•Surface studies of earth-abundant electrocatalysts for artificial photosynthesis•Integrated electrochemistry-surface science apparatus•Surface electrochemical studies of Ni–Mo composites as HER catalysts•Birnessite as heterogeneous analogue of oxygen-evolving complex of Photosystem II•Electrochemically reordered Cu(hkl) from air-oxidized surfacesSurface science research fixated on phenomena and processes that transpire at the electrode-electrolyte 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 single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes results selected from on-going 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.

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.Keywords: hydrogen-evolution reaction; mesoporous catalysts; synthesis of group VI dichalcogenides; synthesis of molybdenum diselenide; wet-chemical synthesis of layered electrocatalysts

Films of CoP have been electrochemically synthesized, characterized, and evaluated for performance as a catalyst for the hydrogen-evolution reaction (HER). The film was synthesized by cathodic deposition from a boric acid solution of Co2+ and H2PO2– on copper substrates followed by operando remediation of exogenous contaminants. The films were characterized structurally and compositionally by scanning-electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman spectrophotometry. The catalytic activity was evaluated by cyclic voltammetry and chronopotentiometry. Surface characterization prior to electrocatalysis indicated that the film consisted of micrometer-sized spherical clusters located randomly and loosely on a slightly roughened surface. The composition of both the clusters and surface consisted of cobalt in the metallic, phosphide, and amorphous-oxide forms (CoO·Co2O3) and of phosphorus as phosphide and orthophosphate. The orthophosphate species, produced by air-oxidation, were eliminated upon HER electrocatalysis in sulfuric acid. The operando film purification yielded a functional electrocatalyst with a Co:P stoichiometric ratio of 1:1. After the HER, the surface was densely packed with micrometer-sized, mesa-like particles whose tops were flat and smooth. The CoP eletrodeposit exhibited an 85 mV overvoltage (η) for the HER at a current density of 10 mA cm–2 and was stable under operation in highly acidic solution, with an increase in η of 18 mV after 24 h of continuous operation. The comparative HER catalytic performance of CoP, film or nanoparticles, is as follows: ηPt < ηCoP film = ηCoP NP, ηNi2P < ηCoSe2 < ηMoS2 < ηMoSe2.

The present article recounts results from studies on the structure, composition and reactivity of polycrystalline and single-crystal magnesium surfaces, previously exposed to the environment and regenerated to pristine conditions in ultrahigh vacuum. Experimental measurements relied on electron spectroscopy, temperature-programmed mass spectrometry and atomic force microscopy. Reaction-chemistry investigations were primarily with gas-phase reagents although selected treatments in aqueous or ethereal electrolyte were also undertaken. The results obtained have relevance in the description of the corrosion characteristics of this industrially important metal. The more significant observations: (i) Air-exposed Mg is invariably encrusted with a surface film composed predominantly of magnesium hydroxide but with a detectable quantity of magnesium bicarbonate. (ii) The exposure of a clean Mg(0001) surface to gradually increased dosages of O2(g) initially yielded a (1 × 1) oxygen layer that suffered surface disorder at the incipient stages of metal–oxide formation; eventually, an epitaxial MgO(100)-on-Mg(0001) adlattice was generated. (iii) Exposure of Mg(0001) to H2O(g), regardless of the dosage applied, resulted in a non-ordered hydroxylated surface. (iv) Upon exposure of a clean Mg(0001) surface to CO2(g), an ordered carbonaceous oxide film was initially found; at much higher dosages, the pseudomorphic adlayer was transformed into a totally disordered film. (v) Exposure of an oxide-coated Mg(0001) surface to H2O(g) yielded a disordered hydroxylated surface. (vi) With pertinence to Grignard chemistry, treatment of the oxided metal with either gaseous or (anhydrous) ethereal HCl enforced an acid–base reaction that led to a substantial reduction in the amount of surface oxide and the concomitant accumulation of a metal-chloride film.Highlights► Structure, composition and reactivity of clean and air-exposed magnesium surfaces. ► Experimental measurements based on electron spectroscopy and atomic force microscopy. ► Air-exposed Mg is encrusted with surface film of Mg(OH)2 and small amount of Mg(HCO3)2. ► Clean Mg prepared in ultrahigh vacuum is quite reactive to gaseous O2, H2O and CO2.

A new strategy in the study of the self-discharge mechanism of metal-hydride electrodes has been developed. The self-discharge behavior of a LaNi3.55Co0.75Mn0.4Al0.3LaNi3.55Co0.75Mn0.4Al0.3 electrode in alkaline solution, with and without ZnO additive, was investigated. Experimental results showed that the self-discharge rate (hydrogen desorption) within the single-phase region is controlled by the difference between the equilibrium hydrogen partial pressure at the electrode and the actual hydrogen partial pressure in the cell. Dissolved oxygen was also found to exert a strong influence on the self-discharge rate in the single-phase region. In the two-phase region, the self-discharge is limited by the rate of phase transformation. A four-step mechanism for the self-discharge process is proposed.

The effect of surface palladium films on the electrochemical properties of hydride-forming metal alloys used as cathode materials in Ni-metal hydride cells has been studied. The results indicate that the presence of surface Pd improved the discharge capacity and rate capability of the hydride electrodes. The enhancement in performance may be explained on the basis of the electrocatalytic effect of Pd on the kinetics of the hydrogen absorption/desorption (hydriding/dehydriding) reactions.

The improvement in the cycle life of a metal-hydride electrode, LaNi3.35Co0.75Mn0.4Al0.3, brought about by the addition of ZnO to the alkaline electrolyte has been investigated using measurements based upon in situ electrical resistance, corrosion, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and inductively-coupled plasma-atomic emission spectroscopy. It was found that Zn is underpotentially deposited on and subsequently alloyed with the subject electrode upon repeated charge-and-discharge cycles. The presence of Zn extends the cycle life of the LaNi3.35Co0.75Mn0.4Al0.3 electrode by inhibiting the disintegration and lowering the corrosion rate of the alloy particles.

Earlier work, based upon ex situ surface analytical measurements, found that a disordered Pd(111) surface can be spontaneously reordered upon exposure, at ambient conditions, to a dilute aqueous solution of iodide. This phenomenon has been re-examined by in situ atom-resolved scanning tunneling microscopy. The results from the present study, while corroborating the chemisorption-induced disorder-to-order surface reconstruction observed earlier, indicate that the kinetics of the structural rearrangements may not be as rapid as initially thought.