Co-reporter:Zixuan Guan;Di Chen
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 34) pp:23414-23424
Publication Date(Web):2017/08/30
DOI:10.1039/C7CP03654J
The oxygen incorporation reaction, which involves the transformation of an oxygen gas molecule to two lattice oxygen ions in a mixed ionic and electronic conducting solid, is a ubiquitous and fundamental reaction in solid-state electrochemistry. To understand the reaction pathway and to identify the rate-determining step, near-equilibrium measurements have been employed to quantify the exchange coefficients as a function of oxygen partial pressure and temperature. However, because the exchange coefficient contains contributions from both forward and reverse reaction rate constants and depends on both oxygen partial pressure and oxygen fugacity in the solid, unique and definitive mechanistic assessment has been challenging. In this work, we derive a current density equation as a function of both oxygen partial pressure and overpotential, and consider both near and far from equilibrium limits. Rather than considering specific reaction pathways, we generalize the multi-step oxygen incorporation reaction into the rate-determining step, preceding and following quasi-equilibrium steps, and consider the number of oxygen ions and electrons involved in each. By evaluating the dependence of current density on oxygen partial pressure and overpotential separately, one obtains the reaction orders for oxygen gas molecules and for solid-state species in the electrode. We simulated the oxygen incorporation current density-overpotential curves for praseodymium-doped ceria for various candidate rate-determining steps. This work highlights a promising method for studying the exchange kinetics far away from equilibrium.
Co-reporter:Liming Zhang, Xiaofei Ye, Madhur Boloor, Andrey Poletayev, Nicholas A. Melosh and William C. Chueh
Energy & Environmental Science 2016 vol. 9(Issue 6) pp:2044-2052
Publication Date(Web):28 Jan 2016
DOI:10.1039/C6EE00036C
Transition-metal-oxide semiconductors are promising photoanodes for solar water splitting due to their excellent chemical stability and appropriate bandgaps. However, in absorbers such as BiVO4, TiO2, α-Fe2O3, and WO3, charge carriers localize as small polarons or become trapped, leading to low minority carrier mobilities. This limits the minority carrier collection efficiency in the quasi-neutral region of the light absorber, and lowers the overall photoactivity. In this work, we demonstrate that modestly elevating the temperature activates minority carrier hopping in monoclinic BiVO4, significantly enhancing the saturation photocurrent without a substantial anodic shift of the onset potential, and is an attractive alternative to employing complex passivation layers and nanostructured templates towards achieving the theoretical photocurrent density. Specifically, using a Mo:BiVO4/SnO2/Si tandem photoanode/photovoltaic, increasing the absolute temperature by 11% from 10 to 42 °C elevates the saturation photocurrent from 1.8 to 4.0 mA cm−2. This strong temperature enhancement, 3.8% K−1, is 5 times greater than that in α-Fe2O3. Concurrently, the onset potential shifts slightly from 0.02 V to 0.08 V versus the reversible hydrogen electrode (or equivalently, from −1.22 V to −1.13 V versus the equilibrium potential of oxygen evolution). Our observation contrasts with the prevailing understanding that the energy conversion efficiency generally decreases with temperature as a result of reduced photovoltage. Thermally-activating minority carrier transport represents a general pathway towards enhancing the photoactivity of light absorbers where hopping conduction limits the minority carrier collection in the quasi-neutral region.
