Making electrocatalytic materials from molecular catalysts

The implementation of earth-abundant metal catalysts in hydrogen fuel cells is necessary in developing sustainable energy schemes. Recently in Chem Catalysis, Chenevier and co-workers investigate the effect of ionomers on the assembly of catalytic layers based on molecular catalysts. They highlight the importance of understanding the layer structuration at the molecular scale to maximize catalytic efficiency at the device-scale. The implementation of earth-abundant metal catalysts in hydrogen fuel cells is necessary in developing sustainable energy schemes. Recently in Chem Catalysis, Chenevier and co-workers investigate the effect of ionomers on the assembly of catalytic layers based on molecular catalysts. They highlight the importance of understanding the layer structuration at the molecular scale to maximize catalytic efficiency at the device-scale. The “terawatt challenge” for global energy consumption necessitates new catalysts for sustainable energy conversions. Besides meeting the requirements of stability and efficiency, the catalysts should be scalable, i.e., based on cost-effective and readily accessible metals.1Bullock R.M. Chen J.G. Gagliardi L. Chirik P.J. Farha O.K. Hendon C.H. Jones C.W. Keith J.A. Klosin J. Minteer S.D. et al.Using nature’s blueprint to expand catalysis with Earth-abundant metals.Science. 2020; 369: eabc3183Crossref PubMed Scopus (76) Google Scholar Although precious-metal catalysts exhibit excellent catalytic activity and stability, their reserves and costs limit their use in large-scale applications in fuel cells and electrolyzers. In nature, most of the redox transformations converting energy and sustaining life are catalyzed by enzymes based on earth-abundant elements. These enzymes evolved toward unique electronic structure, thermochemistry, and kinetics; their exceptional catalytic properties have inspired the design of numerous synthetic catalysts based on earth-abundant metals. The high crustal abundance of these metals would reduce cost and environmental footprint for developing sustainable energy systems. Recently, remarkable progress has been achieved in bio-inspired synthetic catalysts, which now rival and even exceed their precious metal catalysts’ counterparts in selectivity and activity, while also becoming competitive with them in stability.1Bullock R.M. Chen J.G. Gagliardi L. Chirik P.J. Farha O.K. Hendon C.H. Jones C.W. Keith J.A. Klosin J. Minteer S.D. et al.Using nature’s blueprint to expand catalysis with Earth-abundant metals.Science. 2020; 369: eabc3183Crossref PubMed Scopus (76) Google Scholar,2Artero V. Bioinspired catalytic materials for energy-relevant conversions.Nat. Energy. 2017; 2: 17131Crossref Scopus (48) Google Scholar However, this favorable comparison only holds for performance tests carried out in ideal conditions, often in solution or on flat, small electrodes. On the device scale, the electrocatalytic performance of synthetic molecular catalysts is typically lower by orders of magnitude when compared with that of state-of-the-art precious-metals-based fuel cells. The cause for the discrepancy between intrinsic properties of molecular catalysts and their device scale performance is that research endeavors for catalyst integration have almost entirely focused on precious metals whereas the specific requirements for molecular catalysts have received only little attention. Generally, electron, proton, and H2 transport need to be considered when designing the catalytic layer to maximize efficiency (Figures 1A and 1B ). Along with controlling the environment of the catalyst to promote high intrinsic activity, choosing the proper support materials with excellent conductivity is needed to improve electron transfer.3Shi Y. Lyu Z. Zhao M. Chen R. Nguyen Q.N. Xia Y. Noble-Metal Nanocrystals with Controlled Shapes for Catalytic and Electrocatalytic Applications.Chem. Rev. 2021; 121: 649-735Crossref PubMed Scopus (111) Google Scholar Designing a porous gas diffusion layer with high gas permeability is also needed for maximizing H2 transport. In the field of hydrogen fuel cells based on Pt, porous carbon materials have been developed and widely used as conductive support for electrically wiring the catalysts. Precious metal catalysts supported on carbon black were additionally combined with Nafion as an ionomer to accelerate proton transport.4Hou J. Yang M. Ke C. Wei G. Priest C. Qiao Z. Wu G. Zhang J. Platinum-group-metal catalysts for proton exchange membrane fuel cells: From catalyst design to electrode structure optimization.EnergyChem. 