Oxygen Evolution Reaction Catalysis Research Articles

High-efficiency oxygen evolution reaction: Effect of phosphorus doped on the surface reconstruction of high-entropy spinel oxides

High entropy oxides (HEOs) are often used for the oxygen evolution reaction (OER) due to their tunable electronic structure, variable composition and structural stability. However, the catalytic activity of conventional HEOs is limited due to their high crystallinity, resulting in a low number of active sites on the surface. Anion doping, on the other hand, is a method that can effectively increase OER activity. In this study, a series of P-doped HEOs were synthesized and the effect of P doped on HEOs was investigated. It was found that P doped increased the metal (Co, Ni, Mn)-oxygen covalency in HEOs and generated a large number of oxygen vacancies. The electrochemical properties of HEOs with different P doped levels were also compared, and HEO-P-1 showed the best catalytic performance, reaching a current density of 10 mA cm−2 at an overpotential of only 254 mV. Then, the surface reconstruction process of HEO-P-1 was also investigated and it was found that the (oxygen) hydroxides of Co-Ni-Mn play a real catalytic role. Finally, the OER mechanism catalyzed by HEO-P-1 was investigated. This work provides a new idea for effective doping of anions in HEOs for efficient OER catalysis.

Step-by-step enhancing oxygen evolution ability of CoFe2O4 by hybrid structure engineering and fluorine doping

CoFe2O4 is a very promising platform to catalyze oxygen evolution reaction (OER). Herein, the largely enhanced intrinsic performance of CoFe2O4 derived from the ZIF-67 for OER was demonstrated by step-by-step hybrid structure engineering and fluorine doping (F-Co/CoFe2O4@NC). This electrode showed high surface area, rapid charge transfer ability, and catalytic kinetics compared to the CoFe2O4@NC and Co/CoFe2O4@NC catalysts. Specifically, an overpotential of 280 mV was required to drive a kinetic current density of 10 mA cm−2 (loading on glassy carbon electrode, no iR-correction), with a Tafel slope of 49.7 mV dec-1 and remarkable catalytic stability throughout a 50-hour test. Theoretical analysis revealed a charge redistribution among Co, and Fe atoms was triggered by fluorine doping, and a more rapid active phase of CoOOH was generated on the surface during catalysis as confirmed by in-situ Raman spectroscopy and the post-surface chemical state analysis. The stable F atoms were more likely to be doped into CoFe2O4, and the active layer of CoOOH formed over the F-CoFe2O4 surface was more active than other states. Moreover, the electronic interaction induced by F-doping is more conducive to improving the adsorption energy of *OOH species, thereby improving the reaction kinetics and catalytic activity of OER. The current work showed an effective step-by-step approach to boosting the intrinsic activity of traditional catalysts for energy-relevant catalysis reactions.

Competitive Valerate Binding Enables RuO2-Mediated Butene Electrosynthesis in Water.

The (non)-Kolbe oxidation of valeric acid, sourced from a hydrolysis product of cellulose, provides a sustainable synthetic route to access value-added products, such as butene. An essential mechanistic step preceding product formation involves the oxidative and decarboxylative cleavage of a C-C bond. Yet, the role of the electrode surface in mediating this oxidative step remains an open question: the electron transfer can occur either via an inner-sphere or outer-sphere mechanism. Here, we report the electrochemical, in situ spectroscopic, computational, and reactivity studies of RuO2-mediated oxidative decarboxylation of valeric acid to butene in aqueous electrolytes. We find that carboxylates bind to RuO2 anode surfaces at potential values where decarboxylation products are observed. Our results are consistent with a reaction scheme where the competitive and catalytic oxygen evolution reaction (OER) is impeded by these bound carboxylate species while these species are inert toward butene formation. Our results implicate an outer-sphere electron transfer mechanism for decarboxylation where the surface chemistry of the RuO2 electrode serves to enable higher non-Kolbe reaction selectivity by suppressing the parasitic OER. Our findings delineate interfacial design principles for selective electrochemical systems that utilize water as the ultimate oxidant for sustainable decarboxylation.