Co-reporter:Chirranjeevi Balaji Gopal;Farid El Gabaly;Anthony H. McDaniel
Advanced Materials 2016 Volume 28( Issue 23) pp:4692-4697
Publication Date(Web):
DOI:10.1002/adma.201506333
Co-reporter:William E. Gent;Yiyang Li;Sungjin Ahn;Jongwoo Lim;Yijin Liu;Anna M. Wise;Chirranjeevi Balaji Gopal;David N. Mueller;Ryan Davis;Johanna Nelson Weker;Jin-Hwan Park;Seok-Kwang Doo
Advanced Materials 2016 Volume 28( Issue 31) pp:6631-6638
Publication Date(Web):
DOI:10.1002/adma.201601273
Co-reporter:Zhuoluo A. Feng, Chirranjeevi Balaji Gopal, Xiaofei Ye, Zixuan Guan, Beomgyun Jeong, Ethan Crumlin, and William C. Chueh
Chemistry of Materials 2016 Volume 28(Issue 17) pp:6233
Publication Date(Web):July 31, 2016
DOI:10.1021/acs.chemmater.6b02427
Ion insertion at the interfaces of batteries, fuel cells, and catalysts constitutes an important class of technologically relevant, charge-transfer reactions. However, the molecular nature of charge separation at the adsorbate/solid interface remains elusive. It has been hypothesized that electrostatic dipoles at the adsorbate/solid interface could result from adsorption-induced charge redistribution, preferential segregation of charged point defects in the solid, and/or intrinsic dipoles of adsorbates. Using operando ambient-pressure X-ray photoelectron spectroscopy, we elucidate the coupling between electrostatics and adsorbate chemistry on the surface of CeO2–x, an excellent electrocatalyst and a model system for studying oxygen-ion insertion reactions. Three adsorbate chemistries were studied—OH–/CeO2–x (polar adsorbate), CO32–/CeO2–x (nonpolar adsorbate), and Ar/CeO2–x (no adsorbate)—under several hundred mTorr of gas pressure relevant to electrochemical H2/CO oxidation and H2O/CO2 reduction. By integrating core-level spectroscopy and contact-potential difference measurements, we simultaneously determine the chemistry and coverage of adsorbates, Ce oxidation state, and the surface potential at the gas/solid interface over a wide range of overpotentials. We directly observe an overpotential-dependent surface potential, which is moreover sensitive to the polarity of the adsorbates. In the case of CeO2–x covered with polar OH–, we observe a surface potential that increases linearly with OH– coverage and with overpotential. On the other hand, for CeO2–x covered with nonpolar CO32– and free of adsorbates, the surface potential is independent of overpotential. The adsorbate binding energy does not change systematically with overpotential. From these observations, we conclude that the electrostatic dipole at the adsorbate/CeO2–x interface is dominated by the intrinsic dipoles of the adsorbates, with the solid contributing minimally. These results provide an atomistic picture of the gas/solid double layer and the experimental methodology to directly study and quantify the surface dipole.
Co-reporter:Jongwoo Lim;Yiyang Li;Daan Hein Alsem;Hongyun So;Sang Chul Lee;Peng Bai;Daniel A. Cogswell;Young-sang Yu;Xuzhao Liu;Norman Jin;Norman J. Salmon;David A. Shapiro;Martin Z. Bazant;Tolek Tyliszczak
Science 2016 Volume 353(Issue 6299) pp:
Publication Date(Web):
DOI:10.1126/science.aaf4914
Watching batteries fail
Rechargeable batteries lose capacity in part because of physical changes in the electrodes caused by electrochemical cycling. Lim et al. track the reaction dynamics of an electrode material, LiFePO4, by measuring the relative concentrations of Fe(II) and Fe(III) in it by means of high-resolution x-ray absorption spectrometry (see the Perspective by Schougaard). The exchange current density is then mapped for Li+ insertion and removal. At fast cycling rates, solid solutions form as Li+ is removed and inserted. However, at slow cycling rates, nanoscale phase separation occurs within battery particles, which eventually shortens battery life.
