Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces

TLDR: In this article, the effects of the nanostructure of the copper surface and compare the effect of the fcc(111, fcc (100) and fcc-211) facets of copper on the energetics of the electroreduction of CO 2.

About: This article is published in Surface Science.The article was published on 2011-08-01 and is currently open access. It has received 435 citations till now. The article focuses on the topics: Copper.

Figures

Figure 4: Free energy diagram comparing the production of H2 on Cu fcc(111), fcc(110), and fcc(211) crystal facets at 0 V vs RHE.

Figure 1: The (111), (100), and (211) facets of the copper fcc crystal, as viewed directly and from perspective.

Figure 5: Free energy diagram comparing the production of formic acid on Cu fcc(111), fcc(110), and fcc(211) crystal facets at 0 V vs RHE. The ‘C’ indicates the carboxyl (COOH) intermediate, and the ‘F’ indicates the formate (OCHO) intermediate.

Table 1: Calculated adsorption energies (eV) of key adsorbates on the (111), (100) and (211) crystal facets, shown with their optimized binding geometry. A lower number indicates stronger binding. Adsorption energies are relative to the clean slab and reference atoms for H, C, and O of 1/2 H2, graphene, and (H2O - H2), respectively.

Figure 6: The top bars show the onset voltages of the products of CO2 reduction as predicted by the results of this study. The bottom bars, labeled ’Expl.’, display the experimental voltages measured by Hori et al. [10] at which the current reached 5 mA cm−2.

Figure 2: d-orbital density of states of the Cu (111), (100), and (211) surface.

Frequently asked questions

Q1. What are the contributions in this paper?

In this paper, it was shown that the key intermediates in the electrochemical reduction of CO2 are stabilized by the surfaces in the order ( 211 ) > ( 100 ) > ( 111 ), where the adsorbates on the ( 211 ) facet exhibit the greatest stability. 

Q2. What is the limiting potential of the Cu surface?

Since the adsorption of H to the Cu surface is predicted to be energetically limiting, the limiting potential is predicted to be lowest on the (211) surface, and highest on the (111) surface. 

Q3. What is the adsorption energy of the three surfaces?

When a reaction is thermodynamically limited by an adsorption step, the stronger binding facet will catalyze the reaction more effectively; when a reaction is limited by a desorption step, the weaker binding facet will. 

Q4. What is the limiting potential requirement for formic acid on the (211) surface?

As a result of the energetic stabilization of these intermediates, the limiting potential at which Cu catalyzes the electrochemical reduction of CO2 to CH4 is predicted to be lowest on the (211) surface. 

Q5. What is the limiting potential of the 211 step?

as previously noted [3], the active bridge sites for H adsorption on the (211) step are expected to be blocked by hydroxyl (OH) species at neutral potentials, preventing H2 evolution from being observed until the OH is cleared at more negative potentials. 

Q6. What is the effect of the ocho intermediate on the copper surface?

more carefully controlled experimental work that tests the onsets of the various products of CO2 reduction on copper single crystals and other well-characterized surfaces would greatly aid in their understanding. 

Q7. What is the optimum potential requirement for formic acid on the (211) surface?

The calculations in this study indicate that the key intermediates in the electrochemical reduction of CO2 are stabilized by the surfaces in the order (211) > (100) > (111), where the adsorbates on the (211) facet exhibit the greatest stability. 

Q8. What is the current requirement for formic acid on the (211) surface?

As noted earlier, limited experiments have been conducted that directly test the (211) versus (111) and (100) surfaces, and they have been largely focused on the ratio of C2:C1 compounds formed at fixed current densities. [6–10] 

Q9. What is the adsorption strength of the three surfaces?

As is seen in the table, the adsorbates tend to bind most strongly to the (211) surface, most weakly to the (111) surface, and with an intermediate strength to the (100) surface. 

Q10. What is the limiting potential of the CHE model?

the CHE model allows a prediction to be made of the electrical potential at which each thermodynamic pathway becomes exergonic (downhill in free energy); this “limiting potential” will just be numerically equal to the maximum free energy difference between any two steps. 

Q11. What is the favorable pathway to form HCOOH on Cu?

In this case, the destabilization of the strongly-binding formate (OCHO) intermediate makes that pathway more favorable than the more weakly-binding carboxyl (COOH) intermediate, through which the pathway is predicted to proceed on the (211) facet. 

Q12. Why does oxygen bind more strongly to the (211) surface?

This is due to the fact that oxygen does not bind directly at the step site of the (211) surface, but rather binds more strongly in a three-fold site of the terrace, which is geometrically similar to the (111) plane. 

Q13. What is the effect of the OCHO intermediate on the surface of the 211?

On the (211) surface, the OCHO intermediate was calculated to bind very strongly6Structure manuscript v0.11 – AAP – March 7, 2011to the stepped edge; thus, the COOH intermediate led to a lower overall potential requirement [3]. 

Q14. What is the CHE model for forming CO?

Since both adsorbed CO and adsorbed CHO are more stable on the (211) facet than on the (111) or (100) facet, the direction in which the energetics of this elementary step will change amongst the facets is less obvious.