The cobalt-based metal organic frameworks array derived CoFeNi-layered double hydroxides anode and CoP/FeNi2P heterojunction cathode for ampere-level seawater overall splitting

The seawater electrolysis technology powered by renewable energy is recognized as the promising “green hydrogen” production method to solve serious energy and environmental problems. The lack of low-cost and ampere-level current OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) catalysis limits their industrial application. In this work, a unique tri-metal (Co/Fe/Ni) layered double hydroxide hollow array anode catalyst (CFN-LDH/NF) and the CoP/FeNi2P heterojunction hollow array cathode are successfully prepared via one in-situ growth of Co-MOF on nickel foam (Co-MOF/NF) precursor, which exhibits excellent catalytic performance. The η1000 values of 352 and 392 mV are achieved for CFN-LDH/NF (OER catalyst) in 1.0 M KOH and alkaline seawater solution, respectively. The CFNP/NF with a low overpotential of 281 mV is required to reach 1000 mA cm−2 current density for HER in 1.0 M KOH solution, while the η1000 in alkaline seawater solution is 312 mV. The CFN-LDH/NF||CFNP/NF electrolyzer exhibits excellent long-term durability over 100 h, achieving current density of 500 mA cm−2 at 1.825 V in 1.0 M KOH solution. The construction of hollow tri-metal LDH and phosphides heterostructures may open a new and relatively unexplored path for fabricating high performance seawater splitting catalysis.

Rational design of RuO2 composites from hydrogen-bonded organic frameworks for alkaline oxygen evolution reaction

In the past few decades, the task of creating electrocatalysts with consistent costs for facilitating oxygen evolution reactions with minimal overpotential remains a formidable challenge in the realm of chemical energy conversion technologies. In this report, we present a straightforward approach for synthesizing the melamine trithiocynauric acid (MTC) and ruthenium dioxide (RuO2) based, MTC/RuO2 composite, which demonstrates high efficiency in catalyzing oxygen evolution reaction. The physical characterizations for the prepared material are x-ray diffraction (XRD)， scanning electron microscopy (SEM), and x-ray photoelectron spectrometer (XPS) all approve the presence of RuO2 in the MTC composite structure. For oxygen evolution reaction the composite (MTC/RuO2-0.2) exhibited an exceptionally low overpotential of 190 mV for the current density of 10 mA cm−2 and also demonstrated a Tafel slope of 52 mV dec−1. Significant improvement in electrocatalytic activity is observed in alkaline electrolyte (1 M KOH). The outstanding behavior of the catalyst is due to the structural synergistic effect between MTC and RuO2. So far there is no report on RuO2-based hydrogen bonded organic frameworks for oxygen evolution reaction. The catalyst also shows high stability for almost 24 h. Hence, the MTC/RuO2-0.2 composite is highly unique and efficient.

Electronic structure tailoring of CuCo2O4 for boosting oxygen evolution reaction

Electronic structure tuning in metal oxides is a facile and effective strategy on boosting their catalytic oxygen evolution reaction (OER) performance. Here, we demonstrate the electronic structure tuning of CuCo2O4 by phosphorus (P) doping via in-situ diffusion method. The results suggest that due to more electrons transferred from P to the neighboring Co3+, the tuned Co is served as catalytic active sites for the enhanced OER performance. The synthesized P3.85-CCO/NF exhibits an overpotential of 250 mV at a current density of 10 mA cm−2, and a Tafel slope of 27 mV dec−1, which performs an enhanced OER activity than that of IrO2/NF. Moreover, the P3.85-CCO/NF presents stable electrochemical performances upon long-time running for 30 h. Thus, the electronic structure tuning strategy by in-situ P diffusion method emerges as an effective approach on enhancing the catalytic OER performance for metal oxide electrocatalysts.

High Entropy Spinel Oxide (AlCrCoNiFe2)O as Highly Active Oxygen Evolution Reaction Catalysts.

The advancement of water electrolyzer technologies and the production of sustainable hydrogen fuel heavily rely on the development of efficient and cost-effective electrocatalysts for the oxygen evolution reaction (OER). High entropy ceramics, characterized by their unique properties, such as lattice distortion and high configurational entropy, hold significant promise for catalytic applications. In this study, we utilized the sol-gel autocombustion method to synthesize high entropy ceramics containing a combination of 3d transition metals and aluminum ((AlCrCoNiFe2)O). We then compared their electrocatalytic performance with other series of synthesized multimetal and monometallic oxides for the OER under alkaline conditions. Our electrochemical analysis revealed that the high entropy ceramics exhibited excellent performance and the lowest charge transfer resistance, Tafel slope (29 mV·dec-1), and overpotential (η10 = 230 mV). These remarkable results can be primarily attributed to the high entropy effect induced by the addition of Al, Cr, Co, Ni, and Fe, which introduces increased disorder and complexity into the material's structure. This, in turn, facilitates more efficient OER catalysis by providing diverse active sites and promoting optimal electronic configurations for the reaction. Furthermore, the strong electronic interactions among the constituent elements in the metallic spinels further enhance their catalytic activity in the initiation of the OER process. Combined with the reduced charge transfer resistance, these factors collectively play pivotal roles in enhancing the OER performance of the electrocatalysts. Overall, our study provides valuable insights into the design and development of high-performance electrocatalysts for sustainable energy applications. By harnessing the high entropy effect and leveraging strong electronic interactions, electrocatalytic materials can be tailored to improve efficiency and stability, thus advancing the progress of clean energy technologies.

