Manipulating electrochemical performance through doping beyond the solubility limit

Abstract

Improving water splitting efficiency has been the holy grail of hydrogen fuel production. Major efforts have been invested in an attempt to enhance efficiency of a common water oxidation catalyst, α-Fe2O3, through doping and alloying. Recent experiments show that higher efficiency is achieved when niobium (Nb) is added beyond the solubility limit to generate a mixture of two phases: Nb-doped and Nb-alloyed α-Fe2O3. In order to understand why adding high concentrations of Nb is beneficial, we provide a thorough first principles study of the bulk and the surface of pure, Nb-doped, and Nb-alloyed α-Fe2O3 with several surface facets and terminations. We find that the addition of Nb changes the band edge and Fermi level positions. Therefore, we propose a mechanism by which having different Nb doping levels within and above the solubility limit has an advantage: electrons and holes could separate better between doped and alloyed regions that have different band edge positions or between regions with different doping concentrations. Furthermore, the holes' driving force to oxidize water can be increased by placing on the surface the undoped or alloyed phases, since they have a lower valence band maximum. We suggest that obtaining two material phases or gradual doping can be used as a design strategy for next generation catalysts.

The “Rust” Challenge: On the Correlations between Electronic Structure, Excited State Dynamics, and Photoelectrochemical Performance of Hematite Photoanodes for Solar Water Splitting

“…[73] Doping is commonly used in order to improve both the photocurrent and onset potential for water photo-oxidation. [11,18,32,55,60,[74][75][76][77][78][79][80][81][82][83][84] However, the role of doping on the photoelectrochemical performance of hematite photoanodes is not fully understood and has been attributed to a variety of factors including improvement in conductivity, [85] passivation of surface states and grain boundaries [86] , shifting of band edge positions, [32,55,80] reduction in effective mass, [55] and distortion of the crystal structure [82][83][84] which facilitates hopping for both electrons and holes. [59]…”

Enhanced Water Oxidation Catalysis of Nickel Oxyhydroxide through the Addition of Vacancies

“…Developing efficient catalytic materials is vital for obtaining clean energy, particularly through the water-splitting reaction. , Heterogeneous water splitting is based on an electrochemical cell, where water oxidizes and is reduced to produce oxygen gas and hydrogen fuel, respectively . The water oxidation reaction (OER) at the anode is especially challenging because there are very few materials that are able to perform the catalysis efficiently. − …”

Effect of Orientation on Bulk and Surface Properties of Sn-doped Hematite (α-Fe2O3) Heteroepitaxial Thin Film Photoanodes

“…Routes taken by researchers for improving hematite photoanode performance include nanostructuring to reduce the distance needed for minority charge carriers (holes) to reach the electrolyte, 9 optical enhancement using specular substrates for resonance absorption in ultrathin compact films, 10 and doping to improve the electronic properties. 11,12 Another potential route for improvement of hematite photoanodes is crystallographic orientational control. Indeed, theoretical work has predicted anisotropic electronic conduction in hematite with up to 4 orders of magnitude higher conductivity within the basal plane than in directions orthogonal to it.…”

“…The main reason for the limited performance of hematite photoanodes has been attributed to short lifetime of minority charge carriers and low charge carrier mobility, resulting in short diffusion length and significant charge recombination. Routes taken by researchers for improving hematite photoanode performance include nanostructuring to reduce the distance needed for minority charge carriers (holes) to reach the electrolyte, optical enhancement using specular substrates for resonance absorption in ultrathin compact films, and doping to improve the electronic properties. , …”