Water-soluble pH-switchable cobalt complexes for aqueous symmetric redox flow batteries

Abstract

A water soluble cobalt complex with two redox couples that fall within the water splitting window can be applied as both the posolyte and negolyte in an aqueous symmetric redox flow battery.

Axial Redox Tuning at a Tetragonal Cobalt Center

“…Typically, the reduction of π systems leads to distinctive structural alterations, particularly in skeletal bond lengths, which are due to the occupation of a LUMO. 12 For example, according to a previous report, the reduction of 2,2′-bipyridine noticeably decreases the C py −C py bond length from 1.490(3) Å for bpy to 1.431(3) Å for bpy •− , because its SOMO possesses an in-phase π-bonding character between two carbon atoms. 24 To see if this is applicable to our system, we have collected the XRD data for both 4 and 5.…”

“…Redox-active transition metal complexes are currently attracting much attention due to their potential usage in energy conversion and storage related technologies , such as photovoltaic cells , and redox flow batteries. − Having five d orbitals, transition metal complexes capable of undergoing multielectron processes are ideal for such applications. ,, The preparation of stable multielectron transferring metal complexes is, however, rather challenging because the redox activity of a metal ion typically involves ligand association–dissociation coupled with a significant structural change. , These changes in geometry can lead to various undesired chemical reactions. Thus, the design of redox-active metal complexes without significant structural alterations during redox reactions may be ideal for use as electrochemically reversible oxidants and reductants for the applications.…”

Leveraging coordination chemistry in the design of bipolar energy storage materials for redox flow batteries

“…For example, a water-soluble octahedral cobalt complex of intermediate oxidation state [Co II (BCPIP) 2 ] 2+ (BCPIP ¼ 2,6-bis[1-(4carboxyphenylimino)ethyl]pyridine), was deployed as the energy storage compound in both negolyte (cycling between [Co I (BCPIP) 2 ] + /[Co II (BCPIP) 2 ] 2+ ) and posolyte ([Co III (BCPIP) 2 ] 3+ / [Co II (BCPIP) 2 ] 2+ ) in a bipolar aqueous RFB, with coulombic efficiencies > 99% for up to 100 cycles. 8 Some organic solvents furnish larger windows of electrochemical stability compared to water, such as acetonitrile (up to $6 V). 9 This enables operation of cells with larger electromotive forces.…”

pH-Switching of the luminescent, redox, and magnetic properties in a spin crossover cobalt(ii) molecular nanomagnet

“…[3][4][5][6] This goal has been successfully achieved using mononuclear rst-row transition metal complexes as both synthetic and theoretical models of dynamic molecular systems (DMS). [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Hence, this simple class of multifunctional and multiresponsive molecular materials has been found helpful as optical and magnetic sensors for chemical and biological sensing, [7][8][9][10] photoluminescent emitters for light-emitting electrochemical cells (LECs), [11][12][13] electro-and photocatalysts for bioinspired molecular recognition and catalysis, [14][15][16][17] photo-and chemo-responsive contrast agents for magnetic resonance imaging (MRI), 18 electrochemical and electrochromic devices for redox ow batteries (RFB), and capacitors for molecular electronics. [19][20][21] In particular, mononuclear spin crossover (SCO) complexes with potentially electro-or photoactive (noninnocent) ligands are promising candidates for spintronic devices or advanced quantum bits (qubits) and quantum gates for molecular spintronics and quantum information processing (QIP).…”