The (In)Stability of Heterostructures During the Oxygen Evolution Reaction

AbstractThe urgent need for efficient oxygen evolution reaction (OER) catalysts has led to the development and publication of many heterostructured catalysts. The application of such catalysts with multiple phases tremendously increases the material design dimensions, and numerous interface‐related effects can tune the OER performance. In this regard, multiple of these heterostructured electrodes show remarkable OER activities. However, it is not clear if these carefully designed interfaces remain under prolonged OER conditions. Herein, a molecular approach is used to synthesize four different nickel‐iron phosphide (heterostructured) materials and deposit them on fluorine‐doped tin oxide and nickel foam electrodes. The OER performance of the eight electrodes and the reconstruction of the four materials is investigated by in‐situ spectroscopy after one day of operation, enabled by a freeze‐quench approach. The most active electrode is also applied under industrial OER conditions and for the value‐added oxidation of alcohols to ketones. Before catalysis, this electrode comprises crystalline 4 nm nickel phosphide particles on an amorphous iron phosphide matrix. However, after 24 h, a homogenous nickel‐iron oxyhydroxide phase has formed. This work questions to which extent the design of heterostructures is a suitable strategy for non‐noble metal OER catalysis.

Modulation in work function of CoTe as bifunctional electrocatalyst for rechargeable zinc air battery

The sluggish kinetics and inferior stability of oxygen electrocatalyst in rechargeable zinc air battery (ZAB) hamper its industrialization. In this work, we activate cobalt telluride (CoTe) by introduction of metallic cobalt (Co) to modulate the work function to facilitate the electron transfer from Co to CoTe during oxygen catalysis; additionally, the three-dimensional porous carbon nanosheets (3DPC) are invited to reduce the resistance towards electrolyte/oxygen diffusion. Thereby, Co-CoTe@3DPC only demands 280 mV overpotential to reach 10 mA cm−2 under alkaline oxygen evolution reaction (OER) condition, relatively lower than commercial iridium oxides (IrO2); besides, the operando electrochemical impedance spectroscopy (EIS) indicates a better resistance towards surface reconstruction than Co@3DPC leading to a superior stability. A Pt-like oxygen reduction reaction (ORR) performance, half-wave potential associated with kinetic current density, is achieved for Co-CoTe@3DPC. A maximum power density of 203 mW cm−2 is achieved and sustains for 800 h. Furthermore, the all-solid-state ZAB offers 97 mW cm−2. Theoretical calculation suggests that the incorporation of metallic Co to CoTe maintains the superb ORR activity and promotes the OER catalysis.

Spectroscopic Investigations of Complex Electronic Interactions by Elemental Doping and Material Compositing of Cobalt Oxide for Enhanced Oxygen Evolution Reaction Activity

AbstractDoping and compositing are two universal design strategies used to engineer the electronic state of a material and mitigate its disadvantages. These two strategies are extensively applied to design efficient electrocatalysts for water splitting. Using cobalt oxide (CoO) as a model catalyst, it is proven that the oxygen evolution reaction (OER) performance can be progressively improved, first by Fe‐doping to form Fe‐CoO solid solution, and further by the addition of CeO2 to produce a Fe‐CoO/CeO2 composite. X‐ray absorption spectroscopy (XAS) reveals that distinct electronic interactions are induced by the processes of doping and compositing. Fe‐doping of CoO can break down the structural symmetry, changing the electronic structure of both Co and O species at the surface and decreasing the flat‐band potential (Vfb). In comparison, subsequent compositing of Fe‐CoO with CeO2 induces negligible electronic changes in the Fe‐CoO (as seen in ex situ characterizations), but significantly modifies the oxidative transformations of both Co and Fe under OER conditions. The spectroscopic investigations reveal that Fe‐doping and CeO2 compositing play different roles in modifying the electronic properties of CoO in its pristine state and during OER catalysis, in return, providing useful guidance for the design of more efficient electrocatalysts using these two strategies.