Science, this issue p. 566; see also p. 543
Co-reporter:Yezhou Shi, Sang Chul Lee, Matteo Monti, Colvin Wang, Zhuoluo A. Feng, William D. Nix, Michael F. Toney, Robert Sinclair, and William C. Chueh
ACS Nano 2016 Volume 10(Issue 11) pp:9938
Publication Date(Web):November 7, 2016
DOI:10.1021/acsnano.6b04081
Large biaxial strain is a promising route to tune the functionalities of oxide thin films. However, large strain is often not fully realized due to the formation of misfit dislocations at the film/substrate interface. In this work, we examine the growth of strained ceria (CeO2) thin films on (001)-oriented single crystal yttria-stabilized zirconia (YSZ) via pulsed-laser deposition. By varying the film thickness systematically between 1 and 430 nm, we demonstrate that ultrathin ceria films are coherently strained to the YSZ substrate for thicknesses up to 2.7 nm, despite the large lattice mismatch (∼5%). The coherency is confirmed by both X-ray diffraction and high-resolution transmission electron microscopy. This thickness is several times greater than the predicted equilibrium critical thickness. Partial strain relaxation is achieved by forming semirelaxed surface islands rather than by directly nucleating dislocations. In situ reflective high-energy electron diffraction during growth confirms the transition from 2-D (layer-by-layer) to 3-D (island) at a film thickness of ∼1 nm, which is further supported by atomic force microscopy. We propose that dislocations likely nucleate near the surface islands and glide to the film/substrate interface, as evidenced by the presence of 60° dislocations. An improved understanding of growing oxide thin films with a large misfit lays the foundation to systematically explore the impact of strain and dislocations on properties such as ionic transport and redox chemistry.Keywords: ceria; dislocation; strain; yttria-stabilized zirconia
Co-reporter:Yiyang Li;Sophie Meyer;Jongwoo Lim;Sang Chul Lee;William E. Gent;Stefano Marchesini;Harinarayan Krishnan;Tolek Tyliszczak;David Shapiro;Arthur L. David Kilcoyne
Advanced Materials 2015 Volume 27( Issue 42) pp:6591-6597
Publication Date(Web):
DOI:10.1002/adma.201502276
Co-reporter:Yiyang Li;Johanna Nelson Weker;William E. Gent;David N. Mueller;Jongwoo Lim;Daniel A. Cogswell;Tolek Tyliszczak
Advanced Functional Materials 2015 Volume 25( Issue 24) pp:3677-3687
Publication Date(Web):
DOI:10.1002/adfm.201500286
LiFePO4 is a promising phase-separating battery electrode and a model system for studying lithiation. The role of particle synthesis and the corresponding particle morphology on the nanoscale insertion and migration of Li is not well understood, and elucidating the intercalation pathway is crucial toward improving battery performance. A synchrotron operando liquid X-ray imaging platform is developed to track the migration of Li in LiFePO4 electrodes with single-particle sensitivity. Lithiation is tracked in two particle types—ellipsoidal and platelet—while the particles cycle in an organic liquid electrolyte, and the results show a clear dichotomy in the intercalation pathway. The ellipsoidal particles intercalate sequentially, concentrating the current in a small number of actively intercalating particles. At the same cycling rate, platelet particles intercalate simultaneously, leading to a significantly more uniform current distribution. Assuming that the particles intercalate through a single-phase pathway, it is proposed that the two particle types exhibit different surface properties, a result of different synthesis procedures, which affect the surface reactivity of LiFePO4. Alternatively, if the particles intercalate through nucleation and growth, the larger size of platelet particles may account for the dichotomy. Beyond providing particle engineering insights, the operando microscopy platform enables new opportunities for nanoscale chemical imaging of liquid-based electrochemical systems.
Co-reporter:Xiaofei Ye, Jing Yang, Madhur Boloor, Nicholas A. Melosh and William C. Chueh
Journal of Materials Chemistry A 2015 vol. 3(Issue 20) pp:10801-10810
Publication Date(Web):10 Apr 2015
DOI:10.1039/C5TA02108A
Many metal-oxide light absorbing semiconductors, such as α-Fe2O3 (hematite), exhibit localized small polaron carrier conduction. The low electron/hole mobility hinders minority carrier transport, and is not readily modified through doping or nanostructuring. In this work, we demonstrate that thermal energy, which is available in moderately concentrated sunlight, enhances the minority carrier mobility and photoelectrochemical (PEC) water and sulfite oxidation of 30 nm-thick Ti-doped hematite thin-film photoanodes. In NaOH aqueous electrolyte, the instantaneous Tafel slope at 3 mA cm−2 decreases remarkably from 480 mV dec−1 (at 7 °C) to 240 mV dec−1 (at 72 °C) under 9 suns illumination, representing a substantial increase in the fill factor, which also depends on the doping level. Though the photovoltage decreases with temperature expectedly, we show that it can be mitigated by increasing the light intensity. In the presence of a Na2SO3 hole scavenger, the photocurrent at 1.23 V vs. reversible hydrogen electrode increased from 3.1 (at 7 °C) to 5.0 mA cm−2 (at 72 °C) under 8 suns illumination, and the onset potential was shown to depend weakly on the temperature. The strong increase in the photocurrent with temperature in the limit of fast reaction kinetics suggests that it arises from an improvement in the collection of minority carriers in the diffusion region of hematite. We show that room temperature and 1 sun illumination intensity is not the optimal reaction operating condition for hematite photoanodes. The thermally-enhanced minority carrier transport are likely generalizable to other small-polaron-type light absorbers for PEC and solar cells.
