Harnessing the electronic structure of active metals to lower the overpotential of the electrocatalytic oxygen evolution reaction

Abstract

Despite substantial advancements in the field of the electrocatalytic oxygen evolution reaction (OER), the efficiency of earth-abundant electrocatalysts remains far from ideal. The difficulty stems from the complex nature of the catalytic system, which limits our fundamental understanding of the process and thus the possibility of a rational improvement of performance. Herein, we shed light on the role played by the tunable 3d configuration of the metal centers in determining the OER catalytic activity by combining electrochemical and spectroscopic measurements with an experimentally validated computational protocol. One-dimensional coordination polymers based on Fe, Co and Ni held together by an oxonato linker were selected as a case study because of their well-defined electronic and geometric structure in the active site, which can be straightforwardly correlated with their catalytic activity. Novel heterobimetallic coordination polymers were also considered, in order to shed light on the cooperativity effects of different metals. Our results demonstrate the fundamental importance of electronic structure effects such as metal spin and oxidation state evolutions along the reaction profile to modulate ligand binding energies and increase catalyst efficiency. We demonstrated that these effects could in principle be exploited to reduce the overpotential of the electrocatalytic OER below its theoretical limit, and we provide basic principles for the development of coordination polymers with a tailored electronic structure and activity.

Scheme 1. Coordinating and bridging mode of the oxonato ligand in the studied CPs (top). Stereochemistry at the metal center (bottom).

Fig. 1. (a) 3D atomic representation of the CP structures for the systems considered in this work. (b) PXRD patterns of (trans,cis)-NiNi (black), (trans,cis)-NiCo (red), (trans,cis)-NiMn (blue), and (trans,cis)-NiFe (green). (c) PXRD patterns of (cis)-CoCo (black), (cis)-FeMn (red), (cis)-CoFe (blue), and (cis)-CoNi (green) samples.

Fig. 2. (a) Computed (red) vs. experimental (black) UV/Vis spectra for the heterobimetallic CPs, together with an illustrative scheme of the electronic configuration of the d orbitals at the metal centers. For the (trans,cis)-M1M2 case, the computed spectrum is the convolution of the spectra obtained considering both possible cluster environment configurations, i.e., M1(trans)M2(cis) and M1(cis)M2(trans). (b) Computed (red) vs. experimental (black) UV/Vis spectra for the homometallic CPs, together with an illustrative scheme of the electronic configuration of the d orbitals in their pseudo-octahedral ligand field.

Fig. 3. (a) Cyclic voltammetry curves for all synthesized CPs in 1 M KOH with a scan rate of 100 mV s⁻¹ (the reverse scan was omitted for clarity). (b) Experimental overpotentials of CPs obtained at j = 10 mA cm⁻². (c) Tafel plots obtained from LSV curves recorded at a 5 mV s⁻¹ scan rate (see Fig. S3†) and (d) histogram of Tafel slopes.

Fig. 4. (a) Experimental Raman spectrum of embedded (trans,cis)-NiFe (solid black) and the computed one. (b) Top: experimental Raman spectra of embedded (trans,cis)-NiFe (solid grey), after KOH treatment (solid red), after 1 CV (solid blue) and after 50 CV (solid green). Bottom: computed Raman spectra for (trans,cis)-NiFe (dashed grey) and (trans,cis)-NiFe with two OH⁻ coordinated to Fe (dashed red).

Fig. 5. EDX elemental analysis of Fe (a1)–(d1) and Ni (a2)–(d2). (a) NTC matrix + (trans,cis)-NiFe untreated; (b) NTC matrix + (trans,cis)-NiFe treated with KOH; (c) NTC matrix + (trans,cis)-NiFe treated with KOH after 1CV; (d) NTC matrix + (trans,cis)-NiFe treated with KOH after 50 CVs.

Fig. 6. OER free energy profile (left) and spin density evolution along the reaction path (right) for (a) (cis)-CoCo, (b) (trans,cis)-NiFe and (c) (trans,cis)-NiNi. To facilitate the chemical interpretation of the results, the metal oxidation state and local spin configuration are also reported. They were determined using Mulliken spin populations (they were rounded up to the nearest integer). The free energies for the other catalysts considered in this work can be found in Tables S14–S24.

Fig. 7. Free energy difference between the MOOH and MOH intermediates as well as its decomposition into the MOH → MO (orange bar) and MO → MOOH (blue bar) reaction free energies. The horizontal black lines represent the ideal case in which both steps have the same energy. The dashed violet horizontal line represents the theoretical value for the energy difference between MOOH and MOH obtained from linear scaling relationships. The active metal is emphasized in red.