Prussian Blue Analogues as Promising Thermal Power Generation Materials

Abstract

application range of the battery materials from energy storage to energy conversion. The exploration of high-α material with different sign is quite effective to fabricate a high-η thermocell.Among the battery materials, PBAs, whose chemical formula are Li x M[Fe(CN) 6 ] y and Na x M[Fe(CN) 6 ] y (M is transition metal), are promising candidates for the cathode materials in lithium-ion/sodium-ion secondary batteries. [8] For example, thin film of Li 1.6 Co[Fe(CN) 6 ] 0.9 shows a high capacity of 132 mAh/g with a good cyclability. Most of PBAs have the face-centered cubic (fcc) (Fm3m; Z = 4) or trigonal (R3m; Z = 3) structures. [9] They consist of three-dimensional (3D) junglegym-type host framework and guest Li + /Na + ions and H 2 O molecules, which are accommodated in nanopores of the framework. The framework contains considerable [Fe(CN) 6 ] vacancies (10-30 %). The discharge curves of CoÀ and Mn-PBA show characteristic plateaus, whose redox reactions are well assigned by means of X-ray absorption spectroscopy. [10,11] Recently, the α values are reported in Li x Co[Fe(CN) 6 ] 0.71 and Figure 2. Discharge curves of (a) Na x Co[Fe(CN) 6 ] 0.71 (NCF71), (b) Na x Co[Fe (CN) 6 ] 0.9 (NCF90) and (c) Na x Mn[Fe(CN) 6 ] 0.83 (NCF83) films measured at 0.1 C. For convenience of explanation, we defined plateaus I, II, II, IV, and V.

Energy harvesting thermocell with use of phase transition

“…We set T c of the LS-HS transition to just above room temperature, by finely adjusting the Fe concentration (y = 0.82). With increase in 6 ] 0.9 (NCF90) cell steeply increase from 0 mV to ~ 120 mV around 320 K. Our observation indicates that the tertiary battery with use of phase transition is a promising independent power supply for the IT/ IoT devices.…”

“…By means of the X-ray absorption spectroscopy (XAS), Takachi et al 13 indicated that the lower-and higher-lying plateaus of NCF90, which is in the LS phase at 300 K are ascribed to the redox reaction of Fe and Co, respectively. Then, the higher-lying plateau of NCF82 in the LS phase is reasonably ascribed to Co III 6 ] 0.9 . At 323 K in the HS phase, the curve significantly changes.…”

“…Another energy harvesting technology with low cost and high efficiency is the thermocell that consists of cathode and anode materials with different thermal coefficients (α = dV/dT) of the redox potential (V) between the anode (α anode ) and cathode (α cathode ) materials. Several researchers [2][3][4][5][6][7][8] reported that such a thermocell can convert environmental thermal energy to electric energy via the so-called thermal charging effect. Hereafter, we call such devices as "tertiary batteries", because the cell can be charged by thermal energy not by electric energy.…”

“…The thermally-charged energy is converted to the electric energy in the discharge process at T L . Fukuzumi et al 6 fabricated a thermocell, consisting of two types of PBA films, Na x Mn[Fe(CN) 6 ] 0.83 (MCF83) and Na x Co[Fe(CN) 6 ] 0.9 (NCF90), with different α values. The NMF83/ NCF90 cell shows a thermal voltage of V cell ~ 40 mV in the thermal cycle between T L (=286 K) and T H (=313 K).…”

“…One is that the current V cell (several tens mV) is too low to drive the IT/IoT devices. For example, the NMF83/NCF90 cell (α cathode −α anode = 1.7 mV/K) 6 shows a thermal voltage of V cell ~ 40 mV between T L (=286 K) and T H (=313 K). So far, α of wide range of battery materials were reported; α = −0.3-1.4 mV/K in Prussian blue analogues 8 , α = 0.2-1.1 mV/K in several conjugated polymers 9 , α = 0.0−0.9 mV/K in Na x CoO 2 10 and α = 0.9 mV/K in Li x FePO 4…”