Electrochemical Oxidation Of CO Research Articles

Influences of reformate on the performance of high temperature proton exchange membrane fuel cell and its optimization strategy

Due to the challenges of hydrogen production, storage and transport, hydrocarbon reformate-based high temperature proton exchange membrane fuel cell (HT-PEMFC) has wide application opportunities. However, the inevitable water vapor and CO in reformate significantly determines the fuel cell performance and durability. In this work, a three-dimensional, non-isothermal and multiphase model based on agglomerate sub-model is developed to investigate the effects of reformate components and operating conditions, in which multiphase transport, dissolution, diffusion, adsorption, desorption and reaction of H2 and CO are considered. The results show that the fuel cell performance increases with the rising of water vapor content as CO content higher than 2 % (dry basis, the same below) and water vapor content lower 20 % (wet basis). The alleviation effect of water vapor on CO poisoning is discovered due to reduced CO coverage rather than enhanced CO electrochemical oxidation. An elevated temperature leads to improved performance and the effect of CO poisoning (CO<3%) is weak at 180°C.

Optimization studies of intake parameters for direct internal reforming solid oxide fuel cell

Intake parameters are essential factors that are often overlooked, but can easily change the direct internal reforming solid oxide fuel cell (DIR-SOFC) performance. A three-dimensional model is developed to simulate the effects of intake parameters on the performance of an anode-supported flat-plate DIR-SOFC. The two primary chemical reactions, water gas shift reaction (WGSR) and methane steam reforming reaction (MSR), as well as the electrochemical oxidation of H2 and CO, are considered in this model. It was found that the aspect ratio corresponding to the peak current density decreases as fuel increases. A moderate air flow rate makes the fuel-to-air ratio (FAR) corresponding to the drastic change in current density relatively small and wide-ranging, resulting in more desirable temperature distribution and fuel utilization. Under the operating conditions of this paper, adjusting the steam-to-carbon ratio (S/C) to between 1.5 and 2 results in both excellent electrical performance and good temperature uniformity. Additionally, adjusting the flow rate by changing the inlet area or velocity has its own advantages in terms of temperature distribution or current density, which must be taken into account when adjusting the flow rate. This study provides a valuable reference on inlet settings of DIR-SOFC, which would help to promote the development of high-performance SOFCs.

Multi-band Metasurface-Driven Surface-Enhanced Infrared Absorption Spectroscopy for Improved Characterization of in-Situ Electrochemical Reactions.

Surface-enhanced spectroscopy techniques are the method-of-choice to characterize adsorbed intermediates occurring during electrochemical reactions, which are crucial in realizing a green and sustainable future. Characterizing species with low coverage or short lifetimes has so far been limited by low signal enhancement. Recently, single-band metasurface-driven surface-enhanced infrared absorption spectroscopy (SEIRAS) has been pioneered as a promising technology to monitor a single vibrational mode during electrochemical CO oxidation. However, electrochemical reactions are complex, and their understanding requires the simultaneous monitoring of multiple adsorbed species in situ, hampering the adoption of nanostructured electrodes in spectro-electrochemistry. Here, we develop a multi-band nanophotonic-electrochemical platform that simultaneously monitors in situ multiple adsorbed species emerging during cyclic voltammetry scans by leveraging the high resolution offered by the reproducible nanostructuring of the working electrode. Specifically, we studied the electrochemical reduction of CO2 on a Pt surface and used two separately tuned metasurface arrays to monitor two adsorption configurations of CO with vibrational bands at ∼2030 and ∼1840 cm-1. Our platform provides a ∼40-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. A straightforward methodology is outlined starting with baselining our system in a CO-saturated environment and clearly detecting both configurations of adsorption. In contrast, during the electrochemical reduction of CO2 on platinum in K2CO3, CO adsorbed in a bridged configuration could not be detected. We anticipate that our technology will guide researchers in developing similar sensing platforms to simultaneously detect multiple challenging intermediates, with low surface coverage or short lifetimes.

