Electrocatalytic hydrogen production via transition metal complexes offers a promising approach for chemical energy storage. Optimal platforms to effectively control the proton and electron transfer steps en route to H2 evolution still need to be established, and redox-active ligands could play an important role in this context. In this study, we explore the role of the redox-active Mabiq (Mabiq = 2–4:6–8-bis­(3,3,4,4-tetramethlyldihydropyrrolo)-10–15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6) ligand in the hydrogen evolution reaction (HER). Using spectro-electrochemical studies in conjunction with quantum chemical calculations, we identified two precatalytic intermediates formed upon the addition of two electrons and one proton to [CoII(Mabiq)­(THF)]­(PF6) (CoMbq ). We further examined the acid strength effect on the generation of the intermediates. The generation of the first intermediate, CoMbq-H1 , involves proton addition to the bridging imine-nitrogen atom of the ligand and requires strong proton activity. The second intermediate, CoMbq-H2 , acquires a proton at the diketiminate carbon for which a weaker proton activity is sufficient. We propose two decoupled H2 evolution pathways based on these two intermediates, which operate at different overpotentials. Our results show how the various protonation sites of the redox-active Mabiq ligand affect the energies and activities of HER intermediates.

H2 is a promising fuel for sustainable energy conversion and storage. The development of effective earth abundant H2 evolution catalysts is integral to advancing hydrogen‐based technologies. H2 evolution by molecular complexes classically involves the formation of metal hydride intermediates. Recently, the use of redox‐active ligands has emerged as an alternate strategy for electron and proton storage. Herein, we examine the electrocatalytic behavior of [CoII(Mabiq)(THF)](PF6) (CoMbq), containing a redox‐active macrocyclic ligand, in acidic, organic media (using para‐cyanoanilinium (pCA) as the proton source). Cyclic voltammetry (CV) and Rotating Ring Disk Electrode (RRDE) voltammetry evidence a pre‐catalytic process that leads to the formation of a protonated, two‐electron reduced intermediate. This species evolves H2 at potentials negative of −1.1 VFc, as confirmed by On‐line Electrochemical Mass Spectrometry (OEMS). OEMS results further reveal a catalyst deactivation pathway. The electrochemical data denote the involvement of the redox‐active Mabiq ligand in the hydrogen evolution reaction (HER), with implications for the use of such scaffolds in electrocatalytic complexes.

In nature, hydrogen is generated by a family of enzymes known as the hydrogenases, which use Fe and/or Ni as cofactors. Inspired by metalloenzyme cofactors, many earth-abundant hydrogen evolution reaction (HER) catalysts have been developed over the last decade, but their HER activity, chemical stability under operation, and overpotential could still be significantly improved. The H2 evolution mechanisms of many molecular catalysts are based on metal hydride formation, however, the role of redox-active ligands has attracted attention recently. Such systems may operate via alternative H2 evolution pathways involving mainly ligand-assisted and ligand-based routes,1, 2 in which the ligands act as H-atom or hydride reservoirs. Electron or proton storage by redox-active ligands could therewith lead to improved HER kinetics and decreased overpotentials. We will present our studies on the H2 evolution activity and possible HER pathways of a cobalt complex, containing a redox active macrocyclic biquinazoline ligand (Mabiq).3, 4 In the literature an electron transfer series of Co-Mabiq complexes exemplifies ligand-centered reduction among the formally ‘low valent’ compounds.5 The role of the ligand in H2 the evolution activity of a Co-Mabiq complex is the main focus of our study. The electrocatalytic behavior of [CoII(Mabiq)(THF)](PF6) (CoMbq) was studied in organic media (acetonitrile) using para-cyanoanilinium as the proton source.4 Besides the confirmed H2 evolution by CoMbq, the combined on-line electrochemical mass spectrometry (OEMS) and rotating ring disk electrode (RRDE) techniques denote a pre-catalytic process that involves formation of a protonated, two electron reduced intermediate (Figure1). Our results further indicate a competing deactivation pathway that is either time dependent or turnover dependent. The potential intermediate Co-Mabiq complexes in various oxidation states5 each exhibit distinct absorption spectroscopic features. Hence, in our study, bulk electrolysis and UV-Vis absorption spectrometry were further carried out to characterize the active and inactive CoMbq intermediates. Based on these findings, modifications to the ligand backbone may prevent the formation of the inactive intermediates that lead to inhibition of the deactivation pathway, and may therewith enable a prolonged activity for H2 evolution by the CoMbq complex. The combined results of our studies provide an insight into the role of Mabiq and redox-active ligands in general in the HER pathway while also providing a comprehensive toolbox of electrochemical and spectroscopic techniques to allow a deep understanding for this class of molecular H2 evolution catalysts. References: B. H. Solis, A. G. Maher, D. K. Dogutan, D. G. Nocera, and S. Hammes-Schiffer, Proc. Natl. Acad. Sci. USA, 113 (3), 485-492 (2016). A. Z. Haddad, S. P. Cronin, M. S. Mashuta, R. M. Buchanan, and C. A. Grapperhaus, Inorg. Chem., 56 (18), 11254-11265 (2017). E. Muller, G. Bernardinelli, and A. Vonzelewsky, Inorganic Chemistry, 27 (25), 4645-4651 (1988). G. C. Tok, A. T. S. Freiberg, H. A. Gasteiger, and C. R. Hess, ChemCatChem, 11 (16), 3973-3981 (2019). E. V. Puttock, P. Banerjee, M. Kaspar, L. Drennen, D. S. Yufit, E. Bill, S. Sproules, and C. R. Hess, Inorg. Chem., 54 (12), 5864-5873 (2015). Figure 1