A Comprehensive Pyrolysis Mechanism of Binuclear Chromium-Based Complexes for Superior OER Activity.

Transition metal oxides are widely pursued as potent electrocatalysts for the oxygen evolution reaction (OER). However, single-metal chromium catalysts remain underexplored due to their intrinsic activity limitations. Herein, we successfully synthesize mixed-valence, nitrogen-doped Cr2O3/CrO3/CrN@NC nanoelectrocatalysts via one-step targeted pyrolysis techniques from a binuclear Cr-based complex (Cr2(Salophen)2(CH3OH)2), which is strategically designed as a precursor. Comprehensive pyrolysis mechanisms were thoroughly delineated by using coupled thermogravimetric analysis and mass spectrometry (TG-MS) alongside X-ray diffraction. Below 800 °C, the generation of a reducing atmosphere was noted, while continuous pyrolysis at temperatures exceeding 800 °C promoted highly oxidized CrO3 species with an elevated +6 oxidation state. The optimized catalyst pyrolyzed at 1000 °C (Cr2O3/CrO3/CrN@NCs-1000) demonstrated remarkable OER activity with a low overpotential of 290 mV in 1 M KOH and excellent stability. Further density functional theory (DFT) calculations revealed a much smaller reaction energy barrier of CrO3 than the low oxidation state species for OER reactivity. This work reveals fresh strategies for rationally engineering chromium-based electrocatalysts and overcoming intrinsic roadblocks to enable efficient OER catalysis through a deliberate oxidation state and compositional tuning.

Enhancing the Performance of 2D Ni-Fe Layered Double Hydroxides by Cabbage-Inspired Carbon Conjunction for Oxygen Evolution Reactions.

Layered double hydroxide (LDH) nanosheets as one type of two-dimensional materials have garnered increasing attention in the field of oxygen evolution reaction (OER) in recent decades. To address the challenges associated with poor conductivity and limited electron and charge transfer capability in LDH materials, we have developed a straightforward one-pot synthesis method to successfully fabricate a composite material with a microstructure resembling cabbage, which encompasses NiFe-LDH and nanocarbon (referred as NiFe-LDH@C). Atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) revealed that the monolayer NiFe-LDH with a height of ~0.5-0.8 nm is uniformly distributed and closely bonded to the carbon support, leading to a significant enhancement in conductivity and facilitating faster electron and charge transfer. Moreover, the NiFe-LDH@C exhibits a substantial number of surface defect sites, which enhances the interaction with oxygen species. This dual enhancement in charge transfer and oxygen species-mediated transfer greatly improves the catalytic OER performance, which is further corroborated by theoretical calculations. Notably, the Ni10Fe6-LDH@C with the highest concentration of surface oxygen vacancies demonstrated superior water oxidation performance, surpassing commercially available RuO2 catalysts; an OER overpotential of 231 mV@10 mA cm-2 with a Tafel slope of 71 mV dec-1 was achieved.

Facet-Dependent Surface Restructuring on Nickel (Oxy)hydroxides: A Self-Activation Process for Enhanced Oxygen Evolution Reaction.

Unraveling the catalyst surface structure and behavior during reactions is essential for both mechanistic understanding and performance optimization. Here we report a phenomenon of facet-dependent surface restructuring intrinsic to β-Ni(OH)2 catalysts during oxygen evolution reaction (OER), discovered by the correlative ex situ and operando characterization. The ex situ study after OER reveals β-Ni(OH)2 restructuring at the edge facets to form nanoporous Ni1-xO, which is Ni deficient containing Ni3+ species. Operando liquid transmission electron microscopy (TEM) and Raman spectroscopy further identify the active role of the intermediate β-NiOOH phase in both the OER catalysis and Ni1-xO formation, pinpointing the complete surface restructuring pathway. Such surface restructuring is shown to effectively increase the exposed active sites, accelerate Ni oxidation kinetics, and optimize *OH intermediate bonding energy toward fast OER kinetics, which leads to an extraordinary activity enhancement of ∼16-fold. Facilitated by such a self-activation process, the specially prepared β-Ni(OH)2 with larger edge facets exhibits a 470-fold current enhancement than that of the benchmark IrO2, demonstrating a promising way to optimize metal-(oxy)hydroxide-based catalysts.