Unveiling the electrochemical CO oxidation activity on support-free porous PdM (M = Fe, Co, Ni) foam-like nanocrystals over a wide pH range

Binary PdM-based nanocrystals are efficient electrocatalysts for a wide range of renewable and green fuel cell applications; however, their poisoning by CO is a great barrier for commercialization, so it is pivotal to solve this issue. Herein, we fabricated support-free PdM alloys (M = Fe, Co, and Ni) by a prompt one-step aqueous-solution co-reduction with sodium borohydride driven by the coalescence growth mechanism. This forms support-free PdM porous foam-like nanostructures with well-defined compositions from the ICP-OES analysis and clean surface without any hazardous chemicals or multiple reaction steps. The as-prepared PdM foam-like nanostructures were demonstrated for carbon monoxide oxidation (COOxid) electrocatalysis at varied electrolyte pH compared with commercial Pd/C catalyst. The foam-like PdFe nanocrystals achieved an excellent electrochemical COOxid activity that was at least 2.18-times of PdCo, 4.35-times of PdNi, and 1.56-times of Pd/C in both alkaline (KOH) and acidic (HClO4) electrolytes, but PdCo was the best in neutral (NaHCO3) medium. The superior activity of PdM is due to the strain and alloying effect, which promoted the superb CO oxidation durability for 1000 cycles than Pd/C catalyst. This study demonstrated the superiority of support-free bimetallic PdM alloys than Pd/C catalyst in all electrolytes, which may open new gates for understanding CO-poisoning in alcohol-based fuel cells.

Electrocatalytic CO oxidation on porous ternary PdNiO-CeO2/carbon black nanocatalysts: Effect of supports and electrolytes

The catalytic activity and durability of porous ternary Pd-based electrocatalysts are augmented by alloying with other metals and using supports, however, the effect of supports and electrolytes at varied pH on the electrochemical CO oxidation (COOxid) performance is not yet studied. Herein, a PdNiO-CeO2/CB nanosphere, composed of PdNiO nanocrystals sprouted on CeO2 flake-like and carbon black (CB) nanosheets, was synthesized by a sol-gel and impregnation methods. Remarkably, during the impregnation step, CeO2/CB serves as a reactor for the reduction of Pd/Ni precursors and supported the growth of PdNiO nanocrystals without surfactant and/or reducing agent. This endowed the formation of PdNiO-CeO2/CB with multifunctional porous structures, including pore volume (0.29 cm3/g) and specific surface area (151.86 m2/g) and a clean Pd surface. The as-prepared PdNiO-CeO2/CB delivers a superior COOxid activity than those of PdNiO-CeO2, PdNiO/CB, and commercial Pd/C in acidic (HClO4), neutral (NaHCO3), and alkaline (KOH) electrolytes, ascribable to the oxygen-enriched support (CeO2) and its interaction with CB support along with electronic effect of PdNiO, which enhances the CO adsorption/activation and simultaneously activates/splits H2O to produce active oxygenated species required for hastening COOxid kinetics. Interestingly, the COOxid performance of the PdNiO-CeO2/CB in HClO4 electrolyte was higher than those in NaHCO3 and KOH electrolytes. This study reveals that coupling oxygen-enriched support (CeO2) with CB support is preferred for the enhancement of the COOxid activity of PdNiO nanocrystals.

Layered perovskites with exsolved Co-Fe nanoalloy as highly active and stable anodes for direct carbon solid oxide fuel cells

The performance and stability of direct carbon solid oxide fuel cells (DCSOFCs) are severely affected by the electrocatalytic activity of their anode materials. In this work, a cobalt doped layered perovskite, (PrBa)0.95Fe1.8−xCoxNb0.2O5+δ (PBFCoxN, x = 0, 0.1, 0.2, 0.3), is investigated as the anode material for a DCSOFC. Co3Fe7 alloy particles are exsolved on the perovskite substrate under the reduction of carbon, which improves CO chemisorption and electrochemical oxidation. This enhances the electrochemical performance of the anode. The electrolyte-supported cell with a (PrBa)0.95Fe1.6Co0.2Nb0.2O5+δ (PBFCo0.2N) anode achieves a peak power density of 525.6 mW cm−2 at 800 °C, fueled by activated carbon. The performance of this anode material is comparable to or exceeds that of previously reported anodes, indicating that the PBFCo0.2N anode is a promising option for DCSOFCs.

Pt-Based Nanostructures for Electrochemical Oxidation of CO: Unveiling the Effect of Shapes and Electrolytes.

