Elsevier

Electrochimica Acta

Volume 264, 20 February 2018, Pages 269-274
Electrochimica Acta

pH dependent CO adsorption and roughness-induced selectivity of CO2 electroreduction on gold surfaces

https://doi.org/10.1016/j.electacta.2018.01.106Get rights and content

Abstract

Gold belongs to the category of metal electrocatalysts for CO2 reduction reaction (CO2RR) producing CO as a major product due to its inability to bind CO. In this work, we found that when the interfacial pH becomes alkaline, CO, the major product of CO2RR, interacts with gold surfaces to produce minor products like formate and methanol. The course of product formation and CO adsorption on gold was explored by their oxidative voltammetric responses obtained immediately after CO2RR in the forward cathodic scan and SECM studies provided more insights into the effect of interfacial pH on product selectivity. The electrode roughness was found to change the interfacial pH gradients as a result of microheterogeneity of surfaces. The roughness of gold surface was systematically changed from smoothened surface to nanoporous (highly rough) surface. NMR spectroscopic analysis pointed to the presence of methanol and formate being formed on nanoporous Au (np- Au) in significant amounts. Formate production was found increasing as a result of changes in the interfacial pH.

Introduction

Gold nanostructures can be exceptionally active as catalysts for many reactions [[1], [2], [3], [4], [5], [6]]. Depending on the size and the shape, they exhibits catalytic activity for various catalytic oxidation and reduction reactions [[7], [8], [9], [10], [11]]. For many electrochemical oxidation and reduction reactions, gold displays some catalytic activity, e.g., for the oxidation of CO, HCOOH, CH3OH and HCHO [[12], [13], [14]] and the reduction of oxygen [15] and CO2 [16,17]. Chemically and electrochemically modified gold also provides interesting catalytic properties [[18], [19], [20]]. As a substrate, gold promotes the oxygen evolution reaction on cobalt oxide [21]. In many cases, the electrocatalytic activity of gold greatly depends on the surface structure as well as the pH of the solution [12]. In the electrochemical reduction of CO2, gold is highly selective towards CO production (faradaic efficiency, 87%) and produces a small amount of formic acid (0.7%) as a by-product [22]. For the selectivity of CO over HCOO, DFT studies showed that *COOH is the key intermediate for CO production on metal surfaces, whereas *OCHO is reported for HCOO [23]. Au occupies the top of the volcano plot calculated using *COOH binding energy and CO partial current density at −0.9 V vs RHE, whereas it is at the weak binding side of the volcano plot calculated using *OCHO binding energy and HCOO partial current density at −0.9 V vs RHE suggesting that on Au surface the major product is CO and HCOO is a minor product [23].

Recently, various forms of gold have been investigated for the electrochemical reduction of CO2 that include nanoparticles derived from oxidized gold surfaces, monodispersed gold nanoparticles and various gold nanoclusters. Kanan et al. studied the CO2 reduction on gold nanoparticles derived from thick gold oxide layers [17]. They obtained a faradaic efficiency of >98% for CO with a very high current density (6 mA cm−2) at a low cathodic overpotential (−0.4 V vs RHE). Sun et al. studied the CO2RR on gold nanoparticles of different sizes and found that a maximum faradaic efficiency (90% at −0.67 V vs RHE) was achievable using 8 nm sized gold nanoparticles [16]. Interestingly, Au25 nanoclusters were found to electronically interact with the dissolved CO2 to reduce to CO with a faradaic efficiency >95% [24]. Surendranath et al. obtained 99% Faradaic efficiency for CO on meso porous gold film [25].

Generally, bicarbonate solution is used as the electrolyte for CO2RR. Hori et al. reported that the equilibrium that exists between dissolved CO2 and HCO3 can act as buffer of pH 7 which is suitable for CO2 reduction [22]. Recently, Dunwell et al. proposed that the rapid equilibrium exchange between dissolved CO2 and bicarbonate enriches the concentration of reducible CO2 in solution and hence enhances the rate of CO2RR [26]. Further, the cations of the electrolyte play an important role in controlling the selectivity of products generated through CO2RR. The rate of hydrolysis of cations (alkali metal cations) during CO2RR influences the buffer activity of the electrolyte solution and hence controls the interfacial pH. The cations of bigger size have low pKa of hydrolysis and undergo quick hydrolysis during CO2RR due to the polarization of their hydration shell near the cathode, which would have a buffer activity and thus maintain the interfacial pH [[27], [28], [29]]. The distribution of CO2, HCO3 and CO32− in the electrolytes dependent on the pH. If the interfacial pH is higher, the concentration of HCO3 and CO32− become higher than CO2 [27,30]. During the course of CO2 reduction, the pH of the electrolyte at the interface increases due to the formation of OH. When the interface of various metal surfaces was probed by SG-TC mode of SECM during CO2RR using Pt ultramicroelectrode as a probe, the product selectivity varied as a function of pH. At a low pH value of 6.8, CO is produced selectively and at a higher pH (>8) formate is the major product [30].

