Tomas Horsten

Nature, Published online: 29 July 2024; doi:10.1038/d41586-024-02450-1

Nature Communications, Published online: 06 July 2024; doi:10.1038/s41467-024-50081-x

We examine the effect of metal impurities on nickel foam electrodes by electrodepositing Cu, Cr, Mn, Fe, Co and Ni on nickel foam and studying them in OER. Using a variety of methods, we show that even small amounts of impurities can have large effects. This highlights the importance of impurities for electrode design and large-scale application.

Abstract

The energy transition and the implementation of new electrochemical technologies will result in an increased reliance on critical raw materials for electrodes. Their large-scale application will ultimately mean working with lower grade materials. Here we study the influence of metal impurities on nickel foam electrocatalysts. We do this by electrodepositing known amounts of first-row transition metals (Cu, Cr, Mn, Fe, Co and Ni) on nickel foam and studying their performance in the oxygen evolution reaction (OER) as a model reaction. The electrodes’ performance is studied using cyclic voltammetry (CV), linear sweep voltammetry (LSV), Tafel analysis, stepwise chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Combining these results with microscopy analysis, we show that even small amounts of transition-metal impurities have profound effects on the catalytic performance of nickel electrodes. The changes affect the OER onset potential (the energy that the system requires to convert OH^– to O₂) and the Tafel slope of each electrode (the electrode's initial activity in the OER onset region). Our results highlight the implications of such impurities on electrode design and future large-scale application.

The two-electron reduction to low-valent rhodium −I is underpinned by structural rearrangements that defines whether the redox transitions are stepwise or in a bielectronic fashion. Study of a parametrized series of Rh bis(diphosphine) complexes identifies how ligand parameters influence this behaviour, whereas an expedient computed electronic parameter allows correlating electronics and geometrics with redox potentials.

Abstract

Rhodium complexes in the −I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)₂]NTf₂ (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the Rh^I parents, Rh^−I d ¹⁰-complexes were obtained and characterized spectroscopically, including ¹⁰³Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E ⁰ (Rh^I/0) and second E ⁰ (Rh^0/−I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P−Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C−O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

By purely thermolytic treatment of different industrially produced lignins in highly concentrated aqueous KOH either vanillic acid or protocatechuic acid can be accessed in attractive yields and good selectivity. By employing a work-up of the reaction mixture with ion-exchange resins, the products could be obtained, and the media directly reused in further reactions.

Abstract

A new and practical method for the thermal degradation of technically relevant bio-based lignin is presented. By heating a solution of lignin in highly concentrated caustic potash, vanillic acid is almost exclusively obtained in yields up to 10.6 wt %. By altering the reaction parameters, the selectivity of the reaction can be shifted towards the demethylation product, protocatechuic acid, which is obtained in a yield of 6.9 wt %. Furthermore, the procedure was applicable to different types of Kraft and organosolv lignin. To create an economically feasible process, ion exchange resins were used for the work-up of the highly caustic reaction media without neutralizing the complete mixture. By the selective removal of the desired vanillic acid from the caustic potash, this alkaline media could directly be reused for at least 5 further lignin degradations without significant loss of yield.