High-temperature thermoelectric properties of W-substituted CaMnO₃

Abstract

The thermoelectric properties of W-substituted CaMn1-xWxO3-δ (x = 0.01, 0.03; 0.05) samples, prepared by soft chemistry, were investigated from 300 K to 1000 K and compared to Nb-substituted CaMn0.98Nb0.02O3-δ. All compositions exhibit both an increase in absolute Seebeck coefficient and electrical resistivity with temperature. Moreover, compared to the Nb-substituted sample, the thermal conductivity of the W-substituted samples was strongly reduced. This reduction is attributed to the nearly two times greater mass of tungsten. Consequently, a ZT of 0.19 was found in CaMn0.97W0.03O3-δ at 1000 K, which was larger than ZT exhibited by the 2% Nb-doped sample.

W-Doped CaMnO2.5and CaMnO3Electrocatalysts for Enhanced Performance in Oxygen Evolution and Reduction Reactions

“…Specifically, the small amount of doped tungsten with high oxidation state of +6 should greatly induce the mixed valence states of manganese from +4 to +2 in CaMn 1-x W x O 3 to balance the charges, without sacrificing the catalytic sites of manganese. 14 A similar tungsten doping effect can be found in CaMn 1-x W x O 2.5 , where the oxidation states varied from +3 to +2. Thus, the electrons from the low oxidation state of manganese could hop to the adjacent high oxidation state of manganese through the oxygen 2p orbitals, resulting in enhanced electrical conductivity.…”

“…The difference in electrical resistivity between CaMn 1-x W x O 2.5 and CaMn 1-x W x O 3 could be intrinsic between oxygen-deficient and pristine perovskites, where electrical resistivity decreases with the decrease in oxygen content. 12,14,15 We further calculated the ORR kinetic parameters of both CaMnO 2.5 and CaMn 0.97 W 0.03 O 2.5 based on the Koutecky-Levich analysis (K-L plot), as shown in Figure 5d. The slopes of linear regression were obtained at 0.8 V versus RHE, relating to the number of the transferred electrons, which remains constant.…”

“…The mixed valence of manganese results in the increased electrical conductivity via double-exchange mechanism, where the electrons can readily hop from low to high oxidation states of manganese through oxygen 2p orbitals. 14,20,21 These new compounds show improved oxygen electrocatalytic activity in 0.1-M KOH electrolyte solution, where the onset potential and current density for both ORR and OER are greatly enhanced over the untreated compounds of CaMnO y (y = 2.5 and 3.0).…”

“…High valence of W 6+ cation was used to replace the B-site Mncation. [14][15][16][17][18][19] With high oxidation state of tungsten, replacing 1-3% of z E-mail: hy66@illinois.edu manganese cations can effectively change the oxidation states of manganese cation between +2 and +4. The mixed valence of manganese results in the increased electrical conductivity via double-exchange mechanism, where the electrons can readily hop from low to high oxidation states of manganese through oxygen 2p orbitals.…”

Influence of tungsten substitution and oxygen deficiency on the thermoelectric properties of CaMnO3−δ

“…Alfaruq et al demonstrated a significant enhancement of the high-temperature thermoelectric properties of CaMnO 3 due to tungsten substitution. 34 Tungsten is an ideal candidate for B-site substitution as it adds two additional negative charge carriers to the Mn 4þ sublattice 33 and is also more than three times heavier than manganese. Consequently, lattice thermal conductivity can be suppressed significantly by mass-difference impurity scattering.…”