Co-reporter:Cansheng Yuan, Colby Jarrett, William Chueh, Yoshiaki Kawajiri, Asegun Henry
Solar Energy 2015 Volume 122() pp:547-561
Publication Date(Web):December 2015
DOI:10.1016/j.solener.2015.08.019
•Power density mismatch in conventional reactor designs is identified.•A new solar fuel concept is proposed using liquid metal heat transfer fluid.•A novel design of reactor with an array of chamber is proposed.•The reactor design can facilitate more than 80% heat recuperation of sensible heat.•The reactor design allows for control of the gas environment inside of each chamber enabling separate optimization of the two reactions.A new reactor concept for two-step partial redox cycles is presented and evaluated by transient simulation that considers heat and mass transfer along with reaction kinetics. The major difference between the reactor described herein and previous designs is that the conversion from solar to chemical energy is divided into two steps: sunlight-to-thermal energy conversion accomplished with a liquid metal based receiver, and the thermal-to-chemical conversion accomplished with a separately optimized array of reaction chambers. To connect these two conversion steps, liquid metal is used as a high temperature heat transfer fluid that feeds the solar energy captured in the receiver to the reactor. The liquid metal also facilitates efficient heat recuperation (∼80%) between the reaction chambers. The overall thermal-to-chemical efficiency from the thermal energy in the liquid metal to the chemical energy in the hydrogen fuel is estimated to be 19.8% when ceria is employed as the reactive oxygen storage material. This estimated efficiency is an order of magnitude higher than previous designs and the reactor concept discussed herein identifies important insights that apply to solar–fuel conversion in general.
Co-reporter:Zhuoluo A. Feng, Michael L. Machala and William C. Chueh
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 18) pp:12273-12281
Publication Date(Web):09 Apr 2015
DOI:10.1039/C5CP00114E
The efficient electro-reduction of CO2 to chemical fuels and the electro-oxidation of hydrocarbons for generating electricity are critical toward a carbon-neutral energy cycle. The simplest reactions involving carbon species in solid-oxide fuel cells and electrolyzer cells are CO oxidation and CO2 reduction, respectively. In catalyzing these reactions, doped ceria exhibits a mixed valence of Ce3+ and Ce4+, and has been employed as a highly active and coking-resistant electrode. Here we report an operando investigation of the surface reaction mechanism on a ceria-based electrochemical cell using ambient pressure X-ray photoelectron spectroscopy. We show that the reaction proceeds via a stable carbonate intermediate, the coverage of which is coupled to the surface Ce3+ concentration. Under CO oxidation polarization, both the carbonate and surface Ce3+ concentration decrease with overpotential. Under CO2 reduction polarization, on the other hand, the carbonate coverage saturates whereas the surface Ce3+ concentration increases with overpotential. The evolution of these reaction intermediates was analyzed using a simplified two-electron reaction scheme. We propose that the strong adsorbate–adsorbate interaction explains the coverage-dependent reaction mechanism. These new insights into the surface electrochemistry of ceria shed light on the optimization strategies for better fuel cell electrocatalysts.