Direct alcohol fuel cells are deemed as green and sustainable energy resources; however, CO-poisoning of Pt-based catalysts is a critical barrier to their commercialization. Thus, investigation of the electrochemical CO oxidation activity (COOxid) of Pt-based catalyst over pH ranges as a function of Pt-shape is necessary and is not yet reported. Herein, porous Pt nanodendrites (Pt NDs) were synthesized via the ultrasonic irradiation method, and its CO oxidation performance was benchmarked in different electrolytes relative to 1-D Pt chains nanostructure (Pt NCs) and commercial Pt/C catalyst under the same condition. This is a trial to confirm the effect of the size and shape of Pt as well as the pH of electrolytes on the COOxid. The COOxid activity and durability of Pt NDs are substantially superior to Pt NCs and Pt/C in HClO4, KOH, and NaHCO3 electrolytes, respectively, owing to the porous branched structure with a high surface area, which maximizes Pt utilization. Notably, the COOxid performance of Pt NPs in HClO4 is higher than that in NaHCO3, and KOH under the same reaction conditions. This study may open the way for understanding the COOxid activities of Pt-based catalysts and avoiding CO-poisoning in fuel cells.

ReaxFF reactive molecular dynamics study on electrochemistry of H2/CO hybrid fuel in Ni/YSZ anode

Fuel flexibility, e.g. using reformate or syngas fuel directly, is one of the most advantages of solid oxide fuel cells (SOFC). However, the electrochemical mechanism of the H2/CO mixture, which is common and key in hydrocarbon fuel-fed SOFC, is unclear and relatively rarely studied at present. This article makes Reactive Force-Field (ReaxFF) reactive molecular dynamics (RMD) simulations on the electrochemical co-oxidation of H2/CO hybrid fuel in Ni (nickel)/YSZ (yttria-stabilized zirconia) anode. The RMD simulations at different fuel gas components show that in the H2/CO/Ni/YSZ system the H2 conversion rate maintains the maximum (100%), and the conversion rate of CO improves compared with that of pure CO fuel in the case of 30%H2−70%CO. Two ways of CO2 generation are found: one is that the CO(Ni) species formed by the adsorption and activation of CO on the Ni surface is oxidized by the lattice oxygen; the other is the oxidation of CO(Ni) by spillover atomic oxygen, which is adsorbed on the Ni surface. It is worth noting that the effect of oxygen vacancies on the oxidation of CO is more significant than that of H2. A viewpoint is proposed to explain how H2 enhances the oxidation of CO. The results help understand the electrochemistry of H2/CO involved fuel in the context of SOFC operations.

Promoting effective electrochemical oxidation of CO by Cu-doping for highly active hybrid direct carbon fuel cell anode

Hybrid direct carbon fuel cells (HDCFCs) present its inherent and unparalleled advantages in converting chemical energy from organic waste, biomass, and coal directly into clean energy with high efficiency. However, the progress of HDCFC technology continues to be hampered by the sluggish anode reaction kinetics. Herein, an effective design strategy for HDCFC anode is proposed, a Cu-modified Sr2Fe1.3Cu0.2Mo0.5O6-δ (SFCM) perovskite oxide is developed to meet the requirements of intermediate-temperature HDCFC anode. The Cu-doping SFCM anode demonstrates excellent redox structural stability and catalytic activity. In addition, the electrolyte-supported single cell with Cu-doping SFCM anode has improved the peak power density from 285.5 mW cm−2 to 489.3 mW cm−2 at 800 °C with activated carbon as the fuel. The significantly enhanced anode catalytic activity is primarily due to the improved interfacial activity and chemical adsorption of CO. Therefore, the present work shows an effective strategy for designing and developing novel high-performance HDCFCs anode materials.

Closed-Form Formulation of the Thermodynamically Consistent Electrochemical Model Considering Electrochemical Co-Oxidation of CO and H2 for Simulating Solid Oxide Fuel Cells

