The reduction and oxidation (redox) of transition metals allow storing charge/energy in Li-ion batteries and electrochemical capacitors and play an important role in catalysis of electrochemical reactions, such as oxygen reduction reaction (ORR) in fuel cells and metal-air batteries and oxygen evolution reaction (OER) in electrolytic cells. In this review, we present and discuss a universal origin of inductive effect associated with metal substitution in Ni, Co, Fe, Mn-based complexes, (hydr-)oxides, and lithium intercalation compounds by alignment of the electron levels of metal complex redox with partially filled metal d-state and oxygen p-state of oxides on the absolute energy scale. Increased redox potentials of metal complexes and oxides are shown to correlate with the increased electronegativity of the substituting metal, which results in the enhancement of the ORR/OER activity. Such observations provide new insights into potential strategies to optimize ORR/OER catalytic activity by tuning the redox properties of metal sites.

Ruthenium pyrochlores, that is, oxides of composition A2Ru2O7−δ, have emerged recently as state-of-the-art catalysts for the oxygen evolution reaction (OER) in acidic conditions. Here, we demonstrate that the A-site substituent in yttrium ruthenium pyrochlores Y1.8M0.2Ru2O7−δ (M = Cu, Co, Ni, Fe, Y) controls the concentration of surface oxygen vacancies (VO) in these materials whereby an increased concentration of VO sites correlates with a superior OER activity. DFT calculations rationalize these experimental trends demonstrating that the higher OER activity and VO surface density originate from a weakened strength of the M–O bond, scaling with the formation enthalpy of the respective MO x phases and the coupling between the M d states and O 2p states. Our work introduces a novel catalyst with improved OER performance, Y1.8Cu0.2Ru2O7−δ, and provides general guidelines for the design of active electrocatalysts.

Two-dimensional (2D) carbides, nitrides, and carbonitrides known as MXenes are emerging materials with a wealth of useful applications. However, the range of metals capable of forming stable MXenes is limited mostly to early transition metals of groups 3−6, making the exploration of properties inherent to mid or late transition metal MXenes very challenging. To circumvent the inaccessibility of MXene phases derived from mid-to-late transition metals, we have developed a synthetic strategy that allows the incorporation of such transition metal sites into a host MXene matrix. Here, we report the structural characterization of a Mo 2 CT x :Co phase (where T x are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (β-Mo 2 C:Co) through a two-step synthesis: first an intercalation of gallium yielding Mo 2 Ga 2 C:Co followed by removal of Ga via HF treatment. Extended X-ray absorption fine structure (EXAFS) analysis confirms that Co atoms occupy Mo positions in the Mo 2 CT x lattice, providing isolated Co centers without any detectable formation of other cobalt-containing phases. The beneficial effect of cobalt substitution on the redox properties of Mo 2 CT x :Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo 2 CT x catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo 2 CT x :Co to the favorable binding of hydrogen on the oxygen terminated MXene surface that is strongly influenced by the substitution of Mo by Co in the Mo 2 CT x lattice. In addition to the remarkable HER activity, Mo 2 CT x :Co features excellent operational and structural stability, on par with the best performing non-noble metal-based HER catalysts. Overall, our work expands the compositional space of the MXene family by introducing a material with site-isolated cobalt centers embedded in the stable matrix of Mo 2 CT x . The synthetic approach presented here illustrates that tailoring the properties of MXenes for a specific application can be achieved via substitution of the host metal sites by mid or late transition metals.

Development of efficient catalysts for the direct hydrogenation of CO2 to methanol is essential for the valorization of this abundant feedstock. Here we show that a silica-supported Cu/Mo2CTx (MXene) catalyst achieves a higher intrinsic methanol formation rate per mass Cu than the reference Cu/SiO2 catalyst with a similar Cu loading. The Cu/Mo2CTx interface can be engineered owing to the higher affinity of metallic Cu for the partially reduced MXene surface (in preference to the SiO2 surface) and the mobility of Cu under H2 at 500 C.Increasing the reduction time, the Cu/Mo2CTx interface becomes more Lewis acidic due to the higher amount of Cu + sites dispersed onto the reduced Mo2CTx and this correlates with an 2 increased rate of CO2 hydrogenation to methanol. The critical role of the interface between Cu and Mo2CTx is further highlighted by DFT calculations that identify formate and methoxy species as stable reaction intermediates.