Nanoparticulate Ni(OH)₂Films Synthesized from Macrocyclic Nickel(II) Cyclam for Hydrogen Production in Microbial Electrolysis Cells

Abstract

Hydrogen production in microbial electrolysis cells (MECs) is a promising approach for energy harvesting from wastewater. The kinetic barriers toward proton reduction necessitate the use of catalysts to drive hydrogen formation at appreciable rates and low applied potentials. Towards this end, cost effective alternatives to platinum catalysts are of paramount interest. In this study, Ni(OH) 2 films were synthesized by electrophoretic deposition from a Ni(II)cyclam precursor solution at varying concentrations (6 mM, 15 mM, and 23 mM). The films were characterized by scanning electron microscopy and X-ray photo-electron spectroscopy to confirm the deposition of Ni(OH) 2 . The Ni(OH) 2 -modified electrodes were then examined by both traditional electrochemical measurements and in an MEC for hydrogen production. Tafel analysis indicates an exchange current density of ∼0.36 mA cm −2 with a Tafel slope of ∼120 mV decade −1 consistent with a rate determining proton adsoprtion step. The hydrogen production rates increased with increasing Ni(II)cyclam concentration in the precursor solution, with the 23 mM-derived film exhibiting a rate comparable to that of a Pt-based catalyst in MEC tests. Hydrogen is considered to be one of the most promising energy carriers as an alternative fuel because of its high energy density and availability from renewable sources. Over 90% of hydrogen gas is produced by steam reforming and coal gasification, both of which are highly energy-consuming processes.1 Among the newly developed technologies for hydrogen production, microbial electrolysis cells (MECs) are of special interest as a new approach for hydrogen production from organic matter in wastewater and other organic waste. 2,3 In an MEC, organic compounds are degraded by exoelectrogens (electrochemically-active microorganisms), and as a result, electrons are passed to an anode electrode. Hydrogen gas can be formed at the cathode (−0.414 V vs. SHE, standard hydrogen electrode) by reducing protons. This process must be aided by an additional voltage (0.114 V in theory) as a result of the insufficiency of the anode potential (e.g., −0.300 V vs. SHE when acetate is used as an anode substrate) to drive proton reduction. In reality, an external voltage of more than 0.2 V is required to overcome the additional kinetic barriers imposed by the hydrogen formation reaction. 4 This additional voltage is substantially lower than that needed for electrochemical water splitting (theoretically 1.23 V vs. SHE 5 ) making hydrogen production energetically more cost efficient. In addition, recovery of valuable energy content from wastewater/waste also results in environmental benefits.Catalysts are indispensable in accomplishing the hydrogen evolution reaction (HER). Most MEC studies have used platinum-based catalysts for HER catalysis. Platinum is a highly efficient catalyst for HER, but its high cost obstructs its application in MECs, especially for large scale systems that are designed for wastewater treatment. 4,6 Thus, alternative catalysts have bee...

Improved Electrochemical Phase Diagrams from Theory and Experiment: The Ni–Water System and Its Complex Compounds

“…Ni Pourbaix diagrams have been simulated many times over the last fifty years, 1,[30][31][32] where the experimental ∆ f G's are used. However, none of these Ni Pourbaix diagrams are consistent with various electrochemical observations, e.g., NiO and/or Ni(OH) 2 should be stable at pH 5 ∼ 15, [24][25][26]29,[33][34][35][36][37][38][39][40][41][42] while, in those simulated Ni Poubaix diagrams with a moderate aqueous ion concentration ([I] = 10 −6 mol/L), Ni(OH) 2 is only stable at pH 3 of 9 ∼ 13. 1,30,31 These failures have important consequences because solutions with pH > 13 are critically important for the synthesis, characterization, and application of Ni (hydr)oxides.…”

“…Furthermore, the two dissolution boundaries of the metastable NiO are at pH 9.1 and 11.9 (not shown) significantly differ from those obtained at the DFT level. Although Ni(OH) 2 is more frequently reported in experiment, [24][25][26]29,70 NiO and Ni(OH) 2 probably coexist in many electrochemical experiments 36-39 due to their comparable close ∆µ's and the fact that it is challenging to completely differentiate them using conventional experimental methods (e.g., Raman spectroscopy). 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 may contribute to the scatter in the experimental data for the Ni(OH) 2 →NiOOH oxidation potential ( Fig.…”

“…Importantly, NiO, Ni(OH) 2 , or the "NiO+Ni(OH) 2 " complex is observed to be stable at pH 5 ∼ 14.8 in numerous experiments,[24][25][26]29,33,34,[36][37][38][39][40] where pH 14.8 corresponds to the highest alkaline solution ever reported. These observations are much more consistent with the phase domains of NiO and Ni(OH) 2 obtained from our DFT-derived Pourbaix diagram based on calculated ∆ f G's (Fig.…”

“…1c)show the phase domains (electrochemical stabilities) are largely underestimated and do not persist up to such high alkalinity. Furthermore, the average oxidation potentials for the Ni(OH) 2 →NiOOH transition measured in different solutions[24][25][26]29,33,[36][37][38][39] also reside around the phase boundary of Ni(OH) 2 in our DFT Pourbaix diagram. In contrast the oxidation potential for Ni(OH) 2 is systematically lower than the measured ones by ∼ 0.4 V and the oxidants Ni 3 O 4 and Ni 2 O 3 have large phase areas between Ni(OH) 2 and NiOOH in the experimental Pourbaix diagram(Fig.…”

Platinum Group Metal–free Catalysts for Hydrogen Evolution Reaction in Microbial Electrolysis Cells

“…Recently, we synthesized nanoparticulate Ni(OH) 2 films via the electrophoretic deposition of Ni(II)cyclam . Ni(II)cyclam was first prepared solvothermally by refluxing equimolar nickel(II) chloride hexahydrate and 1,4,8,11‐tetraazacyclotetradecane in dimethylformamide at 90 °C for 12 h. The Ni(II)cylcam precursor was then dissolved in 0.1 M NaOH and electrodeposited onto a piece of carbon cloth using CV between 0 V and 1.3 V vs. Ag/AgCl for 150 cycles.…”