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The nature of enhanced activity of layered NiFe hydroxide catalyst in seawater electrolysis
Seawater electrolysis provides a promising path towards green hydrogen generation. Observing NiFe hydroxide catalyst in operation using high- energy X-ray scattering revealed the role of NaCl in the electrocatalytic process, leading to a greater understanding of how the catalyst functions at the molecular level.
The storage of renewable energy is one of the key challenges for a sustainable, carbon-free economy. In this context, NiFe layered double hydroxides (LDHs) are among the most active electrocatalysts for the oxygen evolution reaction (OER) in seawater, the cornerstone reaction for green hydrogen production. The major goal of the concept of seawater electrolysis is the chemically (faradaically) selective splitting into hydrogen and oxygen suppressing undesired competing electrocatalytic processes . This promising application calls for a fundamental understanding of the catalyst/electrolyte interaction, as a number of undesired processes may be triggered inside a water electrolyser, including the formation of biofilms or the
catalytic oxidation of anions. For example, ion exchange effects from the electrolyte on the catalyst structure, and reactivity during cyclic transitions of different NiFe LDH phases associated with repeated (de-)intercalation, have not been studied previously, in particular not under operando catalytic reaction conditions, but they are essential for demystifying the material s enhanced activity. This study fills that knowledge gap and investigates the structural transformations and activity of NiFe LDH in response to varying NaCl concentrations and electrolyte pH at the molecular level.
An operando wide-angle X-ray scattering (WAXS) study of the molecular mechanisms was carried out at beamline ID31. Concurrent electrochemical measurements supported previous observations in a full-cell anion exchange membrane seawater electrolyser : the presence of NaCl and increased pH has a beneficial effect on the catalytic activity of NiFe-LDH (Figure 124). Any enhancement in the current density due to concomitant Cl oxidation reactions and/or the substitution of K+ with Na+ was ruled out based on previous selectivity measurements.
In the non-catalytic resting state at 1 VRHE, the WAXS patterns matched a non-active α-NiFe LDH (Figure 125, open squares), while at 1.6 VRHE, a residual α - and the catalytically active γ-phase emerged (Figure 125, red circles and blue triangles, respectively). The interlayer distance of the γ-phase for the unsupported NiFe LDH at 1.6 VRHE displayed an incomplete contraction. This is related to a limited electrochemical accessibility of the intercalation sites and highlights the importance of the morphology and support to fully unfold their catalytic performance. Furthermore, higher pH values lead to slightly smaller interlayer distances, which translates to a higher percentage of the active γ-NiFe LDH in the composition of the catalyst layer and higher OER activity. No significant positive effect of NaCl on the percentage of γ-NiFe LDH was evident; however, additional scan rate investigations showed a strong correlation of the electrochemical accessibility of NiFe LDH with its history, scan rate and NaCl addition. In particular, the faster and more effective break-in process induced by NaCl addition was proposed as the origin of the enhanced activity at low pH, despite the lower γ-phase percentage. These results strengthen the conclusion that ionic electrolyte additives such as NaCl did not significantly affect the detailed molecular structure of NiFe LDH.
Interestingly, an analysis of the detailed NiFe-LDH phase composition percentages revealed that the set of electrolytes in which the γ-phase exhibited maximum microstrain matched exactly those with the highest γ-phase abundance. This points to an important, not yet understood structural correlation between electrolyte anions, defects, microstrain and the extent of transformation into the
Fig. 124: Cyclic voltammogram in the GID cell of supported NiFe LDH (a) in 0.1 M KOH with and without 0.5 M NaCl and
(b) in 0.5 M NaCl with 0.1 M KOH, 0.5 M KOH, and 1.0 M KOH. Reprinted with permission from S. Dresp et al., ACS Catal. 11, 12, 6800-6809 (2021). Copyright 2021 American Chemical Society.