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Sustainable energy generation calls for a shift away from centralized, high-temperature, energy-intensive processes to decentralized, low-temperature conversions that can be powered by electricity produced from renewable sources. Electrocatalytic conversion of biomass-derived feedstocks would allow carbon recycling of distributed, energy-poor resources in the absence of sinks and sources of high-grade heat. Selective, efficient electrocatalysts that operate at low temperatures are needed for electrocatalytic hydrogenation (ECH) to upgrade the feedstocks. For effective generation of energy-dense chemicals and fuels, two design criteria must be met: (i) a high H:C ratio via ECH to allow for high-quality fuels and blends and (ii) a lower O:C ratio in the target molecules via electrochemical decarboxylation/deoxygenation to improve the stability of fuels and chemicals. The goal of this review is to determine whether the following questions have been sufficiently answered in the open literature, and if not, what additional information is required: What organic functionalities are accessible for electrocatalytic hydrogenation under a set of reaction conditions? How do substitutions and functionalities impact the activity and selectivity of ECH? What material properties cause an electrocatalyst to be active for ECH? Can general trends in ECH be formulated based on the type of electrocatalyst? What are the impacts of reaction conditions (electrolyte concentration, pH, operating potential) and reactor types?

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Electrocatalytic hydrogenation is increasingly studied as an alternative to integrate the use of recycled carbon feedstocks with renewable energy sources. However, the abundant empiric observations available have not been correlated with fundamental properties of substrates and catalysts. In this study, we investigated electrocatalytic hydrogenation of a homologues series of carboxylic acids, ketones, phenolics, and aldehydes on a variety of metals (Pd, Rh, Ru, Cu, Ni, Zn, and Co). We found that the rates of carbonyl reduction in aldehydes correlate with the corresponding binding energies between the aldehydes and the metals according to the Sabatier principle. That is, the highest rates are obtained at intermediate binding energies. The rates of H2 evolution that occur in parallel to hydrogenation also correlate with the H-metal binding energies, following the same volcano-type behavior. Within the boundaries of this model (e.g., compounds reactive at room temperature and without important steric effects over the carbonyl group), the reported correlations help to explain the complex trends derived from the experimental observations, allowing for the correlation of rates with binding energies and the differentiation of mechanistic routes.

Electrocatalytic reduction of benzaldehyde to benzyl alcohol on Pd supported on carbon felt was conducted in the aqueous phase using a continuous flow fixed-bed reactor at room temperature and atmospheric pressure. Methanol, ethanol, or isopropanol was added to the electrolyte to study the impact of alcohol type and concentration on the rates of benzaldehyde electrocatalytic hydrogenation (ECH) and H2 evolution, which is the prevalent side reaction. Whereas the ECH rates and Faradaic efficiency decreased with increasing alcohol concentrations, H2 evolution rates remained constant. The impact of the alcohol on hydrogenation was greater as the length of the alcohol’s hydrocarbon chain increased. Increasing the benzaldehyde concentration allows for high ECH rates and high Faradaic efficiency. The reaction order increased from ∼0.13 to ∼0.66 with half-cell potential increasing from −650 to −1150 mV (vs Ag/AgCl). Kinetic analysis reveals that the changes in reaction order are due to changes in benzaldehyde (and H) surface coverages as a function of half-cell cathodic potential. Thus, the results shown here reveal how the performance of the continuous electrocatalytic operation is affected by the electrolyte composition and half-cell cathodic potential.

Electrocatalytic hydrogenation is a strategy to hydrogenate biogenic compounds under ambient conditions by replacing the thermal and H2 inputs by cathodic potential. This work compares the performances of this approach (in aqueous phase at room temperature) for the conversion of a variety of model oxygenated compounds over a series of metals. The target functionalities were carbonyl groups, aromatic rings, and ether bonds. All of the metals explored (Pt, Rh, Pd, and Cu) are active for the reduction of carbonyl compounds to alcohols. The conversion rate of benzaldehyde increased as a function of the metal as Pt < Rh < Pd (Cu was tested under different conditions). In contrast, only Rh and Pt were active for hydrogenation of aromatic rings (Rh was more active than Pt). In a comparison of the target functionalities, carbonyl groups are more reactive than aromatic rings and ether bonds in phenolic compounds and diaryl ethers on all of the explored metals. This carbonyl reactivity, however, is enhanced by the aromaticity of the molecule. Hence, the reactivity trend of the examined molecules is butyraldehyde < furfural < acetophenone < benzaldehyde. For phenolic compounds, phenol is more reactive than cresol and methoxyphenol. Thus, the presence of substituent groups on the functionality being converted (either carbonyl or aromatic ring) decreases the conversion rate. Ether bonds are cleaved under electrocatalytic conditions, which opens two main pathways for the conversion of aryl ethers: hydrogenation of the aromatic ring and hydrogenolysis of the ether bonds, whereas hydrolysis occurs as a minor pathway. Electrocatalytic hydrogenation competes with the H2 evolution reaction under the conditions of the tests, and therefore, the Faradaic efficiency (the fraction of current utilized in hydrogenation) and hydrogenation rate are correlated. That is, within the potential range explored, increasing hydrogenation rates lead to higher Faradaic efficiencies. The slope of this correlation, however, depends on the potential and on the functionality being hydrogenated.