Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

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A review on fundamentals for designing oxygen evolution electrocatalysts

Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir–Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir–Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an in...

Molecular insight in structure and activity of highly efficient, low-Ir Ir-Ni oxide catalysts for electrochemical water splitting (OER)

The poor activity and stability of electrode materials for the oxygen evolution reaction are the main bottlenecks in the water-splitting reaction for H2 production. Here, by studying the activity–stability trends for the oxygen evolution reaction on conductive M1OxHy, Fe–M1OxHy and Fe–M1M2OxHy hydr(oxy)oxide clusters (M1 = Ni, Co, Fe; M2 = Mn, Co, Cu), we show that balancing the rates of Fe dissolution and redeposition over a MOxHy host establishes dynamically stable Fe active sites. Together with tuning the Fe content of the electrolyte, the strong interaction of Fe with the MOxHy host is the key to controlling the average number of Fe active sites present at the solid/liquid interface. We suggest that the Fe–M adsorption energy can therefore serve as a reaction descriptor that unifies oxygen evolution reaction catalysis on 3d transition-metal hydr(oxy)oxides in alkaline media. Thus, the introduction of dynamically stable active sites extends the design rules for creating active and stable interfaces. Understanding what underpins the activity and stability of oxygen evolution catalysts is an ongoing issue in the field of water splitting. Now, researchers show that balancing the rate of Fe dissolution and deposition over a metal hydr(oxy)oxide host yields dynamically stable Fe active sites, with the Fe–host interaction key to the performance.

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Dynamic stability of active sites in hydr(oxy)oxides for the oxygen evolution reaction

The integration of heterometallic units and nanostructures into metal-organic frameworks (MOFs) used for the oxygen evolution reaction (OER) can enhance the electrocatalytic performance and help elucidate underlying mechanisms. We have synthesized a series of stable MOFs (CTGU-10a1-d1) based on trinuclear metal carboxylate clusters and a hexadentate carboxylate ligand with a (6,6)-connected nia net. We also present a strategy to synthesize hierarchical bimetallic MOF nanostructures (CTGU-10a2-d2). Among these, CTGU-10c2 is the best material for the OER, with an overpotential of 240 mV at a current density of 10 mA cm-2 and a Tafel slope of 58 mV dec-1 . This is superior to RuO2 and confirms CTGU-10c2 as one of the few known high-performing pure-phase MOF-OER electrocatalysts. Notably, bimetallic CTGU-10b2 and c2 show an improved OER activity over monometallic CTGU-10a2 and d2. Both DFT and experiments show that the remarkable OER performance of CTGU-10c2 is due to the presence of unsaturated metal sites, a hierarchical nanobelt architecture, and the Ni-Co coupling effect.

Stable Hierarchical Bimetal-Organic Nanostructures as HighPerformance Electrocatalysts for the Oxygen Evolution Reaction.

A climax in the development of cost-effective and high-efficiency transition metal-based electrocatalysts has been witnessed recently for sustainable energy and related conversion technologies. In this regard, structure-activity relationships based on several descriptors have already been proposed to rationally design electrocatalysts. However, the dynamic reconstruction of the surface structures and compositions of catalysts during electrocatalytic water oxidation, especially during the anodic oxygen evolution reaction (OER), complicate the streamlined prediction of the catalytic activity. With the achievements in operando and in situ techniques, it has been found that electrocatalysts undergo surface reconstruction to form the actual active species in situ accompanied with an increase in their oxidation state during OER in alkaline solution. Accordingly, a thorough understanding of the surface reconstruction process plays a critical role in establishing unambiguous structure-composition-property relationships in pursuit of high-efficiency electrocatalysts. However, several issues still need to be explored before high electrocatalytic activities can be realized, as follows: (1) the identification of initiators and pathways for surface reconstruction, (2) establishing the relationships between structure, composition, and electrocatalytic activity, and (3) the rational manipulation of in situ catalyst surface reconstruction. In this review, the recent progress in the surface reconstruction of transition metal-based OER catalysts including oxides, non-oxides, hydroxides and alloys is summarized, emphasizing the fundamental understanding of reconstruction behavior from the original precatalysts to the actual catalysts based on operando analysis and theoretical calculations. The state-of-the-art strategies to tailor the surface reconstruction such as substituting/doping with metals, introducing anions, incorporating oxygen vacancies, tuning morphologies and exploiting plasmonic/thermal/photothermal effects are then introduced. Notably, comprehensive operando/in situ characterization together with computational calculations are responsible for unveiling the improvement mechanism for OER. By delivering the progress, strategies, insights, techniques, and perspectives, this review will provide a comprehensive understanding of the surface reconstruction in transition metal-based OER catalysts and future guidelines for their rational development.

Recent advances in activating surface reconstruction for the high-efficiency oxygen evolution reaction.