Achieving efficient solid oxide fuel cell operation and simultaneous prevention of degradation effects calls for the development of precise on-line monitoring and control tools based on predictive, computationally fast models. The originality of the proposed modelling approach originates from the hypothesis that the innovative derivation procedure enables the development of a thermodynamically consistent multi-species electrochemical model that considers the electrochemical co-oxidation of carbon monoxide and hydrogen in a closed-form. The latter is achieved by coupling the equations for anodic reaction rates with the equation for anodic potential. Furthermore, the newly derived model is capable of accommodating the diffusive transport of gaseous species through the gas diffusion layer, yielding a computationally efficient quasi-one-dimensional model. This resolves a persistent knowledge gap, as the proposed modelling approach enables the modelling of multi-species fuels in a closed form, resulting in very high computational efficiency, and thus enable the model’s real-time capability. Multiple validation steps against polarisation curves with different fuel mixtures confirm the capability of the newly developed model to replicate experimental data. Furthermore, the presented results confirm the capability of the model to accurately simulate outside the calibrated variation space under different operating conditions and reformate mixtures. These functionalities position the proposed model as a beyond state-of-the-art tool for model supported development and control applications.

Restructuring of 4H phase Au nanowires and its catalytic behavior toward CO electro-oxidation

Au nanowires in 4H crystalline phase (4H Au NWs) are synthesized by colloid solution methods. The crystalline phase and surface structure as well as its performance toward electrochemical oxidation of CO before and after removing adsorbed oleylamine molecules (OAs) introduced from its synthesis are evaluated by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), underpotential deposition of Pb (Pb-upd) and cyclic voltammetry. Different methods, i.e. acetic acid cleaning, electrochemical oxidation cleaning, and diethylamine replacement, have been tried to remove the adsorbed OAs. For all methods, upon the removal of the adsorbed OAs, the morphology of 4H gold nanoparticles is found to gradually change from nanowires to large dumbbell-shaped nanoparticles, accompanying with a transition from the 4H phase to the face-centered cubic phase. On the other hand, the Pb-upd results show that the sample surfaces have almost the same facet composition before and after removal of the adsorbed OAs. After electrochemical cleaning with continuous potential scans up to 1.3 V, CO electro-oxidation activity of the 4H Au sample is significantly improved. The CO electro-oxidation activity is compared with results on the three basel Au single crystalline surfaces reported in the literature, possible origins for its enhancement are discussed.

Dynamic Load Cycle Effects on PEMFC Stack CO Tolerance under Fuel Recirculation and Periodic Purge

This work presents first experimental evidence on the effects of dynamic load cycle on PEM fuel cell system CO tolerance, a topic which to date has not been comprehensively investigated. The experiments were performed with a 1 kW fuel cell system employing components, design, and operation conditions corresponding to automotive applications. To distinguish between the load cycle and other factors affecting the CO tolerance, the experiments were repeated with static and dynamic load cycles, as well as with pure and CO contaminated fuel. The measurement data showed that dynamic load cycle improves the CO tolerance in comparison to static load with the same average current density. Moreover, the cell voltage deviation data indicated that the difference could be explained by higher electrochemical CO oxidation rate under the dynamic load cycle. These results allow us to estimate the effect of the load cycle on CO tolerance and understand its origins, thus giving valuable input for fuel quality standardization and fuel cell system development work.

Titanium Carbide (Ti3C2Tx) MXene Ornamented with Palladium Nanoparticles for Electrochemical CO Oxidation

AbstractTitanium carbide (Ti3C2Tx) MXene possesses various unique physicochemical and catalytic properties. However, the electrochemical CO oxidation performance is not yet addressed experimentally. Herein, Ti3C2Tx (TX=OH, O, and F) ordered and exfoliated two‐dimensional nanosheets ornamented with semi‐spherical palladium nanoparticles (2.5 Wt. %) with an average diameter of (10±1 nm) (denoted as Pd/Ti3C2Tx) is rationally designed for the electrochemical CO oxidation. The fabrication process is based on the selective chemical etching of Ti3AlC2 and delamination under sonication to form Ti3C2Tx nanosheets that are used as a substrate and reducing agent for supporting in situ growth of Pd nanoparticles via impregnation with Pd salt. Interestingly, Pd‐free Ti3C2Tx displayed inferior CO oxidation activity, while Pd/Ti3C2Tx enhanced the CO oxidation activity substantially. This is attributed to the combination of outstanding physicochemical properties of Ti3C2Tx and the catalytic merits of Pd nanoparticles.

