suppressed Na+/vacancy orderings and an overall smoother potential profile. Based on our findings, guidelines to suppress Na+/vacancy orderings in P2-type cathode materials for sodium-ion batteries are postulated and discussed with respect to electrochemical measurements of various transition metal compositions. These guidelines can serve to predict the tendency towards Na+/vacancy orderings for a given cathode composition or to design new cathode compositions for enhanced cycle life based on the absence of Na+/vacancy orderings.

Conversion of CO2 to hard carbon is an interesting technology for the removal of carbon dioxide from the atmosphere. Recently, it was shown that CeO2 can selectively catalyze this reaction, but we still lack information regarding the reaction mechanism. Using density functional theory modeling, we explore possible reaction mechanisms that allow for the polymerization of CO2. According to our computations, the reaction is initialized by the adsorption of CO2 in an oxygen vacancy. Owing to the rich defect chemistry of ceria, a large number of suitable sites are available at the surface. C–C bond formation is achieved through an aldol condensation-type mechanism which comprises the electrochemical elimination of water to form a carbene. This carbene then performs a nucleophilic attack on CO2. The reaction mechanism possesses significant similarities to the corresponding reactions in synthetic organic chemistry. Since the mechanism is completely generic, it allows for all relevant steps of the formation of hard carbon like chain growth, chain linkage, and the formation of side chains or aromatic rings. Surprisingly, ceria mainly serves as an anchor for CO2 in an oxygen vacancy, while all other subsequent reaction steps are almost completely independent from the catalyst. These insights are important for the development of novel catalysts for CO2 reduction and may also lead to new reactions for the electrosynthesis of organic molecules. 

Carbon dioxide reduction reaction (CO2RR) is a promising method for converting CO2 into value-added products. CO2RR over single-atom catalysts (SACs) is widely known to result in chemical compounds such as carbon monoxide and formic acid that contain only one carbon atom (C1). Indeed, at least two active sites are commonly believed to be required for C–C coupling to synthesize compounds, such as ethanol and propylene (C2+), from CO2. However, experimental evidence suggests that iron phthalocyanine (PcFe), which possesses only a single metal center, can produce a trace amount of C2+ products. To the best of our knowledge, the mechanism by which C2+ products are formed over a SAC such as PcFe is still unknown. Using density functional theory (DFT), we analyzed the mechanism of the CO2RR to C1 and C2+ products over PcFe. Due to the high concentration of bicarbonate at pH 7, CO2RR competes with HCO3– reduction. Our computations indicate that bicarbonate reduction is significantly more favorable. However, the rate of this reaction is influenced by the H3O+ concentration. For the formation of C2+ products, our computations reveal that C–C coupling proceeds through the reaction between in situ-formed CO and PcFe(“0”)–CH2 or PcFe(“-I”)–CH2 intermediates. This reaction step is highly exergonic and requires only low activation energies of 0.44 and 0.24 eV for PcFe(“0”)–CH2 and PcFe(“-I”)–CH2. The DFT results, in line with experimental evidence, suggest that C2+ compounds are produced over PcFe at low potentials whereas CH4 is still the main post-CO product. 

Formox, a highly energy-intensive process, currently serves as the primary source of formaldehyde (HCHO), for which there is a crucial and steadily growing chemical demand. The alternative electrochemical production of HCHO from C1 carbon sources such as CO2 and CO is still in its early stages, with even the few identified cases lacking mechanistic rationalization. In this study, we demonstrate that cobalt phthalocyanine (CoPc) immobilized on multiwalled carbon nanotubes (MW-CNTs) constitutes an excellent electrocatalytic system for producing HCHO with productivity through the direct reduction of CO, the two-electron reduction product of CO2. By carefully adjusting both the pH and the applied potential, we identified conditions that enable the production of HCHO with a partial current density of 0.64 mA cm–2 (17.5% Faradaic efficiency, FE) and a total FE of 61.2% for the liquid products (formaldehyde and methanol). A reduction mechanism is proposed.