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This study describes and demonstrates key steps in a carbon-negative process for manufacturing cement from widely abundant seawater-derived magnesium (Mg) feedstocks. In contrast to conventional Portland cement, which starts with carbon-containing limestone as the source material, the proposed process uses membrane-free electrolyzers to facilitate the conversion of carbon-free magnesium ions (Mg 2+ ) in seawater into magnesium hydroxide [Mg(OH) 2 ] precursors for the production of Mg-based cement. After a low-temperature carbonation curing step converts Mg(OH) 2 into magnesium carbonates through reaction with carbon dioxide (CO 2 ), the resulting Mg-based binders can exhibit compressive strength comparable to that achieved by Portland cement after curing for only 2 days. Although the proposed “cement-from-seawater” process requires similar energy use per ton of cement as existing processes and is not currently suitable for use in conventional reinforced concrete, its potential to achieve a carbon-negative footprint makes it highly attractive to help decarbonize one of the most carbon-intensive industries.

Seawater electrolysis has the potential to be a more sustainable means of hydrogen production compared to conventional water electrolysis which relies on highly pure water. This is particularly true for arid coastal regions with access to seawater and ideal conditions for harvesting solar and wind energy, but where fresh water is already scarce.[1] Seawater electrolysis is challenging due to the large concentration of chloride ions, which can be detrimental to electrocatalyst stability. Furthermore, the presence of chloride ions allows the chlorine evolution reaction (CER) to compete with the oxygen evolution reaction (OER) at the anode. Although Cl2 is of industrial value, global hydrogen production already exceeds chlorine production, and demand for hydrogen is projected to grow more rapidly. Additionally, since Cl2 is toxic and harmful to the environment, implementing seawater electrolysis is simplified if pure oxygen is produced and can be safely vented to the atmosphere. Our group has shown that ultrathin semi permeable oxide overlayers can be designed to selectively transport reactants to the active catalyst at the buried interface.[2-3] Importantly for seawater electrolysis, the oxide overlayer selectively rejected chloride ions while allowing for water transport.[4] Thus, the oxide overlayer acts as a membrane, and the composite material can be referred to as a membrane coated electrocatalyst (MCEC). An additional advantage of the MCEC architecture compared to conventional electrocatalysts is enhanced stability.[5] This makes MCECs particularly attractive for stable and selective OER in seawater. This work describes how MCECs can (i) improve catalyst stability and (ii) enable selectivity for OER over CER by impeding transport of chloride ions to the catalyst at the buried interface. This work explores the fundamental relationships between chloride ion transport through different oxide overlayer materials. This knowledge is then applied to prepare MCECs supported on high surface area porous electrodes. References [1] S. Dresp, et al., ACS Energy Lett., 4, 933 (2019). [2] N. Y. Labrador, et al. , ACS Catal. , 8, 1767 (2018). [3] M. E. S. Beatty, et al., ACS Appl. Energy Mater., 3 , 12338 (2020). [4] A. A. Bhardwaj, et al. , ACS Catal. , 11, 1316 (2021). [5] N. Y. Labrador, et al . , Nano Lett. , 16 , 6452 (201 6 ).

RuO2 is a highly active electrocatalyst for the oxygen evolution reaction (OER) but is unstable in acidic environments. We investigated the encapsulation of RuO2 nanoparticles with semipermeable, nanoscopic silicon oxide (SiOx) overlayers as a strategy to improve their stability. SiOx encapsulated RuO2 (SiOx|RuO2) electrodes were prepared by drop-casting RuO2 nanoparticles onto glassy carbon substrates followed by deposition of SiOx overlayers of varying thickness by a room-temperature photochemical deposition process. The best-performing SiOx|RuO2 electrodes consisted of 2-3 nm thick SiOx overlayers on top of RuO2 particles and 3-7 nm thick SiOx on the glassy carbon substrate. Such electrodes exhibited lower overpotentials relative to bare RuO2 due to an improved electrochemically active surface area while also demonstrating an ability to retain OER activity over time, especially at higher overpotentials. Surprisingly, it was found that the SiOx coating was unable to prevent Ru dissolution, which was found to be proportional to the charge passed and independent of the presence or thickness of the SiOx coating. Thus, other possible explanations for the improved current retention of SiOx|RuO2 electrodes are discussed, including the influences of the overlayer on bubble dynamics and the stability of the underlying glassy carbon substrate.