Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

Abstract

We quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using ac oncurrent non-faradaic probe reaction. In the absence of electrocatalysis,n anostructured Pt/C surfaces mediate the reaction of H 2 with cis-2-butene-1,4-diolt of orm am ixture of 1,4-butanediol and n-butanol with selectivity that is linearly dependent on the bulk solution pH value.Weshow that kinetic branching occurs from acommon surface-bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface.W eu sed the pH-dependent selectivity of this reaction to trackchanges in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (> 3units) relative to the bulk value even at modest current densities in well-buffered electrolytes.T his study highlights the key role of interfacial pH variation in modulating inner-sphere electrocatalysis.The efficient interconversion of electrical and chemical energy requires functional interfaces capable of catalyzing complex multi-proton-coupled electron transfer (PCET) reactions of diverse small-molecule substrates. [1][2][3][4][5] Consequently,i nterfacial electrocatalysis is strongly dependent on the proton activity at the electrode surface.I ndeed, there is ag rowing appreciation that the interfacial pH and proton donor environment play ac entral role in determining the kinetics,t hermodynamics,a nd mechanism of elementary PCET steps, [3,4] thereby defining the selectivity and efficiency of key interfacial reactions ranging from CO 2 and O 2 reduction to the oxidation of small-molecule fuels,s uch as MeOH and HCO 2 H. [6][7][8][9][10][11][12] More broadly,t he interfacial pH value has been shown to impact aw ide variety of electrochemical processes,i ncluding metal finishing,c orrosion chemistry,e lectrodeposition, and electro-organic synthesis. [13,14] Clearly,k nowledge of the pH value at the electrode surface and its variation under electrochemical polarization is an essential prerequisite for attaining molecular-level understanding of interfacial reactivity and catalysis.Electrochemical PCET catalysis invariably generates an influx or efflux of protons and thus gives rise to apHgradient, which can perturb the thermodynamics and kinetics of interfacial catalysis.D epending on the reaction conditions and electrode morphology,t his pH gradient can extend 1-10 mmf rom the electrode surface into the reaction diffusion layer. [15,16] Va riations in the pH value in the reaction diffusion layer have been quantified using av ariety of spectroscopic and electrometric methods. [13,14,[17][18][19][20] However, heterogeneous electrocatalytic reactions proceed through inner-sphere mechanisms that require the binding of small-molecule substrates to the electrode surface.T hus,t he pH value within molecular length scales of the surface,r ather than the value remote from the electrode in the reaction diffusion layer, is cr...

Tracking Electrical Fields at the Pt/H2O Interface during Hydrogen Catalysis

“…Only the selectivity for B relative to A changes as the pH or electrolyte strength is altered. In addition, our previous mechanistic analysis 31 confirmed that the Pt surface does not catalyze the conversion of A to B. Taken together, these results indicate that the reaction proceeds through the pHindependent rate-limiting formation of a common surfacebound intermediate, I*, followed by pH-dependent kinetic branching to yield A or B (Figure 4a).…”

“…Importantly, increasing the electrolyte strength to a Na + concentration of 0.64 (I = ∼0.8 M) or 0.78 M (I = ∼1.0 M) with added NaClO 4 supporting electrolyte (Figure 2a, blue and purple) leads to no change in the pH scaling in product selectivity. 31 Together these data indicate that (a) the selectivity of the probe reaction is highly sensitive to the pH of the electrolyte and that (b) this pH dependence is insensitive to the ionic strength of the electrolyte beyond 0.4 M ionic strength.…”

In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO2 Electroreduction

“…26 When the reactions are carried out at a high current density or in electrolytes that have poor buffer action and/or mass transport, the pH near the cathode surface is wellknown to increase compared to the bulk value. 27,28 Although the impact of local conditions on the selectivity of metal electrodes is well recognized, simulated, and reproduced, 18,26,27,29,30 it is still a common practice to test high surface area electrodes in very low buffer capacity solutions. While the surface structure of a catalyst is very important to determine its intrinsic selectivity and activity, the distinction between the effect of surface structure and mass transport effects on the electrocatalytic activity is not explicitly clear and needs to be urgently clarified in order to improve fundamental understanding of reaction mechanisms.…”

Graphite-Conjugated Acids Reveal a Molecular Framework for Proton-Coupled Electron Transfer at Electrode Surfaces

“…Proton-coupled electron-transfer (PCET) reactions are central to energy conversion processes across chemical and biological systems. 1−25 These same reactions take place at the surfaces of materials, 26−28 playing a key role in corrosion, 29 sensing, 30,31 and catalysis. 32−37 Consequently, interfacial PCET reactions underlie a wide array of energy conversion technologies, including flow batteries, 38,39 supercapacitors, 40,41 fuel cells, 42−45 and solar fuels devices.…”