(180.8%) during the six months ended June 30, 2021. We expect higher hydrogen molecule costs to continue at least through 2022.
Expenses
Research and development expense. Research and development (“R&D”) expense includes: materials to build development and prototype units, cash and non-cash stock-based compensation and benefits for the engineering and related staff, expenses for contract engineers, fees paid to consultants for services provided, materials and supplies consumed, facility related costs such as computer and network services, and other general overhead costs associated with our research and development activities.
Research and development expense for the three months ended June 30, 2022 increased $12.3 million, or 109.5%, to $23.6 million, from $11.2 million for the three months ended June 30, 2021. The overall growth in R&D investment is commensurate with the Company’s future expansion into new markets, new product lines, acquisitions and varied vertical integrations.
Research and development expense for the six months ended June 30, 2022 increased $23.0 million, or 109.7%, to $44.0 million, from $21.0 million for the six months ended June 30, 2021. The overall growth in R&D investment is commensurate with the Company’s future expansion into new markets, new product lines, acquisitions and varied vertical integrations.
Selling, general and administrative expenses. Selling, general and administrative expenses includes cash and non-cash stock-based compensation, benefits, amortization of intangible assets and related costs in support of our general corporate functions, including general management, finance and accounting, human resources, selling and marketing, information technology and legal services.
Selling, general and administrative expenses for the three months ended June 30, 2022, increased $57.3 million, or 148.2%, to $96.0 million from $38.7 million for the three months ended June 30, 2021. This increase was primarily related to increased headcount, which resulted in increased salaries and stock-based compensation, as well as branding expenses.
Selling, general and administrative expenses for the six months ended June 30, 2022, increased $112.6 million, or 175.3%, to $176.8 million from $64.2 million for the six months ended June 30, 2021. This increase was primarily related to increased headcount, which resulted in increased salaries and stock-based compensation, as well as branding expenses.
Contingent consideration. The fair value of the contingent consideration related to the Giner ELX, Inc., United Hydrogen Group Inc, Frames, Applied Cryo and Joule acquisitions was remeasured as of June 30, 2022, which resulted in a $5.1 million benefit for the three months ended June 30, 2022 and a $2.6 million benefit for the six months ended June 30, 2022, both of which are reflected in the unaudited interim condensed consolidated statement of operations for the three and six months ended June 30, 2022, respectively.
Interest income. Interest income primarily consists of income generated by our investment holdings, restricted cash escrow accounts, and money market accounts. Interest income for the three and six months ended June 30, 2022 increased $2.4 million and $4.4 million, respectively, as compared to the three and six months ended June 30, 2021. The increase is primarily related to the increase in the investment portfolio during 2022.
Interest expense. Interest expense consists of interest expense related to our long-term debt, convertible senior notes, obligations under finance leases and our finance obligations. Interest expense for the three months ended June 30, 2022 decreased $0.5 million compared to the three months ended June 30, 2021, primarily related to a decrease in long-term debt and an increase in capitalized interest, offset by an increase in finance obligations. Interest expense for the six months ended June 30, 2022 decreased $4.2 million compared to the six months ended June 30, 2021, primarily related to a decrease in long-term debt and an increase in capitalized interest, offset by an increase in finance obligations.