Sandwich-like MnO_<i>x</i>/MnN_0.84/Mn Electrode toward Improved Electrocatalytic Oxygen Evolution in Acidic Media

Developing efficient and robust noble-metal-free electrocatalysts capable of catalyzing water oxidation in acidic media is highly desirable for producing H2 while it remains a great challenge. Herein, a self-supported MnOx/MnN0.84/Mn electrode with a sandwich-like configuration was prepared by consecutive steps involving a nitridation treatment and an in situ electrochemical activation process. The electrode requires overpotentials of ca. 475 and 571 mV at current densities of 10 and 100 mA cm–2, respectively, for the oxygen evolution reaction (OER) in 1.0 M H2SO4. More impressively, the electrode remains stable for over 300 h of continuous operation at a current density of 100 mA cm–2, which is, as far as we know, among the best values reported for Mn-based materials in the field of acidic water electrolysis. It is found that the metallic MnN0.84 layer is not only the precursor for the formation of MnOx nanosheet electrocatalysts as the actual catalyst for the OER but also enables efficient charge transfer between the active sites at the surface and the substrate. Moreover, the anticorrosive MnN0.84 interlayer that acts as the binder between the Mn substrate and the MnOx catalyst can protect the Mn substrate from corrosion in acidic electrolytes, highlighting the importance of interlayer modification in stabilizing electrocatalysts in harsh reaction conditions

Integrating Perovskite Photovoltaics and Noble-Metal-Free Catalysts toward Efficient Solar Energy Conversion and H₂S Splitting

Hydrogen sulfide (H₂S) has been considered as a potential hydrogen source. Identifying efficient solar-driven processes and low-cost materials that can extract hydrogen from H₂S is highly attractive. Herein, for the first time, we reported the establishment of a perovskite photovoltaic-electrolysis (PV-EC) H₂S splitting system by integrating a single perovskite solar cell, noble-metal-free catalysts, and H₂S splitting reaction with the aid of mediators. The as-established system delivered a solar-to-chemical energy conversion efficiency of up to 13.5% during the PV-EC step by using molybdenum–tungsten phosphide (Mo–W–P) as the catalyst for a hydrogen evolution reaction (HER) and a graphite carbon sheet as the catalyst for the oxidation of mediators, respectively. To the best of our knowledge, this is among the highest value ever reported for the artificial conversion of solar to chemical energy using perovskite solar cells. Moreover, upon integration with the PV-EC system, a H₂S splitting reaction with a net energy conversion efficiency of 3.5% can be accomplished, and the overall energy consumption to obtain an equivalent amount of H₂ from H₂S is reduced by ca. 43.3% compared with that from water splitting. This paradigm of producing value-added chemicals by consuming negative value waste products is solely based on low-cost materials and a simpler system configuration, which significantly improves the economic sustainability of the process

Photocatalytic Water Oxidation on BiVO₄ with the Electrocatalyst as an Oxidation Cocatalyst: Essential Relations between Electrocatalyst and Photocatalyst

The oxygen evolution is kinetically the key step in the photocatalytic water splitting. Cocatalysts could lower the activation potential for O₂ evolution. However, the cocatalyst for O₂ evolution has been less investigated, and few effective cocatalysts were reported. This paper reports that the O₂ evolution rate of photocatalytic water splitting under visible light irradiation can be significantly enhanced when the electrocatalyst cobalt–phosphate (denoted as CoPi) was deposited on BiVO₄. The photocurrent density is also greatly enhanced by loading CoPi on BiVO₄ electrode, and this enhancement in performance shows the similar trend between the photocatalytic activity and photocurrent density. We also found that this tendency is true for BiVO₄ loaded with a series of different electrocatalysts as the cocatalysts. These results demonstrate that an effective electrocatalyst of water oxidation can be also an effective cocatalyst for O₂ evolution from photocatalytic water oxidation. By depositing the CoPi as the oxidation cocatalyst and Pt as the reduction cocatalyst on an yttrium-doped BiVO₄ (Bi_0.5Y_0.5VO₄), overall water splitting reaction to H₂ and O₂ was realized. Our work also reveals the essential relations between photocatalysis and electrocatalysis in water splitting reaction

Promoting Charge Separation and Injection by Optimizing the Interfaces of GaN:ZnO Photoanode for Efficient Solar Water Oxidation

Photoelectrochemical water splitting provides an attractive way to store solar energy in molecular hydrogen as a kind of sustainable fuel. To achieve high solar conversion efficiency, the most stringent criteria are effective charge separation and injection in electrodes. Herein, efficient photoelectrochemical water oxidation is realized by optimizing charge separation and surface charge transfer of GaN:ZnO photoanode. The charge separation can be greatly improved through modified moisture-assisted nitridation and HCl acid treatment, by which the interfaces in GaN:ZnO solid solution particles are optimized and recombination centers existing at the interfaces are depressed in GaN:ZnO photoanode. Moreover, a multimetal phosphide of NiCoFeP was employed as water oxidation cocatalyst to improve the charge injection at the photoanode/electrolyte interface. Consequently, it significantly decreases the overpotential and brings the photocurrent to a benchmark of 3.9 mA cm^–2 at 1.23 V vs RHE and a solar conversion efficiency over 1% was obtained

Dynamic Interaction between Methylammonium Lead Iodide and TiO₂ Nanocrystals Leads to Enhanced Photocatalytic H₂ Evolution from HI Splitting

Organic–inorganic hybrid perovskites have been extensively investigated for solar-to-electricity transformation because of their excellent optoelectronic properties. However, these materials have been rarely pursued as photocatalysts for solar-to-fuel conversion because of their intolerance to water and lack of efficient strategies to promote charge transportation in the nanoscale domain. Herein, Pt/TiO₂ nanoparticles, which act as temporary reservoirs for accommodating methylammonium lead iodide (MAPbI₃), were hybridized with MAPbI₃, creating dynamically existing electron-transporting channels between the two components. As a consequence, the charge transportation efficiency for MAPbI₃ nanoparticles was drastically enhanced and the rate of photocatalytic hydrogen evolution from HI splitting was increased by ca. 89 times compared with that of Pt/MAPbI₃. An apparent quantum efficiency of ca. 70% for H₂ evolution at 420 nm and a solar-to-chemical conversion efficiency of ca. 0.86% were obtained