Organic-inorganic hybrid interfaces with π-d electron coupling for preventing metal and sulfur leaching toward enhanced oxygen evolution reaction

Transition metal sulfides (TMSs) catalysts with high catalytic oxygen evolution reaction (OER) activity have been extensively studied, especially Fe and Co-based sulfides. Fe and Co active sites with a strong synergistic effect, which can adjust the electron density distribution and effectively improve the electrocatalytic OER activity. However, TMSs have poor stability in alkaline environment caused by metal ions and sulfur elements are facilitated to dissolve. In this work, TMSs was modified by polyaniline (PANI) to inhibit the precipitation of iron, cobalt, and sulfur elements and enhance its stability under alkaline conditions. Moreover, π-d structure can also be formed by the coating of PANI, which can further adjust its own electronic structure on the basis of stabilizing the TMSs structure, so as to improve the electrochemical performance, rendering them to stably operate at harsh environment for more than 90 h. These findings offer new guidance for improving the electrocatalytic stability of TMSs.

Fe site regulation and activity deciphering by selective phase transformation in the confined FeNi nanoparticles for oxygen evolution reaction

Given its initially low activity and difficulty in active phase formation, Fe site regulation is rarely touched in FeNi-based oxygen evolution reaction (OER) catalysts. Herein, the Fe sites regulation and activity deciphering are demonstrated by selective phase transformation taking nitrogen-doped carbon confined FeNi nanoparticles derived from Fe2Ni-MIL-MOF pyrolysis as an example; driven by the reduction atmosphere and Kirkendall effect assisted by melamine pyrolysis, some Fe and Ni sites were separated to form metallic Fe and FeNi alloy and the subsequent fluorination process induced the ionic compound FeF2 formation rather than NiF2, offering a good platform to evaluate the influence of active Fe phase for OER catalysis. Theoretical calculations combined with ex-situ and in-situ spectral characterization techniques proved the complete convention of FeF2 into a high-valent and reactive intermediate state of FeOOH. Considering the stable metallic state of Ni, the largely improved catalytic performance could be attributed to the Fe site regulation and the subsequent synergistic catalysis effect. Compared to the traditional iron-based catalyst, theoretically, the electronic structure of FeF2 was more conducive to the active FeOOH phase formation and significantly lowered the reaction energy barrier for OER. By addressing the aforementioned problems, low overpotential and good stability for OER catalysts were realized. The current work showed valid proof of the FeNi-based catalysts regulation via Fe sites for energy-relevant catalysis reactions.

Multiwall carbon nanotubes supported rhenium doped Co3O4 as an efficient electrocatalyst for water oxidation in alkaline medium

The recent trends for the development of efficient and low cost transition metal (TMs) based anode catalysts for water splitting is crucial and important. Even though, research is continuously progressing in different directions to design catalyst; however, catalytic OER (oxygen evolution reaction) activity has remained insufficient. Till now, various transition metal oxides (TMOs) have been used as efficient and cost-effective electrocatalyst for the oxygen evolution reaction. The catalytic performance of TMOs could be enhanced by doping with the metal, which increases the catalyst's charge transfer and electronic conductivity. Decoration of TMOs with the multi-wall carbon nanotube (MWCNT) further improves catalytic activity by providing numerous active sites. Here, Co3O4/MWCNT and Rhenium (Re) doped Co3O4/MWCNT composite were prepared by simple hydrothermal process and used as electrocatalyst for OER in alkaline medium. Different concentrations of Re (0.05 mmol, 0.1 mmol, 0.2 mmol, 0.3 mmol) were doped and it was observed that the electrochemical results of the 0.2 mmol Re doped Co3O4/ MWCNT (R0.2CO) was most efficient electrocatalyst which offers the OER overpotential of 298 mV/dec to attain the 10 mA cm−2 current density, lower Tafel slope (70 mV/dec), higher value of turn over frequency (0.95), lower value of the charge transfer resistance (3.10Ω) and good stability, demonstrating the better OER catalytic activity as compared to un-doped Co3O4/MWCNT (R0.0CO) catalyst. The investigated catalyst R0.2CO is more efficient catalyst than most of the reported similar type of catalysts and offers comparable electrocatalytic activity with benchmark electrocatalyst RuO2.

