Hydrogen is a key feedstock for the petrochemical industry and the generation of fertilizers. To date, hydrogen production has relied mainly on fossil fuels: it is obtained from methane reforming, oxidation of oil, or gasification of coal – each of which has a very large carbon footprint.
An alternative is water electrolysis, wherein water is split into hydrogen and oxygen using electricity. When powered using renewable or low-carbon electricity, this offers a route to hydrogen having a low carbon footprint.
In water electrolysis, hydrogen is generated at a cathode, and oxygen evolution reaction (OER) takes place at the anode. Catalysts are required to speed up these reactions and lower the energy input required to drive them.
Today the OER side of this reaction requires a much larger voltage than what thermodynamics mandate. This overpotential leads to a high penalty to the system’s energy efficiency, increasing the cost of producing the hydrogen.
One of the reasons behind the large voltage required to drive the OER is the multiple steps required to assemble oxygen molecules. These entail the adsorption of different oxy-hydroxide reaction intermediates, electron transfer and dehydrogenation, and subsequent coupling into oxygen.
The thermodynamic and kinetic barriers for each of these steps are different, though each one is determined by the electronic interaction of the reaction intermediates with the catalyst. As such, controlling the electronic properties of the catalyst with sufficient accuracy provides a way to improve the performance of OER.
In this paper, we report a design framework to modulate the properties of OER catalysts.1 Our approach relies on the homogeneous mixture of metal atoms with different electronic properties. We predict which elements lower the energy barriers for OER; and we incorporate them in such a way as to boost catalyst performance.
We turned our attention to first row 3d transition metals such as Ni, Fe, and Co, already used in high-performance alkaline electrolyzers.2 These elements share too similar an electronic nature to offer the needed tunability.3 To overcome this, we added high-valence transition-metal modulators such as W, Mo, Nb, Ta, and Re. Our hypothesis was that this would enable accessing a wider design space for electronic modulation, leading to improved OER.
We began by building a computational model to consider the local electronic structure and OER cycle of different compositions and atomic arrangements. This helped us identify the catalytic sites slowing down the reaction; and equipped us to overcome these barriers by modifying judiciously their electronic properties.
From a materials perspective, it is challenging to incorporate atoms with different electronic structure into materials that have the desired composition and atomic arrangement. We employed a sol-gel technique in which different metal salts are mixed and processed from solution, to a wet gel and final solid. This technique is important to achieve the homogeneous incorporation of all dopants.4
To characterize the properties of the resulting catalysts, we carried out a suite of X- and Gamma-ray spectroscopy at the Canadian Light Source (CLS) and the Shanghai Synchrotron Radiation Facility (SSRF). This allowed us to probe the chemical, structural, and electronic configuration at the atomic level during catalyst operation. This is important, as electrocatalysts change while they are on the job compared to before (the precatalyst) and after (after use and now ex situ, catalysts change (for example, the oxidation state of metals near the surface evolves) in light of the ambient conditions during storage and sample characterization).5,6
Using operando techniques, we monitored OER cycling of catalytic sites and found that the addition of high valence metals does indeed modulate the energetics of catalytically active sites.
We then characterized the performance of the OER catalysts, correlating the observed overpotentials with composition and structure. We found that the best-performing doped samples - NiFeMoW and FeCoMoW - required less energy input to drive OER, and exhibited ~20 times higher mass activity compared to NiFe and FeCo controls, and stability over 120 h.
We implemented the salient catalysts in commercial industrial electrolyser (Peric) operating in alkaline conditions (30% KOH) and mild temperature (85°C).7 The electrolyser, equipped with these catalysts, delivered a current density of 300 mA/cm-2 at ~1.7 V over 12 h, surpassing the performance of commercial Raney Ni catalysts.
The results contribute to the framework for the design of catalysts that, with tunable electronic and chemical properties, exhibit improved performance for the OER.
The topic is an important one not only for water electrolysers and the production of clean hydrogen, but also for CO2 electrolysers – systems that directly upgrade CO2 at the cathodic site. Further work remains to decrease further the overpotential of the OER – a bottleneck in water electrolysis-based technologies— and, very importantly, to improve further catalyst stability at high current densities.8
- Zhang, B. et al. High-valence metals improve oxygen evolution reaction performance by modulating 3d metal oxidation cycle energetics. Nat. Catal. 1–8 (2020). doi:10.1038/s41929-020-00525-6
- Roger, I., Shipman, M. A. & Symes, M. D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nature Reviews Chemistry 1, 1–13 (2017).
- Gerken, J. B., Shaner, S. E., Massé, R. C., Porubsky, N. J. & Stahl, S. S. A survey of diverse earth-abundant oxygen evolution electrocatalysts showing enhanced activity from Ni-Fe oxides containing a third metal. Energy Environ. Sci. 7, 2376–2382 (2014).
- Zhang, B. et al. Homogeneously dispersed, multimetal oxygen-evolving catalysts. Science 352, 333 (2016).
- Seh, Z. W. et al. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 355, (2017).
- Dionigi, F. et al. In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution. Nat. Commun. 11, 1–10 (2020).
- Integrated water electrolysis hydrogen production equipment-Water electrolysis hydrogen production equipment-the 718th research institute of CSIC. Available at: http://www.peric.ac.cn/en/cpzsfb.asp?id=1091. (Accessed: 21st October 2020)
- Spöri, C., Kwan, J. T. H., Bonakdarpour, A., Wilkinson, D. P. & Strasser, P. The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angewandte Chemie - International Edition 56, 5994–6021 (2017).