Operando optical studies of sulfur contamination in syngas operation of solid oxide fuel cells

Near-infrared thermal imaging, voltammetry, and chronocoulometry have been used to examine electrochemical oxidation mechanisms in solid oxide fuel cells (SOFCs) operating with model syngas mixtures (CO + H2) at 800 °C with and without a sulfur-containing contaminant. Questions persist regarding the electrochemical oxidation of H2 vs. CO especially on the role of CO activation and CO mobility in porous SOFC cermet anodes. Chronocoulometry, electrochemical impedance spectroscopy, and operando optical methods are also used to characterize contaminant effects of SOFC operation at 800 °C in the presence of 50 ppmv sulfur to gain insight into its effect on syngas SOFC operation. The chronocoulometry results indicate that in the absence of sulfur contaminants, electrochemical oxidation of adsorbed CO does not contribute to current production, possibly because of rapid carbon formation at the anode surface. This discovery suggests that initially adsorbed H2 is the primary source of charge at early times in the electro-oxidation of syngas fuels. Furthermore, in the presence of sulfur contaminants, chronocoulometry data show that S competitively occupies ~30% of the electroactive sites at the triple phase boundary and contributes to the total initial charge likely via electrochemical oxidation of S to SO2.

Numerical Simulation of Fluidized Bed Gasifier Coupled with Solid Oxide Fuel Cell Fed with Solid Carbon

A model of a fluidized bed coupled with direct carbon solid oxide fuel cell (SOFC) is developed to explore the effect of coupling between fluidized bed and solid oxide fuel cell. Three gas–solid flow regimes are involved including fixed bed, delayed bubbling bed and bubbling bed. The anode reaction of SOFC is treated as the coupling processes of Boudouard gasification of carbon and electrochemical oxidation of CO. The effects of inlet velocity of the fluidizing agent CO2, carbon activity, channel width and coupling extent on the system performance are investigated. The results show that the inlet velocity of CO2 can promote the gasification rate in the anode, but too high velocities may lower CO molar fraction. The gasification rate generally increases with the increase of the channel width and carbon activity. The overlapping area between the anode surface and the initial carbon bed, gas–solid regime and carbon activity have a significant influence on the gasification rate and the maximum current density the system can support. Overall, the mass transport in the anode is dramatically enhanced by the expansion of the carbon bed, back-mixing, solid mixing and gas mixing, especially for the delayed bubbling bed and bubbling bed. This indicates that the adopted coupling method is feasible to improve the anode performance of direct carbon solid oxide fuel cell.

Cobalt anchored CN sheet boosts the performance of electrochemical CO oxidation* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11574167 and 11874033).

Single-atom catalysts (SACs) have attracted great interest due to their significant roles played in applications of environmental protection, energy conversion, energy storage, and so on. Using first-principles calculations with dispersion-correction, we investigated the structural stability and catalytic activity of Co implanted CN sheet towards CO oxidation. The adsorption energy of CO and O2 on the catalysts Co@CN and 2Co@CN are close, thus preventing CO poisoning. Among three possible CO oxidation mechanisms, termolecular Eley-Rideal is the most appropriate reaction path, and the corresponding rate-limiting reaction barriers of the two systems are 0.42 eV and 0.38 eV, respectively.

Electrochemical preparation of Ni(OH)2/CoOOH bilayer films for application in energy storage devices

CoOOH/Ni(OH)2 and Ni(OH)2/CoOOH bilayer films were obtained through electrochemical deposition of Ni(OH)2 and Co(OH)2 single layers onto steel plate substrate, with subsequent electrochemical oxidation of Co(OH)2 to form CoOOH. The films were structurally characterized using X-ray diffraction and Raman spectroscopy techniques. Cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy techniques were used to perform the electrochemical characterization of the prepared bilayers. It was verified that during the electrochemical oxidation, CoOOH formation occurs without Ni(OH)2 oxidation. Ni(OH)2 films deposited on CoOOH (CoOOH/Ni(OH)2) showed a superior electrochemical performance, with 324.27 mA h g−1 and 96.82% at 1.0 mA cm−2 for the specific capacity and capacity retention respectively. It was also determined that the relaxation time constant under this condition is 17.05 s. These results suggested that the CoOOH/Ni(OH)2 bilayer films might be potential candidates for application as electrode in energy storage devices.

