CoSe₂ and NiSe₂ Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting

Abstract

Catalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) are central to key renewable energy technologies, including fuel cells and water splitting. Despite tremendous effort, the development of low-cost electrode catalysts with high activity remains a great challenge. In this study, we report the synthesis of CoSe2 and NiSe2 nanocrystals (NCs) as excellent bifunctional catalysts for simultaneous generation of H2 and O2 in water-splitting reactions. NiSe2 NCs exhibit superior electrocatalytic efficiency in OER, with a Tafel slope (b) of 38 mV dec(-1) (in 1 M KOH), and HER, with b = 44 mV dec(-1) (in 0.5 M H2SO4). In comparison, CoSe2 NCs are less efficient for OER (b = 50 mV dec(-1)), but more efficient for HER (b = 40 mV dec(-1)). It was found that CoSe2 NCs contained more metallic metal ions than NiSe2, which could be responsible for their improved performance in HER. Robust evidence for surface oxidation suggests that the surface oxide layers are the actual active sites for OER, and that CoSe2 (or NiSe2) under the surface act as good conductive layers. The higher catalytic activity of NiSe2 is attributed to their oxide layers being more active than those of CoSe2. Furthermore, we fabricated a Si-based photoanode by depositing NiSe2 NCs onto an n-type Si nanowire array, which showed efficient photoelectrochemical water oxidation with a low onset potential (0.7 V versus reversible hydrogen electrode) and high durability. The remarkable catalytic activity, low cost, and scalability of NiSe2 make it a promising candidate for practical water-splitting solar cells.

Straightforward synthesis of nitrogen-doped carbon nanotubes as highly active bifunctional electrocatalysts for full water splitting

“…Electrochemical water splitting in alkaline media includes two half reactions: the hydrogen evolution reaction (HER, 4H 2 have shown high activity toward OER in acidic and alkaline media [4,5], and among the non-noble metal OER electrocatalysts, nickel (Ni) based electrocatalysts have shown very promising performance for OER in alkaline media [6][7][8][9][10]. For HER, Pt based materials are known as the most efficient HER catalysts in both acidic and alkaline media [4,11]. However, platinum group metals suffer from high costs and scarcity resulting in difficulties in the long-term availability.…”

“…In order to reduce/replace noble metal based electrocatalysts, various non-precious metal water-splitting catalysts have been developed [12]. In recent years, clear progress has been made in the development of efficient electrocatalysts with earth-abundant materials for HER [13,14] and OER [3,4,15] in alkaline media. Substantial research efforts have been put on developing versatile bifunctional catalysts with satisfying activity toward both HER and OER, enabling operation in the same electrolyte [8,16].…”

“…However, developing such active bifunctional electrocatalysts is challenging due to the incompatibility of pH ranges in which catalysts are stable and active. In this view, various efficient bifunctional catalysts based on transition-metal based compounds of Ni [6,8,[17][18][19][20], Co [4,16,21] and doped heteroatoms [22][23][24] in alkaline media have been proposed. However, the transition metal oxide materials suffer from intrinsically low conductivity limiting their performance at high current densities.…”

Nanostructured p-Type Semiconductor Electrodes and Photoelectrochemistry of Their Reduction Processes

“…Since H 2 is attainable as the reduction product of H + cation it is then mandatory to focus the attention towards p-type electrodes having photoelectrocatalytic properties [62][63][64] towards the reduction of the proton either in the pristine state or with the surface modified by a dye-absorber/photocatalytic agent [65,66]. A comprehensive list of p-type semiconductor cathodes for the photoelectrochemical production of H 2 has been given recently in [67] when the design of optimized photoelectrochemical water splitting cells [68,69] was considered, and solar radiation represented the primary energy source for the activation of this conversion device [70]. One of the main concerns at the basis of the development of PECs for water splitting [71] which produce molecular H 2 at the (photo)cathode and O 2 at the (photo)anode is the replacement of precious metals like Pt or Ru photoelectrocatalysts with durable semiconducting electrodes of p-type based on cheaper metal oxides, phosphides and sulfides/disulfides or Si, in order to photoactivate HER (hydrogen evolution reaction).…”

“…A comprehensive list of p-type semiconductor cathodes for the photoelectrochemical production of H2 has been given recently in [67] when the design of optimized photoelectrochemical water splitting cells [68,69] was considered, and solar radiation represented the primary energy source for the activation of this conversion device [70]. One of the main concerns at the basis of the development of PECs for water splitting [71] The Ru-complex O22 immobilized on NiO surface is the dye-sensitizer acting as an electron donor towards the co-catalyst CodmgBF2, and as an electron acceptor from NiO.…”