Deciphering Ligand Controlled Structural Evolution of Prussian Blue Analogues and Their Electrochemical Activation during Alkaline Water Oxidation

Abstract

Herein, we have demonstrated the control over the structure of precatalysts to tune the properties of the active catalysts and their water oxidation activity. The reaction of K 3 [Fe(CN) 6 ] and Na 2 [Fe(CN) 5 (NO)] with Co(OH) 2 @CC produced precatalysts PC-1 and PC-2, respectively, with distinct structural and electronic features. The replacement of the À CN group with strong π-acceptor À NO modulates the electronic and atomic structure of PC-2. As a result, a facile electrochemical transformation of PC-2 into active catalyst FeÀ Co-(OH) 2 -Co(O)OH (AC-2) has been attained only in 15 CV cycles while 600 CV cycles are required for the electrochemical activation of PC-1 into AC-1. The X-ray absorption studies reveal the contraction of the CoÀ O and FeÀ O bond in AC-2 because of the presence of a higher amount of Co 3 + and Fe 3 + than in AC-1. The high valent Co 3 + and Fe 3 + modulates the electronic properties of AC-2 and assists in the OÀ O bond formation, leading to the improved water oxidation activity.

Iron(iii) ion-assisted transformation of ZIF-67 to a self-supported FexCo-layered double hydroxide for improved water oxidation

“…3c). 12 The CV of Co-LDH@NF showed a peak at a higher potential (1.39 V vs RHE) than that in Fe 0.4 Co-LDH@NF. The Fe( iii ) ion in Fe 0.4 Co-LDH@NF induces the draining of the electron density from Co( ii ), facilitating the oxidation of Co( ii ) to Co( iii ) and hence Co( iv ).…”

“…The Co 2p 3/2 and 2p 1/2 peaks at 783.1 and 797.39 eV were ascribed to Co 2+ and the peaks at 779.68 eV and 795.47 eV were attributed to Co 3+ . 12 The Co 2p 3/2 peak of Fe 0.4 Co-LDH showed a positive shift of 0.6 eV compared to that of Co-LDH, indicating an increase in the positive charge density on the Co atom due to Fe introduction into the structure of the catalyst. In addition, the Co 2p 3/2 –Co 2p 1/2 spin–orbit coupling spacing value was decreased from 15.83 eV (in Co-LDH) to 15.32 eV in Fe 0.4 Co-LDH.…”

“…Recently, we and other groups have also explored MOFs as the precursors of LDHs to achieve exceptional electrochemical properties. 9–12…”

“…The facile formation of Co IV = O species in Fe 0.4 Co-LDH@NF helps in the attack of nucleophile OH − to promote the O–O bond formation. 12…”

“…Recently, we and other groups have also explored MOFs as the precursors of LDHs to achieve exceptional electrochemical properties. [9][10][11][12] The treatment with Fe(III)-salt results in the complete destruction of the structure of ZIF-67 and produces hierarchical nanosheets of Fe x Co-LDH@NF. To realize the effect of the introduction of the strong Lewis acid Fe(III) in the structure of LDH, we have prepared two more catalysts-Co-LDH [prepared by the reaction of ZIF-67 with Co(II)] and ZnCo-LDH [prepared by the reaction of ZIF-67 with Zn(II)].…”