Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations

ARTICLE https://doi.org/10.1038/s41467-020-19729-2 OPEN Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations 6, 1234567890():,; Mikaela Görlin 1,2 ✉, Joakim Halldin Stenlid 1, Sergey Koroidov1, Hsin-Yi Wang 1, Mia Börner1, Mikhail Shipilin1, Aleksandr Kalinko 3,4, Vadim Murzin 4,5, Olga V. Safonova 6, Maarten Nachtegaal Abdusalam Uheida7, Joydeep Dutta 7, Matthias Bauer3, Anders Nilsson1 & Oscar Diaz-Morales 1,8 ✉ Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni2+/(3+δ)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH > NaOH ≈ KOH > RbOH > LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect. 1 Department of Physics, AlbaNova University Center, Stockholm University, SE-106 91 Stockholm, Sweden. 2 Department of Chemistry - Ångström laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden. 3 Department of Chemistry and Center for Sustainable Systems Design (CSSD), University of Paderborn, Warburger Strasse 100, D-33098 Paderborn, Germany. 4 Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22607 Hamburg, Germany. 5 Bergische Universität Wuppertal, Gaußstraße 20, D-42119 Wuppertal, Germany. 6 Paul Scherrer Institute, CH-5232 Villigen, Switzerland. 7 Functional Materials, Department of Applied Physics, School of Engineering Sciences, KTH Royal Institute of Technology, Hannes Alfvéns väg 12, SE-114 19 Stockholm, Sweden. 8 Applied Electrochemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden. ✉email: ; NATURE COMMUNICATIONS | (2020)11:6181 | https://doi.org/10.1038/s41467-020-19729-2 | www.nature.com/naturecommunications 1 ARTICLE T NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19729-2 o slow down the growth of the steadily increasing carbon footprint, a transition to renewable energy is imperative1. Electrochemical water splitting (2 H2O → 2 H2 + O2) offers a zero-carbon route to hydrogen (H2) from water2, where the main cause of energy loss and cost in this process is the anodic oxygen evolution reaction (OER)3. To make water a viable source for H2, efforts therefore need to be focused on more efficient OER electrocatalysts4. Here, we investigate the OER activity and redox-activity using X-ray absorption spectroscopy (XAS) of a Ni–Fe oxyhydroxide (OOH) electrocatalyst in the presence of alkali metal cations (Li+, Na+, K+, Rb+, Cs+), which is one of the best performing catalysts in alkaline media5,6. Electrolyte cations are known to impact the oxygen evolution activity of various oxide-derived electrocatalysts7–12, where the activity seemingly follows the trend in cation size, typically increasing from small (Li+) to large (Cs+) cations. However, the role of cations in OER is not entirely understood due to reoccurring discrepancies from the trend in cation size. Moving down the alkali metal group, the cation size increases from Li+ to Cs+, along with modifications of several parameters such as decreasing Lewis acidity and electronegativity13, increasing molar conductivity14 and proton affinity15, and increasing basicity of the corresponding alkali hydroxide16. Small cations (Li+) generally form stronger noncovalent interactions with water compared to large cations (Cs+), and water becomes more structured around the central cation17. Smaller cations are therefore referred to as “structure makers” and bigger cations as “structure breakers”. The strong noncovalent interactions between small cations and water disrupt the native H-bonded network and result in large solvation shells17, which is usually associated with slower reorientation times and slower kinetics14, and observed to alter redox-kinetics of metalcenters18. However, there are several unexpected deviations from the cation size trend. Michael et al.19. showed that the activity of the NiOOH catalyst increased from LiOH to CsOH, whereas the activity of Ni(Fe)OOH was lower in CsOH compared to both NaOH and KOH. Zaffran et al.7 demonstrated using density functional theory (DFT) that electrolyte cations modify the adsorption energies of OER intermediates (*OH, *O, *OOH) of the Ni–Fe catalyst, where especially small and strongly acidic cations are not beneficial for OER. Garcia et al.8 found using surface ennhanced Raman spectroscopy that large cations promote peroxo-like “active oxygen” species (O− or O2−) in NiOOH to a larger extent than small cations, which could explain the higher OER activity in the presence of large cations. Yet, several inconsistencies in the activity trends put the mechanism by which the alkali metal cations modify the OER activity at question. The catalytic site in the Ni–Fe catalysts has been characterized using in situ XAS, where several studies reveal contradicting information regarding the impact of Fe on the redox-activity of the Ni-site20–27. According to several DFT studies, the Fe-site plays a significant role as a low overpotential-site in the bimetallic Ni–Fe active site and provides optimal adsorption energies for the OER intermediates28–31, which is also supported by experiments32. In addition, Fe at coordinatively unsaturated sites such as edge sites or defect sites are predicted as more reactive33–35. Recent studies employing in situ soft XAS at the O K-edge also confirmed anionic redox-activity involving the lattice oxygens in the Ni–Fe catalyst36,37, most likely related to the “active oxygen” earlier identified in Raman spectroscopy38–41. Here, we employ in situ XAS at the Ni and Fe K-edges to probe the local atomic structure and metal redox-states of an electrodeposited Ni65Fe35(OOH) catalyst in the presence of alkali metal cations at alkaline pH (i.e. LiOH, NaOH, KOH, RbOH, CsOH). We further utilize DFT to explore the correlations between the OER activity and three reactivity properties: The local electron 2 attachment energy E(r)42, the local average ionization energy Ī(r)43, and the electrostatic potential V(r), to predict how electrolyte cations influence the local Lewis acidity/basicity of the Ni–Fe(OOH) lattice sites44–46. In short, our data conclude (...truncated)