Two-Dimensional Conjugated Metal–Organic Frameworks for Electrocatalysis: Opportunities and Challenges

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties. Show more

Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm(-2) per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can determine electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alkaline solutions: CoO(x), CoPi, CoFeO(x), NiO(x), NiCeO(x), NiCoO(x), NiCuO(x), NiFeO(x), and NiLaO(x). The oxygen-evolving activity of an electrodeposited IrO(x) catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alkaline solution, every non-noble metal system achieved 10 mA cm(-2) current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrO(x) was unstable under oxidative conditions in acidic solutions. Show more

Objective comparisons of electrocatalyst activity and stability using standard methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, we use a standard protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochemically active surface area (ECSA) of 18 electrocatalysts for the hydrogen evolution reaction (HER) and 26 electrocatalysts for the oxygen evolution reaction (OER) under conditions relevant to an integrated solar water-splitting device in aqueous acidic or alkaline solution. Our primary figure of merit is the overpotential necessary to achieve a magnitude current density of 10 mA cm(-2) per geometric area, the approximate current density expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm(-2) with overpotentials <0.1 V in acidic and/or alkaline solutions. Among OER catalysts in acidic solution, no non-noble metal based materials showed promising activity and stability, whereas in alkaline solution many OER catalysts performed with similar activity achieving 10 mA cm(-2) current densities at overpotentials of ~0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alkaline solutions, while most HER catalysts showed much lower specific activity than Pt in both acidic and alkaline solutions. For select catalysts, additional secondary screening measurements were conducted including Faradaic efficiency and extended stability measurements. Show more