Ion Transport and Competition Effects on NaTi₂(PO₄)₃ and Na₄Mn₉O₁₈ Selective Insertion Electrode Performance

Abstract

We evaluate the efficiency and capacity of electrochemically reversible insertion electrodes for use in targeted ion removal applications in aqueous solutions. The relative attributes of insertion material chemistry are evaluated by comparing the performance of two different sodium insertion materials, NaTi(PO) and NaMnO, in different electrolyte environments. We performed experiments over a range of solution compositions containing both sodium and other non-inserting ions, and we then developed mechanistic insight into the effects of solution concentration and composition on overpotential losses and round trip Coulombic efficiency. In dilute aqueous streams, performance was limited by the rate of ion transport from the bulk electrolyte region to the electrode interface. This leads to slow rates of ion removal, large overpotentials for ion insertion, parasitic charge loss due to water electrolysis, and lower round trip Coulombic efficiencies. This effect is particularly large for insertion electrodes with redox potentials exceeding the water stability window. In solutions with high background concentrations of non-inserting ions, the accumulation of non-inserting ions at the electrode interface limits inserting ion flux and leads to low ion removal capacity and round trip Coulombic efficiency.

Theory of Water Desalination with Intercalation Materials

“…Although our general theory below could be applied to arbitrary intercalation compounds, adsorbing cations or anions, we will focus on the more common case of cation intercalation. Examples of such materials are nickel hexacyanoferrate (abbreviated as NiHCF, with chemical structure NiFe(CN) 6 ), which is a Prussian Blue analogue (PBA) [5][6][7][8][9][10][11], sodium manganese oxide (NMO) [12][13][14][15][16], and iron or titanium phosphates [16,17] (which are also used in Li-ion batteries). In response to an applied voltage, cations intercalate, or are reversibly inserted, into the these redox-active host materials.…”

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…As for the specific removal of a particular cations among other cations in solution such as Na + , K + , Mg 2+ , Ca 2+ , NH 4 + , etc., sodium manganese oxides, [39,62] NASICON-type phosphates, [301] and metal hexacyanometalates [40,51,61] have been investigated. NMO and NTP both show affinity for Na + over K + and some divalent ions (Ca 2+ and Mg 2+ ).…”

“…[39,62,301] For example, Kim et al [62] determined that the NMO electrode is 13 and 6-8 times more selective toward Na + compared to K + and Mg 2+ /Ca 2+ respectively in a mixed electrolyte containing equal concentrations of each ions (Figure 28c). NTP has been also evaluated, [301] where its affinity toward Na + removal is nearly an order of magnitude over K + , Ca 2+ , and Al 3+ . Depending on the lattice geometries of NMO and NTP, the favorability of ion insertion into the crystal lattice may be associated with losing their hydration shells.…”

“…NMO and NTP both show affinity for Na + over K + and some divalent ions (Ca 2+ and Mg 2+ ). [ 39,62,301 ] For example, Kim et al. [ 62 ] determined that the NMO electrode is 13 and 6–8 times more selective toward Na + compared to K + and Mg 2+ /Ca 2+ respectively in a mixed electrolyte containing equal concentrations of each ions (Figure 28c).…”

Desalination with porous electrodes : Mechanisms of ion transport and adsorption

“…During charging, there is an input of electrical energy, which can partially be recovered during discharge [140]. Many CDI architectures are possible including the use of flowing electrodes [5,77,79], chemically modified electrodes [151], redox-active materials [91,152], and the addition of ion-exchange membranes [37,38,71]. The most commonly used CDI architecture is the flow-by cell, which employs film electrodes with a spacer channel placed in between, through which the solution flows along the electrodes [2].…”