Tracking oxygen atoms in electrochemical CO oxidation - Part II: Lattice oxygen reactivity in oxides of Pt and Ir

Electrochemical oxidation of carbon monoxide (CO oxidation) is often used as a model reaction to investigate the surface of metallic electrocatalysts, most notably in CO stripping experiments. In this report, we use chip-based electrochemistry mass spectrometry with 18O isotope-labeled oxides Pt and Ir to investigate the involvement of lattice oxygen in the electrochemical oxidation of water (the oxygen evolution reaction, OER), adventitious carbon, and CO. For Pt, we find that the labeled oxygen from Pt18Ox is incorporated into the CO2 resulting from CO oxidation at the potential at which oxygen at the electrode surface is reduced to hydroxyl (*OH), confirming that *OH is the reactive species in the Langmuir-Hinshelwood electrochemical oxidation of CO. For Ir we find that lattice oxygen in Ir18O2 is similarly involved in electrochemical CO oxidation, but only if it is first activated by a reductive sweep. The labeled CO2 signal is transient, indicating that activated lattice oxygen provides the “ignition sites” for the Langmuir-Hinshelwood electrochemical oxidation of CO on Ir. We also confirm the previously reported result that electrochemically prepared, amorphous, Ir18Ox incorporates much more lattice oxygen in O2 evolved during OER than does rutile Ir18O2, but we also quantify the amount and show that in all cases the labeled O2 is a very small portion of the total O2 evolved, and that more lattice oxygen is released in CO2 when oxidizing CO and adventitious carbon than is released in O2 when oxidizing water. Through these results, we demonstrate that EC-MS in concert with isotope labeling and CO as a probe molecule can provide insight into lattice oxygen reactivity, extending the utility of CO oxidation to the study of noble metal oxides used in e.g. PEM electrolyzer anodes.

Operando Optical Studies of Sulfur Contamination in Syngas Operation of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are attractive devices for power generation because they can operate under a variety of fuels, including H2, hydrocarbon fuels, biogas, and syngas (CO + H2). Syngas is an important fuel since it is a major product of reforming hydrocarbon logistics fuels (i.e. JP-8, JP10, S8, etc.). However, the mechanism of electrochemical oxidation of CO in SOFCs is still not well understood. This is due, in part, to the difficulty in probing CO consumption and product formation under operating conditions in real time. Herein, the operando methods Fourier transform infrared emission spectra (FTIRES) of the SOFC headspace together with thermal imaging data of the Ni-YSZ anode surface to provide data relevant to understanding these reaction mechanisms. The studies are carried out on anode-supported SOFCs (Ni-YSZ/GDC-LSC button cells) operating on simulated syngas mixtures. The contamination and subsequent degradation of SOFC operation at operational temperatures of 600° C to 800° C, in the presence of sulfur concentrations below 30 ppm, is followed using potentiostatic spectroelectrochemistry, chronocoulometry, electrochemical impedance spectroscopy, and operando optical methodologies to provide a comprehensive picture of the processes occurring in the fuel oxidation. Together, the data obtained indicate that sulfur contaminants 1) competitively inhibit formation of carbon over CO catalytic and electrocatalytic sites, and 2) induce changes of the Ni-YSZ anode permanently perturbing their catalytic and electrocatalytic function.

Facet-Dependent Cobalt Ion Distribution on the Co3O4 Nanocatalyst Surface.

Co3O4 is an important catalyst widely used for CO oxidation or electrochemical water oxidation near room temperature and was also recently used as support for single-atom catalysts (SACs). Co3O4 with a spinel structure hosts dual oxidation states of Co2+ and Co3+ in the lattice, leading to the complexity of its surface structure as the exposure of Co2+ and Co3+ has a significant impact on the performance of the catalysts. Although it is acknowledged that different facets exhibit varied catalytic activities and different abilities in hosting single atoms to provide active centers in SACs, the Co3O4 surface structure remains under-investigated. In this study, major facets of {111}, {110}, and {100} were studied down to subangstrom scale using advanced electron microscopy. We noticed that each facet has its own most stable surface configuration. The distribution of Co2+ and Co3+ on each facet was quantified, revealing a facet-dependent distribution of Co2+ and Co3+. Co3+ was found to be preferentially exposed on {100} and {110} as well as surface steps. Surface reconstruction was revealed, where a subangstrom scale shift of Co2+ was confirmed on facets of {111} and {100} due to polarity compensation and oxygen deficiency on the surface. This work not only improves our fundamental understanding of the Co3O4 surface structure but also may promote the design of Co3O4-based catalysts with tunable activity and stability.