Electrochemical and chemical cycle for high-efficiency decoupled water splitting in a near-neutral electrolyte.

Green hydrogen produced by water splitting using renewable electricity is essential to achieve net-zero carbon emissions. Present water electrolysis technologies are uncompetitive with low-cost grey hydrogen produced from fossil fuels, limiting their scale-up potential. Disruptive processes that decouple the hydrogen and oxygen evolution reactions and produce them in separate cells or different stages emerge as a prospective route to reduce system cost by enabling operation without expensive membranes and sealing components. Some of these processes divide the hydrogen or oxygen evolution reactions into electrochemical and chemical sub-reactions, enabling them to achieve high efficiency. However, high efficiency has been demonstrated only in a batch process with thermal swings that present operational challenges. This work introduces a breakthrough process that produces hydrogen and oxygen in separate cells and supports continuous operation in a membraneless system. We demonstrate high faradaic and electrolytic efficiency and high rate operation in a near-neutral electrolyte of NaBr in water, whereby bromide is electro-oxidized to bromate concurrent with hydrogen evolution in one cell, and bromate is chemically reduced to bromide in a catalytic reaction that evolves oxygen in another cell. This process may lead the way to high-efficiency membraneless water electrolysis that overcomes the limitations of century-old membrane electrolysis.

Understanding hydrazine oxidation electrocatalysis on undoped carbon.

Carbons are ubiquitous electrocatalytic supports for various energy-related transformations, especially in fuel cells. Doped carbons such as Fe-N-C materials are particularly active towards the oxidation of hydrazine, an alternative fuel and hydrogen carrier. However, there is little discussion of the electrocatalytic role of the most abundant component - the carbon matrix - towards the hydrazine oxidation reaction (HzOR). We present a systematic investigation of undoped graphitic carbons towards the HzOR in alkaline electrolyte. Using highly oriented pyrolytic graphite electrodes, as well as graphite powders enriched in either basal planes or edge defects, we demonstrate that edge defects are the most active catalytic sites during hydrazine oxidation electrocatalysis. Theoretical DFT calculations support and explain the mechanism of HzOR on carbon edges, identifying unsaturated graphene armchair defects as the most likely active sites. Finally, these findings explain the 'double peak' voltammetric feature observed on many doped carbons during the HzOR.

Template-Free Formation of Regular Macroporosity in Carbon Materials Made from a Folded Polymer Precursor.

Porous carbon materials attract great interest in a wide range of applications such as batteries, fuel cells, and membranes, due to their large surface area, structural and compositional tunability, and chemical stability. While micropores are typically obtained when preparing carbon materials by pyrolysis, the fabrication of mesoporous, and especially macroporous carbons is more challenging, yet important for enhancing mass transport. Herein, template-free regular macroporous carbons are prepared from a mixture of unfolded (linear) and folded (single-chain nanoparticles, SCNP) polyvinylpyrrolidone chains. While having the same chemical composition, the different molecular architectures lead to phase separation even before pyrolysis, creating a dense cell architecture, which is retained upon carbonization. Upon increasing the SCNP content, the homogeneity of the pore network increases and the specific surface area is enlarged 3-5-fold, until ideal properties are obtained at 75% SCNP, as observed by high-resolution scanning electron microscopy and N2 physisorption porosimetry. The materials are further investigated as hydrazine oxidation electrocatalysts, demonstrating the link between the evolving morphology and current density. Importantly, this study demonstrates the role of polymer architecture in macroporosity templating in carbon materials, providing a new approach to develop complex carbon architectures without the need for external templating.

Fe-N-C electrocatalysts in the oxygen and nitrogen cycles in alkaline media: the role of iron carbide.

Fe-N-C electrocatalysts hold a great promise for Pt-free energy conversion, driving the electrocatalysis of oxygen reduction and evolution, oxidation of nitrogen fuels, and reduction of N2, CO2, and NOx. Nevertheless, the catalytic role of iron carbide, a component of nearly every pyrolytic Fe-N-C material, is at the focus of a heated controversy. We now resolve the debate by examining a broad range of Fe3C sites, spanning across many typical size distributions and carbon environments. Removing Fe3C selectively by a non-oxidizing acid reveals its inactivity towards two representative reactions in alkaline media, oxygen reduction and hydrazine oxidation. The activity is assigned to other pre-existing sites, most probably Fe-Nx. DFT calculations prove that the Fe3C surface binds O and N intermediates too strongly to be catalytic. By settling the argument on the catalytic role of Fe3C in alkaline electrocatalysis, we hope to spur innovation in this critical field.