Layered transition metal oxides like NCAs (LiNixCoyAlzO2, with x+y+z=1) and NCMs (LiNixCoyMnzO2, with x+y+z=1) are used as cathode active materials (CAMs) for high energy Li-ion batteries due to their high capacity. However, at high upper cut-off potentials, those CAMs suffer from structural instabilities, resulting in severe capacity fading and thus limiting the accessible capacity that can be obtained. Possible causes for the capacity fade at high cut-off potentials and high state-of-charge (SOC) include the (electro)chemical oxidation of the electrolyte oxidation and transition metal (TM) dissolution from the CAM surface.1 Furthermore, layered TM-oxides are known to release lattice oxygen from the near-surface region at high SOC (i.e., at ≈80% SOC when referenced to the total amount of lithium), resulting in reactive oxygen species that induce electrolyte oxidation and HF formation.2 This release of lattice oxygen results in a surface reconstruction from the pristine layered structure to a more resistive spinel- or rocksalt-like structure, thereby inducing an impedance build-up on the cathode. Diffusion of dissolved transition metals to the anode and their subsequent deposition on the anode active material particles can also have a severe effect on cell aging, as the accumulation of metal species on the graphite anode has shown to catalyze the degradation of the protective anode solid/electrolyte interphase (SEI), eventually resulting in the loss of active lithium and in an anode impedance growth. Since the dissolution of manganese is considered to have the most detrimental effect on the anode SEI compared to cobalt and nickel,3 manganese-free NCAs (e.g., LiNi0.8Co0.15Al0.05O2) might have an advantage over manganese-containing NCMs. In this study, we will examine the potential-dependent dissolution of Ni and Co in NCA/graphite cells using operando XAS, and compare it to the potential-dependent dissolution of Ni, Co, and Mn from LiNi0.6Co0.2Mn0.2O2 (NMC622) that we had determined previously by operando XAS.4 Owing to the specially designed geometry of the operando XAS cell,5 we can spectroscopically access and independently investigate the concentration and oxidation state of transition metals, both dissolved in the electrolyte and deposited within the graphite anode. This is illustrated for an NCA/graphite cell in Figure 1. We will also examine the effect of lattice oxygen release from NCA on the NCA/graphite full-cell performance by applying different techniques: We employ a three-electrode Swagelok® type T-cell with a gold wire micro reference electrode (µ-GWRE)6 to quantify the anode and the cathode impedance over the course of 100 cycles as a function of the upper cutoff voltage. In addition, on-line electrochemical mass spectrometry (OEMS)7 is applied to detect the onset SOC for the release of lattice oxygen. From these comparisons, we aim to get a detailed understanding about the influence of transition metal dissolution from NCA on capacity fade and cycle life. References: J. A. Gilbert, I. A. Shkrob, and D. P. Abraham, Journal of The Electrochemical Society, 164 (2), A389-A399 (2017). R. Jung, M. Metzger, F. Maglia, C. Stinner, and H. A. Gasteiger, Journal of The Electrochemical Society, 164 (7), A1361-A1377 (2017). S. Solchenbach, G. Hong, A. T. S. Freiberg, R. Jung, and H. A. Gasteiger, Journal of The Electrochemical Society, 165 (14), A3304-A3312 (2018). R. Jung, F. Linsenmann, R. Thomas, J. Wandt, S. Solchenbach, F. Maglia, C. Stinner, M. Tromp, and H. A. Gasteiger, Journal of The Electrochemical Society, 166 (2), A378-A389 (2019). J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger, and M. Tromp, Journal of Materials Chemistry A, 4 (47), 18300-18305 (2016). S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, and H. A. Gasteiger, Journal of The Electrochemical Society, 163 (10), A2265-A2272 (2016). N. Tsiouvaras, S. Meini, I. Buchberger, and H. A. Gasteiger, Journal of The Electrochemical Society, 160 (3), A471-A477 (2013). Figure 1