The validity of the electrochemical series for metal sulfides decomposition in their standard state has been tested experimentally at 1500 K for La2S3, Cu2S, MoS2, and ReS2 in a molten electrolyte with the following molar composition: (BaS)54-(Cu2S)31-(La2S3)15 (electrolyte B). Voltammetry measurements indicated the presence of faradaic reactions in the investigated electrolyte with and without the addition of MoS2 and/or ReS2. Electrolysis experiments showed that the addition of La2S3 to BaS-Cu2S increases the faradaic efficiency for liquid copper production with respect to a previously studied (BaS)54-(Cu2S)46 electrolyte, and enabled isolation of elemental sulfur as the anodic product. Electrochemical measurements suggested the need to take into account the activity of dissolved Cu2S in order to explain the observed cell voltage during electrolysis. Electrolysis in the presence and absence of ReS2 and/or MoS2 confirmed their relative stability as predicted by assuming decomposition in their standard states. Analysis of the metal products electrowon from an electrolyte containing Cu2S, MoS2, and ReS2 indicates that simultaneous production of solid and liquid phases with nonequilibrium compositions.

Surface science research fixated on phenomena and processes that transpire at the electrodeelectrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical-analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined singlecrystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; i.e., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surfacescience protocols remain unchanged but the experimental capabilities have been escalated by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes results selected from ongoing studies of earth-abundant electrocatalysts for the reactions that underpin artificial photosynthesis: nickel-molybdenum alloys for the hydrogen evolution reaction, calcium birnessite as a heterogeneous analogue for the oxygen-evolving complex in natural photosynthesis, and single-crystalline copper in relation to the carbon dioxide reduction reaction.

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