Electrochemical reforming of ethanol with acetate Co-Production on nickel cobalt selenide nanoparticles

Highlights

• Ni_xCo_ySe₂ nanoparticles with controlled Ni/Co ratio are produced.

• They were used for ethanol conversion into a valuable chemical and hydrogen.

• Optimized electrocatalysts deliver 0.34 mmol cm^-2h⁻¹ of acetate from ethanol oxidation.

• Co enhances ethanol adsorption and decreases the dehydrogenation energy barrier.

Abstract

The energy efficiency of water electrolysis is limited by the sluggish reaction kinetics of the anodic oxygen evolution reaction (OER). To overcome this limitation, OER can be replaced by a less demanding oxidation reaction, which in the ideal scenario could be even used to generate additional valuable chemicals. Herein, we focus on the electrochemical reforming of ethanol in alkaline media to generate hydrogen at a Pt cathode and acetate as a co-product at a Ni_1-xCo_xSe₂ anode. We first detail the solution synthesis of a series of Ni_1-xCo_xSe₂ electrocatalysts. By adjusting the Ni/Co ratio, the electrocatalytic activity and selectivity for the production of acetate from ethanol are optimized. Best performances are obtained at low substitutions of Ni by Co in the cubic NiSe₂ phase. Density function theory reveals that the Co substitution can effectively enhance the ethanol adsorption and decrease the energy barrier for its first step dehydrogenation during its conversion to acetate. However, we experimentally observe that too large amounts of Co decrease the ethanol-to-acetate Faradaic efficiency from values above 90% to just 50 %. At the optimized composition, the Ni_0.75Co_0.25Se₂ electrode delivers a stable chronoamperometry current density of up to 45 mA cm⁻², corresponding to 1.2 A g⁻¹, in a 1 M KOH + 1 M ethanol solution, with a high ethanol-to-acetate Faradaic efficiency of 82.2% at a relatively low potential, 1.50 V vs. RHE, and with an acetate production rate of 0.34 mmol cm⁻² h⁻¹.

Introduction

Molecular hydrogen is a convenient carbon-free energy carrier and a key component in the chemical industry. While molecular hydrogen does not exist in nature, it can be produced by water electrolysis using renewable energy sources such as wind, hydropower, and solar.[1] However, while water electrolysis systems have been commercialized for several years, their high cost has prevented extensive deployment, and most molecular hydrogen is still being produced by the partial oxidation and steam reforming of natural gas and coal gasification.[2] To turn this trend around, cost-effective strategies for the production of hydrogen from renewable sources need to be developed. In this context, biomass-derived products provide an excellent alternative source of hydrogen owing to their renewable character and their net-zero CO₂ footprint.[3] Besides, the partial oxidation of an organic molecule offers a less energy demanding anodic oxidation reaction, which can contribute to overcoming the energy efficiency limitation of water electrolysis associated with the sluggish reaction kinetics of the oxygen evolution reaction (OER).[4], [5], [6] An additional advantage of the production of hydrogen from the reforming of waste or biomass-derived organics is the potential co-generation of valuable organic chemicals, which can both improve process economics and further diminish the dependence on fossil resources that are currently used to produce them.[7], [8], [9] On top of these advantages, the replacement of OER by an organic oxidation reaction enables the implementation of coupled cost-effective waste abatement processes. Thus, overall, the clean electrochemical conversion of organic waste and biomass-derived products into value-added chemicals driven by renewable energy has both high fundamental interest and enormous potential for socio-economic and environmental impact.[10], [11], [12], [13].

In this scenario, (bio)ethanol is a paradigmatic and particularly interesting hydrogen source. Ethanol is a key commodity in the chemical industry, used as a precursor or building block for the synthesis of a plethora of chemicals, including formaldehyde, acetic acid, and plastics.[14], [15] Ethanol can be renewably generated from the reduction of CO₂ and several biomass-derived feedstocks and organic residues such as sewage sludge.[12], [13], [14] As a liquid, ethanol can be easily stored and transported. Besides, bioethanol aqueous solutions can be directly used in electrocatalytic processes, without the need for purification. Compared with water splitting, the production of hydrogen from ethanol is thermodynamically advantageous (ΔG⁰ = +237 kJ·mol⁻¹ for water oxidation vs. ΔG⁰ = +9.6 kJ·mol⁻¹ for ethanol oxidation to acetate), which decreases the energy input required.[16] The reforming of ethanol also prevents the H₂ and O₂ back reaction. Besides, compared with the complete ethanol reforming down to CO₂, the production of H₂ and acetate could have higher economical profitability associated with the high economic value of acetate as a side product.[17].

In terms of catalysts, while OER generally requires high-cost noble metal electrocatalysts, the electrooxidation of organic molecules, such as methanol or ethanol, in alkaline media can be activated at a lower cost using abundant 3d transition metal-based electrocatalysts, e.g. nickel, iron and cobalt.[17], [18], [19] To improve the electrocatalytic performance, multimetallic transition metal-based nanoparticles have been also developed and studied. Among them, nickel–cobalt compounds have demonstrated particularly high activity towards OER,[20], [21], [22] and excellent performance for the conversion of methanol to formate,[23], [24], [25], [26]. Thus, we hypothesize nickel–cobalt selenides to be excellent electrocatalysts for the ethanol oxidation catalysts, but this reaction and its mechanism is yet to be studied on these materials.

Herein, we detail the preparation of bimetallic Ni-Co selenides nanoparticles (NPs) over a full range of compositions using a solution-based strategy. This set of materials is tested for the partial electrooxidation of ethanol in an alkaline electrolyte. Finally, the material is optimized to minimize the overpotential and maximize the activity and Faradaic efficiency for ethanol to acetate conversion.