Metal-substituted ZrB2 and HfB2 as low-cost and high-performance bifunctional electrocatalysts for water splitting

Growing global energy demand, descending amount of fossil fuels, and severe climate changes have encouraged researchers to develop clean, sustainable, and efficient energy sources. Currently, hydrogen energy is one of the most promising solutions as the ideal energy carrier by virtue of the highest gravimetric energy density of any known fuel, zero carbon footprint, and earth abundance. In high purity hydrogen gas production, electrocatalytic water splitting has come to the fore; in this regard, earth-abundant, cost-efficient, corrosion-resistant, and bifunctional materials with remarkable electrocatalytic performances are needed. In this thesis, transition metal substituted ZrB2 and HfB2 families were investigated as promising electrocatalysts. They were synthesized by the carbothermal reduction method and employed as bifunctional catalyst in 1 M KOH alkaline medium for both hydrogen and oxygen evolution reactions (HER and OER, respectively). The family of ZrB2-based catalysts was studied with a general formula of Zr(1-x)TMxB2 (x = 0.05, 0.1, 0.2 and 0.3) (TM = Fe, Co, and Ni). In the case of OER, Zr0.8Ni0.2B2 sample led to an onset potential of 1.58 V at 10 mA cm–2 current density, indicating a remarkable performance with a very low overpotential of 350 mV. Besides, Zr0.8Ni0.2B2 displayed a value of 56.6 mV dec–1 regarding the Tafel slope, which was smaller than the commercial RuO2 (66.2 mV dec–1). In the case of HER, Zr0.8Co0.2B2 with 420 mV overpotential and 101.6mV dec–1 showed the best performance. The family of HfB2-based catalysts with a general formula of Hf(1-x)TMxB2 (x = 0.1, 0.2 and 0.3) (TM = Co, and Ni) was also investigated. In parallel with the ZrB2 case, Ni substitution enhanced the OER performance and Hf0.8Ni0.2B2 sample leading to an onset potential of 1.55 V at 10 mA cm–2 current density with a very low overpotential of 320 mV and infinitesimal value of 39.5 mV dec–1 as Tafel slope. For HER, Hf0.8Co0.2B2 with 430 mV overpotential and 105.4mV dec–1 showed the best performance. Both catalysts were examined for their long-term stability, resulting in excellent durability after 12 -20 h. As-prepared transition metal substituted diborides have great potential to be implemented in green energy applications.