Co-reporter:Dr. Johanna NelsonWeker;Yiyang Li;Dr. Rengarajan Shanmugam; Wei Lai; William C. Chueh
ChemElectroChem 2015 Volume 2( Issue 10) pp:1576-1581
Publication Date(Web):
DOI:10.1002/celc.201500119
Abstract
State-of-charge maps of a LiFePO4 electrode in standard nonaqueous electrolyte are recorded in operando with a resolution of tens of nanometers by using hard X-ray spectroscopic microscopy. We observe spatially inhomogeneous charging and discharging of the composite electrode, with predominately Li-rich and Li-poor micron-scale agglomerates. There appears to be no correlation between the agglomerate size and the (de)lithiation pathway. Additionally, we observe that the electrochemical heterogeneity exhibits a “last to delithiate, first to lithiate” behavior, in which the regions that are the last to charge are also the regions that are the first to discharge. These observations suggest that the nanoscale pore structure within the agglomerates dominates lithium-ion transport.
Co-reporter:William C. Chueh, Farid El Gabaly, Joshua D. Sugar, Norman C. Bartelt, Anthony H. McDaniel, Kyle R. Fenton, Kevin R. Zavadil, Tolek Tyliszczak, Wei Lai, and Kevin F. McCarty
Nano Letters 2013 Volume 13(Issue 3) pp:866-872
Publication Date(Web):January 30, 2013
DOI:10.1021/nl3031899
The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.
Co-reporter:Xiaofei Ye, John Melas-Kyriazi, Zhuoluo A. Feng, Nicholas A. Melosh and William C. Chueh
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 37) pp:15459-15469
Publication Date(Web):11 Jul 2013
DOI:10.1039/C3CP52536H
Photoelectrochemical cells (PECs) have been studied extensively for dissociating water into hydrogen and oxygen. Key bottlenecks for achieving high solar-to-hydrogen efficiency in PECs include increasing solar spectrum utilization, surmounting overpotential losses, and aligning the absorber/electrochemical redox levels. We propose a new class of solid-state PECs based on mixed ionic and electronic conducting (MIEC) oxides that operates at temperatures significantly above ambient and utilizes both the light and thermal energy available from concentrated sunlight to dissociate water vapor. Unlike thermochemical and hybrid photo-thermochemical water-splitting routes, the elevated-temperature PEC is a single-step approach operating isothermally. At the heart of the solid-state PEC is a semiconductor light absorber coated with a thin MIEC layer for improved catalytic activity, electrochemical stability, and ionic conduction. The MIEC, placed between the gas phase and the semiconductor light absorber, provides a facile path for minority carriers to reach the water vapor as well as a path for the ionic carriers to reach the solid electrolyte. Elevated temperature operation allows reasonable band misalignments at the interfaces to be overcome, reduces the required overpotential, and facilitates rapid product diffusion away from the surface. In this work, we simulate the behavior of an oxygen-ion-conducting photocathode in 1-D. Using the detailed-balance approach, in conjunction with recombination and electrochemical reaction rates, the practical efficiency is calculated as a function of temperature, solar flux, and select material properties. For a non-degenerate light absorber with a 2.0 eV band-gap and an uphill band offset of 0.3 eV, an efficiency of 17% and 11% is predicted at 723 and 873 K, respectively.
Co-reporter:Xiaofei Ye, John Melas-Kyriazi, Zhuoluo A. Feng, Nicholas A. Melosh and William C. Chueh
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 37) pp:NaN15469-15469
Publication Date(Web):2013/07/11
DOI:10.1039/C3CP52536H
Photoelectrochemical cells (PECs) have been studied extensively for dissociating water into hydrogen and oxygen. Key bottlenecks for achieving high solar-to-hydrogen efficiency in PECs include increasing solar spectrum utilization, surmounting overpotential losses, and aligning the absorber/electrochemical redox levels. We propose a new class of solid-state PECs based on mixed ionic and electronic conducting (MIEC) oxides that operates at temperatures significantly above ambient and utilizes both the light and thermal energy available from concentrated sunlight to dissociate water vapor. Unlike thermochemical and hybrid photo-thermochemical water-splitting routes, the elevated-temperature PEC is a single-step approach operating isothermally. At the heart of the solid-state PEC is a semiconductor light absorber coated with a thin MIEC layer for improved catalytic activity, electrochemical stability, and ionic conduction. The MIEC, placed between the gas phase and the semiconductor light absorber, provides a facile path for minority carriers to reach the water vapor as well as a path for the ionic carriers to reach the solid electrolyte. Elevated temperature operation allows reasonable band misalignments at the interfaces to be overcome, reduces the required overpotential, and facilitates rapid product diffusion away from the surface. In this work, we simulate the behavior of an oxygen-ion-conducting photocathode in 1-D. Using the detailed-balance approach, in conjunction with recombination and electrochemical reaction rates, the practical efficiency is calculated as a function of temperature, solar flux, and select material properties. For a non-degenerate light absorber with a 2.0 eV band-gap and an uphill band offset of 0.3 eV, an efficiency of 17% and 11% is predicted at 723 and 873 K, respectively.