Recent studies by Jaramillo et al. showed that gold can produce methanol (to a small extent of 0.01% faradaic efficiency) also from CO2 electroreduction [31]. Motivated by their observation, we investigated the possible routes of methanol generation on gold surfaces by changing the surface roughness. Since the amount of methanol production is very low and a majority of CO2 reduction products can be oxidized on the gold surface itself due to their electroactive nature and the major product CO itself catalyzes the oxidation of methanol on gold surfaces [32], we attempted identification of the products instantaneously after their formation by recording the oxidative voltammograms of the CO2 reduction product(s) on the same gold surface. In addition, we also used substrate generation-tip collection (SG-TC) mode of SECM to understand the surface processes. From these studies, we show (i) the voltammetric (SECM and oxidative voltammetry) features that aid identification of possible electroactive products; and (ii) the spectroscopic analysis of the products of bulk electrolysis to elucidate the CO2RR pathways in relation to the surface roughness; and (iii) the role of surface roughness and interfacial pH in controlling product selectivity.

Section snippets

Materials and methods

KHCO3 (Sigma-Aldrich, 99.95%), HCOOK (Merck, ≥ 99.0%), Na2HPO4 (Merck, > 98%), NaH2PO4 (Merck, > 98%), Ferrocene methanol (Aldrich, 97%), KCl (Merck, ≥ 99.0%), HCl (SRL, 35%), H2SO4 (Merck, 98%), KOH (Merck, 99.9%), CH3OH (Merck, ≥ 99.8%), CH3COONa (Sigma-Aldrich, ≥ 99%), CH3COOH (Merck, 96%), (NH4)2FeSO4.6H2O (Sigma-Aldrich, 99%), and EDTA (Sigma-Aldrich, ≥98.5%) were used as received.

All the voltammetric experiments were carried out by Autolab potentiostat using 2 mm diameter gold disk as a

Results and discussions

The procedures for preparing gold electrodes of different roughness is described below. A very smooth surface (roughness factor, Rf = 1.4) was obtained by exposing the mechanically polished gold (mp-Au) surface to the Fenton reagent [18,19]. A very rough surface (Rf = 28.15) was produced by chemically reducing the electrochemically oxidized gold surface (nanoporous, np-Au). The mp-Au cycled in 0.1 M H2SO4 solution exhibited low roughness (Rf = 2.68), whereas the mp-Au electrodes cycled in

Conclusions

In this work, we have demonstrated that the roughness of gold substrates affects the selectivity of CO2RR. Roughness of the gold surface seems to be responsible for the extent of interfacial pH change during the CO2RR; the interfacial pH is highly alkaline on nanoporous gold (np-Au) than that of other surfaces used. In addition to CO and formate, methanol is also formed as shown by its presence in the NMR analysis of the electrolysis products. Oxidative voltammetric experiments provided more

Acknowledgements

Authors acknowledge Department of Science and Technology, India and DAAD, Germany for providing financial support under DST- DAAD collaborative project (DST- DAAD- PPP; INT/FRG/DAAD/P-247/2015 & I.D. 57129302) and thank Dr Radhakrishnan for professional help in NMR work. Sreekanth thanks UGC for the research fellowships.

References (43)

  • M. Haruta et al.

    J. Catal.

    (1989)
  • G. Hutchings

    Catal. Today

    (2005)
  • M. Haruta

    Catal. Today

    (1997)
  • A.K. Santra et al.

    Electrochim. Acta

    (2002)
  • T. Izumi et al.

    J. Electroanal. Chem.

    (1991)
  • R. Gisbert et al.

    Electrochim. Acta

    (2011)
  • J. Zeng et al.

    Nano Lett.

    (2010)
  • T. A Baker et al.

    Phys. Chem. Chem. Phys.

    (2011)
  • A. Wittstock et al.

    Phys. Chem. Chem. Phys.

    (2010)
  • P.A. Sermon et al.

    J. Chem. Soc. Faraday. Trans.

    (1979)
  • J.H. Shim et al.

    J. Phys. Chem. C

    (2011)
  • W. Chen et al.

    Angew. Chem. Int. Ed.

    (2009)
  • J. Hernandez et al.

    J. Phys. Chem. C

    (2007)
  • G.L. Beltramo et al.

    ChemPhysChem

    (2005)
  • P. Rodriguez et al.

    Phys. Chem. Chem. Phys.

    (2014)
  • J. Zhang et al.

    J. Phys. Chem. C

    (2007)
  • C. Jeyabharathi et al.

    J. Solid State Electrochem.

    (2014)
  • W. Zhu et al.

    J. Am. Chem. Soc.

    (2013)
  • Y. Chen et al.

    J. Am. Chem. Soc.

    (2012)
  • A.M. Nowicka et al.

    Angew. Chem. Int. Ed.

    (2010)
  • P.E. Karthik et al.

    Chem. Commun. Chem. Commun.

    (2014)
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