Reducing the noble metal loading and increasing the specific activity of the oxygen evolution catalysts are omnipresent challenges in proton-exchange-membrane water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements. However, proper verification of the stability of these materials is still pending. Here we introduce a metric to explore the dissolution processes of various iridium-based oxides, defined as the ratio between the amounts of evolved oxygen and dissolved iridium. The so-called stability number is independent of loading, surface area or involved active sites and provides a reasonable comparison of diverse materials with respect to stability. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to the formation of highly active amorphous iridium oxide, the instability of which is explained by the generation of short-lived vacancies that favour dissolution. These insights are meant to guide further research, which should be devoted to increasing the utilization of highly durable pure crystalline iridium oxide and finding solutions to stabilize amorphous iridium oxides.

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The stability number as a metric for electrocatalyst stability benchmarking

Simultaneous determination of succinate-dehydrogenase-inhibitor fungicide traces in cereals by QuEChERS preparation and UPLC-MS/MS analysis.

In this work, Fe3O4 nanoparticles (NPs) were synthesized by reverse microemulsion method. After that, in a strong alkaline environment, the Fe3O4 NPs were coated with β-cyclodextrin (β-CD). Then γ-glycidoxypropyltrimethoxysilane (KH560) was used to modified Fe3O4@β-CD NPs to obtain Fe3O4@β-CD-g-KH560 NPs. Through the click reaction between the epoxy group of KH560 and the amino group of Penicillin G acylase (PGA), the covalent immobilization of PGA was realized under mild conditions. The results showed that the enzyme activity (EA), enzyme activity recovery (EAR) and maximum loading (ELC) of immobilized PGA were 28242 U/g, 84 % and 112 mg/g, respectively. Its enzyme activity was kept at 43 % after repeated use for 4 times. Afterwards, in the process of β-CD coating Fe3O4 NPs, Mn2+ doped, and Scanning electron microscopy - Energy dispersive spectrometer (SEM-EDS) was used to scan the metal ion doped sample. The results showed that Mn2+-Fe3O4@β-CD-g-KH560 NPs performed higher EAR and reusability (Ra (%)) than Fe3O4@β-CD-g-KH560 NPs. The immobilized PGA performed higher EAR and reusability, i.e. its EA, EAR and ELC were 31528 U/g, 94.2 % and 105 mg/g, respectively, and 72 % of the original enzyme activity was remained after repeated use for 11 times. Furthermore, the structures of the magnetic composites were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Vibrating sample magnetometer (VSM) and Transmission electron microscopy (TEM). 

Nanoarchitectonics of penicillin G acylase with Mn2+ doped β-cyclodextrin/Fe3O4 for enhanced catalytic activity and reusability

Synthesis of polymer-functionalized β-cyclodextrin, Mg2+ doped, coating magnetic Fe3O4 nanoparticle carriers for penicillin G acylase immobilization

A reliable high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was established for the simultaneous detection of zearalenone-14-glucoside (ZEN-14G) and its metabolite, zearalenone (ZEN), in the plasma, urine, and various tissues of rats. The performance of the developed method was validated by determining the selectivity, linearity (R2 > 0.99), sensitivity (lower limit of quantification, 0.1–1 μg/L), recovery (80.7 ± 3.0–112.3 ± 3.1%), precision (0.6–16.5%), and stability (81.7 ± 1.7–104.1 ± 3.9%). Through use of the methodological advances, the subsequent kinetics and distribution after administration of ZEN-14G by gavage were thoroughly investigated. ZEN-14G and ZEN exhibited similar trends in the plasma, and reached their peak concentrations at 10 min and then rapidly decreased. ZEN-14G could be quantified in the stomach, small intestine, and large intestine 24 h after administration, while ZEN was detectable in all tested tissues. Interestingly, ZEN-14G (7.6 ± 3.0 μg/L) and ZEN (977.5 ± 98.0 μg/L) were also detected in the urine 24 h after administration, indicating that ZEN-14G was prone to be slowly and continuously hydrolyzed into ZEN to be absorbed into the plasma and distributed to various tissues, thus leading to a cumulative exposure. Continuous attention should be paid to the co-exposure of ZEN and ZEN-14G, which might pose additional health risks to humans and animals.

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Zhizhong Li

Papers

The stability number as a metric for electrocatalyst stability benchmarking

Simultaneous determination of succinate-dehydrogenase-inhibitor fungicide traces in cereals by QuEChERS preparation and UPLC-MS/MS analysis.

Nanoarchitectonics of penicillin G acylase with Mn2+ doped β-cyclodextrin/Fe3O4 for enhanced catalytic activity and reusability

Synthesis of polymer-functionalized β-cyclodextrin, Mg2+ doped, coating magnetic Fe3O4 nanoparticle carriers for penicillin G acylase immobilization

Kinetics and Distribution of Zearalenone-14-Glucoside and Its Metabolite Zearalenone in Rat, Determined by a Reliable HPLC-MS/MS Method