Co-reporter:Zhuoluo A. Feng, Michael L. Machala and William C. Chueh
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 18) pp:NaN12281-12281
Publication Date(Web):2015/04/09
DOI:10.1039/C5CP00114E
The efficient electro-reduction of CO2 to chemical fuels and the electro-oxidation of hydrocarbons for generating electricity are critical toward a carbon-neutral energy cycle. The simplest reactions involving carbon species in solid-oxide fuel cells and electrolyzer cells are CO oxidation and CO2 reduction, respectively. In catalyzing these reactions, doped ceria exhibits a mixed valence of Ce3+ and Ce4+, and has been employed as a highly active and coking-resistant electrode. Here we report an operando investigation of the surface reaction mechanism on a ceria-based electrochemical cell using ambient pressure X-ray photoelectron spectroscopy. We show that the reaction proceeds via a stable carbonate intermediate, the coverage of which is coupled to the surface Ce3+ concentration. Under CO oxidation polarization, both the carbonate and surface Ce3+ concentration decrease with overpotential. Under CO2 reduction polarization, on the other hand, the carbonate coverage saturates whereas the surface Ce3+ concentration increases with overpotential. The evolution of these reaction intermediates was analyzed using a simplified two-electron reaction scheme. We propose that the strong adsorbate–adsorbate interaction explains the coverage-dependent reaction mechanism. These new insights into the surface electrochemistry of ceria shed light on the optimization strategies for better fuel cell electrocatalysts.
Co-reporter:Xiaofei Ye, Jing Yang, Madhur Boloor, Nicholas A. Melosh and William C. Chueh
Journal of Materials Chemistry A 2015 - vol. 3(Issue 20) pp:NaN10810-10810
Publication Date(Web):2015/04/10
DOI:10.1039/C5TA02108A
Many metal-oxide light absorbing semiconductors, such as α-Fe2O3 (hematite), exhibit localized small polaron carrier conduction. The low electron/hole mobility hinders minority carrier transport, and is not readily modified through doping or nanostructuring. In this work, we demonstrate that thermal energy, which is available in moderately concentrated sunlight, enhances the minority carrier mobility and photoelectrochemical (PEC) water and sulfite oxidation of 30 nm-thick Ti-doped hematite thin-film photoanodes. In NaOH aqueous electrolyte, the instantaneous Tafel slope at 3 mA cm−2 decreases remarkably from 480 mV dec−1 (at 7 °C) to 240 mV dec−1 (at 72 °C) under 9 suns illumination, representing a substantial increase in the fill factor, which also depends on the doping level. Though the photovoltage decreases with temperature expectedly, we show that it can be mitigated by increasing the light intensity. In the presence of a Na2SO3 hole scavenger, the photocurrent at 1.23 V vs. reversible hydrogen electrode increased from 3.1 (at 7 °C) to 5.0 mA cm−2 (at 72 °C) under 8 suns illumination, and the onset potential was shown to depend weakly on the temperature. The strong increase in the photocurrent with temperature in the limit of fast reaction kinetics suggests that it arises from an improvement in the collection of minority carriers in the diffusion region of hematite. We show that room temperature and 1 sun illumination intensity is not the optimal reaction operating condition for hematite photoanodes. The thermally-enhanced minority carrier transport are likely generalizable to other small-polaron-type light absorbers for PEC and solar cells.