Electron-deficient Co7Fe3 induced by interfacial effect of molybdenum carbide boosting oxygen evolution reaction

Developing a high-activity and low-cost catalyst to reduce the anodic overpotential is essential for hydrogen production from water splitting. In this work, a hetero-structured Co7Fe3/Mo2C@C catalyst has been developed to efficiently catalyze oxygen evolution reaction (OER), the overpotential (ƞ10) of Co7Fe3/Mo2C@C-catalyzed OER with current density of 10 mA/cm2 is about 254 mV, substantially lower than the counterparts of Co7Fe3@C-catalyzed OER (ƞ10, 308 mV) and Mo2C@C-catalyzed OER (ƞ10, 439 mV), close to that of OER catalyzed by commercial RuO2. The mechanistic studies reveal that the distinct electron transfer across the Co7Fe3/Mo2C interface results in electron-deficient Co7Fe3, which has been identified as the highly active catalytic sites. Density functional theory (DFT) calculations manifest that Mo2C induces a distinct decrease in electron density on Co7Fe3 and upgrades the d-band centers of Co and Fe in Co7Fe3 towards Fermi energy level, thus substantially lowering the energy barrier of the rate-determining reaction step and conferring significantly improved OER activity on the Co7Fe3/Mo2C@C catalyst.

Co-Ni co-sputter deposited bimetallic thin film catalysts for alkaline hydrogen and oxygen evolution reactions

Co-Ni bi-metallic thin films with different stoichiometries have been deposited by magnetron co-sputtering technique on Ni foam (NF), Si and carbon paper substrates. The structural and morphological characterisations of these films have been done by X-ray diffraction, X-ray absorption spectroscopy and Field Emission Scanning Electron Microscope measurements. The performances of these films for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been evaluated through electrochemical measurements like Linear Sweep Voltammetry, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy measurements. Operando X ray absorption near edge spectroscopy measurements have also been carried out during HER and OER to decipher the redox reaction mechanisms. It has been observed that the HER and OER performance of all the Co Ni bi-metallic thin films on NF are found to be better than that of bare NF. The CoNi sample with composition of 58.7 % Co and 41.3 % Ni shows the best catalytic activity and stability towards HER and OER reactions. Though the HER activity is not the best but is comparable to many results reported in the literature, however the catalyst shows excellent OER activity with overpotential of 214 mV for 10 mA/cm2 current density. This is much better than that of the OER benchmark catalyst RuO2 and most of those reported in the literature. The OER is kinetically slower than HER and requires more overpotential, hence a significant improvement in the catalytic OER activity will definitely enhance the overall water splitting activity and hydrogen production efficiency. Thus the Co-Ni bi-metallic catalyst prepared by an easily scalable and highly reproducible magnetron co-sputtering technique has significant potential to emerge out as a technologically viable water splitting catalyst.

Prediction of Feasibility of Polaronic OER on (110) Surface of Rutile TiO2.

The polaronic effects at the atomic level hold paramount significance for advancing the efficacy of transition metal oxides in applications pertinent to renewable energy. The lattice-distortion mediated localization of photoexcited carriers in the form of polarons plays a pivotal role in the photocatalysis. This investigation focuses on rutile TiO2, an important material extensively explored for solar energy conversion in artificial photosynthesis, specifically targeting the generation of green H2 through photoelectrochemical (PEC) H2O splitting. By employing Hubbard-U corrected and hybrid density functional theory (DFT) methods, we systematically probe the polaronic effects in the catalysis of oxygen evolution reaction (OER) on the (110) surface of rutile TiO2. Theoretical understanding of polarons within the surface, coupled with simulations of OER at distinct titanium (Ti) and oxygen (O) active sites, reveals diverse polaron formation energies within the lattice sites with strong preference for bulk and surface bridge (Ob) oxygen sites. Moreover, we provide the evidence for the facilitative role of polarons in OER. We find that hole polarons situated at the equatorial oxygen sites near the Ti-active site, along with bridge site hole polarons distal from the Ob active site yield a small reduction in OER overpotential by ~0.06 eV and ~0.12 eV, respectively. However, subsurface, equatorial, and bridge site hole polarons significantly reduce the Ti-active site OER overpotential by ~0.4 eV through the peroxo-type oxygen pathway. We also observe that the presence of hole polarons stabilizes the *OH, *O, and *OOH intermediate species compared to the scenario without hole polarons. Overall, this study provides a detailed mechanistic insight into polaron-mediated OER, offering a promising avenue for improving the catalytic activity of transition metal oxide-based photocatalysts catering to renewable energy requisites.

Surface oxidation/spin state determines oxygen evolution reaction activity of cobalt-based catalysts in acidic environment

Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin CoII are found to be not active due to their unfavorable water dissociation to form CoIII-OH species. By contrast, the presence of low-spin CoIII is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism.