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Linking capacity loss and retention of nickel hexacyanoferrate to a two-site intercalation mechanism for aqueous Mg and Ca ions
Prussian blue analogues (PBAs) are promising cation intercalation materials for electrochemical desalination and energy storage applications. Here, we investigate the mechanism of capacity fade and degradation of nickel hexacyanoferrate (NiHCFe) during galvanostatic cycling in aqueous electrolytes t...
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| Published in: | Physical chemistry chemical physics : PCCP 2019-09, Vol.21 (36), p.2177-2188 |
|---|---|
| Main Authors: | , , |
| Format: | Article |
| Language: | |
| Online Access: | Get full text |
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| Summary: | Prussian blue analogues (PBAs) are promising cation intercalation materials for electrochemical desalination and energy storage applications. Here, we investigate the mechanism of capacity fade and degradation of nickel hexacyanoferrate (NiHCFe) during galvanostatic cycling in aqueous electrolytes that are rich in either Mg
2+
or Ca
2+
. We combine experimental characterization, first principles electronic structure calculations, statistical mechanics and lattice-percolation modeling of electron transfer to elucidate the mechanisms responsible for the degradation of NiHCFe and its partial retention of capacity. Electrochemical characterization of porous NiHCFe electrodes suggests a two-site intercalation mechanism, while spectroscopy reveals the presence of Ni
2+
and Fe(CN)
6
3−
ions in the electrolyte post cycling in Mg
2+
(aq)
. Using simple coprecipitation reactions, we show that Mg
2+
and Ni
2+
can coexist in the lattice framework, forming stable PBAs. Galvanostatic cycling of these PBAs shows that the presence of Mg
2+
in the lattice framework results in the dissolution of Mg
1.5
Fe
III
(CN)
6
in water during oxidation. We propose that Mg
2+
can partially substitute Ni
2+
ions in the lattice framework during galvanostatic cycling, displacing the substituted Ni
2+
ions into interstitial sites. Based on differential capacitance analysis we show that Mg
2+
intercalates into interstitial sites at ∼0.45 V
vs.
Ag/AgCl and it displaces Ni
2+
in the lattice framework at ∼0.05 V
vs.
Ag/AgCl. Substitution of Ni
2+
leads to Fe(CN)
6
3−
and Ni
2+
ions being removed into the electrolyte during oxidation. Using first principles density functional theory (DFT) calculations combined with a statistical mechanics model, we verify the thermodynamic feasibility of the proposed reaction mechanism and predict the fraction of Ni
2+
ions being substituted by Mg
2+
during intercalation. Further, analysis of the electron density distribution and local density of states indicates that Mg
2+
ions can act as insulating defects in the lattice framework that render certain Fe ions electrically inactive and likely contribute to capacity fade along with dissolution of Fe(CN)
6
3−
.
Partial substitution of Ni
2+
in the host lattice of nickel hexacyanoferrate by Mg
2+
or Ca
2+
from aqueous electrolytes leads to rapid capacity fade during galvanostatic cycling, while capacity is retained by intercalation into interstitial sites. |
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| ISSN: | 1463-9076 1463-9084 |
| DOI: | 10.1039/c9cp04115j |