Thermodynamically controlled photo-electrochemical CO₂ reduction at Cu/rGO/PVP/Nafion multi-layered dark cathode for selective production of formaldehyde and acetaldehyde

Highlights

• Photoanode assisted hybrid cathode for the formaldehyde and acetaldehyde production.

• Multi-electron accumulation and transfer control through rGO layer.

• CO₂ activation and minimization of reaction overpotential via PVP coating.

• Reduction potential tuning via conduction band, bias, and open circuit potential.

• Possible reaction pathway for CO₂ activation and electron-coupled proton transfer.

Abstract

Transforming greenhouse gases such as CO₂ to energy rich carbon-based chemicals is considered as one of the most efficient technologies for environmental and energy sustainability. However, CO₂ is highly stable molecule and difficult to reduce due to its linear structure. The rate of reduction and the nature of fuel product depend on the kinetics and thermodynamics of involved reactions. While the overall reaction kinetics depends on the energy of activated CO₂ molecule and its subsequent transition states along with reduction dynamics. Here we demonstrate that by activation of thermodynamically stable CO₂ molecule through complexation or coordination with suitable activator such as N-heterocyclic polymers (e.g., poly(4-vinyl)pyridine, PVP), both the kinetics and thermodynamics of photoelectrochemical CO₂ reduction reaction can be controlled by proper choice of electrode materials and bias potential. We present a solar light driven photoelectrochemical process for producing formaldehyde and acetaldehyde selectively on multi-layered Cu/rGO/PVP/Nafion hybrid cathode.

Introduction

Substantial increase of atmospheric CO₂ concentration is considered as the principal reason for current global warming and subsequent climate change [1], [2], [3], [4], [5]. While a collective global effort for the reduction of CO₂ emission is in place, conversion of ambiental CO₂ into useful hydrocarbon fuel is beneficial not only to keep in check the global warming, but also as useful contribution towards current energy demand. Several methods such as thermo-catalytic hydrogenation under high temperature and pressure [6], photocatalytic (PC) reduction [7], [8], [9], [10], photothermal reduction [11], [12], electrochemical (EC) reduction [13], [14], [15], [16], [17], biological and photoelectrochemical (PEC) reduction [18], [19], [20] have been utilized recently for the conversion of CO₂ into useful hydrocarbons. However, these methods further modified by using MOF assisted catalysis materials [21], [22], [23]. Among them, thermo-catalytic hydrogenation and electrochemical processes, although deemed efficient, suffer from high input energy cost and lack of product selectivity. On the other hand, in natural biological processes such as in plants, while the CO₂ reduction occurs under optimum reaction conditions to produce glucose, the reaction is not kinetically controlled (solar energy conversion efficiencies of most of the plants is less than 1%) [24], [25]. The slow kinetics of CO₂ reduction in the natural photosynthesis process even with perfect thermodynamic control over product selectivity is associated to the reaction of plant (also of microbes such as bacteria) enzymes with CO₂ molecules at lower collision frequencies [25]. Therefore, it is necessary to study the reaction kinetics of CO₂ reduction in optimum thermodynamic conditions to enhance the solar to fuel (STF) conversion efficiency in artificial photosynthesis processes.

Several research groups have reported the EC reduction of CO₂, both theoretically and experimentally, proposing possible reaction pathways [26], [27], [28], [29], [30], [31], [32]. However, the main problems associated with EC reduction of CO₂ are the high overpotential and lack of product selectivity. The high overpotential required for CO₂ reduction is the reason behind the lower Faradaic efficiency (FE) or current efficiency of electrochemical CO₂ reduction processes [26], [27], [28]. Lack of product selectivity and low Faradaic efficiency of electrochemical CO₂ reduction are the result of simultaneous occurrence of two inter-competing reactions, i.e. generation of hydrogen and reduction of CO₂ in the cathode compartment [33], [34]. On the other hand, solar photocatalytic CO₂ reduction is a simpler process. However, as the only energy source involved in solar photocatalytic reduction process is the external one (solar radiation), product selective reduction of CO₂ is almost impossible. Moreover, the efficiency of solar CO₂ reduction producing liquid fuel in solar photochemical process is limited by water splitting. In contrast, in PEC process, as the reduction potential can be controlled through external bias, we can overcome such limitation to enhance CO₂ reduction efficiency. Therefore, PEC is considered to be one of the most convenient processes for product-selective reduction of CO₂. The generation of CO [35], methane [36], methanol [37], formic acid and formaldehyde [19], [38] have been demonstrated through PEC reduction of CO₂.

Recently, Aguirre et al. reported multi-layered photo-electrochemical CO₂ reduction towards methanol with FTO/Cu/Bi₂Se₃-Se/Cu₂O as a photocathode [39]. However, in most of the cases these photocathodes suffer from long term stability during photocatalytic reaction. Utilizing such multi-layered electrodes as dark cathode seems to be the best option to overcome this problem. Recently Kang et al. reported the utilization of rGO-based dark cathodes showing their good performance in CO₂ reduction reaction for producing alcohol selectivity [20], [37]. Yet, lack of product selectivity and low conversion efficiency are the two aspects which require further attention. To attend these problems, it is necessary to develop innovative cathode materials or multi-component cathodes containing layers of specific functionalities, which not only can improve the product selectivity of the process, but also can enhance the CO₂ reduction rate by lowering the activation energy, especially for the first electron transfer (which is the rate determining step) from cathode to CO₂ molecules adsorbed at their surfaces.

In this study, we demonstrate a novel strategy for selective and controlled production of formaldehyde and acetaldehyde through PEC reduction of CO₂. Utilizing calcium (Ca) and iron (Fe) co-doped TiO₂ (TiO₂:Ca-Fe) films as photoanode and Cu/rGO/PVP/Nafion multi-layered electrode as cathode, formaldehyde and acetaldehyde could be generated selectively at cathode surface by tuning anode bias potential. Product selectivity in the CO₂ reduction process was monitored in situ utilizing gas chromatography (GC). The first electron transfer process, i.e., the formation of CO₂^-• radical at cathode surface was monitored through in situ electron paramagnetic resonance (EPR). Formation of CO₂ reduction intermediates during the process was monitored by in situ ATR-IR and Raman spectroscopies. The 4-electron transfer involved in CO₂ reduction process was probed by time-resolved chronoamperometry using Au ultra-microelectrode. Considering all the thermodynamic aspects, possible reaction pathways for formaldehyde and acetaldehyde generation have been outlined.