Unraveling the Most Relevant Features for the Design of Iridium Mixed Oxides with High Activity and Durability for the Oxygen Evolution Reaction in Acidic Media.

Proton exchange membrane water electrolysis (PEMWE) is the technology of choice for the large-scale production of green hydrogen from renewable energy. Current PEMWEs utilize large amounts of critical raw materials such as iridium and platinum in the anode and cathode electrodes, respectively. In addition to its high cost, the use of Ir-based catalysts may represent a critical bottleneck for the large-scale production of PEM electrolyzers since iridium is a very expensive, scarce, and ill-distributed element. Replacing iridium from PEM anodes is a challenging matter since Ir-oxides are the only materials with sufficient stability under the highly oxidant environment of the anode reaction. One of the current strategies aiming to reduce Ir content is the design of advanced Ir-mixed oxides, in which the introduction of cations in different crystallographic sites can help to engineer the Ir active sites with certain characteristics, that is, environment, coordination, distances, oxidation state, etc. This strategy comes with its own problems, since most mixed oxides lack stability during the OER in acidic electrolyte, suffering severe structural reconstruction, which may lead to surfaces with catalytic activity and durability different from that of the original mixed oxide. Only after understanding such a reconstruction process would it be possible to design durable and stable Ir-based catalysts for the OER. In this Perspective, we highlight the most successful strategies to design Ir mixed oxides for the OER in acidic electrolyte and discuss the most promising lines of evolution in the field.

Active and durable R2MnRuO7 pyrochlores with low Ru content for acidic oxygen evolution.

The production of green hydrogen in water electrolyzers is limited by the oxygen evolution reaction (OER). State-of-the-art electrocatalysts are based on Ir. Ru electrocatalysts are a suitable alternative provided their performance is improved. Here we show that low-Ru-content pyrochlores (R2MnRuO7, R = Y, Tb and Dy) display high activity and durability for the OER in acidic media. Y2MnRuO7 is the most stable catalyst, displaying 1.5 V at 10 mA cm-2 for 40 h, or 5000 cycles up to 1.7 V. Computational and experimental results show that the high performance is owed to Ru sites embedded in RuMnOx surface layers. A water electrolyser with Y2MnRuO7 (with only 0.2 mgRu cm-2) reaches 1 A cm-2 at 1.75 V, remaining stable at 200 mA cm-2 for more than 24 h. These results encourage further investigation on Ru catalysts in which a partial replacement of Ru by inexpensive cations can enhance the OER performance.

Immuno-battery: A single use self-powered immunosensor for REASSURED diagnostics.

In this work, we present a novel self-powered approach totally independent from any external energy source. We have developed a self-powered paper-based immunosensor that generates energy in the presence of the biomarker in the sample. In particular, the device - which has been labeled as Immuno-Battery - makes use of magnesium as anode and the widely employed HRP-labeled antibody as cathodic catalyst to detect C-reactive protein (CRP) presence in artificial samples. Feasibility of self-powered sensing is proved by submitting the immuno-battery to a resistive load. In this regime, the sensor provides operation voltages above 1.55 V and maximum power densities from 40 to 571 µW cm-2 that allow for future implementation of an electronic readout circuit. We have demonstrated that sensitivity of the system is not compromised by the self-powered mode operation, as the LOD value delivered by our battery (20 ± 2 ng mL-1) is compliant with LOD values reported for protein detection in paper-based electrochemical immunoassays with chronoamperometric methods. Moreover, as a case study, a LOD of 269 ± 39 ng mL-1 is obtained for CRP detection, in accordance with available commercial high-sensitivity CRP detection kits. This proof-of-concept opens the path towards the development of digital diagnostic devices in a sustainable and affordable manner.

Nanoceria dissolution at acidic pH by breaking off the catalytic loop.

This manuscript proves the reproducibility and robustness of cerium oxide nanoparticles, nanoceria, employed as a chemical reagent with oxidizing capacity (as an electron sink) at acidic pH. Unlike nanoceria multi-enzyme-mimetic capabilities at neutral or high pH, nanoceria can behave as a stoichiometric reagent at low pH where insoluble Ce4+ ions transform into soluble Ce3+ in the nanocrystal that finally dissolves. This behaviour can be interpreted as enzyme-like when nanoceria is in excess with respect to the substrate. Under these conditions, the Ce3+/Ce4+ ratio in the NPs can easily be estimated by titration with ferrocyanide. This procedure could become a rapid assessment tool for evaluating nanoceria capacity in liquid environments.