Amine-based CO 2 chemisorption has been a longstanding motif under development for CO 2 capture applications, but large energy penalties are required to thermally cleave the N-C bond and regenerate CO 2 for subsequent storage or utilization. Instead, it is attractive to be able to directly perform electrochemical reactions on the amine solutions with loaded CO 2. We recently found that such a process is viable in dimethyl sulfoxide (DMSO) if an exogenous Li-based salt is present, leading to formation of CO 2-derived products through electrochemical N-C bond cleavage. However, the detailed influence of the salt on the electrochemical reactions was not understood. Here, we investigate the role of individual electrolyte salt constituents across multiple cations and anions in DMSO to gain improved insight into the salt's role in these complex electrolytes. While the anion appears to have minor effect, the cation is found to strongly modulate the thermochemistry of the amine-CO 2 through electrostatic interactions: 1 H NMR measurements show that post-capture, pre-reduction equilibrium proportions of the formed cation-associated carbamate vary by up to five-fold, and increase with the cation's Lewis acidity (e.g. from K + → Na + → Li +). This trend is quantitatively supported by DFT calculations of the free energy of formation of these alkali-associated adducts. Upon electrochemical reduction, however, the current densities follow an opposing trend, with enhanced reaction rates obtained with the lowest Lewis-acidity cation, K +. Meanwhile, molecular dynamics simulations indicate significant increases in desolvation and pairing kinetics that occur with K +. These findings suggest that, in addition to strongly affecting the speciation of amine-CO 2 adducts, the cation's pairing with-COOin the amine-CO 2 adduct can significantly hinder or enhance the rates of electrochemical reactions at moderate overpotentials. Consequently, designing electrolytes to promote fast cation-transfer appears important for obtaining higher current densities in future systems.

Amine-based CO 2 chemisorption has been a longstanding motif under development for CO 2 capture applications, but large energy penalties are required to thermally cleave the N-C bond and regenerate CO 2 for subsequent storage or utilization. Instead, it is attractive to be able to directly perform electrochemical reactions on the amine solutions with loaded CO 2. We recently found that such a process is viable in dimethyl sulfoxide (DMSO) if an exogenous Li-based salt is present, leading to formation of CO 2-derived products through electrochemical N-C bond cleavage. However, the detailed influence of the salt on the electrochemical reactions was not understood. Here, we investigate the role of individual electrolyte salt constituents across multiple cations and anions in DMSO to gain improved insight into the salt's role in these complex electrolytes. While the anion appears to have minor effect, the cation is found to strongly modulate the thermochemistry of the amine-CO 2 through electrostatic interactions: 1 H NMR measurements show that post-capture, pre-reduction equilibrium proportions of the formed cation-associated carbamate vary by up to five-fold, and increase with the cation's Lewis acidity (e.g. from K + → Na + → Li +). This trend is quantitatively supported by DFT calculations of the free energy of formation of these alkali-associated adducts. Upon electrochemical reduction, however, the current densities follow an opposing trend, with enhanced reaction rates obtained with the lowest Lewis-acidity cation, K +. Meanwhile, molecular dynamics simulations indicate significant increases in desolvation and pairing kinetics that occur with K +. These findings suggest that, in addition to strongly affecting the speciation of amine-CO 2 adducts, the cation's pairing with-COOin the amine-CO 2 adduct can significantly hinder or enhance the rates of electrochemical reactions at moderate overpotentials. Consequently, designing electrolytes to promote fast cation-transfer appears important for obtaining higher current densities in future systems.

We present a combination of first-principles calculations and the Boltzmann transport theory to understand the carrier transport and thermoelectric performance of mixed halide perovskite alloys CsPb­(I1–x Br x )3 with different Br compositions. Our computational results correlate the conduction band splitting in CsPb­(I1–x Br x )3 to the significant anisotropy in their carrier transport properties, such as effective masses and deformation potential constants. Such band splitting originates from the symmetry-broken crystal structures of CsPb­(I1–x Br x )3 polymorphs: with residue stresses/strains in asymmetric CsPb­(I1–x Br x )3, nondegenerate orbitals reconstruct the conduction band and reduce the Pb-halide antibonding character along certain directions. While the Seebeck coefficient (S) and the relaxation time-normalized electrical conductivity (σ/τ) show weak directional anisotropy, the carrier relaxation time (τ) is highly direction-dependent. The reconstruction of the conduction band finally leads to significantly anisotropic and enhanced thermoelectric power factors (PF = S 2σ) in CsPb­(I1–x Br x )3 compared to those in pure CsPbI3 and CsPbBr3, showing anomalous nonlinear alloy behavior. A delicate balance between S 2σ and combined measurement of the carrier effective mass and deformation potential constant, m*E DP, is confirmed. The lattice thermal conductivities of CsPb­(I1–x Br x )3 are significantly suppressed compared to those of their pure counterparts due to strong mass disordering and strain fields upon halogen substitution. As a result, symmetry breaking in CsPb­(I1–x Br x )3 leads to anisotropy in carrier transport, high PF, and scattered phonon transport (ultralow thermal conductivity), concurrently contributing to their promising thermoelectric figures of merit (ZT) up to 1.7 at room temperature. The principles behind the asymmetry-induced factors would serve as new design concepts to tailor the thermoelectric properties of alloys, mixtures, superlattices, and low-dimensional materials.

Strain engineering of metal halide perovskites shows promise for better stability and device performance, but the impact on thermoelectric performance remains elusive. We demonstrate that the electronic structures and carrier transport properties in halide perovskites CsPb­(I1–x Br x )3 can be tailored synergetically through the practical biaxial strain-engineering strategies. For the pure halide perovskite CsPbI3, the lattice geometry and electronic structures are basically retained under strains (from −6 to 8%), leading to moderately varied transport properties. Interestingly, under a −8% compressive strain, sharp changes in the carrier transport properties are observed in CsPbI3 because of the dramatically increased contribution of iodine electrons to the conduction band minimum. For the mixed halide perovskites, we find that CsPbI3/2Br3/2 is the thermodynamically most stable CsPb­(I1–x Br x )3 as determined by the generalized quasi-chemical approximation method. The band gap, carrier effective mass, and other carrier transport properties of CsPbI3/2Br3/2 change dramatically in response to high external strains (≤−6 or ≥6%), accompanied by the ultralow thermal conductivities. Such abnormal phenomena originate from the distorted lattice geometry that is caused by the non-uniform internal stress distribution under high external strains. In addition, external strains can also tailor the optimal carrier concentration needed to achieve the maximum figure of merit (ZT), providing a new avenue to tackle the longstanding challenge in heavy-doping perovskites. Finally, the ZT values are very sensitive to the magnitude of strains, especially for mixed halide perovskites, showing enhanced ZT from ∼0.1 without strain to ∼0.9 under a −6% compressive strain at 300 K. This work provides practical biaxial strain-engineering strategies to enhance the thermoelectric performance and also to optimize the doping process in mixed halide perovskites.

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