Article pubs.acs.org/JPCC
Mechano-Electrochemistry and Fuel-Forming MechanoElectrocatalysis on Spring Electrodes Drazenka Svedruzic* and Brian A. Gregg* National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: Each material, in principle, possesses a continuum of electrochemical and electrocatalytic properties that can be reversibly tuned by mechanical stress over its elastic range. As an initial test of this hypothesis we investigate stainless steel extension springs as electrodes. Stretching the springs reversibly doubles the heterogeneous rate constant for electron transfer to a redox species in solution, Ru(NH3)6Cl3, while the charge transfer rate through a surface film of Ni(II/III) oxy-hydroxide increases ∼4fold. Straining the springs near their elastic limit in 1 M NaOH increases the electrcatalytic hydrogen evolution current by ∼50% and the oxygen evolution current by ∼300%. Thus, even the small elastic strain (∼0.1% lattice deformation) that can be applied by stretching a spring leads to significant and reversible increases in the rates of: 1) electron transfer to a redox couple in solution, 2) charge transport through a surface film, and 3) electrocatalysis.
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INTRODUCTION A single material normally has just one set of electronic properties. However, if the material can be physically deformed by mechanical stressaltering its bond lengths and anglesits electronic structure will change accordingly. In principle, each material possesses a continuum of electronic properties that are reversibly tunable over its elastic range. This hypothesis builds on the ancient field of mechanochemistry which is currently enjoying a renaissance. Mechanochemistry includes any means of causing or promoting chemical reactions via mechanical force and its converse, generating mechanical force from chemical energy.1−5 Some of the best known examples are found in living systems: hearing, touch, muscle contraction, cell motility, etc. Enzyme active sites, for example, are subject to both static and dynamic mechanical forces, and these may be essential for the catalytic process in some cases (the entatic state hypothesis).6−13 The electronic properties of semiconductors can be tuned in strained-lattice devices14−18 where a thin semiconductor film is grown epitaxially on a lattice larger than its native lattice. This narrows the bandgap and increases charge carrier mobilities. Qualitative changes, such as the transition from indirect-todirect bandgap in germanium, can also occur.15,16 Strained metal monolayers are also obtained by growth on incommensurate substrates. These biaxially strained films show altered surface energetics and catalytic behavior.19−21 In a similar vein, molecular catalysts are being synthesized with strained bonds to potentially improve their catalytic performance.22−24 It should be possible to strain molecular catalysts and metal films dynamically and reversibly, although this has not yet been accomplished with molecular catalysts and has only recently been approached with metal and metal oxide films.21 This © 2014 American Chemical Society
capability would provide access to the entire spectrum of electronic properties available from each strained material, rather than just the few discrete data points we have now. Our initial approach to this problem is to study an electrochemical system based on stainless steel (SS) springs as electrodes. The effects of stress in SS in the context of corrosion have been studied, particularly via scanning electrochemical microscopy (SECM).25−29 The results were particular to the type of steel and its previous thermal and chemical treatments. Films typically showed a breakdown of the passive layer upon straining SS near its elastic limit. In the experiments reported here the passive layer has been removed by a chemical etch. We explore the reversible changes in the rates of charge transfer and electrocatalytic (multielectron, multiproton) fuel-forming reactions as a function of stress applied to the spring electrode.
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RESULTS AND DISCUSSION Spring Electrodes. We employ commercial springs made from SS 302 (∼70% Fe, 18% Cr, 9% Ni) and activate them with a hot citric acid etch (see Supporting Information, SI). The etch results in reproducible electrochemical behavior typified by overpotentials for the hydrogen and oxygen evolution reactions (HER and OER) that are each decreased by 50−60 mV compared to the unetched spring. Stainless steel and related alloys are some of the better elastomeric metals. The experimental setup for measuring electrochemistry and varying the spring length is shown in Figure 1. The top hook of the spring and its electrical contact to the stretcher are kept above the solution. The bottom hook is coated with vacuum Received: June 24, 2014 Revised: July 17, 2014 Published: July 30, 2014 19246
dx.doi.org/10.1021/jp506279q | J. Phys. Chem. C 2014, 118, 19246−19251
The Journal of Physical Chemistry C
Article
no significant effect on its subsequent mechano-electrocatalytic behavior (Figure S3, SI). All electrochemical data are corrected for solution resistance except where noted; this affects mainly the high-current watersplitting experiments. Although ideally SS can be strained by up to ∼20% and still return to its original shape, most of the strain is inelastic (plastic).34 Inelastic strain energy is irreversibly dissipated via dislocations moving through the lattice (equivalent to atoms shifting to neighboring lattice sites). This process acts as a stress relief mechanism and is not expected to chemically change the surface, and we show below that it results in no measurable changes. Only 1% or less of the strain energy in SS is elastic, and only this results in reversible expansion of the atomic lattice on the surface in the direction of the tensile stress.34 The surface area should not change significantly in this process. More experimental details are given in the SI. Heterogeneous Electron Transfer. Initial experiments probed the cyclic voltammetric response of 20-coil SS-302 springs to a kinetically fast one-electron redox species, Ru(NH3)6Cl3. The electrochemical reaction is Ru III(NH3)6 + e− ↔ Ru II(NH3)6
(1)
Figure 2 shows cyclic voltammograms, CVs, vs scan rate at pH 4 for a relaxed spring and for the same spring strained to ∼83% Figure 1. Photo of electrochemical setup showing the calibrated spring stretcher, reference electrode (behind), and the working spring electrode. The platinum mesh counter electrode that encircles the stretcher has been removed for clarity.
grease to prevent including its electrochemical response. The surface properties of SS adjust to the chemical and applied potential environment, thus specific protocols were employed for the water oxidation and reduction experiments in order to minimize these time-dependent effects. The electrical resistance of the 20-coil springs was R = 3.3 ± 0.1 Ω and did not change upon stretching; for 3-coil springs, R = 0.7 Ω. The maximum length springs could be stretched reversibly was Elim = 36 mm and Elim = 9.5 mm for 20-coil and 3-coil springs, respectively (the hooks account for ∼8 mm of the length). Here we report strain as %Elim and generally stretch the springs
