Abstract
Electrocatalytic water splitting is a possible route to the expanded generation of green hydrogen; however, a long-term challenge is the requirement of fresh water as an electrolyzer feed. The use of seawater as a direct feed for electrolytic hydrogen production would alleviate fresh water needs and potentially open an avenue for locally generated hydrogen from marine hydrokinetic or off-shore power sources. One environmental limitation to seawater electrolysis is the generation of chlorine as a competitive anodic reaction. This work evaluates transition metal (W, Co, Fe, Sn, and Ru) doping of Mn-Mo-based catalysts as a strategy to suppress chlorine evolution while sustaining catalytic efficiency. Electrochemical evaluations in neutral chloride solution and raw seawater showed the promise of a novel Mn-Mo-Ru electrode system for oxygen evolution efficiency and enhanced catalytic activity. Subsequent stability testing in a flowing raw seawater flume highlighted the need for improved catalyst stability for long-term applications of Mn-Mo-Ru catalysts. This work highlights that elements known to be selective toward chlorine evolution in simple oxide form (e.g., RuO2) may display different trends in selectivity when used as isolated dopants, where Ru suppressed chlorine evolution in Mn-based catalysts.
Highlights
The electrocatalytic splitting of water to produce hydrogen can be used to store renewable electricity as either a chemical fuel or as a feedstock
A primary limiter of energy efficiency is the sluggishness of the reaction that counters the hydrogen evolution reaction (HER): the oxygen evolution reaction (OER), which in seawater competes with the hazardous chlorine evolution reaction (CER): OER, alkaline: 4[OH]−→2 H2O + O2 + 4e−
Considering abundance and a promising price point [24], we focus here on Mn-based oxides, which are stable and have demonstrated good Faradaic efficiency in a wide range of electrolyte pH
Summary
The electrocatalytic splitting of water to produce hydrogen can be used to store renewable electricity as either a chemical fuel or as a feedstock. This process currently requires an ultra-pure water feed [1], which would strain drinking water supplies at hydrogen production levels required for a fleet of fuel cell vehicles [2]. Precious metal oxides (e.g., IrO2) are highly active for OER in acidic media, such materials are highly active for CER in chloride-containing media [14,15]
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