Improved understanding of Ni based catalysts could enable large scale energy storage using hydrogen fuel cells

16-11-2015

In-situ measurements of Ni(OH)2/NiOOH materials in electrochemical cells lead to an in-depth understanding of the oxygen evolution reaction

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Hydrogen fuel cells are one of the most promising technologies to store energy created using solar cells or wind turbines. The philosophy is that energy produced via an unpredictable intermittent supply is used to electrolyse water, creating a supply of hydrogen and oxygen fuel that can be stored and used at will (H2O → H2 + ½ O2). However, to make the electrolytic process energy efficient requires catalysts. Considerable research effort is being made to develop cost effective and highly active catalysts that can be scaled up to enable large scale energy storage and replace the first generation expensive and scarce ruthenium and iridium oxide materials.

The most promising oxygen evolution catalysts are based on nickel alloys. Nickel based catalysts oxidize water at alkaline pH with an activity comparable to the benchmark RuO2 and IrO2 materials. Ni(OH)2/NiOOH material has been used in direct photoelectrochemical water splitting cells, but in the past few years, a new form of this catalyst has been developed that consists of a nickel-borate (NiBi) material. This is believed to be composed of nanosized clusters/domains of NiOOH, resulting in a much larger fraction of surface-exposed, catalytically active, nickel centers, relative to NiOOH. However, the activity of NiBi has recently been shown to be mainly due to the Fe impurities and not the Ni sites, a result that is also consistent with previous findings of the beneficial role that Fe plays in the catalytic activity of pure NiOOH.

To understand the mechanism of the catalytic activity in these Ni(Fe)OOH /Ni(Fe)-Bi materials (and hence optimize them) Bartek TrzeĊ›niewski, David Vermaas and Wilson A Smith from Delft University of Technology, together with Oscar Diaz-Morale and Marc Koper from Leiden University and Alessandro Longo and Wim Bras from DUBBLE, have studied the structural and electronic changes of Ni(Fe)OOH and Ni(Fe)-Bi catalysts during the electrochemical water oxidation reaction using a combination of UV-vis, Raman and XANES spectroscopy. The in-situ XANES performed on BM26A with Alessandro Longo revealed the transformation of Ni2+ into active (transient) Ni3+ and Ni4+ species. XANES spectra of both anodized Ni(Fe)OOH and Ni(Fe)-Bi collected at various applied potentials look very similar which supports the idea that the nickel-borate catalyst in its active state is a form of γ-NiOOH. The increase in the oxidation state of nickel in NiOOH induces changes in its local chemical environment and a crystallographic phase change from trigonal/rhombohedral to monoclinic occurs.

The γ-NiOOH structure was modeled with periodically alternating repetitions of edge-shared octahedra arranged into higher ordered layers

intercalated Ni.png

Figure 6. Sketch of the swollen γ NiOOH phase. For sake of clarity, the direct Ni-O and Ni-Ni distance are not showed but only the two three-body configurations are evidenced in black and in blue lines. reference: http://pubs.acs.org/doi/pdfplus/10.1021/jacs.5b06814

 

This model agrees with previous work by Risch et al. that proposed that the structure of Ni-Bi may consist of fragments of layered γ-NiOOH separated by water and borate molecules

 

The main conclusion of the work is that they find virtually no difference between Ni(Fe)-Bi and Ni(Fe)OOH catalyst materials. Differences in catalytic activity are ascribed to the formation of negatively charged NiOO- “active oxygen” sites via a deprotonation process that is strongly dependent on pH which form precursors for the oxygen evolution reaction. They also show that this effect is independent of that arising from the presence of Fe impurities; the deprotonation process is crucial at moderate pHs, as even Fe-doped Ni-based catalysts show negligible activity in neutral pH. The improved understanding of this class of catalysts should lead to better large-scale electrolysis cells for water.

 

References

Bartek J TrzeĊ›niewski, Oscar Diaz-Morales, David A. Vermaas, Alessandro Longo, Wim Bras, Marc T.M. Koper, and Wilson A Smith; “In-situ observation of active oxygen species in Fe-containing Ni-based oxygen evolution catalysts: the effect of pH on electrochemical activity”; Journal of the American Chemical Society Just Accepted Manuscript (2015) DOI: 10.1021/jacs.5b06814

Risch, M.; Klingan, K.; Heidkamp, J.; Ehrenberg, D.; Chernev, P.; Zaharieva, I.; Dau, H. Chemical communications 2011, 47, 11912.