2020; 2: 100023Crossref Scopus (79) Google Scholar Intense efforts in the optimization over the past decades established Pt-based catalytic layers showing exceptional catalytic properties for H2 oxidation (up to around 2 A cm−2 at minimal overpotential). Recently in Chem Catalysis, Artero, Chenevier, and co-workers propose the first systematic study for the integration of bio-inspired molecular catalysts based on earth-abundant elements in a catalytic layer for hydrogen fuel cells. Nickel(II) bisdiphosphine complexes (the so-called DuBois catalysts) (Figure 1C) for H2 oxidation were mixed with Nafion and multi-walled carbon nanotubes and drop cast on a gas diffusion layer. The authors then used a combination of electrochemical, scanning-electron microscopy, and small-angle neutron scattering methods, to reveal how the formulation of the layer embedding the catalysts influences its structuration and thus its performance.5Coutard N. Reuillard B. Huan T.N. Valentino F. Jane R.T. Gentil S. Andreiadis E.S. Le Goff A. Asset T. Maillard F. et al.Impact of ionomer structuration on the performance of bio-inspired noble-metal-free fuel cell anodes.Chem Catalysis. 2021; https://doi.org/10.1016/j.checat.2021.01.001Abstract Full Text Full Text PDF Google Scholar They specifically studied layers assembled from three distinct DuBois catalysts that differ in their linking unit, but are identical in their catalytic center (Figures 1B and 1D). First, a previously reported pyrene functionalization of the DuBois catalyst was used for tethering to bare carbon nanotubes via π-stacking. Second, a DuBois catalyst functionalized with activated esters was used for covalent amide linkage to amine-modified carbon nanotubes. Third, a positively charged arginine-modified DuBois catalyst was immobilized via electrostatic interactions on carboxylate functionalized carbon nanotubes. Surprisingly, these three systems, despite differing only in their attachment chemistry, greatly differ in their catalytic performance when formulated into a catalytic layer:(1)π-stacking on bare carbon nanotubes. Excessive interactions between the Nafion ionomer and the bare carbon nanotubes induced the displacement and detachment of the DuBois catalysts immobilized via π-stacking. As a result, the electron transfer between the catalyst and the support was cut off, preventing catalysis of the H2 oxidation reaction. These new insights explain the low performance of previously reported fuel cells based on pyrene-DuBois catalysts.(2)Covalent attachment on amine-modified carbon nanotubes. The amine-modified carbon nanotubes were intended for robust covalent tethering of the catalysts. However, the negatively charged Nafion ionomer densely wraps via electrostatic interaction around the positively charged carbon nanotubes modified with the DuBois catalyst. The coverage of the catalysts by the Nafion film severely hinders the gas diffusion to the active center and decreases the overall performance of the catalytic layer.(3)Electrostatic adsorption on negatively charged carbon nanotubes. In the case of carboxylate modified carbon nanotubes, the negatively charged Nafion ionomer imposes marginal influence on the anchoring of the catalysts. The efficiency of the electron transfer remains unaffected. Meanwhile, the ionomer forms a relatively independent water-rich matrix network that facilitates proton transport. The resulting porous structure further benefits gas and electrolyte diffusion. Altogether, the use of this catalytic layer composite in a fuel cell led to an open circuit voltage of 820 ± 5 mV, a maximum current density of 28 mA cm−2 and an impressive maximum power output of 325 ± 2 μW cm−2 at 415 mV. This performance is an enhancement by a factor of 15 in comparison to the state-of-the-art bio-inspired Ni-based catalysts integrated into an operational proton-exchange membrane fuel cell. This breakthrough exposes how features that are apparently unrelated to catalysis, such as the linking unit, have tremendous effect on the structuration of the catalytic layer. This means that the catalytic layer might require re-engineering for each type of molecular catalyst. Although the optimization of the layer formulation seems a challenging task, progress in the general understanding of its design principle, as pioneered in the paper by Chenevier and co-workers, will guide and greatly simplify future engineering for targeted reactions and for specific features of a given catalyst. This work will be of great interest for those working on the implementation of other molecular catalysts in electrocatalysis for which material-based catalysis holds the current benchmark in device performances. In particular, molecular catalysts made impressive progress for CO2 reduction6Boutin E. Merakeb L. Ma B. Boudy B. Wang M. Bonin J. et al.Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes.Chem. Soc. Rev. 2020; 49: 5772-5809Crossref Google Scholar and for N2 reduction7Shen H. Choi C. Masa J. Li X. Qiu J. Jung Y. Sun Z. Electrochemical Ammonia Synthesis: Mechanistic Understanding and Catalyst Design.Chem. 2021; https://doi.org/10.1016/j.chempr.2021.01.009Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar whereby the selectivity will need to be maintained or even improved upon device integration. This study is also instructive for wiring other highly active but potentially fragile catalysts such as enzymes. For instance, hydrogenase shows higher activity per active site than platinum even when immobilized on carbon materials suitable for high performance fuel cells.8Mazurenko I. Monsalve K. Infossi P. Giudici-Orticoni M.-T. Topin F. Mano N. Lojou E. Impact of substrate diffusion and enzyme distribution in 3D-porous electrodes: a combined electrochemical and modelling study of a thermostable H2/O2 enzymatic fuel cell.Energy Environ. Sci. 2017; 10: 1966-1982Crossref Google Scholar Nevertheless, additional considerations are necessary for preventing the denaturation of the enzyme during immobilization on the support and for achieving long-term stability in the aggressive environments of an operating fuel cell. Previously reported protection strategies using a redox matrix can effectively stabilize the hydrogenase and significantly increase its operational life-time (up to weeks).9Li H. Münchberg U. Oughli A.A. Buesen D. Lubitz W. Freier E. Plumeré N. Suppressing hydrogen peroxide generation to achieve oxygen-insensitivity of a [NiFe] hydrogenase in redox active films.Nat. Commun. 2020; 11: 920Crossref PubMed Scopus (15) Google Scholar,10Li H. Buesen D. Dementin S. Léger C. Fourmond V. Plumeré N. Complete Protection of O2-Sensitive Catalysts in Thin Films.J. Am. Chem. Soc. 2019; 141: 16734-16742Crossref PubMed Scopus (26) Google Scholar However, mass transport and electron transfer limitations emerge as trade-offs when increasing film thickness for protection considerations.10Li H. Buesen D. Dementin S. Léger C. Fourmond V. Plumeré N. Complete Protection of O2-Sensitive Catalysts in Thin Films.J. Am. Chem. Soc. 2019; 141: 16734-16742Crossref PubMed Scopus (26) Google Scholar Satisfying the catalyst protection criterium while achieving efficient transport and wiring is the next challenge in engineering the catalytic layer to enable practical implementation of a variety of bioelectrochemical reactions for energy conversion and electrosynthesis. In conclusion, the results in the Chem Catalysis paper present a step change in understanding the requirement for the catalytic layer and the potential for its optimization toward order of magnitude enhancement in performance. In particular, requirements for catalyst tethering and catalytic layer assembly might be mutually exclusive as illustrated by the displacement of the pyrene-bound catalyst from the carbon support by Nafion. Ultimately, the design principle of the catalysts might need to consider the requirement for the catalytic layer to avoid extensive research investment eventually leading to a dead end at the catalyst integration stage. The Chem Catalysis paper also exposes that catalytic layer technology developed for Pt catalyst is not necessarily compatible with the emerging new classes of sustainable catalysts. Future development of ionomers, electrode materials, and membranes specifically designed for molecular and biological catalytic systems will become an important field of research. Further efforts toward optimized catalyst integration, as opened in the Chem Catalysis paper, will be essential because industrial interest will not be reached before the device-scale performance compare or outperform existing technology. Unless niche markets rapidly appear to motivate the engineering of the catalytic layer, fundamental research will be a much needed driver to better understand the requirements for integrating sustainable catalysts and enable their global use in fuel cells and electrolyzers. Impact of ionomer structuration on the performance of bio-inspired noble-metal-free fuel cell anodesCoutard et al.Chem CatalysisJanuary 28, 2021In BriefMolecular-engineered materials based on bio-inspired catalysts hold promises for the development of noble-metal-free PEMFCs, but attention should be paid to surface functionalization because it directs the structuration of ionomer and affects mass transport and therefore the performance of the catalytic layers. Full-Text PDF